切削刀具技术：工业手册 cutting tool technology industrial handbook

切削刀具技术：工业手册 cutting tool technology industrial handbook Cutting Tool Technology Previous books for Springer Verlag by the author: Advanced Machining: (1989) CNC Machining Technology series: Book 1: Design, Development and CIM strategies Book 2: Cutting, Fluids and Workholding Technologies Book 3: Part Programming Techniques (1993) CNC Machining Technology: Library Edition (1993) Industrial Metrology: Surfaces and Roundness (2002) Graham T. Smith Cutting Tool Technology Industrial Handbook 123 Graham T. Smith, MPhil (Brunel), PhD (Birmingham), CEng, FIMechE, FIEE Formerly Professor of Industrial Engineering Southampton Solent University Southampton U. K. ISBN 978-1-84800-204-3 e-ISBN 978-1-84800-205-0 DOI 10.1007/978-1-84800-205-0 British Library Cataloguing in Publication Data Smith, Graham T., 1947– Cutting tool technology: industrial handbook 1. Metal-cutting 2. Metal-cutting tools I. Title 671.3'5 ISBN-13: 9781848002043 Library of Congress Control Number: 2008930567 ? Springer-Verlag London Limited 2008 Apart from any fair dealing for the purposes of research or private study, or criticism or review, as permitted under the Copyright, Designs and Patents Act 1988, this publication may only be reproduced, stored or transmitted, in any form or by any means, with the prior permission in writing of the publishers, or in the case of reprographic reproduction in accordance with the terms of licences issued by the Copyright Licensing Agency. Enquiries concerning reproduc- tion outside those terms should be sent to the publishers. absence of a speci,c statement, that such names are exempt from the relevant laws and regula- tions and therefore free for general use. information contained in this book and cannot accept any legal responsibility or liability for any errors or omissions that may be made. Cover illustration: eStudio Calamar S.L., Girona, Spain Printed on acid-free paper 9 8 7 6 5 4 3 2 1 springer.com Preface Just over twenty years ago I began writing a book, the Multi-functional tooling. Here, the chip control de- forerunner to this present volume for Springer Verlag, velopment is facilitated by both chip-narrowing and entitled: Advanced Machining – -vectoring, being achieved by computer-generated in- ting Technology . sert design, to position raised protrusions–‘embossed the topics discussed here, but in a more general and dimples’, on the top face. Further, some cutting insert less informative manner. Since this previous volume toolholders are designed for controlled elastic compli- was published, many of the tooling-related topics are ance – giving the necessary clearance as the tool is vec- now more popular, or have recently been developed. tored along and around the part’s pro,le, allowing a Typical of these latter topics, are both High-speed range of plunge-grooving and forming operations to and Hard-part machining that have now come to the be simultaneously undertaken by just this one tool. fore. While Micro-machining and Arti,cial Intelli- Coating technology advances have enabled signi,cant gence (AI) coupled to neural network tool condition progress to be made in both Hard-part machining and monitoring have become important, the latter from for that of either abrasive and work-hardened compo- a research perspective. nents. Some coating techniques today approach the topics, plus many others have been included herein, hardness of natural diamond, particularly the aptly- but only in a relatively concise manner. It would have named ‘diamond-like coatings’ (DLC). Recently, one been quite possible to write a book of this length just major cutting tool company has commercially-intro- concerned with say, drilling techniques and associated duced an ‘atomically-modi,ed coating’, such is the tooling technologies alone. level of tool coating sophistication of late. With the concerns raised on the health hazards to Potential problems created by utilising faster cut- operational personnel exposed to cutting ,uid mists ting data o in the atmosphere, the permissible exposure levels ant in cutting technology applications, has had an in- (PEL’s) have been signi,cantly reduced recently. Fur- ,uence on the resulting machined surface integrity of ther, with the advent of Near-dry and Dry-machining the component. strategies, they have played a important role of late, guised, or not even recognised as a problem, until the particularly as their disposal and attendant costs have part catastrophically fails in-service – as a result of the become of real consequence. Tool management issues instability produced by the so-called ‘white-layering previously discussed in the ‘Advanced Machining’ e,ect’. While another somewhat unusual factor that book have hardly changed, because when I wrote this has become of some concern, is in either handling, or chapter over two decades ago, most of today’s tooling measuring miniscule components produced by Micro- issues by then had been addressed. However, the tool- machining techniques. O presetting machines and associated so production of such diminutive machined parts could far more advanced and sophisticated than was the case easily be ,tted into a small shoebox! then, but the well-organised and run tool preparation All of these previously mentioned tooling-related ‘rules’ are still applicable today. challenges and many others have to a certain extent, One area of cutting tool development that has now become a reality. While other technical and ma- seen signi,cant design novelty, is in the application of chining factors are emerging that must be techni- cally-addressed, so that cutting tool activities continue trial specialist journals. I make no apology for liberally to expand. It is a well acknowledged fact that if one quoting many of these industrial and research sources was to list virtually all of our modern-day: domestic; within the text. However, I have attempted – wherever medical; industrial; automotive; aerospace, etc; com- possible – to acknowledged their contributions when ponents and assemblies, they would to some extent applicable, in either the references, or in the associated rely on machining operations at a certain stage in their diagrammatical and pictorial ,gures herein. Further, it subsequent manufacturing process. is hoped that the ‘machining practitioner’ can obtain ing manufactured components clearly show that there additional information and some solutions and expla- is a substantive machining requirement, which will nations from the relevant appendices, where amongst continue to grow and thus be of prime importance for other topics, are listed a range of ‘trouble-shooting the foreseeable future. guides’. ‘Cutting Tool Technology – Indus- Finally, it is hoped that this latest book: ‘Cutting trial Handbook’, has been written in a somewhat prag- Tool Technology – Industrial Handbook’ will o,er the matic manner and certain topics such as ‘Machining ‘machining practioner’ the same degree of support Mechanics’ have only been basically addressed, as they as the previous book (i.e. Advanced Machining – are well developed elsewhere, as indicated by the ref- Handbook of Cutting Technology) achieved, from the erenced material at the end of each chapter. Any book signi,cant feed-back obtained from practitioners and that attempts to cover practical subject matter such as readers who have contacted me over the past decades. that of cutting technology, must of necessity, heavily rely on information obtained from either one’s own Graham T. Smith machining and research experiences, or from indus- Fortuna, Murcia, Spain Acknowledgements First and foremost, I would like to express my sincere Dorries Scharmann; DMG (UK) Ltd; Giddings and thanks to my wife Brenda for her support and for the Lewis; Starrag Machine Tool Co; and E. Zoller GmbH time I have taken, whilst writing this book: Cutting and Co KG. While other tooling-based and associated Tool Technology – Industrial Handbook. I could not companies have also provided considerable informa- have achieved such an in-depth treatment and rea- tion, including: Renishaw plc; Kistler Instrumente AG; sonably comprehensive account of the subject matter Taylor Hobson plc; Mahr/Feinpruf; Cimcool; Kuwait without her unstinting co-operation and help. Petroleum International Lubricants; Edgar Vaughan; A book that relies heavily on current industrial Pratt Burnerd International; Lion Precision; Westwind practices could not have been produced without the Air Bearings Ltd; unconditional support from speci,c tooling manufac- Handling; Tooling University. turers and the machine tool industries. I would like to I have listed the main companies above, rather than particularly single-out one major cutting tool company, attempting to name individuals within each company, to genuinely thank everyone at Sandvik Coromant who otherwise the list would be simply vast. However, have provided me with both relevant and signi,cant: I would like to express my gratitude to each one of information; photographic; and diagrammatic support them, personally. I would also like to acknowledge the – the book would have been less relevant without their breadth and depth of information obtained from in- indefatigable co-operative help and discussion. Like- dustrially-based journals, such as: Cutting Tool Engi- wise, other tooling companies have been of much help neering; American Machinist; Metalworking Produc- and assistance in the preparation of this book, such as: tion; Machinery and Production Engineering. Seco Tools; Kennametal Hertel and Kennametal Inc; Iscar Tools; Ingersoll; Guhring; Sumitomo Electric patient with me as I have attempted to meet extended Hardmetal Ltd; Mitsubishi Carbide; Horn (USA); She- deadlines for the manuscript, for which I am indebted fcut Tool and Engineering Ltd; Rotary Technologies to and can only o,er my sincerest thanks. Lastly, if any Corp; Diashowa Tooling; Centreline Machine Tool Co unfortunate mistakes have inadvertently slipped into Ltd; DeBeers – element 6; Walter Cutters; Widia Va- the text, or misinterpretations in the draughting of any lenite; TRW – Green,eld Tap and Die; Triple-T Cut- line diagrams have occurred, it is solely the author’s ting Tool, Inc; Hydra Lock Corp; Tooling Innovations; fault and does not represent any of the companies, or and Microbore Tooling Systems. Several machine tool their products, nor that of the individuals mentioned. companies have been invaluable in providing informa- tion, notably: Cincinnati Machines; Yamazaki Mazak; Graham T. Smith Contents 1Cutting Tool Materials ................. 2.2 History of Machine Tool Development 1.1 Cutting Technology – an Introduction ... 2 1 and Some Pioneers in Metal Cutting ... 1.1.1 Rationalisation ................. 2 2.2.1 Concise Historical Perspective 50 1.1.2 Consolidation .................. 4 of the Development of Machine 1.1.3 Optimisation ................... 4 Tools ........................ 50 1.2 Evolution of Cutting Tool Materials 2.2.2 Pioneering Work in Metal 1.2.1 Plain Carbon Steels ............. 7 Cutting – a Brief Resumé ...... 7 1.2.2 2.3 Chip-Development ................... 54 High-Speed Steels .............. 7 51 1.2.3 2.4 Tool Nose Radius ..................... 62 Cemented Carbide .............. 8 1.2.4 2.5 Chip-Breaking Technology ........... 66 Classi,cation of Cemented Carbide Tool Grades ............ 12 2.5.1 Introduction to Chip-Breaking 66 1.2.5 Tool Coatings: Chemical 2.5.2 Principles of Chip-Breaking 68 Vapour Deposition (CVD) ...... 14 2.5.3 Chip-Breakers 1.2.6 Diamond-Like CVD Coatings .. 14 and Chip-Formers ............ 69 2.5.4 1.2.7 Tool Coatings: Physical Helical Chip Formation ....... 71 2.5.5 Vapour Deposition (PVD) ...... 17 Chip Morphology ............ 75 1.2.8 2.5.6 Ceramics and Cermets .......... 19 Chip-Breaker Wear ........... 79 2.6 Multi-Functional Tooling ............. 79 1.2.9 Cermets – Coated .............. 23 1.2.10 Cubic Boron Nitride (CBN) and Poly-crystalline Diamond 3Drilling and Associated Technologies (PCD) ......................... 25 3.1 Drilling Technology ................. 88 87 1.2.11 Natural Diamond ............... 29 3.1.1 Introduction to the Twist Drill’s Development ........... 88 2Turning and Chip-breaking Technology 3.1.2 Twist Drill Fundamentals ..... 88 2.1 Cutting Tool Technology ................ 34 33 3.1.3 Dynamics 2.1.1 Turning – Basic Operations ..... of Twist Drilling Holes ........ 96 3.1.4 2.1.2 34 Indexable Drills .............. 103 3.1.5 Turning – Rake and Clearance Counter-Boring/Trepanning .. 107 2.1.3 3.1.6 Angles on Single-point Tools .... 34 Special-Purpose, or Customised 2.1.4 Cutting Insert Edge Preparations 36 Drilling and Multi-Spindle and Oblique .................... 39 Drilling ...................... 110 Tool Forces – Orthogonal 2.1.5 3.1.7 Plan Approach Angles .......... 41 Deep-Hole Drilling/ 2.1.6 Cutting Toolholder/Insert Gun-Drilling ................ 113 3.1.8 Selection ....................... 43 Double-Tube Ejector/ Single-Tube System Drills ..... 115 3.1.9 Deep-Hole Drilling – 5.4 Dies ....................... 189 Cutting Forces and Power ..... 117 5.5 Turning – Introduction ........ 191 5.5.1 3.2 Boring Tool Technology – Introduction 117 Radial Infeed Techniques ..... 193 Single-Point Boring Tooling ... 118 5.5.2 3.2.1 Helix Angles, for Single-/Multi-Start 195 Boring Bar Selection of: 3.2.2 5.5.3 Insert Inclination ... Toolholders, Inserts 5.5.4 195 and Cutting Parameters ....... 122 3.2.3 5.5.5 Pro,le Generation ..... 198 Multiple-Boring Tools ........ 124 Cutting Data and Other 3.2.4 Turning – Boring Bar Damping ......... 126 Important Factors ............ 200 3.2.5 ‘Active-suppression’ 5.6 Milling ....................... 203 of Vibrations ................. 127 3.2.6 5.7 Rolling – Introduction ........ 206 Hard-part Machining, 5.7.1 Rolling Techniques ... 209 Using Boring Bars ............ 128 3.3 Reaming Technology – Introduction .. 133 3.3.1 Reaming – Correction 6Modular Tooling of Hole’s Roundness Pro,les ... 135 3.3.2 and Tool Management ............... Radially-Adjustable Machine Reamers ............. 139 6.1 211Modular Quick-Change Tooling ....... 212 3.3.3 Reaming – Problems 6.2 Tooling Requirements and Remedies ........... 142 for Turning Centres .................. 216 3.4 Other Hole-Modi,cation Processes .... 142 6.3 Machining and Turning Centre Modular Quick-Change Tooling ............... 221 6.4 Balanced Modular Tooling – 4Milling Cutters for High Rotational Speeds ............ 230 and Associated Technologies ......... 6.5 Tool Management .................... 233 4.1 Milling – an Introduction ............. 150 6.5.1 Tool Management 149 4.1.1 Basic Milling Operations ...... 151 Infrastructure ................ 238 6.5.2 4.1.2 Milling Cutter Geometry – Insert Creating a Tool Management Axial and Radial Rake Angles 155 and Document Database ...... 240 6.5.3 4.1.3 Milling Cutter – Approach Overall Bene,ts of a Tool Management System .......... 244 Angles ....................... 158 4.1.4 6.5.4 Tool Presetting Equipment Face-Milling Engagement – Angles and Insert Density ..... 160 and Techniques for 4.1.5 Peripheral Milling Cutter Measuring Tools .............. 245 Approach Angles – 6.5.5 Tool Store and its Presetting eir A,ect on Chip 163 Facility – a Typical System .... 261 6.5.6 4.1.6 Spindle Camber/Tilt – Computerised-Tool when Face-Milling ............ 166 Management – a Practical Case 4.2 Pocketing, Closed-Angle Faces, for ‘Stand-alone’ Machine Tools 264 in-Walled and Milling Strategies ..................... 169 7Machinability and4.3 Rotary and Frustum-Based Milling Integrity ............................. 269 Surface Cutters – Design and Operation ....... 172 7.1 Machinability ........................ 270 4.4 Customised Milling Cutter Tooling .... 177 7.1.1 Design of Machinability 4.5 Mill/Turn Operations ................. 177 Tests and Experimental Testing Programmes .......... 270 5 Technologies ............. 7.2 Machined Roundness ................ 285 5.1 ............................. 182 181 7.2.1 Turned Roundness – 5.2 Hand and Machine Taps .............. 182 Harmonics and Geometrics ... 291 7.3 Chatter in Machining Operations ..... 294 5.3 Fluteless Taps ........................ 189 7.3.1 Chatter and Chip Formation – 8.8.1 Product Mixing – Preparation Signi,cant Factors In,uencing of a Aqueous-Based Cutting its Generation ................ 297 Fluids ........................ 410 8.8.2 7.3.2 Chatter – Important Factors Monitoring, Maintenance A,ecting its Generation ....... 297 and Testing of Cutting Fluid – 7.3.3 Stability Lobe Diagrams ....... 300 in Use ........................ 411 7.4 Milled Roundness – Interpolated 8.9 Multi-Functional Fluids .............. 417 Diameters ........................... 301 8.10 Disposal of Cutting Fluids ............ 417 7.5 Machined Surface Texture ............ 305 8.11 Health and Safety Factors – Concerning 7.5.1 Parameters for Machined Cutting Fluid Operation and Usage .... 418 Surface Evaluation ............ 308 8.11.1 Cutting Fluid-Based 7.5.2 Health Issues ................. 420 Machined Surface Topography 317 8.12 Fluid Machining Strategies: Dry; 7.5.3 Manufacturing Process Envelopes .................... 324 Near-Dry; or Wet .................... 425 7.5.4 Ternary Manufacturing 8.12.1 Wet- and Dry-Machining – Envelopes (TME’s) ............ 326 the Issues and Concerns ....... 425 7.6 Machining Temperatures ............. 326 8.12.2 Near-Dry Machining ......... 426 7.6.1 Finite Element Method (FEM) ....................... 328 7.7 Tool Wear and Life ................... 330 9Machining and Monitoring Strategies 7.7.1 Tool Wear .................... 331 431 9.1.1 HSM Machine Tool Design 7.7.2 Tool Life ..................... 337 9.1 High Speed Machining (HSM) ........ 432 Considerations ............... 434 7.7.3 Return on the Investment (ROI) 342 7.8 Cutting Force Dynamometry .......... 3439.2 HSM Dynamics – Acceleration 7.9 Machining Modelling and Simulation 350 and Deceleration ..................... 445 9.2.1 HSM Dynamics – Servo-Lag .. 446 7.10 Surface Integrity of Machined 9.2.2 E,ect of Servo-lag Components – Introduction .......... 360 and Gain on Corner Milling ... 7.10.1 Residual Stresses 9.2.3 448 in Machined Surfaces ......... 360 E,ect of Servo-Lag and Gain Paths ........................ 448 Whilst Generating Circular 8Cutting Fluids ....................... 381 9.2.4 CNC Processing Speed ........ 449 8.1 Historical Development 9.3 HSM – with Non-Orthogonal Machine of Cutting Fluids ..................... 382 Tools and Robots ..................... 451 8.2 Primary Functions of a Cutting Fluid .. 383 9.4 HSM – Toolholders/Chucks ........... 458 9.4.1 Toolshank Design 8.3 High-Pressure Jet-Assisted Coolant and Gripping Pressures ....... 458 Delivery ............................. 383 9.4.2 Toolholder Design 8.4 Types of Cutting Fluid ................ 387 8.4.1 Mineral Oil, Synthetic, and Spindle Taper ............ 465 or Semi-Synthetic Lubricant? .. 392 9.5 Dynamic Balance of Toolholding 8.4.2 Assemblies ........................... 467 Aqueous-Based Cutting Fluids 395 9.5.1 HSM – Problem of Tool Balance 8.4.3 Water Quality ................ 397 8.5 Cutting Fluid Classi,cation – According 9.5.2 469 Machine Application .......... 472 to Composition ...................... 398 HSM – Dynamic Balancing 9.6 HSM – Research Applications ......... 474 8.6 Computer-Aided Product Development 398 8.6.1 Cutting Fluid – Quality Control 404 9.6.1 Ultra-High Speed: Face-Milling 8.7 Selecting the Correct Cutting Fluid .... 407 Design and Development ..... 474 9.6.2 8.7.1 Factors A,ecting Choice ...... 407 Ultra-High Speed: 8.7.2 Selection Procedure ........... 408 Turning Operations ........... 480 9.6.3 8.8 Care, Handling, Control and Usage – Ultra-High Speed: Trepanning of Cutting Fluids ..................... 409 Operations ................... 484 9.6.4 Artefact Stereometry: 9.10.3 Nano-Machining for Dynamic Machine Tool and Machine Tools ........... 526 Comparative Assessments ..... 486 9.11 Machine Tool Monitoring Techniques 531 9.7 HSM: Rotating Dynamometry ........ 493 9.11.1 Cutting Tool Condition Monitoring .................. 531 9.8 Complex Machining: 9.11.2 Adaptive Control and Machine of Sculptured Surfaces ................ 496 Utilising the Correct Tool Tool Optimisation ............ 535 9.8.1 for Pro,ling: Roughing 9.11.3 Arti,cial Intelligence: and Finishing ................. 496 AI and Neural Network 9.8.2 Integration ................... 538 Die-Cavity Machining – 9.11.4 Tool Monitoring Techniques – Retained Stock ............... 498 9.8.3 a ‘Case-Study’ ................ 538 Sculptured Surface Machining – with NURBS ................. 502 9.8.4 Sculptured Surface Machining – Appendix ................................. 549 Cutter Simulation ............. 505 9.9 Hard-Part Machining ................ 507 About the Author ......................... Hard-Part Turning ............ 508 9.9.1 Hard-Part Milling ............ 511 9.9.2 587 9.10 Ultra-Precision Machining ............ 516 9.10.1 Micro-Tooling ................ 518 Subject Index ............................. 589 9.10.2 Micro-Machine Tools ......... 525 1 Cutting Tool Materials ‘What is the use of a book’, thought Alice, ‘without pictures or conversations?’ LEWIS CARROLL (1832–1898) [A lice in W onderland, C hap. 1] requirements for a speci,c manufacturing environ- 1.1 Cutting Technology–ment, can be initially addressed by employing the fol- lowing tooling-related philosophy – having recently undertaken a survey of the current status of tooling within the whole company: an Introduction Previously, many of the unenlightened manufactur- • Rationalisation ing companies, having purchased an expensive and Consolidation • sophisticated new machine tool, considered cutting Optimisation • tool technology as very much an a plied little ,nancial support, or technical expertise to NB ese three essential tool-related factors in es- purchase these tools. Today, tooling-related technolo- tablishing the optimum tooling requirements for gies are treated extremely seriously, as it is here that the current production needs, will be brie,y re- optimum production output, consistency of machined viewed. product and value-added activities are realised. Of- ten companies feel that to increase productivity – to o,set the high capital investment in the plant and to amortise such costs (i.e. pay-back), is the most advan- 1.1.1 Rationalisation tageous way forward. necks’ and disrupt the harmonious ,ow of production In order to be able to rationalise the tools within the at later stages within the manufacturing environment. current production facility, it is essential to conduct Another approach might be to maximise the number a thorough appraisal of all the tools and associated of components per hour, or alternatively, drive down equipment with the company. costs at the expense of shorter tool life, which would will be both time-consuming and costly, because it increase the non-productive idle time for the produc- necessitates a considerable manpower resource and tion set-up. Here, the prime tooling factor should not needs a means of identifying all the tools and inserts be for just a marginal increase in productivity and currently utilised, in some logical and tabulated man- tion. If ‘bottlenecks’ in component production occur, ing a primitive but e,cient tool-card indexing system they can readily be established by piles of machined in the e,ciency, nor the perfection of any particular opera- ner. Such,rst instance. Details, such as: tool surveys are o type and its parts sitting on the shop ,oor awaiting further valued- tooling manufacturer, quantity of tools in use and the added activities to be undertaken. current levels of stock in the tool store, their current production problems need to be addressed by achiev- location(s), feeds and speeds utilised, together with ing improved productivity across the whole operation, any other relevant tool-related details are indexed on perhaps by the introduction of a Taguchi-type com- such cards. Once these tooling facts have been estab- ponent ,ow analysis system within the manufactur- lished, then they can be loaded into either a comput- ing facility. No machine erized tool management system database, or recorded is an island’ (i.e. for part production) and that manu- onto an uncomplicated tooling database for later in- facturing should be thought of as ‘One big harmonious terrogation. machine’ and not a lot of independent problems, will Having established the current status of the tool- create a means by which increases in productivity can ing within the manufacturing facility, this allows for be achieved. a tooling rationalisation campaign to be developed. Tool rationalisation (Fig. 1) consists of looking at the of tooling inventory, inappropriate tools/out-dated results of the previous tooling survey and signi,cantly tooling, or not enough tools for the overall operational reducing the number of tooling suppliers for particular types of tools and inserts. policy has the twin bene,ts of minimising tooling sup- pliers with their distinct varieties of tools, while en- abling bulk purchase of such tools from the remaining Tooling refers not only to non-consumable items such as: cut- suppliers, at preferential ,nancial rates of purchase. ting tools and inserts, tool holders, tool presetters, screws, Moreover, by using less tooling companies whilst pur- washers and spacers, screwdrivers/Allen keys, tool handling chasing bulk stock, this has the bonus of making you equipment, but also consumable items, such as hand wipes, one of their prime customers with their undivided at- grease/oils employed in tool kitting and cutting ,uids, etc. . Figure 1. Rationalisation of cutting inserts, can have a dramatic e,ect on reducing the tooling and workholding inventory. [Courtesy of Sandvik Coromant] tention, should the need for later ‘tool problem-solv- high-performance grades, they must be worked hard ing’ of manufacturing clichés in production occur. and pushed to their fullest capabilities. When tool life is reduced by increasing the depth of cut, there are several ways that a such loss can be 1.1. Consolidation minimised. For example, it is known that the size of the insert’s nose radius has a pronounced e,ect on tool For any tooling that remains a life, so by doubling the depth of cut this can, in the exercise, these should be consolidated, by reducing the main, allow for a larger nose radius – assuming that number of insert grades, by at least half – which o the component feature allows access. If an increase proves to have little e,ect on production capability. in nose radius cannot be utilised, then increasing the By grouping inserts by their respective sizes, shapes insert’s size will help to o,set any higher wear rates, and say, nose radius for example, this will eliminate by providing a better heat dissipation for the action of many of the less-utilised inserts, enabling the poten- cutting. tial for bulk purchase from the tooling supplier, with an attendant reduction in tool costs. From this con- out, is that no more than half the insert’s cutting edge solidation activity, it may now be possible to purchase length should be utilised, because as the depth of cut higher-performance grade cutting inserts, that meet a approaches this value, a larger insert is recommended. wider application range, enabling the consolidation to Where large depths of cut are used in combination be even more e,ective. Furthermore, such improved with high feedrates, a roughing grade insert geometry inserts, will probably have a longer tool life and can be promotes longer tool life, than a general-purpose in- utilised at higher speeds, which probably negates their sert. O extra cost, over the previously used inserts. If fewer a double-sided one for roughing cuts, this has the grades of insert are stocked, the tooling/application twin bene,ts of increased productivity and longer tool engineers will be acquainted with them much more life (in terms of machined parts per edge). Normally, thoroughly and this will result in a added e,ectiveness single-sided inserts are recommended whenever the and a consistent application, for the production of ma- depth of cut and feedrate are so high that the surface chined components – more will be said on this latter speed must be reduced below the grade’s normal range, point in the next section on Optimisation. in order to maintain an adequate tool life. Such inserts should be considered if erratic insert breakage occurs. Later to be discussed in the chapter on Machin- 1.1. Optimisation ability and Surface Integrity, is the fact that the highest temperature region on the tool’s rake face is not at the By consolidating the tooling, it allows productivity cutting edge; but in the vicinity on the chip/tool inter- to be boosted by optimisation of the cutting insert face where chip curling occurs – this is some distance grades. For example, in turning operations, the depth back and where the crater is formed. of cut can probably maximised and, as a result, the this highest isothermal region can vary, depending number of passes along, or across the part can be mi- upon the feedrate. For example, if the feedrate is in- nimised. It can be argued that increasing the depth of creased, the highest temperature zone on the insert’s cut leads to a reduction in the subsequent tool life (in face will move away from the cutting edge; conversely, terms of minutes of cutting per edge). However, there if the feedrate is now reduced, this region moves to- are fewer cuts per part, so each machined workpiece ward the cutting edge. requires less overall cutting and as a result, many more if the feedrate is too low for the chosen insert geom- parts per edge can be produced. More important, are etry and edge preparation, heat will be concentrated that the cycle times for roughing operations be re- too near the cutting edge and insert wear will be ac- duced: a reduction in the number of roughing passes celerated. from three to one, results in a 66% reduction in the a,ect of moving the maximum heat zone away from cycle time. any potential decrease in tool life, on the basis that it terms of minutes of actual cutthe insert’s edge and is so doing, extends tool life -time per edge. As a re– in- could reduce, or eliminate a potential ‘bottleneck’ in sult, each machined part will be produced in less time latter production processes of the part’s manufacture. and at higher feeds, so the tool life in terms of parts To extract the maximum productivity from today’s per edge will also increase. As a result of the inappropriate use of cutting data, failure, which on a ,nishing cut, would probably result such as incorrect feedrates employed for the chosen in- in scrapping the part. When exceedingly large parts sert geometry, this can produce a number of undesir- are to be machined, the objective may simply be to able symptoms. complete just one part per insert, or in an even more extremely shortened tool life, edge chipping and insert extreme situation, just one pass over the part. When breakage are likely if feedrates are too high, whereas small parts are being produced, then the tool life can when feeds are too low, chip control becomes a prob- be controlled in order to minimise dimensional size lem. Once the insert grades have been consolidated variation with in-cut time. with their associated geometries, it is relatively easy to trol, reduces the need for frequent adjustment of tool determine the feedrates for a selected grade of work- o,set compensations in the CNC controller. However, piece materials. Tooling suppliers can recommend a one idea shared by all of these strategic production ap- potential insert grade for particular component part proaches, is that by optimising the surface speed, the material, with an initial selection of insert grade, such manufacturing objectives will be realised. As a con- surface speeds being indicated in the Appendix. sequence of this approach to production, there is no inserts can be optimised by ‘juggling’ the grades and correct surface speed for any speci,c combination of geometries marginally around the speci,ed values, this material and insert grade, the optimum surface speed may allow feedrates to be increased and should provide depends upon the company’s manufacturing require- a signi,cant pay-o, in terms of improved productivity, ments at this time. at little, or no additional capital expenditure. When long production runs occur, these are ideal If the cutting speed is increased rather than the because it allows cutting data experimentation to dis- feed, a point is reached where any increase in surface cover the optimum speed for a particular production speed will result in a decrease in productivity. In other cycle. Sometimes it is not possible to ,nd the speed to words, cutting too fast will mean spending more time exactly meet the production demands and, a change of changing tools than making parts! Equally, by cutting insert grades, to one of the higher-technology materi- too slowly, the tool will last much longer, but this is als may be in order. If short production runs are neces- at the expense of the number of machined parts pro- sary, this can o duced per shi insert grades, but by consultation with a ‘cutting tool expert’, or reference to the published cutting literature is the ‘right’ surface speed? discussed more fully. the answer may be found to the problem of insert op- If we return to the theme previously mentioned, timisation. However a cautionary note, care must be namely: ‘No machine is an island’ and treat the pro- taken when utilising published recommendations, as duction shop as: ‘One big machine’, it can be stated they should only be employed as guidelines, to help that every shop should determine its own particular initiate the job into production. manufacturing objectives – when considering both Comparison with a known starting point within cutting speeds and tool life. Typical objectives for tool the recommended range for speci,c production con- life might be the completion of a certain number of ditions, namely for: large depths of cut, high feedrates, parts before indexing the insert, or adopting a ‘sister very long continuous cuts, signi,cant interrupted tool’ , or alternatively, insert indexing a cuts, workpiece surface scale and the absence of cool- of a shi ant, would all suggest that reductions in surface speed chined, the main goal is to avoid catastrophic insert should be initially considered. Conversely, production conditions that result in: short lengths of cut, shallow depths of cut, low feedrates, smooth uninterrupted cuts, clean pre-turned, or bright-drawn wrought workpiece materials and ,ood coolant, having a very ‘Sister tooling’ is the term that refers to a duplicate tool (i.e. rigid setup, suggests that the recommended ranges for having the same tool o,sets) held in the turret/magazine and can be automatically indexed to this tool, to minimise the insert could be exceeded , while still maintaining an down-time when changing tools. Such a ‘sister tool’ , can be acceptable tool life. pre-programmed into the CNC controller of the machine It should be remembered that the main requirement tool, to either change a is for an overall increase in production output and not produced, or if the tool life has been calculated, then when perfection. A the feed function on the CNC has decremented down to this preset value, then the ‘sister tool’ is selected. tory has been consolidated, there will be fewer and more versatile insert grades and geometries that need to be considered. a detailed appreciation of how to optimise the speeds and feeds in combination with depths of cut more ef- fectively, for the desired production objectives. By op- timisation here of the machining parameters, this al- lows full utilisation of the capital equipment, with the result that large improvements in overall manufactur- ing e,ciency can be expected. It is evident from this discussion concerning opti- misation, that the parameters of: tool life, feedrate and cutting speed form a complex relationship, which is il- lustrated in Fig. 2a. Consequently, if you change one parameter, it will a,ect the others, so a compromise has to be reached to obtain the optimum performance from a cutting tool. Preferably, the ideal cutting tool should have superior performance if ,ve distinct areas (see Fig. 2b): • Hot hardness – is necessary in order to maintain sharp and consistent cutting edge at the elevated temperatures that are present when machining. NB If the hot hardness of the tooling is not su,- cient for the temperature generated at the tool’s tip, then it will degrade quickly and be useless. • Resistance to thermal shock – this is necessary in order to overcome the e,ects of the continuous cycle of heating and cooling that is typical in a mill- ing operation, or when an intermittent cutting op- eration occurs on a lathe (e.g. an eccentric turning operation). NB If this thermal shock resistance is too low, then rapid wear rates can be expected, typi,ed in the past, by ‘comb cracks’ on High-speed steel (HSS) milling cutters. • Lack of a,nity – this condition should be present between the tool and the workpiece, since any de- gree of a,nity will lead to the formation of a built- up edge (BUE) – see the chapter on Machinability and Surface Integrity. NB is BUE will modify the tool geometry, lead- ing to poorer chip-breaking ability, with higher forces generated, leading to degraded workpiece surface ,nish. Ideally, the cutting edge should be inert to any reaction with the workpiece. . Figure 2. The main factors a,ecting cutting tool life, under ‘steady-state’ cutting conditions • Resistance to oxidation – a cutting edge should mately 5 m?min . have the desirable condition of having a high resis- had quench cracks present which severely weakened – tance to oxidation. the cutting edge, as a result of water hardening at quenching rates greater than 1000?C/second (i.e. nec- NB is oxidation resistance of the cutting tool is essary to exceed the critical cooling velocity – to fully necessary, in order to reduce the debilitating wear harden the steel), upon manufacture. By 1870, Mushet that oxidation can produce when machining at el- (working in England), had introduced a more com- evated temperatures. plex steel composition, containing: 2% carbon, 1.6% • manganese, 5.5% Tungsten and 0.4% chromium, with Toughness – allows the cutting edge of the insert to the remainder being iron. absorb the cutting forces and shock loads produced developed steel was that it could be air-hardened, whilst machining, particularly relevant when inter- this was a signi,cantly less drastic quench than using mittent cutting operations occur. a water quenchant. Mushet’s steel had greater ‘hot- hardness’ and could be utilised at cutting speeds up NB If an insert is not su,ciently tough, then when to 8 m?min . unwanted vibrations are induced, this can result in was retained until around 1900, but with the level of – either premature failure, or worse, a shattered cut- chromium gradually superseding that of manganese. ting edge. Cutting tool manufacturers, by careful balancing of 1. . High-Speed Steels these ,ve factors for the ideal cutting tool, can produce grades of inserts which distinctly vary, allowing a wide Around the turn of the century in the United States, range of workpiece materials to be machined through fundamental metallurgical work was being undertaken the selection of the correct insert grade for a particular by F.W. Taylor and his associate M. White and by 1901, material. In recent years, tooling manufacturers have these researchers had greatly improved the overall produced wider ranges of workpiece-cutting ability tool steel and slightly modifying its composition with from fewer types of inserts, across a diverse range of a material that was to be known as High-speed steel – speeds and feeds, allowing tooling inventories to be (HSS), enabling cutting speeds to approach 19 m?min . reduced even further. High-speed steel was not a new material, but basically how and in what manner correct tooling can be used an innovative heat treatment procedure. to increase production output, needs to be considered metallurgical composition of HSS was: 1.9% carbon, against the current situation of advances in cutting tool 0.3% manganese, 8% tungsten, and 3.8% chromium, materials and their selection – this will be the theme of with iron the remainder. Taylor and White’s tool steel the next section. mainly di,ered from that of Mushet’s by an increased amount of tungsten and a further replacement of man- ganese by chromium. By 1904, the content of carbon had been reduced, allowing for more ease in forging 1.2 The Evolution this HSS. Further rapid development of the HSS oc- curred over the next ten years, with tungsten content of Cutting Toolincreased to improve its ‘hot-hardness’. Around this time, Dr J.A. Matthews found that vanadium additions Materials 1. .1 Plain Carbonhad improved the material’s abrasion resistance. By 1910, the content of tungsten had increased to 18%, Steels Prior to 1870, all turning tooling materials were pro- with 4% chromium and 1% vanadium, hence the well- duced from plain carbon steels, with a typical compo- known 18:4:1 HSS had arrived, its metallurgical com- sition of 1% carbon and 0.2% manganese – the remain- position continued with only marginal modi,cations der being iron. Such a tool steel composition meant over the next 40 years. Of the modi,cations to HSS that it had a low ‘hot-hardness’ (i.e, its ability to retain during this time, of note was that in 1923 the so-called a cutting edge at elevated temperatures), as such, the ‘super’ HSS was developed, although this variant did cutting edge broke down at temperatures approaching not become commercially viable until 1939, when Gill 250?C, this in reality kept cutting speeds to approxi- reduced the tungsten content to enable the tool steel to be successfully hot-worked. Around 1950 in the rial, but this is a complex subject and more will be said United States, M2 HSS was introduced, having some on this subject shortly. of the tungsten content replaced by that of molybde- num. cal composition as: 0.8% C, 4% Cr, 2% V, 6% W and 1. . Cemented Carbide 5% Mo – Fe being the remainder. In this form, the M2 HSS could withstand machining temperatures of Possibly the widest utilised cutting tool materials today up to 650?C (ie the cutter glowing dull red) and still are the cemented carbide family of tooling, of which maintain a cutting edge. the group derived from tungsten carbide is most read- In 1970, Powder Metallurgy (P/M) by metallurgical ily employed. Prior to discussing the physical metal- processing via hot isostatic pressing (HIP), was intro- lurgy and expected mechanical/physical characteris- duced for the production of HSS, with careful control tics of cemented carbides, it is worth looking into the of elemental particle size; a complex task of insert selection. uct is forged then hot-rolled. In Fig. 4, just a small range of the material types, ing gave a uniformly distributed elemental matrix, grades, shapes of inserts and coatings by a leading overcoming the potential segregation and resulting cutting tool company is depicted. Highlighting the non-homogenous structure that would normally oc- complex chip-breaker geometries, necessary to both cur when ingot-style HSS forging. Such P/M process- develop and break chips and evacuate them e,ciently ing techniques enable the steel-making company to from the workpiece’s surface region. To give a sim- ‘tailor’ and specify the exact metallurgical composition pli,ed impression of just some of the tooling insert of alloying elements, this would allow the newly-de- variations and permutations available from a typical veloped sintered/forged HSS tooling to approach that tooling manufacturer, if 10 insert grades are listed, in of the performance of cemented carbides, in terms of 6 di,erent shapes, with 12 di,ering chip breakers and inherent wear resistance, hardness and toughness. In ,ve nose radii in the tooling catalogue, this equates to Fig. 3, a comparison of just some of the tooling materi- 10 × 6 × 12 × 5, or 3,600 inserts. In reality, there are a als is highlighted, here, fracture toughness is plotted number of other important features that could extend against hardness to indicate the range of in,uence of this cutting insert permutation to well over ,ve signi,- each tool material and the comparative relative mer- cant ,gures – for potential insert selection. When the its of one material against another, with some of their permutated insert number reaches this level of com- physical and mechanical properties tabulated in Fig. plexity, selecting the optimum combination of insert 3b. A typical sintered micro-grained HSS of today, characteristics becomes more a matter of luck than might contain: 13% W, 10% Co, 6% V, 4.75% Cr andskill . 2.15% C – Fe the remainder. One reason for the ‘keen’ Tungsten (synonym Wolfram, hence the chemical cutting edge that can be retained by sintered micro- symbol W), is the heaviest metal in the group VIB in grained HSS, is that during P/M processing the rapid Mendeleev’s Period Chart (i.e. atomic number 74). It atomisation of the particles produces extremely ,ne was named a carbides of between 1 to 3 µm in diameter – which mineral wolframite – as it was derived from the term fully support the cutting edge, whereas HSS produced wolf rahm, because the ore was said to interfere with from an ingot, has carbides up to 40 µm in diameter. tin smelting – supposedly devouring the tin. Whereas By way of illustration of the bene,ts of the latest mi- the term tungsten, was coined from the Swedish tung cro-grained HSS – in the uncoated condition – when sten, meaning heavy stone. Hence, in 1923, the Ger- compared to its metallurgical competitor of cemented man inventor K. Schröter produced the ,rst metal ma- carbide, HSS has a bend, or universal tensile strength trix composite, known today as cemented carbides. In of between 2,500 to 6,000 MPa – this being dependent these ,rst cemented carbides, Schröter combined tung- on metallurgical composition, whereas cemented car- sten monocarbide (WC) particles embedding them in bide tooling has a bend strength of between 1,250 to a cobalt matrix – these particles acted as a very strong 2,250 MPa. - binder. Cemented carbide is a hard transition metal niques have signi,cantly improved sintered micro- carbide ranging from 60% to 95% bonded to cobalt, this grained HSS enabling for example, high-performance being a more ductile metal. drilling, reaming and tapping to be realised. from having hexagonal structures, to a solid solution Coating by either single-, or multiple-coating has of titanium, tantalum and niobium carbides to that of been shown to signi,cantly enhance any tooling mate- a NaCl structure. Tungsten carbide does not dissolve . Figure 3. Cutting tool materials – toughness versus hardness – and their typical material characteristics. [Courtesy of Mitsubishi Carbide] any transition metals, but it can melt those carbides sten powder using hydrogen from chemically puri,ed found in solid solution. Powder metallurgy processing ore. Ore reduction can be achieved by the manipula- route – liquid-phase sintering – is utilised to produce tion of the processing conditions, enabling the grain cemented carbides, as melting only occurs at very high size to be controlled/modi,ed as necessary. temperatures and there is a means of reducing tung- form grain sizes of tungsten carbide today can range from 0.2 to 7 µm – enabling a ,nal sintered product to in the ,nal cutting insert condition. be carefully controlled. Moreover, by additions of ,ne porosity in the ,nal product is the result of ‘wetting’ cobalt at a further processing stage, then wet milling by the liquid present upon sintering. the constituents, allows for precise and uniform con- ‘wetting’ during liquid-phase sintering, being depen- trol of the grain size – producing a ,ne powder. Prior dent upon molten binder metal dissolving to produce to sintering, the milled powder can be spray-dried giv- a pore-free cutting insert, this has excellent cohesion ing a free-,owing spherical powder aggregate, with between the binder and the hard particles (see Fig. 5, the addition of lubricant to aid in its consolidation for typical cemented carbide powders and resulting (i.e. pressing into a ‘green compact’). Sintering nor- microstructures). It should be stated that most of the mally occurs at temperatures of 1500?C in a vacuum, ‘iron-group’ of metals can be ‘wetted’ by tungsten car- which reduces the porosity from about 50% that is in bide, forming sintered cemented carbide with excel- the ‘green state’, to less than 0.01% porosity by volume lent mechanical integrity. . Figure 4. Cutting inserts indicating the diverse range of: shapes, sizes and geometries available, with compositions varying from: cemented carbide, ceramics, cermets, to cubic boron nitride deriva- tives. [Courtesy of Sandvik Coromant] . Figure 5. Cemented carbide powders and typical microstructures after sintering. [Courtesy of Sandvik Coromant] • P(blue) – highly alloyed workpiece grades bide to be tough and readily sintered, also cause it to for cut- • easily dissolve in the iron, producing the so-called ting long-chipping steels and malleable irons, ‘straight’ cemented carbide grades. M (yellow) – lesser alloyed grades for grades normally contain just cobalt and have been used cutting fer- • to predominantly machine cast iron, as the chips eas- rous metals with long, or short chips, cast irons and ily fracture and do not usually remain in contact with non-ferrous metals, the insert, reducing the likelihood of dissolution wear. K (red) – is ‘conventional’ tungsten Conversely, machining steel components, requires al- carbide grades ternative carbides such as tantalum, or titanium car- Under this previous ISO system (Fig. 6), both steels for short-chipping grey cast irons, non-ferrous bides, as these are less soluble in the heated steel at the and cast irons can be found in more than one category, metals and non-metallic materials. cutting interface. Even these ‘mixed’ cemented carbide based upon their chip-formation characteristics. Each grades will produce a tendency to dissolution of the grade within the classi,cation is given a number to tool material in the chip, which can limit high speed designate its relative position in a continuum, rang- machining operations. Today, the dissolution tool ma- ing from maximum hardness to maximum toughness. terial can be overcome, by using cutting insert grades modi,ed over based on either titanium carbide, or nitride, together the years by many tooling manufacturers, introducing with a cobalt alloy binder. Such grades can be utilised more discretion in their selection and usage. Typi- for milling and turning operations at moderate cutting cal of this manufacturer’s modi,ed approach, is that speeds, although their reduced toughness, can upon found by just one American tooling company, forming the application of high feed rates, induce greater plas- a simple colour-coding matrix, such as the three des- tic deformation of the cutting edge and induce higher ignated manufacturer’s chip-breaker grades (such as: tool stresses. F, M and R) and three workpiece material grades (i.e. much the product of the past and today, virtually all Steel, Stainless steel and Cast iron) – producing a nine- such tooling inserts are multi-coated to signi,cantly cell grid. While another manufacturer in Europe, has reduce the e,ects of dissolution wear and greatly ex- produced a more discerning matrix, based upon add- tend the cutting edge’s life – more will be said on such ing the ‘machining di,culty’ into the matrix, produc- coating technology later. ing a 3 × 3 × 3 matrix – producing a twenty seven cell grid. In this instance, the tooling manufacturer uses the workpiece material to determine the tool material 1. . Classification ofneeded. to the type of machining operation to be undertaken, Cemented while the insert grade is determined by the application Carbide Tool Grades Most cemented carbide insert selection guides group conditions – whether such factors as interrupted cuts insert grades by the materials they are designed to cut. occur, forging scale on the part are present and the de- sired machining speed being designated as: good, av- carbide cutting of workpiece materials is: ISO 513- erage, or di,cult. 1975E Classi,cation of Carbides According to Use – NB of subwhich has a colour-groups. In its original form, this ISO 513 code sert selection process will get a user to approximately-coding for ease of identi,cation utilises 3 broad letter-and-colour classi,cations (see 90% of optimum, with the ‘,ne-tuning’ (optimisation) Fig. 6 for the tabulated groupings of carbides and their requiring both technical appreciation of information various colours, designations and applications): from the manufacturer’s tooling catalogue/recommen- dations from ‘trouble- shooting guides’ and any previ- ous ‘know-how’ from past experiences – as necessary. tive chip production characteristics and certain metallurgical characteristics, such as casting condition, hardness and tensile strength. ISO 1832–1991 has clesignations: ‘P’ (Steels, low-alloy); ‘M’ (Stainless steels); ‘K’ (Cast irons); ‘N’ (Aluminium alloys); ‘H’ (Hardened steelas) . Figure 6. Classi,cation of carbides according to use. *Courtesy of Seco Tools] 1. . Tool Coatings: Chemicaltooling, then the previously used methane is substi- tuted by a nitrogen/hydrogen gas mixture. For example, if a simple multi-coated charge is Vapour Deposition Rather quaintly, the idea of introducing a very thin required for the tooling, it is completed in the same (CVD) coating onto a cemented carbide cutting tool origi- cycle, by ,rstly depositing TiC using methane and nated with the Swiss Watch Research Institute, using then depositing TiN utilising a nitrogen/hydrogen gas the chemical vapour deposition (CVD) technique. In mixture. As the TiN and TiC are deposited onto the the 1960’s, these ,rst hard coatings were applied to tooling, they nucleate and grow on the carbides pres- cemented carbide tooling and were titanium carbide ent in the exposed surface regions, with the whole (TiC) by the CVD process (Fig. 7 shows a schematic CVD coating process taking approximately 14 hours, view of the CVD process) at temperatures in the range consisting of 3 hours for heating up, 4 hours for coat- 950 to 1050?C. Essentially, the coating technique con- ing and 7 hours for cooling. sists of a commercial CVD reactor (Fig. 8a) with cutcoating- ting tools, or inserts to be hard-coated placed on trays this being the subject of: various gaseous constituents (depicted in Fig. 8b). and their respective ,ow rates, coating temperature is a function of the reaction concentration, Prior to coating the tooling situated on their re- and the soaking time at this temperature. spective trays, these tools should have a good surface process is undertaken in a vacuum together with a ,nish and sharp corners should have small honed protective atmosphere, in order to minimise oxidation edges – normally approximately 0.1 mm. With the of the deposited coatings. However it should be noted CVD technique, if these honed tool cutting edges are that, in the case of high-speed steel (HSS) tooling such too large, they will not adequately support the coat- as when coating small drills and taps, the elevated ing, but if they are even greater, the cutting edge will coating temperatures employed, necessitate post-coat- be dulled and as a result will not cut e,ciently. ing hardening heat treatment. tooling trays (Fig 8b) are accurately positioned one above another, being pre-coated with graphite and are then loaded onto a central gas distribution column (i.e 1. . Diamond-Like CVD Coatings tree). placed inside a retort of the reactor (Fig. 8a). Crystalline diamond is only grown by the CVD process tained tooling within the reactor, is heated in an inert on solid carbide tools, because of the high temperatures atmosphere until the coating temperature is reached involved in the process, typical diamond coating tem- and the coating cycle is initiated by the introduction of peratures are in the region of 810?C. Such diamond- titanium tetrachloride (TiCl ) together with methane like tool coatings (Fig. 9), make them extremely useful (CH ) into the reactor. is a cloud of volatile when machining a range of non-ferrous/non-metallic vapour and is transported into the reactor via a hy- workpiece materials such as: aluminium-silicon alloys, drogen carrier gas (H ), whereas CH is introduced metal-matrix composites (MMC’s), carbon compos- directly. ites and ,breglass reinforced plastics. Although such surfaces and the chemical reaction in say, forming a workpiece materials are lightweight, they have hard, TiC as a surface coating, is: abrasive particles present to give added mechanical strength, the disadvantage of such non-metallic/me- TiCltallic inclusions in the workpiece’s substrate are that  + CH ? + TiC + 4HCl charged from the reactor onto a ‘scrubber’, where it is neutralised. When titanium is to be coated onto the Some limitations in the CVD process are that residual tensile stresses of coatings can concentrate around sharp edges, pos- sibly causing coatings to crack in this vicinity – if edges are not su,ciently honed – prior to coating. Additionally, the elevated temperatures cause carbon atoms to migrate (dif- Graphite shelves are most commonly employed, as it is quite fuse) from the substrate material and bond with the titanium. inexpensive compared to either stainless steel, or nickel-based Hence, this substrate carbon de,ciency – called ‘eta-phase’ is shelving, with an added bene,t of good compressive strength very brittle and may cause tool failure, particularly in inter- at high temperature. rupted-cut operations. . Figure 7. A PVD-coating, with coated tooling, plus a schematic representation of the CVD and PVD coating processes. [Courtesy of Sandvik Coromant] . Figure 8. Modern insert/tooling coating plant. [Courtesy of Walter Cutters] they become extremely di,cult to machine with ‘con- medium temperatures (250 to 750?C), as a result of ventional tooling’ and are a primary cause of heat gen- lower PVD temperatures found than by the CVD pro- eration and premature face/edge wear. Here, the high cess,no eta-phase forms. Secondly, the PVD technique tool wear is attributable to both the abrasiveness of the is a line-of-sight process, by which atoms travel from hard particles present and chemical wear promoted by their metallic source to the substrate on a straight corrosive acids created from the extreme friction and path. By contrast, in the CVD process, this creates an heat generated during machining. omni-directional coating process, giving a uniform Such diamond-coated tooling is expensive to pur- thickness, but with the PVD technique the fact that a chase, but these coatings can greatly extend the tool coating may be thicker on one side of a cutting insert life by up to 20 times, over uncoated tooling, when than another, does not a,ect its cutting performance. machining non-metallic and certain plastics, this more than compensates for the additional cost premium. ent at sharp corners in the CVD coated tooling, are Such diamond-like coated tools, combine the (almost) compressive in nature by the PVD technique. Com- high hardness of natural diamond, with the strength pressive stresses retard the formation and propagation and relative fracture toughness of carbide. of cracks in the coating at these corner regions, allow- ing tooling geometry to have the pre-honing operation enable the e,ective machining of non-ferrous/non- eliminated. Fourthly, the PVD process is a clean and metallic materials and, by way of an example of their pollution-free technique, unlike CVD coating meth- respective hardness when compared to that of a PVD ods, where waste products such as hydrochloric acid titanium aluminium nitride coated tool, they are three must be disposed of safely a times as hard (see Fig. 3a). Although, these diamond- In general, there have been many di,ering PVD like coatings do not have the hardness properties of coating techniques that have been utilised in the past crystalline diamond, they are approximately half their to coat tooling, brie,y some of these are: • micro-hardness value. Diamond-like coatings can Reactive sputtering – being the range from 3 to 30 ,m in thickness (see Fig. 9 – bot- oldest PVD coat- tom), with the individual crystal morphology present ing method, it utilises a high voltage which is posi- measures between 1 to 5,m in size (Fig. 9 – top). tioned between the tooling to be coated (anode) and Recently, a diamond-coating crystal structure called say, a titanium target (cathode). ‘nanocrystalline’ has been produced by a specialised barded with an inert gas – generally argon – which CVD process. frees the titanium ions, allowing them to react with tals measuring between 0.01 to 0.2 ,m (i.e. 10 to 200 the nitrogen, forming a coating of TiN on the tools. nanometres), with a much ,ner grain structure and the TiN to the tool’s surface – hence the coating will smoother surface to that of ‘conventional’ diamond- grow, • like coatings. Reactive ion plating – relies morphology presents less opportunity for workpiece upon say, titanium material built-up edge (BUE) at the tool/chip inter- ionisation using an electron beam to meet the tar- face, signi,cantly improving both the chip-,ow across get, which forms a molten pool of titanium. the rake face of the tool and simultaneously giving a titanium pool then vaporises and reacts with the better surface ,nish to the machined component. nitrogen and an electrical potential accelerates to- • ward the tooling to subsequently coat it to the de- Arc evaporation – utilises a sired thickness. 1. . Tool Coatings: Physicalcontrolled arc which vaporises say, the titanium source directly onto the Vapour Depositioninserts – from solid. In 1985 the main short-comings resulting from the As with the CVD process, all of the PVD coating pro- (PVD) CVD process were overcome by the introduction of duction methods are undertaken in a vacuum. Fur- the physical vapour deposition process (Fig. 7), when ther, the PVD coatings tend to have smoother and less the ,rst single-layer TiN coatings were applied to ce- mented carbide. PVD and CVD coating processes and their resulting coatings. Firstly, the PVD process occurs at low-to- dimpled surface appearance, than are found by the per -glide’ coating is molybdenum disulphide (MoS ) ‘blocky-grained’ surface by the CVD technique. A typ- which is normally applied by the PVD modi,ed mag- ical tooling tungsten carbide substrate that has been netron sputtering process (see Fig. 11 for a schematic PVD multi-coated is depicted in Fig. 10a. Such multi- of a typical MoS ‘super-glide’ coating). ple coating technology allows for a very exotic surface uum coating process is performed at a relatively low metallurgy to be created, which can truly enhance tool temperature (200?C). cutting performance. In general and in the past, CVD process prevents the substrate from annealing, while coatings tended to be much thicker than their PVD maintaining dimensional stability. alternatives, having a minimum coating thickness of ‘super-glide’ coating has a micro-hardness of between between 6 to 9 µm, whereas PVD coatings tended to be 20 to 50 HV; it is deposited 1 µm thick, typically over in the range: <1 to 3 µm. Today, by employing sophis- a previous titanium nitride (TiN) coating, or a ‘bright’ ticated coating plant technology with lateral rotatingtool. coatings can have over 1,200 applied arc cathodes, it is possible to have a nano-composite molybdenum disul,de layers present, each measuring coating, typical of these coatings on the tooling, might a few angströms (i.e. one angström – denoted by the be a nano-crystalline AlTiN coating embedded in an symbol ‘Å’ – is equal to one 10-millionth of a mm). amorphous silicon nitride (Si N ) matrix. composite structure creates an enormously compact coating, has a dendritic crystal structure, being simi- and resistance surface structure, not unlike that of a lar to graphite and has weak atomic bonds between honeycomb. the crystal layers, allowing easy movement of the adja- been proven to deliver a coating hardness of between cent planes of the crystalline layers (Fig. 11). Such an 40 to 50 gigaPascals (i.e. 1 GPa equals 100 HV) and a MoS  coating, tends to reduce the likelihood of adhe- heat resistance of up to 1,100?C, enabling the tooling sive wear and seizure, yet allowing sharp edges to the to be employed on dry, high-speed machining opera- coated tooling. tions. An advantage of using a nano-composite sur- face structure, is that they can provide both hardness and toughness to nano-layers without the complexity 1. . Ceramics and Cermets and precision required to apply individual nano-layer coatings. 100,000 BC and were ceramic (,ints), as stone-aged lic coatings applied to tooling is simply vast and ever- people used these specially-prepared broken ,ints to changing and is outside the present remit of this book. cut and work into hunting tools such as arrowheads, However, it is worth mentioning just one of the newly- spears and for knives when eating their hunted prey. developed ‘super-glide’ coatings that are currently utilised by tooling manufacturers today. ics as cutting tools occurred in the 1940’s. glide’ coatings have a hardness that is comparable to ceramic tools had the promise of retaining their hard- chalk, or talc and acts as a solid lubricant coating on ness at elevated temperatures, while being chemically the hard-coated substrate. inert to the ferrous workpieces they were originally really well when dry machining of: aluminium alloys, designed to machine. alloyed steels, nickel-based super-alloys, titanium al- mented carbide tools, allowed them to exploit higher loys and copper. In particular, the more demanding cutting speeds that were now becoming available on machining operations such as small-diameter drilling the newly-developed machine tools of that time. and reaming, deep-hole drilling and tapping, etc, are ceramic tools o,ered virtually negligible plastic defor- particularly suited to such ‘so mation, with the cutting edge being inert to any disso- lution wear. tooling was that they lacked toughness and resistance to both mechanical and thermal shock (see Fig. 2b). Smoother surfaces present in the PVD processes, create less thermal cracking which might lead to potential chipping and premature edge failure, while improving the resistance to re- Dendritic derives from the Greek word for ‘tree-like’ (i.e. den- peated mechanical and thermal stresses thereby minimising dron), hence its appearance as a crystalline structure. interface friction, resulting in lower ,ank wear rates. . Figure 10. Multi-coatings applied to cemented carbides and cermets, together with tool geometries of cermet cutting inserts. [Courtesy of Sandvik Coromant] . Figure 11. A typical ‘super-glide’ coating of molybdenum disul,de (MoS2) applied to a hard-coating on a tool’s sub- strate – weak bonds between crystal layers allow easy movement of the planes. [Courtesy of Guhring] Moreover, ceramic tools at this juncture, were only re- dies , with subsequent sintering, the fused alumina ally employed for turning operations and in particular, in ‘stable machining’, where interrupted/intermittent particulates are sintered together, thereby signi,- cutting operations did not occur. cantly decreasing porosity. With the recent advances in powerful and very been known in the past as ‘pure oxide’, or ‘cold- rigid CNC machine tools, this has opened-up the pos- pressed’ ceramics. sibility of utilising ceramic tooling, either in a purely ceramics is their low thermal conductivity, making sintered monolithic tooling insert, or more recently as them highly susceptible to thermal shock (i.e. the a multi-coated variant – more will be said on this topic hot and cold thermal cycles that can occur when in- shortly. Returning to the monolithic ceramic cutting terrupted cutting takes place). tool materials, they have normally been available in three distinct grades, which will now be mentioned. • ‘Pure’ ceramic – this is the traditional tooling in- or ‘,oating’ die sets from the admixed powders, are termed sert material, consisting of aluminium oxide. ‘green compacts’ and are friable, that is having very limited alumina is white in colour and is produced by cold mechanical strength and must be gently handled, prior to sin- pressing powder in the desired insert geometry tering – therea problems are exacerbated by short machining cycle particles will react with the yttria forming a liquid. times, variable depths of cut and higher machining speeds. ‘Pure’ alumina inserts can be improved by so depending upon the relative proportions of the additions of zirconia (Zr) to greatly increase the reactants, the resultant ‘Sialon’ formed may have toughness somewhat, but such cutting tool material, either of the following atomic arrangements: beta has been widely superseded today, by ‘mixed grade’silicon nitride, or alpha silicon nitride. It is possible ceramics, or cermets – to shortly be discussed, to produce a very complex cutting insert material, • Black, or mixed ceramics – tend to minimise the having both ‘beta-’ and ‘alpha-Sialons’ in atten- dance. e,ects of thermal shock on the cutting insert, by having additions of titanium carbide added to the alumina, this causes the insert to turn black. A A typical ‘beta-Sialon’ might be composed of: problem with these earlier ‘black ceramics’ was that they did not sinter as readily as the former ‘pure’ Si .ZAlZOZN .Z ceramic inserts. ditional ‘hot pressing’ operation to achieve the de- Where: ‘Z’ represents the degree of substitution of sili- sired densities, which tended to limit the geomet- con and nitrogen by aluminium and oxygen. ric shapes for such inserts. A later development Conversely, an ‘alpha-Sialon’ can consist of: of these cutting tool materials was termed ‘mixed ceramics’, these had additions of titanium nitride, Mx(Si, Al) (O,N) which improved thermal shock still further, with the sintered inserts tending to be brown, or choco- Where: ‘M’ is the metal atom, such as yttrium. late in colour – the term ‘black’ for these later in- serts, became irrelevant. All this sounds quite confusing, but basically the ‘Si- had good hot hardness, enabling them to machine alon’ microstructure consists of a crystalline nitride harder steel components, or chilled cast irons and phase, held in a glassy, or partially crystallised matrix. at greater temperatures, where the combinations of ese crystalline grains can be either ‘beta-Sialon’, or higher cutting forces and greater chip/tool interface a mixture of ‘alpha’ and ‘beta’, but generally it can be temperatures would have induced cutting insert said that as the ‘alpha’ phase increases, the hardness plastic deformation in their previous counterparts. of the ‘Sialon’ becomes greater. • Cermets – the original cermet was developed by mechanical changes, result in a higher ‘hot-hardness’ Lucas under the trade name ‘Sialon’ which was a for the cutting insert when in-cut. An additional and silicon nitride based material, having a very low co- probably greater bene,t is gained by the signi,cant e,cient of thermal expansion. improvement in insert toughness, which can rival that rate when in-cut, tends to reduce the stresses be- of cemented carbide of equal hardness. One limitation tween the hotter and cooler isothermal zones of the in the past to such cermets, was that they could not insert, giving very high thermal shock resistance. satisfactorily machine steels, owing to their poor per- Originally, it was di,cult to sinter these inserts to formance in resisting solution wear. However, these full density, although by substituting some of the earlier cermets when machining nickel-based alloys, silicon and nitrogen with aluminium and oxygen, or cast irons they performed very well, but even the   the new material ‘Sialon’ it had the added bene,ts ‘mixed’ ceramics based on alumina, having 25% addi- of: ease of pressing and sintering, with equally as tions of carbide (i.e. ‘whisker-reinforcement’) within emental addition was that of yttria (Ygood thermal shock resistance. A notable later el O ), which- the insert’s substrate are a direct competitor to such cermets. aided sintering performance and during sintering. ) on the surface of the silicon nitride Sialon, this name was coined for the insert, as it represented the chemical symbols for the constituent elements: Si, Al, O and N. Today, with the more complex material technology HV and the surfaces of such coatings tend to be very cermet insert grades (Fig. 10b), they can easily ma- smooth and having a total thickness of less than 3 µm chine ferrous 0 -based workpieces at high cutting speeds, thick – allowing around 2,000 durable layers. One of tool lives and excellent surface ,nishes. Complex pow- the key factors in successfully applying these com- der particulates are utilised for the current turning plex metallurgy multiple coatings, has been the de- inserts, such powders may have a large core of TiCN, velopment of ‘super-lattice technologies’ at medium surrounded by TiN – for superior hardness, adjacent temperatures, which do not compromise the thermal particulates having a small core of niobium (Nb), sur- properties of the substrate. rounded by tungsten (W) and titanium (Ti) – for supe- rior toughness. lower than its equivalent cemented carbide grade, this a very complex substrate, which is further enhanced accounts for the fact that at present, in turning opera- by subsequent multi-coating. tions in Japan 35% of all the inserts utilised for a range Typical turning data for a high-performance steel machining steel grades tend to be cermets, whereas, in product that can be rough-to-,nish turned using the Europe less than 5% of cermets are employed. Cermets same insert on a 34CrMo4 grade workpiece has been are considerably more wear and heat resistant than shown to be: tungsten carbide-based cutting materials. By way of il- – Cutting data: cutting speed (Vc) 140 m min, feed lustration for the reason for edge failure of tungsten – (f) 0.2 mm rev , depth of cut (DOC) 1.0 mm and with carbide inserts, is the heat generated at the tool/chip ,ood coolant. interface – at high cutting speeds. For example, if one In interrupted cutting trials with the cutting data considers the pre-sintering temperature for a typical mentioned on this workpiece material (i.e. having tungsten carbide material it is in the region of 1,150?C 4 equally-spaced splines around its periphery), the and, if turning a: 0.48% C, 0.8% Mn medium carbon – cermet insert’s edge withstood over 7,000 impacts per steel workpiece at 200 m min , this equates to the edge. highest isothermal edge temperature of 1,000?C – cre- the hardness, shock resistance and life of the latest ating the potential for localised thermal so such cermet tooling materials. edge failure. While an equivalent multi-coated Cermet, can readily turn alloy carbon steels at a depth of cut (DOC ) of up to 3 mm, with cutting speeds of between 1. . Cermets –200 to 300 m min– , with feedrates ranging from 0.1 to 0.3 mm rev . Moreover, as less ,ank wear takes place, Coated – To enable a wider range of machining applications the dimensional size of subsequent components in a while improving still further the original cermet batch will not signi,cantly ‘statistically-dri , produc- grades available, tool coatings were introduced and ing much less tolerance variation (i.e. reliable size-for- ometries (see Fig. 10b and c).with sophisticated high-technology cutting inser t geincreased multi- size consistency) in the completed turned parts.-coated cermet tool life, allows for an for indexable cutting inserts have individual ‘nano- excellent surface ,nish and dimensional consistency, coatings’ and are extremely hard, approaching 4000 whether cut wet, or dry. In general, the multi-coated cermet cutting tool materials, can be consolidated (i.e. pressed) in com- pound die-sets with very complex tool geometries and have integrated chip-breakers present – as illustrated in Fig. 10c. Such inserts, have seen a slow take-up in Europe and o,er considerable economical advantages when in particular, turning hardened steel parts. 0 Cermet is derived from the two words ceramic and metallic and, the clear distinction between this and other cutting tool materials, such as cemented carbide and ceramic tooling has become somewhat ‘blurred’ , with one tooling manufacturer claiming it was developed in 1929, which is ‘at odds’ with the A comparison of the hardness of di,erent popular coatings patented ‘Sialon’ product developed by the Lucas company may be applicable here, as TiCN coating has a hardness of – previously discussed. around 2,700 HV and TiAlN coating has a hardness of ap- One nanometre is equal to 10 m, or one millionth of a mm. proximately 3,200 HV. – . Figure 12. Ultra-hard cutting tool materials – cubic boron nitride (CBN). [Courtesy of DeBeers – element 6] . Table 1. Cutting tool materials – with some important physical properties –3 Density [g cm ] 4.28 14.7 4.28 3.52 17 17 Knoop hardness [GPa] 27.5 30 Young’s modulus *GPa, 587 649 390 593 ? Fracture toughness [MPam 2.94 10.48 3.7 5.90 ] –6 –1 7.8 5.4 4.6 4.7 Thermal expansion [10K ] –1 –1 44 Thermal conductivity [WmK ] 9.0 100 85 1. .10 Cubic Boron Nitride (CBN) andcation of very high temperatures and pressures , it can be transformed into the cubic structure of diamond (Fig. 12aii) – this transformation does not occur easily. Poly-crystalline Diamond As boron and nitrogen are two elements on either side Cubic Boron Nitride (CBN)/Synthetic (PCD) of carbon in the Periodic Table, it is possible to form a Diamond – compound of boron nitride, that exhibit’s a hexagonal Ex trac tion and Sintering Cubic boron nitride (CBN) is one of the hardest ma- boron nitride (HBN) as depicted in Fig. 12bi, having terials available and for machining operations it can the characteristics of being both slippery and friable. be considered as a ultra-hard cutting tool, it was f rst HBN can be transformed in a similar fashion to that synthesised in the late 1950’s. In many ways, CBN and of CBN (Fig. 12 bii). In practice, to facilitate the rate natural diamond are very similar materials, as they of transformation in the reaction chamber, additions both share the same atomic cubic crystallographic of solvents/catalysts are utilised for synthesis at more structure (see Fig. 12a and b). Both materials exhibit easily obtainable levels: pressures of approximately a high thermal conductivity, although they have pro- 60 GPa and temperatures 1,500?C. As this transforma- foundly dif erent properties. For example, diamond is tion proceeds in the reaction volume of a high pres- prone to graphitisation and will readily oxidise in air, sure system, the CBN/synthetic diamond grows, being reacting to ferrous workpieces at high temperatures, embedded in a portion of reaction mass and extracted conversely, CBN is stable to higher temperatures and af erward from this special-purpose press. By dissolv- can ef ortlessly machine ferrous components. CBN ing away the unwanted matrix, the CBN/synthetic dia- can therefore machine ferrous materials, such as: tool mond can be liberated and recovered for subsequent steels, hard white irons, surface hardened steels, grey processing. Grain sizes vary from large dimensions cast irons, (some) austempered ductile irons and hard- of approximately 8 µm, down to sub-micron sizes, for facing alloys. Normally, CBN tools should be used on f ne-grain tooling. workpiece materials with hardnesses greater than 48 Once the synthesised CBN/diamond has been ex- HRC, because if workpieces are less hard than this, the tracted, it is possible to sinter together these crystals cutting edge will result in excessive tool wear. of CBN, or diamond, with the aid of a ceramic binder, In graphite, the carbon atoms are arranged in a hex- to produce polycrystalline masses. Commercially, in agonal layered structure (Fig. 12ai) and, by the appli- To transform hexagonal graphite into the cubic diamond structure, requires exceedingly high temperatures > 2000?C and applied pressures > 60 GPa, to enable the conversion to take place. order to speed-up the rate of sintering, additions of a the physical properties of various comparable cutting solvent/catalyst are utilised (i.e. normally metals, or tool materials. metal nitrides), but during sintering the whole mass In order to produce the required tool geometries, must be held in the ‘cubic region’ of the respective pres- both the polycrystalline layer of CBN and polycrystal- sure/phase diagram – to prevent these hard crystals line diamond (PCD) are bonded to a thick tungsten reverting back to their original sof hexagonal form. carbide backing layer, then cutting inserts are wire- By sintering these hard particles together, it is possible cut out of this large blank – obtaining the maximum to form a conglomerate of CBN/diamond, in which number of insert shapes per blank (see Fig. 13a). T ese randomly orientated crystals are combined to produce CBN/PCD inserts are either full-size, or smaller tips a large isotropic mass. A very wide range of poly- that are then brazed onto suitably-shaped blanks, to f t crystalline products can be produced, utilising either the desired tool holder (as illustrated in Fig. 13b). CBN, or synthetic diamond as a base. For example, by Both CBN and PCD cutting tools can successfully changing the: grain size (see Figs. 12 c and d), solvent/ machine: super-alloys (ie with low iron content), grey catalyst employed, degree of sintering and particle size cast iron and non-ferrous metals, but show distinct distribution and, the presence/absence of inert f llers, dif erences when other workpiece materials are to be this will have a profound ef ect on the mechanical and productively machined – as depicted in Fig. 14. physical properties of the f nal product – Table 1 lists Polycrystalline diamond cutting tools are not utilised for machining ferrous workpieces, this is be- cause when machining under the high temperatures and sustained pressures that occur during cutting, the diamond has a tendency to revert back to graphite, Isotropic materials can be considered to have the same prop- erties in diferent directions. af er only a few seconds in-cut. T is reversion, does . Figure 13. Cutting tool materials: Cubic Boron Nitride (CBN) and Polycrystalline Diamond (PCD) . [Courtesy of DeBeers – ele- ment 6] . Figure 14. A diagram illustrating how Cubin Boron Nitride ther machining highly abrasive components, or high tempera- (CBN) and Polycrystalline Diamond (PCD) applications are tures in the cutti ng vicinity grouped, by workpiece materials. Their e,ectiveness when ei- . Figure 15. Turning operations with Cubic Boron Nitride (CBN) and Polycrystalline Diamond (PCD). [Courtesy of DeBeers – element 6] not take place when machining many non- ferrous and mond is used correctly in a very rigid machine-tool- non-metallic workpiece materials. Although CBN is workpiece setup for materials that require the best synthesised in a similar fashion to that of PCD cutting possible surface f nish, then there is simply no alterna- tool products, it is not as hard as PCD and is therefore tive. By way of illustration of this fact, if production less reactive with ferrous metals, as long as the cut- turning high-silicon content aluminium pistons with ting temperature is less than 1,000?C, it will not revert polycrystalline diamond (PCD) tooling, the best sur- to its sof er hexagonal form and oxidise in air. Tis face f nish that can be obtained will be in the region means that CBN can machine many ferrous parts and of 0.4 µm, conversely using an SCD tool this will give cast iron grades. Te complementary nature of both a surface f nish of better than 0.15 µm. If one really CBN and PCD is clearly depicted in Fig. 14, where the wants the ultimate surface f nish currently obtainable ‘cross-over’ between these ultra-hard cutting tool ma- by machining – in the ‘nano-range’, then a monolithic terials is shown. diamond tool, mounted in a special-purpose diamond In both CBN and PCD machining applications, turning lathe is the only manner in achieving such su- an excellent machined surface f nish can be obtained perb ‘mirror-f nish’ surfaces. SCD tool edges are pro- (see Figs. 15a and b). In the case of many PCD opera- duced as either razor sharp edges, or are made with tions, the cutting tool must not only machine widely a perfect radius being chip-free, imparting machined – 0 dif ering materials that are situated adjacent to one ‘mirror-f nishes’ of just a few angströms (i.e. 10 m). another in many passes over such a diverse material Te optical industries in particular f nd that the latest workpiece, but produce an excellent machined surface blemish-free ultra-sharp cutting edges of SCD, means f nish, which really ‘challenges’ the tool. Tool life can that diamond (paste) polishing af er machining has be extended greatly by utilising either CBN, or PCD been virtually, if not completely eliminated, this fact in tooling, of en tool lives can be increased by 50 to 200 particular being a very big production cost for the f - times that of the previous cemented carbide alterna- nal manufacture of large monolithic astronomical mir- tives. Tis boost in output, makes their additional rors. A cautionary note, is that to use SCD tooling for purchase price irrelevant, when considered against the anything other than as a f nishing cut is totally uneco- massive productive gains that are to be made by their nomic, as these precision components to be machined, adoption. should have been roughly conf gured to the desired Today, both CBN and PCD can of en be found shape, prior to diamond machining. Terefore, SCD as either thin-coated layers on tooling (see Fig. 3 for tools should be employed for exceedingly light f nish their relative tool insert hardnesses/toughnesses), or cuts of no deeper than 0.0008 m. as a ‘sandwich’ between metallic backing layers. Tese Natural diamond is a truly remarkable material, ‘sandwiched’ tool edges, permit brazing on both sides that exhibit’s a diverse range of mechanical and physi- of the hardened product, which are then accurately cal properties. For example diamond has the highest positioned and held onto a tungsten carbide shank, known: bulk hardness, thermal conductivity, while making them an ideal alternative for many micro- having a very low coef cient of friction and will not drilling operations. Such compound drilling edge corrode, these properties make it an ideal tool material technology, gives considerably improved edge reten- for the highest precision and accuracy machined com- tion and resistance to any abrasive particles present in ponents. Of these properties, hardness is probably the the workpiece and its severely work-hardened swarf, most important characteristic in machining operations typically found with the latest metal matrix composites and, when measured by the Knoop indentor (MMC’s). Such ultra-hard tooling, can be readily used of comparison of ultra-hard cutting tool materials, the on high-silicon aluminium alloys used in the automo- following two examples may prove informative: . By way tive industries, while not discounting the wide range of workpiece composites employed by the aerospace industries and the resin-based components utilised in the furniture industry. Knoop indentors produce a wedge-shaped indentation in the 1. .11 Naturalform of a parallelogram, with one diagonal seven times lon- Diamond ger than the adjacent one. Te Knoop test method is generally Monolithic, or single-crystal diamond (SCD), is the considered the optimum technique, for crystalline solids – hardest material available today. If such natural dia- having crystallographic directionality (i.e. anisotropy). – • Natural diamond – has a hardness of Fretty, P. Grade Wise. Cutting Tool Engg., 46–50, Feb., 2000. (ie diamond orientation and test conditions): Dia- 9,000 kg mm Gough, P. Tool Life Boosted by Titanium Nitride Coat. Ma- mond (111) surface, <110> direction, 500g load, • chinery and Prod. Engg., 52–53, Feb. 1983. Cubic boron nitride (CBN) – has a hardness of – 4,500 kg mm , (111) surface, <110> direction, 500g Gummeson, P.U. and Stosuy, A. Iron-carbon Behaviour dur- load. ing Sintering. In: Source Book on Powder Metallurgy, ASM Pub., 49–61, 1979. One of the main limitations of natural diamond is that Hanson, K. Lowering your Grades. Cutting Tool Engg., 54–60, Jan., 2000. it has distinct cleavage planes (111) . Tis consistent Heath, P.J. Ultra-hard Materials. European J. of Engg. Ed., cleavage plane makes it ideal for jewellery-makers to Vol. 12 (1), 5–20, 1987. cleave the beautiful facets demanded of diamond jew- Israelsson, J. A Progress Report on Cutting Tool Materials. ellery, but this means that monolithic diamonds must American Machinist, 39–40, Dec., 1992. be mounted in their respective tool holders in exactly Jindal, P.C. et al., PVD Coatings for Turning, Cutting Tool the correct orientation/plane, so avoiding any poten- Engg., 42–52, Feb., 1999. tial cleavage in-cut. Kennedy, B. Making the Grade – PCBN Applications. Cut- SCD tool cost is a draw-back, because these tools ting Tool Engg., 22–30, June 2002. cost in the region of four times more than the equiva- Lewis, B. Fast Times in HSS. Cutting Tool Engg., 28–32, lent PCD tool. However, despite this very high cost July 2001. dif erence, SCD can reduce the overall operating costs and signif cantly improve productivity, when appliedLewis, B. Conf dence Game – Grades and Geometries. Cut- ting Tool Engg., 46–52, Dec., 2002. to the correct machining process. Expensive tooling such as SCD, must be handled with care, because alMiel- ert, W. Coating for Speed. Cutting Tool Engg., 40–44, Feb., 1996. though it is the hardest material known, it is also very brittle and subject to thermal shock, the problem being Mirchandani, P.K. Making a Better Grade – Composite Car- exacerbated with its very sharp tool edges. Terefore, bide Substrates. Cutting Tool Engg., 58–61, Jan., 2005. Mitoraj, L. T e Coating Edge. Cutting Tool Engg., 51–55, it is essential that sudden impacts to the tool’s edge Feb., 2000. must be avoided, through either inappropriate cutting Novak, D. Single Minded – Single Crystal Diamond. Cutting applications, or by rough handling. Tool Engg., 38–41, June 2002. Raymond, M.K. Ceramics Ease Up the Machining of High- hardness Parts, American Machinist, May 1996. References Raymond, M.K. Coatings Keep Cutting Tools Sharp. Ameri- can Machinist, 40–42, May 1996. Journal and Conference Pa pers Richter, A. Raising Al – AlTiN Coatings. Cutting Tool Engg., 42–46, Jan., 2003. Boller, R. Crystal Clear – DLCoatings. Cutting Tool Engg., Richter, A. Top Coat. Cutting Tool Engg., 36–41, Dec., 36–40, May 2002. 2003. Craig, P. Behind the Carbide Curtain. Cutting Tool Engg., 26–41, Aug., 1997. Sanders, E.H. Understanding Coated Carbides. Cutting Tool Engg. 3–7, Sept./Oct., 1977. Dzierwa, R. Slippery when Blue – Coatings. Cutting Tool Engg., 36–41, Jan., 2003. Sprout, W. PVD Today. Cutting Tool Engg., 52–4057, Feb., 1994. Eastman, M. Inserts Show their True Colors. Cutting Tool Engg., 30–36, April 1999. Taylor, F.W. On the Art of Cutting Metals. Trans. of ASME Feir, M. Post-treatment of PM Parts. Metal Powder Report, 28, 31–350, 1907. 28–30, Jan., 1981. Talmann, R. Cracking the Code – Carbide Classif cations. Cutting Tool Engg., 34–43, June 1995. Vasilash, G.S. T e Superfard Coatings: More than Meets the Eye. 52–54, Production, Dec., 1995. Weiner, M. Coatings Move Forward. Cutting Tool Engg., Miller indices determine the crystalline orientation for a plane 22–29, Feb., 1999. in an atomic structure and for natural diamond it is normally Woods, S. Coat, Please. Cutting Tool Engg., 50–56, Oct., on the (111) plane, although some cleavage has been observed 2004. on the (110) plane. Books, Booklets and Guides Lenel, F.V. Powder Metallurgy, Principles and Applications. Metal Powder Inds. Princetown (NJ), April 1980. Balshin, M.Y. and Kiparisov, S.S. General Principles of Pow- Metals Handbook – Powder Metallurgy. ASM Pub. 8th Ed., der Metallurgy. MIR (Moscow) Pub., 1980. Vol. 3, 1967. Chin, G.Y. Advances in Powder Metallurgy. ASM Pub., Modern Metal Cutting – A Practical Handbook. AB Sandvik 1982. Coromant Pub., 1994. Dieter, G.E. Mechanical Metallurgy. McGraw-Hill Kogaku- Reed-Hill, R.E. Physical Metallurgy Principles. Van Nos- nd sha 2 Ed., 1976. trand Reinhold (NY) Pub., 1973. Dowson, G. Powder Metallurgy – Te Processes and its Trent, E.M. Metal Cutting. Oxford: Butterworth Heine- Products. Adam Hilger (Bristol) Pub., 1990. mann (3 Ed.), 1991. Kalpakjian, S. Manufacturing Processes for Engineering Ma- rd rd terials (3 Ed.). Addison Wesley, 1997. 2 Turning and Chip-breaking Technology ‘Machines are the produce of the mind of Man; and their existence distinguishes the civilized man from the savage.’ WILLIAM COBBETT (1762–1835) [L etter to the L uddites of Nottingham] including: forming , while others such as drilling, bor- 2.1 Cutting Tool ing, screw-cutting, of internal features, and forming Technology In the following sections a review of a range of Turnand screw- few of the traditional operations und-cutting of external features, to name just aertaken. ing-related technologies and the importance of chip- With the advent of mill/turn centres, by hav- breaking technology will be discussed. ing CNC control of the headstock and rotational, or ‘driven-/live-tooling’ to the machine’s turret, this al- lows prismatic features to be produced (i.e. ,ats, slots, 2.1.1 Turning – Basicsplines, keyways, etc.), as well as drilled and tapped holes across and at angles to the major axis of the work- Operations Turning can be broken-down into a number of basic piece, or o,-axis. Even this explanation of mill/turn cutting operations and in e,ect, there are basically centres is far from complete, with regard to today’s four such operations, these are: sophisticated machine tools. As machine tool builders 1. Longitudinal turning today, can o,er a vast array of machine con,gurations, including: co-axial spindles (ie twin synchronised in- (Fig. 16a), 3. Taper turning – not line headstocks), ,tted with twin turrets with X- and 2. Facing (Fig. 16b), Y-axes simultaneous, but separate control, having pro- shown, grammable steadies (i.e. for supporting long slender 4. Pro,ling – not shown. NB workpieces), plus part-catchers , or overhead gantries brie,y reviewed. for either component load/unload capacity, to multi- axes robots feeding the machine tool. In its most simple form, turning generates cylindrical chine tool exists and has multi-axes CNC controllers forms using a single-point tool (Fig. 1.16a). Here, a tool to enable the machine’s down-time to be drastically is fed along the Z-axis slideway of the lathe (CNC), or reduced and in this manner achieving high productive a turning centre, while the headstock rotates the work- output virtually continuously. piece (i.e. the part is held in either: a chuck, on a man- drel, face-plate, or between centres – when overhang is too long), machining the component and thereby 2.1.2 Turning – Rake and Angleson Single-point Tools generating a circular and cylindrical form of consistent Clearance diameter to the turned part machining operation that is undertaken (Fig. 16b) and In order for a turning tool to e,ectively cut and pro. Facing is another basic - in this case, the tool is fed across the X-axis slideway duce satisfactory chips, it must have both a rake and while the part rotates, again, generating a ,at face to clearance angle to the tool point (Fig. 17). Today’s sin- the part, or a sharp corner at a shoulder, alternatively it gle-point cutting tools and inserts are based upon de- can be cutting the partial, or ,nished part to length (i.e cades of: past experience, research and development, facing-o, ) . Taper turning can be utilised to produce looking into all aspects of the tool’s micro-geometry short, or long tapers having either a fast taper (i.e. with at the cutting edge. Other important aspects are an ef- small included angle a large included angle), or slow taper (i.e. having a ,cient chip– o , such critical control of the ,exure (i.e. elastic behaviour) of-breaking technology, in certain insta nces as a Morse taper). the actual tool insert/toolholder combination for the that can be achieved on a CNC lathe/turning centre, latest multi-functional tooling is essential – more will be said on some of these topics later in the chapter. the cutting edge and can vary according to the work- ied, as can rotational speeds. Facing operations can also be used to produce either curved Forming can be achieved in a number of ways, ranging from convex, or concave surface features to the machined part – complex free-form features (externally/internally) on the ma- here the surface is both generated and formed, requiring si- chined part, to simply plunging a form tool to the required multaneous programmed feeding motions to the Z- and X- depth. axes. . Figure 16. Typical turning operations with the workpiece orientation shown in relation to the cutting insert, for either: (a) cylin- drical turning, (b) facing. [Source: Boothroyd 1975] piece material being machined. In general, for ductile 1. Chamfer – which simply breaks the corner – illustrated, not materials, the rake inclination is a positive angle, as to be low, so a weaker wedge angle (i.e. the angle be-ance side to various lengths on the rake face (see the shearing characteristics of these materials tends 2. Land – stretching back negatively from the clear- tween the top face and the clearance angle) will su,ce. Fig. 18b), For less ductile, or brittle workpiece materials, the top 3. Radius – around the actual corner (see Fig. rake inclination will tend toward neutral geometry, 4. Parabolic – has unequal levels of honing 18c) , whereas for high-strength materials the inclination faces (see Fig. 18d). on two will be negative (see Fig. 17), thereby increasing the wedge angle and creating a stronger cutting edge. Even here, more o stronger cutting edge has the disadvantage of requir- of these four edge preparations are utilised, so that the ing greater power consumption and needing a robust cutting forces are redirected onto the body of the rake’s tool-workpiece set-up. Machining high-strength mate- face, rather than directed down against the more frag- rials requires considerable power to separate the chip ile cross-section of the edge. from the workpiece, with a direct relationship existing are o between the power required for the cutting operation etry of the contoured surface. Typical T-lands range in and the cutting forces involved. Cutting forces can size from 0.07 to 0.50 mm, having angles varying from be calculated theoretically, or measured with a dyna- 5 to 25? o, of the rake face (Fig. 18b). mometer – more will be said on this subject later in Honing which is the ‘rounding’ of the cutting edge, the text. Both side and front clearances are provided can be performed in one of several ways. Probably to the cutting edge, to ensure that it does not rub on the oldest technique for honing, utilises mechanical the workpiece surface (see Fig. 17). If the tool’s clear- means, which employs a vibrating tub ,lled with an ance is too large it will weaken the wedge angle of the abrasive media, such as aluminium oxide – to ‘break’ tool, whereas if too small, it will tend to rub on the the corner on these inserts. A variation in this de- machined surface. Most tools, or inserts have a nose sign, uses an identical abrasive, except here the inserts radius incorporated between the major and minor cut- are held by centrifugal force to the inside of a rotat- ting edges to create strength here, while reducing the ing tank. While yet another method of honing using height of machined cusps , with some inserts having a an abrasive media, involves spraying the inserts with ‘wiper’ designed-in to improve the machined surface ,ne abrasive particles – to hone the edges of the in- insert integrated features later. cutting insert honed edges, uses brushes made from ,nish still further – more will be mentioned on these serts. Probably the most popular method for obtainingextruded nylon impregnated with diamond (see Fig. 18a). 2.1.3 Cutting Insert Edge Preparations individual carriers and rotate as they all revolve the brushes, thereby applying equal hones to all insert under O edges. Depending upon the amount of desired honing, 18b, c and d) is created onto the sharp cutting edge of these brushes can be either raised, or lowered, or alter- the insert, this imparts additional strength to the out- natively, the inserts can make multiple passes through ermost corners of the cutting edge, where the rake and the machine. All of the above honing techniques pro- clearance faces coincide. duce a hone that is roughly equal on both the ,ank in which the honed edge preparation is fashioned, and rake faces – what is termed a ‘round hone’ (Fig. these are: 18c). Yet another honing pro,le termed the parabolic hone (i.e. sometimes this honed edge is known as: Machined cusps result from a combination of the feedrate and the nosed radius of the tool. If a large feedrate occurs with a radius is sometimes termed ‘edge rounding’ (i.e. denoted small nose radius then the resultant cusp height will be high by the letters ‘ER’) – o and well-de,ned, conversely, if a small feedrate is utilised in enabling the cutting forces to be directed on to the stronger conjunction with a large nose radius, then cusp height is mini- part of the insert. mised, hence the surface texture is improved. . Figure 17. Typical turning ‘,nishing’ insert/toolholder geometry and the insert’s edge chamfering, in relation to the workpiece P-hone, oval, or waterfall), is produced by a machine cutting insert edge preparation, the P-hone directs the with a so cutting forces into the body of the insert. Honing can be speci,ed in a number of sizes, usu- serts, it tends to extend slightly over the inserts sides, ally being determined by the amount of time these producing a hone of uneven proportions between the insert spend in the honing device. two insert faces (Fig. 18d). As in the case of the T-land dard for honing was established in the United States by . Figure 18. A honing machine (i.e. brush-style) and several types of honing edge preparations. [Courtesy of Ingersoll] the American National Standards Institute (ANSI) in . Table 2. Typical in-cut shear strengths of various 1981, which included dimensions and expected toler- materials ances for these three basic hones. Today, many cutting tool manufacturers have expanded upon this Stan- Iron 370 dard, or adopted their own – specifying hone manu- facturing and identi,cation methods. Hones must be 480 0.13% C. steel applied prior to the application of coatings. Inserts 690 that are destined to receive a CVD coating, must have Ni-Cr-V steel a minimum hone to strengthen the edge, in order to 630 Austenitic stainless steel counteract the e,ects of this high temperature coating process. Conversely, PVD coatings, can be equally ap- Nickel 420 plied either over fully-honed insert edges, or on an un- 250 honed cutting edge. In recent years, the cutting toolCopper (annealed) manufacturers have an emphasis toward providing Copper (cold-worked) 270 honed edges of greater consistency and repeatability. Cartridge brass (70/30) 370 Aluminium (99.9% pure) 97 2.1.4 Tool Forces – Orthogonal Magnesium 125 and Oblique 36 Lead tion, its removal and chip-breaking actions, with the immense pressure and friction in this process produc[Source: Trent ( 1984)]- ing forces acting in various directions. Stresses at the rake face tend to be mainly compressive in nature, al- though some shear stress will be present (see Table 2, NB Both of these cutting force models are heav- by way of illustration of the machining shear stresses ily in,uenced by the: cutting tool/insert orientation for various materials), this is due to the fact that the to workpiece, tool’s direction of cut and its applied rake is rarely ‘normal’ to the main cutting direction. feedrate . est to the cutting edge, with the area of contact between Oblique Cutting Forces the chip and rake face being directly related to the ge- ometry here, hence the need for tooling manufacturers Fig.1.19a, can be seen a model of the three-dimen- to optimise the geometry in this region. sional cutting force components in an oblique turn- ing operation, when the principal cutting edge is at an machining operations concerning single-point cutting angle to the main workpiece axis (i.e. Z-axis). tools/inserts (see Fig. 19), these are: component forces can be separated into the: • 1. Orthogonal cutting forces – two forces Tangential force (FT) – which is (ie tangen- greatly in,uenced 2. Oblique cutting forces – three forces (i.e. tial and axial – see Fig. 19b), by the contact and friction between both the work- tangen- piece and tool, as well as the contact conditions tial, axial and radial – see Fig. 19a). between the chip and the rake face of the cutting edge. is the greatest of these three component forces and contributes to the torque, which in turn, in,uences As well as the tool/chip interface temperatures being up to 1,000?C, the interface pressures can reach a maximum of Feedrates play a major role in determining the axial force in 3,000 MPa, these being sterile smooth surfaces makes them single-point cutting operations, in association with the tool’s ‘ideal’ conditions for the occurrence of ‘pressure-welding’/sei- orientation to the part being machined. zure. . Figure 19. The two- and three-force models of orthogonal and oblique cutting actions, with the component forces approximately scaled to give an indication of their respective magnitudes the power requirement for cutting. Fundamentally, has been neutralised – as indicated by the fact that the the product of the tangential force and the cutting resultant force shows no X-axis of set). If any radial speed represent the power required for machining. force was present, this would create either a ‘candle- Te speci,c cutting force is a unit expression for stick ef ect’, or ‘barrelling’ to the overall turned length. the tangential cutting force, being closely related to In reality, there will always be some form of nose ra- lected feedrate, some degree of ‘push-of ’, depending upon the size of the material’s undeformed chip thickness and se- dius, or chamfer to the tool point, which will have • Axial force (FA) – the magnitude of this force will this incorporated nose feature – creating a ‘certain de- vary depending on the selected feedrate and the gree’ of radial component force af ect. approach angle’, or ‘entering angle’, – more will chosen tool geometry and in particular, the ‘plan be said on this topic later. Its direction is from the 2.1.5 Plan Approach Angles feeding of the tool, along the direction of workpiece machining, Te manner in which the cutting edge contacts the • Radial force (FR) – is directed at right angles to the workpiece is termed the ‘plan approach angle’ (Fig. tangential force from the cutting point. Te ‘plan 20a), being composed of the entering and lead angles approach angle’ and the size of the nose radius, will for the selected tool geometry. In ef ect for single- inf uence this force. point turning operations, the tool’s orientation of its plan approach, is the angle between the cutting edge NB Tese three component forces are signif cantly and feeding direction. When selecting a tool geometry inf uenced by the rake angle, with positive rakes for turning specif c workpiece feature – such as a 90: producing in general, lower cutting forces. Tere- shoulder – it is important as it will not only af ect the sultant force, its magnitude and angle, will be af- machined part geometry, but has an inf uence on con- fected by all three component forces, in conjunc- sequent chip formation and the direction and magni- tion with the tool’s geometry and the workpiece tude of the component cutting forces, together with material to be cut. the length of engagement of the cutting edge (see Fig. 20b). In single-point turning (Fig. 20b), the depth of cut (DOC Or thogonal Cutting ) , or ‘cutting depth’ is the dif erence between Forces In Fig. 19b the two-dimensional model for orthogonal ence in the un-cut and cut diameter (i.e. the diame- an un-cut and cut surface, this being half the dif er- cutting is depicted, once again, for comparison to the ter is reduced by twice the DOC in one pass along the oblique cutting model, in a single-point turning op- workpiece). Tis DOC is always measured at 90: to the eration. For simplicity, if one assumes that the point of tool’s feed direction, not the cutting edge. Te manner the tool is inf nitely sharp and that the tool is at right in which the cutting edge approaches the workpiece is angles to the workpiece axis having no def ection pres- termed the ‘entering angle’ (i.e. plan approach angle), ent, then the two component forces are the tangential this being the angle between the cutting edge and feed force and axial force (i.e. previously mentioned above). direction (Fig. 20a – shown here in a cylindrical turn- In this case, this tool geometry-workpiece conf gura- ing operation). Moreover, the plan approach angle tion, allows long slender bars to be turned, as there is not only inf uences the workpiece features that can be less likelihood of tool ‘push-of ’ (i.e. as the radial force produced with this cutting geometry, it also af ects the formation of chips and the magnitude of the compo- nent forces (Fig. 20b). Te ‘entering angle’ af ects the length of the cut- ting edge engaged in-cut, normally varying from 45? In reality, the speci,c cutting force is a better indication of to 90?, as illustrated in the four cases of dif ering plan the power requirement, as it is the force needed to actually approach angles shown in Fig. 20b. Here, in ‘case I’ an deform the material prior to any chip formation. It will vary and is inf uenced by the: undeformed chip thickness, feedrate, and yield strength of the workpiece material. For example, if the cutting conditions are kept the same and only the material changed, then if a nickel-based alloy is machined, the initial In single-point turning operations, the depth of cut chip forming force (i.e. specif c cutting force) will be more (DOC) is than ten times greater than when cutting a pure aluminium sometimes referred to by the term: ‘undeformed chipworkpiece. thick- ness’. . Figure 20. Insert approach angle geometry for turning operations entering angle of 45? and lead angle of 45? is utilised, creasing the tool’s nose radius, this will inf uence other giving rise to equal axial and radial component forces. factors, which in turn could have a great impact on the: In ‘case II’, the entering angle has changed to 75? and type of machined surface f nish produced, expected lead angle is now 15?, these altered angles change the tool life and the overall power consumption during the component forces, with an increase in the axial force operation. In fact, the main factors that inf uence the while reducing the radial force. In ‘case III’, an or- application of tooling for a specif c turning operation thogonal cutting action occurs, with only a 90? enterare:- ing angle (i.e. the lead angle reduces to zero), showing I.Workpiece material – machinability, condition (i.e. internal/external), mechanical and physical properties, etc., a large increase in the axial force component at the 0 zero . In ‘case IV’, an oblique cutting action has re- II. Workpiece design – shape, dimensions and expense of the radial force component which is now ma- turned (i.e. as in ‘cases I and II’), but here the entering chining allowance, angle has changed to -15?, with the lead angle 75?, this III. Limitations – accuracy and precision require- produces a large axial component force, but the radial ments, surface texture/integrity, etc., similar to the geometry of a light turning and facing V. Stability – loop stif ness/rigidity (i.e. component force direction has now reversed. Tis last IV. Machine tool – type, power, its tool, allowing cylindrical and facing operations to be cutting edge to its foundations), from the condition and usefully undertaken – but the tool’s point is somewhatVI. Set-up – tool accessibility, workpiece tool plan approach angle geometry (i.e. ‘case IV’), is specif cations, weaker that the others, with the tool points becoming clamping of increased strength from right to lef . Terefore, in VII. Tool programme – the correct/specif ed and toolholding, tool changing, ‘case I’, for a given feedrate and constant DOC, the cut and its tool of sets, etc., tool length/area is greater than the other ‘cases’ shown and VIII. Performance – cutting data, anticipated tool-with this geometry, it enables the tool to be employed and economics, life for heavy roughing cuts. Returning to ‘case III’, if this IX. Quality – tool delivery system and service. tool is utilised for f nish turning brittle-based work- piece materials, then upon approaching the exit from a In order to gain an insight into the complex and im- cut, if the diameter is not supported by a larger shoul- portant decisions that have to be made when select- der diameter, then the axial component force /pressure, ing tooling for the optimum production of either part will be likely to cause edge break-out (i.e. sometimes batch sizes, or for continuous production runs, then termed ‘edge frittering’), below the machined surface the following section has been incorporated. diameter at this corner (i.e. potentially scrapping the machined part). In mitigation for this orthogonal cut- ting tool geometry, if longer slender workpieces re- 2.1.6 Cutting Toolholder/Insert Selection quire cylindrical turning along their length, then with the radial force component equating to zero, it does not create signif cant ‘push-of ’ and allows the part to When deciding upon the correct selection of a tool- be successfully machined. A single-point turning geometry is subject to very of diverse factors must be considered, as indicated inholder/cutting insert for a given application, a range complex interactions and, as one geometric feature is Fig. 21. As can be seen by the diagram (Fig. 21) and modif ed such as changing the entering angle, or in- associated text and captions, there are many other variables that need to be considered prior to selection of the optimum toolholder/insert. Generally, the f xed conditions cannot be modif ed, but by ‘juggling’ with 0 In all of these cases, it is assumed – for simplicity – that there is the variable conditions it is possible to accomplish the no nose radius/chamfer on the tool and it is inf nitely sharp. best compromise toolholder /insert geometry, to opti- In order to minimise the ef ects of the radial force component mise these cutting conditions for the manufacture of when cylindrically turning long slender workpieces with ‘Case a specif c workpiece and its intended production re- I and II’ tool geometries, the use of a programmable steady, quirements. Whenever toolholders and cutting inserts or a ‘balanced turning operation’ (i.e. utilising twin separately are required for a specif c manufacturing process, it programmable turrets on a turning centre, with tools situ- is important to view the tooling selection procedure ated virtually opposite each other running parallel during the as a logical progression, in order to optimise the best turning operation – see Fig. 41), will reduce this ‘push-of ’. . Figure 21. The factors that must be considered prior to commencing a turning operation, when utilising indexable inserts possible tools/inserts for the job in hand. Perhaps the part access, as a toolholder is def ned by its: ef ective following selection strategy for a ‘start point’ in choice entering and point angles , together with the insert’s and application of turning tools, can be undertaken shape (see Fig. 22). according by the following step-by-step approach: Toolholders should be the largest possible size for the turning centre’s tool turret, this requirement is vi- tal, as it reduces the ‘tool overhang ratio’ – providing Start Point ? Edge clamping rigidity and integrity to stabilise the insert’s cutting system, edge. ? Toolholder size NB Appendix a shows the ISO ‘Code Key’ – and type, for Ex- ? Appendix b shows the ISO ‘Code Key’ – ternal Toolholders. Insert shape, for Solid ? Appendix c shows the ISO ‘Code Key’ – Boring Bars. Insert size, ? for Car- tridges. Nose radius, Inser t ? Shape Insert type, T e insert shape should be selected relative to the en- ? tering angle needed for the tool’s accessibility, or ver- Tool material, satility. Here, the largest suitable point angle should ? be chosen for strength and economy (see Fig. 23). In Cutting data ? Final Tool- Fig. 23, is illustrated a practical example of how chang- holder and ing only one variable – insert geometry (shape) – can Insert Selection inf uence an insert’s turning application. T e shape of an insert will determine its inherent weakness, or strength, which is of particular relevance if rough- Edge Clamping turning operations are necessary. Furthermore, insert System Initially, the tool holder clamping system should be shapewill inf uence whether it is prone to vibration, or selected to provide optimum performance in dif er- not and its predictable tool life. Hence, if one is con- ent applications over a wide range of workpiece geom- cerned about vibrations of either the tool, workpiece, etries. T e type of machining operation and to a lesser or both, then a weaker insert such as a light turning extent, the workpiece size determines tool holder se- and facing geometry with less cutting edge length ex- lection. For example, roughing-out operations on big posed in-cut, might be more suitable. Variable condi- components will make considerably dif erent demands, tions such as the selection of insert’s geometric shape to that of f nishing passes on small components. can af ect other machining parameters and, this is valid for other insert factors, so acompromise will al- NB Pin, clamp and lever are just three of the insert ways occur in any machining application. clamping systems available – consultation with the tool suppliers at this point might be benef cial. To olholder Size and Ty pe Efective entering angles (κ ) must be carefully selected when Once the clamping system has been selected, the size the operation involves prof ling, or copying. T e maximum and type of toolholder must be determined, with its prof ling angle (β) is recommended for each tool type – if selection being inf uenced by: feed directions (i.e. see ‘workpiece fouling’ is to be avoided. Fig. 22 for turning insert shapes and feed directions), NB κ  = κ + β (for plunging into a surface), whereas κ  = κ – β (for size of cuts, workpiece and toolholder situated in the ramping-out of a surface), κ  = κ (β = 0?) for cylindrical turn- machine for accessibility requirements. T e work- ing, Where: ef ective entering angle (κ ), entering angle (κ), piece’s shape plays a decisive role if surface contouring maximum in-copy angle (β). Always select the smallest enter- is necessary, this is particularly relevant for machining ing angle that the part geometry will allow. . Figure 22. Tool paths in fnish turning operations. [Courtesy of Sandvik Coromant] . Figure 23. Selecting indexable inserts for turning operations. [Courtesy of Stellram] NB Appendix d shows the ISO ‘Code Key’ – for In- In roughing-out operations, the largest cutting dexable Inserts. for a given toolholder, will inf uence the insert size. depth For any insert, the ef ective cutting length has to be determined (see Fig. 20b), as the entering angle will Inser t inf uence the size of the insert selected. If the ef ective Size An indexable insert size is directly related to the tool- cutting edge length is less than the depth of cut (D), OC holder selected for the operation, with the entering a larger insert should be chosen, or the Dshould OC angle and insert shape having previously been estab- be reduced. Sometimes in more demanding turning lished. Only the matching-shaped insert can be f tted operations, a thicker insert – of the same geometric into the seat of a particular toolholder, as its shape shape – gives extra reliability. and size are predetermined by the seating dimensions. Cutting speed (VC) Nose VC = πDN/1000 (m/min) Radius Of particular relevance in any turning operation is the insert’s tool nose radius (rε – see Fig. 17), as it is the keyWhere: factor with regard to: D = workpiece diameter (mm) • inherent strength in roughing operations, N = workpiece rotational speed (rpm) the resulting surface texture from ,nishing opera- • tions. Speci,c cutting force (kC): kC = FT/A (N/mm ) Further, the size of the nose radius a,ects vibrational 2 tendencies (see Fig. 23) and in certain instances, theWhere: feedrates. transition between the A = cutting area (mm ) major and minor cutting edges, which determines the 2 strength, or weakness of the point angle (see Figs. 16a For example, for ,nishing operations, with the nose and 17), therefore it is an imperative factor to get right. radius in combination with the feedrate (i.e. pre-se- In general, roughing-out should be undertaken with lected), this will a,ect the surface texture and part ac- the largest possible nose radius, as it is the strongest curacy, in the following manner: tool point (see Fig. 23). Further, a larger tool nose ra- dius permits higher feedrates, although it is important Machined surface texture (Rt): to monitor any possible vibrational tendencies. Later in the relevant section, more will be said on the in,u- (Rt, this parameter being: maximum pro,le height) 2 ence that the insert’s tool nose radius plays in the ,nal Rt /8 × r= fε x (µm) machined surface texture, but it is worth mentioning 1000 here that the feedrate for roughing operations shouldWhere: be set to approximately half the size of the nose radiusf = feedrate per revolution (mm/rev) utilised. 2 (mm) power consumed in turning in conjunction with therε = nose radius material’s yield strength and chip-forming ability, parNB- Rt’, can be con- ticularly in rough-turning operations. verted into other surface texture parameters – as nec- material removal rate (MMR) can be obtained by aessary. combination of high feedrate, together with a moder- ate cutting speed, with other limiting factors, such as By utilising either: larger turning insert tool nose ra- depth of cut (DOC), tool’s nose radius, under consider- dius, ‘wiper insert’ (yet to be discussed), a more posi- ation. O tive plan approach angle, or in certain circumstances, can sometimes be a limiting factor when mmR is the a higher cutting speed, the surface texture can be im- requirement and, in such circumstances the cutting proved. In general, the coordination of the tool’s nose speed is usually lowered somewhat. For a given nose radius and the pre-selected feedrate in ,nishing op- radius and cutting insert geometry, the power can be erations, indicates that the feed should be kept below derived, to ensure that the machine tool will be able a certain level to achieve an acceptable machined sur- to cope with this pre-selected mmR, in the following face texture value. manner: Inser t Ty Machine tool’s power requirement (P): pe P = tangential force (FT) x cutting speed (VC) P = FT × VC by the previously selected geometry – see Appendix 1d P = kC × A × VC for the selection of indexable inserts. In reality, vari- , P = kC × f × aP × VC (kW) ous cutting conditions and workpiece materials make di,erent demands on the insert’s cutting edge. For ex- Where: ample, when machining hardened steel parts, this will f = feed/rev (mm/rev) be completely di,erent from that to the machining of aP = depth of cut (mm) aluminium components. Once the insert shape has been established in con- rial, or combination of materials that constitute the nection with its plan approach angle together with the cutter’s tool edge. Today, manufacturers of tooling nose radius dimension, this just leaves the type of ge- have a strategy for continuous improvement with varia- ometry to be found. In this instance, the type of insert tions in both tool matrices and coatings being consid- geometry refers to the ‘working area’ (i.e. nominally erable. Not only are cutting tool material research and found by its depth of cut and feedrate – more will be development an on-going intensive activity, but their said concerning this topic later, when ‘chip-breaking application for wider ranges of machining applica- envelopes’ will be discussed). Additional factors can tions are being considerably enhanced. A brief review inf uence the type of cutting geometry choice, such of just some of the current tool materials and coatings as: machine tool’s condition, its power, the stability of have been previously mentioned in Section 1.2, with the workpiece-tool-machine set-up, other factors that the main range of cutting tool materials being: ce- could af ect geometry selection include: whether con- mented carbides, coated cemented carbides, ceramics, tinuous, or intermittent cutting occurs, any tendency cermets, cubic boron nitride, polycrystalline diamond toward vibration while machining. Turning operations and monolithic (i.e. natural) diamond. can be separated into a number of ‘working areas’, be- ing based upon the removal of workpiece material and NB A good ‘start-point’ for most machining opera- the generation of accurate machined component di- tions, is to consider coated carbides initially, then if mensions, in combination with specif c surface texture these grades prove unsatisfactory, for whatever reason, requirements – as shown in Table 3. select one of the other materials – perhaps af er con- When establishing an insert type, the feedrate sultation with a cutting tool manufacturer, or af er a and depth of cut should be identif ed with one of the machinability testing procedure. ‘working ranges’ (i.e. from Table 3), as the various in- sert types to be chosen relate to this chart. It should be borne in mind that the most suitable ‘working area’ Cutting selected, will Data vary, in combination with such factors as the insert’s: size, shape and nose radius. Once all of the physical, metallurgical and geometrical factors for the cutting tool have been established for the machining operation, then it is necessary to set, or To ol Material calculate the cutting data – of en these criteria can be T e penultimate evaluation to be made concerning found from tooling manufacturers recommendations tooling decision-making is the choice of insert mate- and cutting data tables. Certain variable factors such as feedrate should have already been made, allowing the cutting speed to be calculated, from the well-known expression (below): . Table 3. Typical working areas for external turning tionsopera - VC = πDN/1000 (m min ) – Where: – VC = cutting speed (m min) Extreme f nishing 0.05 to 0.15 0.25 to 2.0 = Workpiece diameter (mm) D Finishing 0.1 to 0.3 0.5 to 2.0 N = rotational speed (rpm) 0.2 to 0.5 2.0 to 4.0 Light roughing Roughing 0.4 to 1.0 4.0 to 10.0 >1.0 6.0 to 20 Heavy roughing Extremely heavy roughing 8 to 20 >0.7 In the case of drilling, reaming and tapping operations, it is the diameter of the cutting tool that is used in the calculation. (mm) (mm) For any other internal machining operations – such as in bor- ing, it is the initial hole diameter that is employed in the cut- [Courtesy of Sandvik (UK) Ltd] ting speed calculation. Once again, manufacturers data tables are of en useful by the Cincinnati Screw and Tap Company in 1884. ‘starting-points’ for estimating the initial cutting pa- T is ‘Cincinnati machine’ was a direct forerunner of rameter information. Considerable care must be taken today’s manual controlled knee-type milling machine if the material has either a high work-hardening ten- tools. Of particular note was the ergonomic grouping dency, or intrinsic bulk (i.e. workpiece material) hard- of the controls centrally for a more ef cient hand con- ness, as this can inf uence the numerical data selected. trol by the skilled operator. At this time the machine Moreover, the plan approach angle also has an ef ect tool still utilised the Vee-form screw thread, with the on the numerical value for the parameter, for example, Acme-form (ie having the ability to take-up backlash) oblique machining allows a higher value than for or- still someway of development. thogonal machining. Steady development and ref nement of a range of machine tools continued into the the f rst half of the 20 curred. T is signif cant development was the ‘modern’ th 2.2 History of Machinenumerically-controlled (NC) machine. Around the late century until the next major ‘milestone’ oc- 1940’s, the ‘recirculating ballscrew ’ was designed so Tool that it could take-up backlash in both directions of rotation for machine tool axes. T ese early ‘ballscrews’ Development andwere f tted to a converted Cincinnati Milling Machine Company’s ‘Hydro-Tel’ die-sinking machine tool, Some 2.2.1 Concise Historical Perspectiveat MIT (Massachusetts Institute of Technology). T is military research-funded project having been Pioneers in Metalcommissioned by the United States Air force – who of the Development of required complex free-form aeronautical parts to be Machine Cutting Toward the end of the 1700’s, any high-quality machin- automatically machined for the latest aircraf . T is Tools ing at the time meant tolerances of 0.1mm being con- research was undertaken by MIT, in association with sidered as ‘ultra-precision’, with this level of tolerance ‘Cincinnati’ and the Parsons Tool Company. T e having steadily improved from the beginning of thebinary -coded punched-paper tape, controlled the Industrial Revolution. Pioneers in machine tool devel- simultaneous machine tool axes using alpha-numeri- opment such as John Wilkinson (1774), developed the cal characters (ie the forerunner of today’s programs f rst boring machine, this being capable of generating using ‘G- and M-coded’ CNC controllers), through a a bored hole of 1270 mm in diameter, with a error of about 1 mm. A contemporary of Wilkinson, namely Henry Maudslay (1771–1831), invented many preci- sion machine tools, but he was particularly noted for Who, when and where ‘recirculating ballscrew’ design and the design and development of the f rst engine lathe. development took place is open to some debate. As propo- Slightly later, Sir Joseph Whitworth (1803–1887), de- nents in the UK say it was Alfred Herbert and Sons, whereas veloped the f rst modern-day Vee-form screwthread in the United States, the Parsons Tool Company are of en and nut (i.e. 55? included angle – ‘Whitworth thread’), quoted as the originators. However, what is not in question, thereby enabling precision feed-motion to be achieved is that with its unique ‘Gothic’ arch’ (i.e. Ogival geometry), via suitable gear trains on such machine tools. T ese having point contacts between the screw and the adjacent re- early fundamental advances in machine design, al- circulating balls, allows the assembly to be pre-loaded in-situ, lowed others and in particular, Joseph R. Brown thereby eliminating any appreciable backlash allowing accu- (1852) to design the ‘dividing engine’. T is newly-de- rate control of these axes. veloped equipment, allowed precision engraving of NB T e previous Acme taper thread (i.e. 29? included angle) the hand dials on machine tool axes, enhancing them f tted to ‘conventional’ machine tools had an ef ciency of with much better machinist’s judgment in both rotary no better that 40% – with backlash present, whereas today’s hardened ‘ballscrews’ have ef ciencies of ~90%, coupled to and linear control, in combination with consistent an impressive rigidity (~900 N μm ) and minimal ‘stick-slip’ , repeatability by the skilled operative. Shortly af er – these developments, Eli Whitney produced the origi- therefore minimising the so-called ‘ballscrew wind-up’ due to nal milling machine, which was ref ned still further the action of torque-ef ects in combination with the cutting forces. valve-driven hydraulically-servo controlled ‘computer’ In 1873, Hartig tabulated research into metal cutting called ‘Whirlwind’. in a book, which was the f rst authoritative work on In the late 1970’s, with the advent of microproces- the subject. A more practical metal cutting description sor technology, these later NC machine tools were was given by Tresca (1878), ustilising visio-plasticity converted to Computer Numerical Control (CNC)models . In 1881, a presentation at the Royal Society, of ering a signif cant stride forward in operator-us- of London by Lord Rayleigh of Mallock’s metal cut- ability, via on-board editing – without the costly and ting research f ndings was given. Mallock’s scientif c timely re-punching of NC paper tapes each time a study of carefully etched specimens of the workpiece minor modif cation occurred to the NC program. and attached chip for both ferrous and non-ferrous Today, CNC machine tools have fast multiple-proces- metals, where he observed them using a microscope sor controls, with on-line computer graphics, enabling (magnif cation: x5). Mallock correctly surmised from new programs to be written and ‘prove-out’ while the his investigation of his ‘models’ that the cutting pro- machine tool cuts other components, or the programs cess was basically one involving shearing and, that can be automatically down-loaded by a Direct Nu- friction occurred in forming the chip, emphasizing merical Control (DNC) data-link from the CAD/CAM the importance of this friction along the cutting tool’s workstation, or via remote satellite-linkage from other face – between the chip and the tool. T e sharpness sites either locally, or internationally. T e design and of the cutting edge was also mentioned and the rea- development of some of today’s and the future machine sons for instability of the cutting process, leading to tools, utilise ultra-fast CNC microprocessors, coupled unwanted vibrations, or ‘chatter’. Moreover, Mallock to orthogonal multi-axes linear-induction motor- employed basic lubricants in this work, noting that driven slideways, that can be precisely monitored via the application of lubrication reduced chip/tool inter- laser-controlled positional encoders, with ultra-fast face friction. T ese general observations by Mallock co-axial spindles. Moreover, non-orthogonal-axes mentined above, of er a surprisingly close approxi- controlled machine tools are under development, us- mation to today’s theories on the ‘mechanics of metal ing simultaneous mulitple-axes slideway control, with cutting’, although his equations for the work done in hybrids having tool spindles that incorporate multiple internal shearing and chip and tool friction were in- angular orientation together with their linear slideways correct, surprisingly, he was unaware of the ‘plasticity for truly sculptured free-form surface machining capa- models’ by Tresca and his theory of ‘plastic heating’. bilities. Even today, operations carried out by several To compound the metal cutting problems still further, machine tools are now being incorporated into one in 1900, an unfortunate ‘step backward’ in the under- hybid machine tool, with such as: turning, milling and standing of the metal cutting process was taken by grinding at one set-up. In the near future, the machine Reuleaux. He suggested that a crack occurred ahead of tools will have slideway acceleration/decelerations of the tool’s point and likened the cutting action to that faster >5g’s, with these machines having the ability to: of splitting wood, regrettably having popular support rough-turn, mill, heat-treat, grind critical features, all for some years. remotely-controlled via satellite from the CAD/CAM In 1907, a seminal paper by the now-famous Amer- designer, signif cantly speeding-up the product devel- ican researcher Taylor, who published his 26 years of opment process time-to-market. Tresca’s visio-plasticity models, involved scoring a grid of 2.2.2 Pioneering Work inaccurate closely-spaced lines onto the edge of a specimen Metal of metal to be machined, then partially cutting it at a preset Cutting – adepth of cut and leaving the chip attached. He then investi- Basic research into metal cutting did not commence Brief Resumé gated the plastic deformation that had taken place as these until approximately 70 years af er the f rst machine tool grids were distorted and buckled by the action of machining. was introduced. In 1851, early research by Cocquilhat Both lighter and deeper cut depths were investigated in this was into the work required to machine a given volume manner, across a range of metal specimens. Tresca noted that of material by drilling. By 1870, the terms ‘chip’ and f ner depths of cut introduced greater plastic deformation than larger cut depths, stating that stif er and more powerful ma- ‘swarf ’ were introduced by the Russian engineer Time, chine tools were needed to benef t from these recommended where he attempted to explain how chips were formed. greater depths of cut (i.e. undeformed chip thickness). . Figure 24. The formation of a continuous chip, based upon the ‘deck of cards’ principle. *After: Piispanen, 1937, practical experience into investigation and research cutting conditions to be attained. By establishing op- fndings in metal cutting. Taylor, was fascinated by timum cutting data for metal cutting operations and the application oftime-and-motion studies that could employing ‘piece-work systems’ at the company, Taylor be applied within the machine shop and in particular, was able to increase the Bethlehem Steel Company’s ‘piece-work systems’ . In order to enable the progres- output by 500%. Of particular note, was the fact that sion through optimisation of these time-and-motion the empirical law governing the cutting tool and its studies, new cutting tool materials were employed, anticipated tool life is still used today, in the study in particular high-speed steels (HSS). Taylor investi- of machining economics – more will be said on this gated the efect that tool materials and in particular, topic later in Chapter 7 (Machinability and Surface cutting conditions had, on tool life during roughingIntegrity). operations, in order to assist in the application ofthese Notable in the years prior to World War Two, were time-and-motion studies. His principal objective was the contributions made into the generation ofdata on to establish empirical laws that would enable optimum cutting forces and tool life, initially by Boston (1926) Piece-work systems are where a set time allowance is given for Taylor’s machinability work produced a fundamental dis- a particular job, or a batch and, a bonus is agreed if the worker covery, namely, that the interface temperature existing at the performs this task within the allotted time. tool’s cutting edge controlled the tool-wear rate. . Figure 25. Variations in chip morphological surfaces at dif erent cutting speeds, giving an indication of the various shearing mechanisms. [Source: Watson & Murphy, 1979] and later, by Herbert (1928). Around this time, the cut- material being cut in a somewhat similar manner to ting speeds were steadily improving with the arrival of that of a pack of cards sliding over one another, with new cutting tool materials, such as cemented carbide. the free surface an angle, which corresponded to the In 1937, Piispanen introduced his so-called ‘Deck of shear angle (,). So, as the tool’s rake face moves rela- Cards’ principle as an explanation of the cutting pro- tive to that of the workpiece, it ‘engages’ one card at cess (see Fig. 24 for Piispanen’s idealised model, with a time, causing it to slide over its adjacent neighbour, Fig. 25 depicting sheared chips at a range of cutting this process then repeats itself ‘ad f nitum’ – during speeds). Here, Piispanen’s model depicts the workpiece the remainder of the cutting process. Some important limitations are present with Piispanen’s model, namely picted schematically in Fig. 26. Te overall machining that it: process is well concealed behind a amalgamation of: • exaggerates strain in homogeneity, workpiece material, high speeds and feeds, elevated shows tool face friction as elastic rather than plastic temperatures and enormous pressures • . Te actual in nature, cutting dynamics in contemporary machining opera- • considers shearing takes place on a completely f at tions, utilises just a few millimetres of physical contact plane, between the tool and the chip of a precisely-shaped • assumes that BUE does not occur, cutting edge geometry in an exotic mixture of tool ma- takes an subjectively assumed shear angle, terial to ef ciently machine the workpiece – this being • takes no account of either chip curling, or predic- an impressive occurrence worthy of note. • tion of chip/tool length. In the early work on machining, it was thought that the chip was formed by deformation along a shear NB Piispanen’s model is easily understood and does plane, elastically in the f rst instance, then plastically contain the major concepts in the chip-forming as the evolving chip passed through a stress concentra- process – admittedly for simple shear in the main.tion. Te Piispanen model (i.e. Fig. 24) illustrates this point, where workpiece material is being cut by pro- By way of further information concerning chip mor- gressive slip relative to the tool point, an angle which phology: the micrographs of chip surfaces illustrated corresponded to that of the shear plane. Here (i.e. Fig. in Fig. 26 show in these cases, that the morphology 24), it shows how each chip segment forms a small, but indicates a semi-continuous chip form. Tese chip very thin parallelogram, with slippage occurring along forms point towards the fact that noticeable periodic its shear plane. variations have occurred, perhaps as the result of theIn an orthogonal cutting process stress becoming unstable, rather than resulting from material approaches this ‘shear plane’ it will not be- any vibrational ef ects produced by the machine tool. gin to deform until it reaches the ‘shear plane’. Here, , as the workpiece Any such instability, has the ef ect of causing minute it is transformed from that of simple shear, as it moves oscillations (i.e. backward and forward motion) in across a thin shear zone, with the minute amount of the shear zone, while the machining takes place. Te secondary shear being virtually ignored, as is the case dif erences in segment shapes shown and their fre- for tertiary shear – this being the equivalent of a slid- quency occurring at dif ering cutting data in these ing friction but having a constant coef cient of fric- micrographs, are thought to be dependent upon the tion. Chip deformation in reality, is produced over a frequency of the shear plane’s oscillation relative to the zone of f nite width, usually termed the ‘primary shear cutting speed. zone’ (see Fig. 26). As the chip evolves, the back of the A considerable volume of fundamental work on chip tends to be roughened, due to the plastic strain machining research has been undertaken over the being inhomogeneous in nature (see Fig. 25). Tis last few years, but during World War Two (i.e. from shearing action creates a particular chip morphology a European perspective), Ernst and Merchant (1941) as a result of the either, stress concentrations, or by produced another signif cant paper dealing with the presence of points of weakness in the workpiece be- mechanics of the machining process – some of these research f ndings will be brief y dealt with in the chap- ter on Machinability and Surface Integrity, along with other contributions to this subject. Interface pressures between the chip and the tool are nor- mally exceedingly high, typically of the order of 1,000 to 2,000 2.3 Chip-N mm , with temperatures in certain instances at the tool’s – Development face reaching approximately 1100?C. Orthogonal machining, is when the cutting tool’s edge (i.e. rake Most metallic materials can be considered as rela- face – see Fig. 19b) is presented ‘normal’ to the evolving chip and tively hard to machine and this is evident from all of thus, to the workpiece, at 90? to the relative cutting motion. Tat the reported literature on the subject of metal cutting, is, little if any, side shearing action occurs, while the chip is be- indicating that shearing occurs in a concentrated re- ing formed as it progresses up the tool’s rake face – ef ectively gion between the chip and tool, this ef ect being de- created by two distinct cutting forces: tangential and axial. . Figure 26. Schematic representation of a sing-point stock removal process, during the continuous cutting of ductile metals ing machined . Once the chip deformation begins, it physical properties within the chip/tool region, as the 0 will continue within this ‘zone’, as though here in this various deformation zones are distinctly altered. In ef- negative strainvicinity, the workpiece material is exhibiting a form of f-hardening. rake’s inclination), this can have a profound af ect onect, due to rake angle modif cation (i.e. changing the Te oblique cutting process presents a dif erent the: cutting forces, frictional ef ects, power require- and much more complex analytical problem, which has ments and machined surface texture/integrity. been the subject of a lot of academic interest over the Te chips formed during machining operations can years. Even here, the whole cutting dynamics change, vary enormously in their size and shape (see Fig. 35a). when the tool’s top rake surface is not f at, which is the Chip formation involves workpiece material shearing, normal status today, with the complex contoured chip- from the vicinity of the shear zone extending from the breaker geometries nowadays employed (typically il- tool point across the ‘shear plane’ to the ‘free surface’ lustrated in Figs. 4, 10 and 27a). at the angle (,) – see Fig. 26. In this region a consider- Actual chips are normally severely work-hardened, able amount of strain occurs in a very short time in- in particular with any strain-hardening materials (for terval, with some materials being unable to withstand example: high-strength exotic alloys employed for this strain without fracture. For example, grey cast heat-resistance/aerospace applications) as they evolve, iron being somewhat brittle, produces machined chips by the combined action of: elevated interface tempera- that are fragmented (i.e. termed ‘discontinuous ’), con- tures, great pressures and high frictional ef ects. Such versely, more ductile workpiece materials and alloys machined action of the combined ef ects of mechanical such as steels and aluminium grades, tend to produce and physical work, produce a ‘compressive chip thick- chips that do not fracture along the ‘shear plane’, as ness’ , which is on average, dimensionally wider than a result they are continuous. A continuous chip form the original undeformed chip thickness (see Fig. 26). may adopt many shapes, either: straight, tangled, or Te rake angle depicted in Fig. 26 is shown as posi- with dif erent types of curvature (i.e. helices – see Fig. tive, but its geometry can tend to the neutral, right 35a). As such, continuous chips have been signif cantly through to the negative in its inclination. As the rake worked, they now have considerable mechanical angle changes, so will the complete dynamic cutting strength, therefore ef ciently controlling and dealing behaviour also change, modifying the mechanical and with these chips is a problem that must be overcome (see the section on Chip-breaking Technology). Chip formation can be classif ed in a number of distinct ways , these chip froms will now be brief y reviewed: • Continuous chips – are normally the result of high 0 As the shear plane passes through a particular stress concen- tration point, it will deform more readily and at a lower stress cutting speeds and/or, large rake angles (see Figs. value, than when one of these ‘points’ is not present. 26 and 27b). Te deformation of workpiece mate- Oblique machining, is when the rake face has a compound an- rial occurs along a relatively narrow primary shear gle, that is it is inclined in two planes relative to the workpiece, zone, with the probability that these chips may de- having both a top and side rake to the face, creating a three- velop a secondary shear zone at the tool/chip inter- force model (see Fig. 19a), where the cutting force mathemati- face, caused in the main, by frictional ef ects. Tis cal dynamics are extremely complex and are of en produced secondary zone is likely to deepen, as the tool/chip by either highly involved equations, or by cutting simulations. friction increases in magnitude. Deformation can Tis latter simulated treatment is only brief y mentioned later also occur across a wide primary shear zone with and is outside the remit of this current book. However, this information on dynamic oblique cutting behaviour can be gleaned, from some of the more academic treatment given in some of the selected books and papers listed at the end of this chapter. Compressive chip thickness is sometimes known as the: chip One of the major cutting tool manufacturer classif es chips in thickness ratio (r)* – being the dif erence between the unde- seven basic types of material-related chip formations, these formed chip thickness (h ) and the width/chip thickness of the are: Continuous, long-chipping – mostly steel derivatives, La- chip (h ). mellar chipping – typically most stainless steels, Short-chip- *Chip thickness ratio (r) = h /h  ** (i.e. illustrated in Fig. 26). ping – such as many cast irons, Varying, high-force chipping ** h = W/ρwl – many super alloys, Sof , low-force chipping – such as alu- Where: W = weight of chip, ρ = density of (original) work- minium grades, High pressure/temperture chipping – typif ed piece material – prior to machining, w = chip width (i.e DOC ), by hardened materials, Segmental chipping – mostly titanium l = length of chip specimen. and titanium-based alloys. . Figure 27. Chip-breaking inserts and chip control whilst turning – in action. [Courtesy of Iscar Tools] . Table 4. Strength and hardness of chips when turning mild steel. Rake angle (γ?): 45 35 27 10 10 10 –1 Feed (f - mm rev 0.30 0.30 0.20 0.20 0.20 0.20 ) 50 50 168 168 168 76 Cutting speed ( VC - m min ) None None Soluble oil None Soluble oil None –1 91 93 75 84 92 95 Tensile strength (UTS – kg mm ) Cutting f uid: –2 272 289 302 320 314 325 Vickers hardness number (Hv) 3.4 3.3 3.4 3.4 3.6 3.5 HV/UTS Shear strain (Pa-s) 1.1 1.7 2.1 2.9 3.1 4.0 [Source: Nakayama and Kalpakjian 1997] curved boundaries, with the lower boundary being• Continuous chips with a built-up edge (BUE) – when below the machined surface (Fig. 26), which may machining ductile workpiece materials, a built-up distort a sof er workpiece’s machined surface – par- edge (BUE) can form on the tool’s tip. Tis BUE con- ticularly with small rake angles and at low speeds. sists of gradually deposited material layers from the Strain-hardening of this type of chip, results in it workpiece, hence the term ‘built-up’ (see Fig. 28). becoming harder than the bulk hardness of the As cutting continues, the BUE becomes larger and original workpiece material (see Fig. 28c, where the more unstable, eventually partially breaking away, bulk workpiece hardness is 230 HK and the work- with some fragments being removed by the under- hardened chip is ?350 HK). Tis increase in the side of the chip, while the remainder is randomly chip’s strength and hardness will depend upon thedeposited on the workpiece’s surface (Fig. 28a). shear strain (see Table 4 for details of the Rheololog- Tis process of BUE formation, shortly followed by ical status, related to the inclination of the tool’s its destruction, is continuously repeated during the rake angle). Terefore as the rake angle decreases, whole cutting operation. Te BUE deposited on the the shear strain will increase, causing this con- workpiece will adversely af ect the machined sur- tinuous chip to become both harder and stronger face texture. Te BUE modif es the cutting geom- – behaving in a similar manner to that of a rigid, etry, creating a large cutting tip radius (Fig. 28a and perfectly plastic body. In order to satisfactorily deal b). Due to the BUE being severely work-hardened with long continuous work-hardened chips, that by the action of successive deposits of workpiece could either wrap around the machined workpiece, material, the BUE’s hardness signif cantly increases potentially spoiling the surface texture, or become by around 300% over the bulk component hardness ensnarled around tooling, or even, reduce ef cient (Fig. 28c). At this severely work-hardened level, the coolant delivery to the cutting edge, with integrated BUE becomes in ef ect a modif ed cutting tool. Nor- tool chip-breakers having been designed and devel- mally, an unstable BUE is undesirable, conversely, a oped – see Fig. 27b. thin stable BUE is as a rule, regarded as desirable, as it protects the top rake surface. Te formation mechanism for the BUE is thought to be one of ad- hesion of workpiece material to the tool’s rake face, with the bond strength being a function of the af- f nity of the workpiece to that of the tool material. Rheology is a branch of science dealing with both the f ow Tis adhesion, is followed by the successive build- and deformation of materials, with the shear strain rate, of en up of adhered layers forming the BUE. Yet another termed just the shear rate. (i.e usually quoted in Pascals-sec- factor that contributes to the formation of a BUE, onds ‘Pa-s’). . Figure 28. The development of a continuous chip with Built-Up Edge (BUE), its typical hardness distribution and its af ect on the machined surface is the strain-hardening tendency of the workpiece increase the probability that such defects occur in the material. Terefore, the greater the strain-harden- cutting zone, thereby aiding discontinuous chip for- ing exponent, the higher will be its BUE formation. mation. While, faster cutting speeds result in higher From experiments conducted on BUE formation, it localised temperatures, causing greater ductility in the would seem that the higher the cutting speed, the chip, lessening the tendency for the formation of dis- less is the tendency for BUE to form. Whether this continuous chips. If the magnitude of the compressive lack of BUE formation at higher cutting data is the stresses in the both the primary and secondary shear consequence of increased strain rate, or the result zones signif cantly increase, the applied forces aid in of higher interface temperatures is somewhat open discontinuous chip formation, this is because of the to debate. However, it would seem that a paradox fact that the maximum shear strain will increase, due exists, because as the speed increases, the tempera- to the presence of an increased compressive stress. ture will also increase, but the BUE decreases. Te propensity for BUE formation can be lessened by:NB Due to the nature of discontinuous chip I. Changing the geometry of the cutting edge – tion, if the workpiece-tool-forma- by either increasing the tool’s rake angle, or de- ciently stif , this will generate vibrational and chatter machine loop is not suf - creasing the D , or both,OC tendencies, which can result in an excessive tool wear II. Utilising a smallerregime, or machined component surface damage. cutting tip radius , • Segmented chips – are sometimes termed: in-, or III. Using an ef ective cutting f non-homogeneous chips, or serrated chips. Tis uid,or • Discontinuous chips – consist of adjacent work- chip form has the characteristic saw-toothed pro- IV. Any combination ofpiece chip segments that are usually either loosely f le which is noted by zones of low and high shear these factors . attached to each other, or totally fragment as they strain (Fig. 30). Tese workpiece materials possess are cut (Fig. 29). Te formation of discontinuous low thermal conductivity, as such, when machined chips usually occur under the following machining their mechanical strength will drastically decrease conditions: with higher temperatures. Tis continuous thermal I. Brittle workpiece materials –cycle of both fracture and rewelding in a very nar - row region, creates the saw-toothed prof le, being these materials particular relevant for titanium and its alloys and do not have the machining capability to un- certain stainless steel grades. For example, to ex- dergo the high shear strains, plain what happens in realistic machining situation, II. Hard particlesand impurities – materials the specimen Fig. 30a is displayed, for an austenitic with stainless steel quick-stop micrograph. Tis micro- these in their matrix, will act as ‘stress-raisers’ graph being the result of a less than continuous ma- and actively encourage chip breakage, chining process (1), utilising a 5? top rake-angled III. Very high, or low cutting speeds – turning insert. Here, variations in the cutting pro- chip veloc- IV. Low rake angles/large D OC’s – either cess have created f uctuations in the cutting forces, ity at both ends of the cutting spectrum, will resulting in waviness of the machined surface (2). small top result in lack of adherence/fragmentation of Prior to the material yielding, then the shearing rakes and heavy DOC’s will decrease the adher- the chip segments, V. Inefective cutting f uid – poor lubricity, process occurring, the workpiece material has de- ence of the adjacent chip segments, formed against the cutting edge (3). To explain how com- changing the top rake angle inf uences the resul- bined with a meagre wetting ability, will en- VI. Inadequate machinetool stif ness – creating tant chip formation for an identical stainless steel courage discontinuity of chip segments, vibrational tendencies and cutting instability, workpiece material, Fig. 30b is shown. Machining leading to disruption of the machining dynam- has now been undertaken with a 15? top rake, pro- ics and loosening of chip segments. moting a more continuous machining process than was apparent with the 5? tool (i.e. illustrated in Fig. As mentioned in ‘Roman II’ above, the hard particles 30a). Tis more ef cient cutting process, results in and impurities tend to act as crack nucleation sites, smaller variations in the cutting forces (1 and 2). therefore creating discontinuous chips. Large D’s OC Te chip is seen to f ow over the rake face in a more . Figure 29. Discontinuous chip formation. [Courtesy of Sumitomo Electric Hardmetal Ltd.] consistent manner (3). It was found with this work- 2.4 Tool Nosepiece material in an experimental cutting proce- dure, that the tangential cutting force component, Radius was closer to the actual cutting edge than when Te insert’s nose radius has been previously mentioned similar machining was undertaken on unalloyed in Section 2.1.6, concerning: Cutting Tool holder/In- steel specimens. sert Selection. Moreover, the top rake geometry of the cutting insert will signif cantly af ect the chip forma- NB Te cutting data for machining the stain- tion process, particularly when prof le turning. In Fig. less steel specimens in Figs. 30 a and b, were: 31a, a spherically-shaped component is being ‘prof le – – 180 m min cutting speed, 0.3 mm revfeedrate, machined’ using a large nose-radiused turning insert. 3 mm DOC. Here, as the component nears its true geometric cur- vature, the cutting insert forces will f uctuate continu- . Figure 30. Segmented chip formation, resulting from machining stainless steel and the work-hardening zone – which is af ected by the sharpness of the insert’s edge. *Courtesy of Sandvik Coromant, . Figure 31. The cutting insert’s tool nose radius when either profling, or general turning, will modify both the profle and diameter as fank wear occurs. [Courtesy of Sandvik Coromant] ously as the insert progresses (i.e circular interpolates drif ’ which could af ect the process capability of the with the X- and Z-axes of the machine tool) around overall parts produced. Tis f ank wear ‘VB’ can be cal- the curved prof le. If the geometry of the tool was not culated and utilised to determine the anticipated tool’s itself of round geometry, then the ‘point-contact’ could life (ie, in-cut), this important factor in production not be maintained, leading to signif cant variations in machining operational procedure, will be discussed in chip formation. If this lack of tool-work contact were due course. not to occur, then the machined prof le would be Wiper blades (Fig. 32) are not a new insert geom- compromised and due to insuf cient chip control, the etry concept, they have been used for face milling op- actual cut surface prof le would not have a consistent erations for quiet a long time, but only in recent years and accurate surface texture. are they being utilised for component f nish turning. Te machined surface texture generated by the pas- Te principle underlying a wiper insert for turning op- sage of the cutting insert’s geometry, is to a large extent erations, concerns the application of a modifed ‘tool the product of the relationship, between the nose ra- nose radius’ (see Fig. 32 – bottom lef and right dia- dius and the feedrate and, to a lesser degree the cutting grams). When a ‘standard’ tool nose geometry insert speed and its tool wear pattern. Te size of the tool is used (i.e. Fig. 32 – bottom lef ), it creates a series of nose radius will have quite an ef ect on the surface tex- peaks and valleys (i.e. termed ‘cusps’ ture produced, if the feedrate is set, then a small nose sage of the ‘insert nose’ over the machined surface. radius will create a dif erent workpiece surface texture Conversely, a cutting insert with ) af er the paswiper blade - geom- to that of a larger one (see Fig. 31b). Moreover, if a etry (i.e. Fig. 32 – bottom right), has trailing radii that large nose radius is selected for a lighter DOC, or if the blends – beyond the tangency point – with the tool feed is equal to the nose radius, then this larger nose nose radius which remains in contact with the work- geometry will be superior to that of a smaller tool nose piece, allowing it to wipe (i.e. smooth) the peaks, leav- radius. Tis is because the ‘larger nose’ of ers a smaller ing a superior machined surface texture. plan approach angle, having the pressure of the cut In the past, wiper insert geometries were only em- distributed across a longer cut length, creating an en- ployed for surface improvement in f nishing opera- hanced surface texture. Tere are several disadvan- tages to utilising a larger tool nose radius geometry, these are that the: • Chip formed becomes more dif cult to bend and ef ectively break, • Radial cutting forces are greater, Power consumption increases, • Process capability denoted by ‘CP’ , is a measure of the quality Rigidity of the set-up is necessary – leading to pos- • of the parts produced, which is normally found by the follow- sible vibrational tendencies on either weaker, or ing simple relationship: *CP = Drawing specif cation tolerance/6 σ unstable workpieces. Where: σ = a statistical measure, termed the ‘standard devia- tion’ for the particular production process. , CP values of < .0Tool wear (i.e. denoted by ‘?’ in Figs. 31ci and cii) and in particular f ank wear , can signif cantly inf uence denote low process capability, CP values of between .0 andthe resulting machined component dimensional accu- . are moderate process capability, CP values of > . racy (Fig. 31cii), which on a batch of components cut NB Today, process capabilities of .0 are of en are with the same insert, will result in some level of ‘tool demanded for termed as high process capability. high-quality machined parts for the automotive/aerospace sectors of industrial production, reducing likelihood of part scrappage. Cusps are the product of the partial geometry of the tool nose radius geometry, positioned at regular intervals related to the selected feedrate. Te cusp height (i.e. the dif erence in height between the peak and valley), will inf uence the machined surface texture of the component, in the following relation- ship:   Flank wear is normally denoted by specif c ‘zones’ – more will Rmax = fn × 250/rε (µm) Where: Rmax= maximum peak -to-valley height within the sam- be said on this topic later – but, in this example, the tool’s in- – ) rε = tool nose radius (mm). sert wear ‘VB’ is shown in both Figs. 31ci and cii. pling length. fn = feedrate (m min . Figure 32. The application of wiper insert geometry on the resulting surface texture when f ne turning. [Courtesy of Iscar Tools] tions. With recent advancement in wipergeometry, tool-changing and setting, signif cantly increasing ma- this has allowed them to be used at double the previ- chine tool utilisation rates. Even when conventional ous feedrates for semi-f nishing/roughing operations, turning inserts are employed, for heavy roughing cuts without degrading the surface texture. Te wiper ge- (Fig. 33a), where feedrates are high as are the large ometry being in contact with the workpiece’s surface DOC’s, ef cient control of the chip must be achieved. To for longer than equivalent standard insert nose radius enable excellent control of chip-breaking with rough- tends to wipe – hence its name, or burnish the ma- ing cuts (Fig. 33b), a similar overall insert geometry chined surface, producing a smoother surface texture. is shown to that in the previous example, but here the Due to the fact that a ‘wiper’ has an extended edge, the rake face embossed dimples/chip-breakers dif er sig- cutting forces are distributed across a longer tool/chip nif cantly. Finally, for light f nishing cuts (Fig. 33c), contact region. Te wiper portion of the insert, being chips are broken in a totally dif erent manner to that of somewhat protected, enables these wiper inserts to in- the previous examples. Hence, with all of these dif er- crease tool life by up to 20% more than when using ing types of turning operations on workpieces, control conventional tool nose geometries. of the chip is vital, as it can drastically impair the over- Wiper blades have their clearance lengths care- all production rates and af ect part quality, if not given fully designed, if they are too long, the insert gener- due consideration. ates too much heat, on the contrary, they need to be Chip formation is chief y inf uenced by the follow- long enough to cope with relatively large feeds, while ing factors: • still smoothing over the surface cusps. Wipers with Workpiece material composition – positive turning insert geometries, they can cope with its heat treat- – • ment (i.e. if any), which af ects the chip’s strength, feedrates of 0.6 mm rev at DOC’s of up to 4 mm. For Insert’s cutting geometry – rake and example, with steel component hardnesses of 65HRc, clearances, as • this of en negates the need for any successive precision well as any chip-formers present, the geometry be- grinding operations. By designing wiper geometries ing associated with the work piece material, • with the cutting edge and nose radii to improve ma- Plan approach angle – depending chined surface f nish, while increasing tool life, can be upon whether considered as outstanding tool design. roughing, or f nishing cuts are to be taken, • Nose radius – this being linked to the feedrate and 2.5 Chip-Breaking here, to a lesser extent, the surface texture require- ments, Undeformed chip thickness (i.e.Note: Another important factor that can also play a Technology D OC) – this will af- 2.5.1 Introduction to Chip-Breakingsignif cant role in chip formation, is the application of fect the chip curling aspect of the chip’s formation – coolant and its supply velocity. more will be said on this topic in the following sec- Te technology of both chip-forming and chip-break- tion. ing has been one of the major areas of advancement Te shear angle has some ef ect on the contact length in recent years. A whole host of novel toolholders and between workpiece and the rake face and, it is in this cutting inserts has been developed to enable the cut- vicinity that cutting forces and machining-induced ting process to be under total chip control, allowing temperatures predominantly af ect the cutting insert. some toolholder/inserts combinations to machine Moreover, the insert’s rake is signif cant, in that as the multiple component features with just one tool, re- rake angle increases the contact length decreases, the moving at a ‘stroke’ the non-productive aspects of more positive the rake, the shorter the contact length. Actual chip formation is primarily dependent upon several factors: DOC, feedrate, rake angle, together with the workpiece’s mechanical strength, noting that the chip starts forming in the primary deformation zone Some tooling manufacturers have re-named wiper inserts as (see Fig. 26). Tus, the chip is subsequently formed high-feed inserts, as they have demonstrated in production by the bending force of the cutting action, ef ectively conditions to promote higher component output, without the ‘pivoting’ from the chip’s roughen ‘free top surface’, recourse to expensive capital outlay. . Figure 33. Turning cuts and associated insert geometries for forming and shearing of a chip. [Courtesy of Sandvik Coromant] this being a somewhat shorter length than that of the ing equipment, such as overhead gantries, or dedi- ‘shiny’ underside at the tool/chip interface. cated robotic loading devices. Many theories have been given for the actual ‘cause and ef ect’ of preliminary chip formation which is In terms of priority for these swarf control factors, pos- schematically illustrated Fig. 33d – ‘A’- one such, be- sibly the most important one is that the swarf should ing that any formation is related to the cutting speed. f ow smoothly away from the cutting area, as with the A large insert rake angle normally means that there is latest chip-breakers f tted to today’s cutting inserts, less tendency for chip curling through a larger radius, chips can be readily broken and controlled , this will 0 but it will have lower cutting forces. In Fig. 33d – ‘B’, be theme of the following section. cutting process, which can be expressed via the simpleis depicted a somewhat ‘idealised’ view of the actual 2.5.2 The Principles of Chip-relationship of ‘λ’ and ?X/?Y. Breaking NB: In this schematic representation: ‘h ’ represents In machining, the cutting edge’s primary function is DOC and, ‘,’ is the ‘shear plane angle’. to remove stock from the workpiece. Whether this is achieved by forming a continuous chip, or by the When utilising CNC machine tools and in particu- f ow of elemental chips will depend upon several fac- lar turning centres, a major problem is the variety of tors, including the properties of the workpiece mate- continuous chip forms created and the large quantity rial, cutting data employed and coolant type and its and volume of swarf produced. Te manner to which delivery. Te terms ‘long-chipping’ and ‘short-chipping’ swarf af ects machining operations depends upon the are utilised when considering the materials to be ma- operating conditions, but fundamentally there are sev- chined. Short-chipping materials such as most brasses eral requirements in any form of swarf control, these and cast irons, do not present a chip-breaking problem are: for swarf disposal, so this section will concentrate on • Te swarf must f ow freely away from the cutting the long-chipping workpiece materials, with particu- zone, without impairing the cutting action’s ef - lar focus on ‘steel family’ grades. Steels are produced ciency, in a wide variety of specif cations and this allows • Swarf must be of convenient size and shape to fa- their properties to be ‘tailored’ to the specif c indus- cilitate handling manually, or in swarf conveyors trial applications. In addition, these steels methods of (i.e. if f tted), together with any future large-volume primary processing, such as: casting, forging, rolling, storage, then transportation and subsequent dis- forming and sintering, together with the type of subse- posal, quent heat treatment, creates still further metallurgical • Any swarf should drop away into the machine’s variations that may have an even greater inf uence on swarf tray, without snarling around, the workpiece, the workpiece’s chip-breaking ability. Te workpiece’s tool, or interfering with other functions such as: strength and hardness values describe the individual automatic tool-changing magazine/turret, in-situ material’s character to some extent, but it should be touch-trigger inspection probes, component load- borne in mind that it is the chip’s mechanical strength that determines whether it can be broken with ease. No absolute correlation exists between a steel com- Individual chips when in any great volume are generally termed swarf. It is important to be able to manage this swarf 0 volume and, satisfactory chip control can be determined by Today, many high -volume manufacturing companies have re- ‘Lang’s chip-packing ratio’ , this being denoted by the letter alised the benef t of the value of clean and briquetted swarf, ‘R’ , in the following manner: as opposed to oily scrap swarf, which sells at just ‘fractions’ R = Chip volume (mm )/Equivalent volume of uncut work- of this value. At present, briquetted and cleaned aluminium piece material (mm swarf can be sold for approaching ?1,000/tonne, moreover, ) the coolant/oil can be reclaimed, further driving down the NB: ‘R’ ranges from values of 3-to-10, where an R-value of overall machining costs. For other non-ferrous ‘pure’ metals 4 gives satisfactory chip-breaking control, producing neatly and others, such as copper alloys and brasses, the economic curled ‘6 and 9-shaped’ chips. savings are even greater. ponent’s strength and the mechanical strength of the NB Te helical formation of this chip-curling behav- chip, illustrating that a complex metallurgical and cut- iour will shortly be mentioned, but prior to this, chip- ting tool geometric relationship exists whilst machin- breakers/formers will be discussed. ing occurs. In particular for turning operations, a convention- ally-turned chip is a rather frail product of serrated 2.5.3 Chip-Breakers and Chip-Formers appearance (see Figs 25 and 34a and b). In order to promote good chip-breaking tendencies, thus enabling Chip-breakers have been utilised on turning tools for short elements to be formed, it is necessary to encour- many years, initially introduced in the 1940’s in the age this basic character by causing these serrations to form of an abutment, or step, situated behind the rake be as deep as possible and the chip sections in between face of the tool. Hence, with this type of early chip- to be rigid. Tis chip occurrence causes the chip to be breaker, as the continuous chip moves across the rake inf exible, which can then subsequently be broken. face it collides with this step and breaks. Tis origi- Tere are several distinct ways in which chips can then nal form of chip-breaker geomtery was relatively in- be broken, these include: ef cient as the resultant force direction changed with • Self-breaking – this is when the chip’s mechanical the programmed tool path, this meant that the step strength is not great enough to hold the chip seg- would be approached by the chip from dif ering di- ments together and they consequently break upon rections making chip-breaking less controlled. Such exiting the machining region (Fig. 31a), chip-breakers were superseded in the 1970’s by in- • Chip collision with the workpiece – as the chip is built ‘wavy-shaped’ chip-breakers sintered into the in- steered towards an obstacle such as the workpiece’s sert’s top face (Fig 34 bottom lef -hand photograph). surface this provides the breaking force (see Figs. Recent developments in designing chip-breaker geom- 33 and 34b), etries by computer-generated (i.e. CAD) techniques, • Chip is stopped by the tool – here the chip-curling has shown a signif cant step-forward in both chip- behaviour comes into play, this being a function of former design enabling chip control and reduction in the: tool’s nose radius geometry, depth of cut and frictional forces across the rake face at a range of cut- feedrate employed (see Fig. 34 bottom lef -hand ting data to be achieved. Such ‘automatic’ chip breaker photograph), the latter two functions af ecting the geometry forces the chip to def ect at a narrower angle, chip cross-section, or chip thickness . causing it to break of , either immediately, or just af er the free end of the chip has hit either the tool’s f ank or, the workpiece before the f rst coil has formed. If such a collision does not take place, the result would be a smaller diameter spiral chip and, it can be anticipated that the chip would still break, but only when it be- Chip thickness is inf uenced by the plan approach angle utilised and the D , in association with the selected feedrate.OC came slightly longer – this later chip breakage is due to Te chip thickness is measured across the cutting edge, per- the increasing chip mass and the ef ect of gravity upon pendicular to the cut (i.e. along the main cutting edge). Te it, with, or without any further collision. chip width and thickness are the dimensions that def ne the Chip f ow direction will depend upon several fac- theoretical cut of the edge into the workpiece material. Hence, tors, such as the: chip-breaker prof le, back rake and the chip thickness will vary with the size of the plan approach angle according to the relationships involving: feedrate, DOC setting angles, nose radius, DOC and feedrate – these and the ef ective cutting depth. Te chip thickness is related to latter three factors require further discussion. Te the plan approach angle and this af ects the amount of pressure relationship between the nose radius, DOC and feedrate bearing upon the cutting edge. Hence, the thinner the chip, will of en change during vectored tool paths in any the smaller the distributed pressure along the edge and the less machining operation. Even though the insert’s nose power consumed, conversely, the thicker the chip, the greater will be the machine tool’s power consumption. A thicker chip radius is preset, its inf uence on the chip direction is generally advantageous for an increased tool life, because of dif ers for dif erent DOC’s, depending on how much the improved contact between the chip and its cutting edge. corner rounding is represented by the total engaged Furthermore, if the plan approach angle is too small and chip edge length (Fig. 34c). Further, the feedrate also af- thickness is thin, this will reduce tool life, however, this can fects the chip thickness: at dif erent DOC’s and with a be compensated for by increasing the feedrate, to produce a thicker chip. constant feedrate, the form of chip cross-section (i.e. . Figure 34. The principles of chip-breaking and chip-breaking envelopes for ‘coma-shaped swarf ’ control and insert edge preparations the ratio of chip width-to-thickness), will change and the insert’s f ank face (see Fig. 27b). Only today’s very this has a deleterious ef ect on the insert’s chip-break- complex chip-breaker designs can reduce these out- ing ability. ward-curling helical chips. Although such chip helices produced by combinations of the feeds and DOC’s that result in the chip width being too small in relation to 2.5.4 Helical Chipits thickness must be avoided. Formation Conventional Tu Grooving and rning Recessing For the general turning operations, such as sliding (i.e. In conventional turning operations, it is signif cantly Z-axis tool feeding) and facing (i.e. X-axis tool path easier to form a manageable chip, than for features re- motions), the chip is rolled into a helix, simply because quiring either grooving, or recessing. Te chip formed the chip edges are formed from dif erent rotation radii during plunge grooving counter-rotates in relation (Fig. 34d). Here, the two edges of the chip consume to the workpiece, whereby it does not experience the dif erent quantities of workpiece material, creating dif- same twisting force as chips produced by either Z-, or fering edge lengths, coupled to the fact that a varia- X-axes turning operations. When grooving, ideally the tion in cutting speed is present, these relationships will chip resembles a ‘watch spring’, indicating that the chip result in a helical chip formation. Te appearance of is curling back onto itself and will ultimately break in the chip’s helix depends upon the workpiece’s diameter several distinct ways: such as at the completion of the and its metallurgical specif cation/condition, which grooving cycle, or due to friction between the chip and means the chip helices are extremely dif cult to quan- its groove side walls – as the chip diameter becomes tify. greater. In grooving operations, three signif cant fac- Most common types of helical chip diameters are tors af ect chip control, these are: determined either directly by the initial curvature from its origin, or are the result of additional bending, (i) Insert geometry – applied to the rake face, can be introduced by the chip-breaker. For example, the heli- classif ed into distinct groupings: • cal chip type shown in Fig. 34c (lef ), has its chip seg - Radial-ground top rake (not shown), ments turned inwards, this being a desirable chip form producing when not fully developed, that is prior to the f rst coil the desired ‘watch-spring’ chip formation. Tis being completed. Whether, or not the chip is of this grooving insert geometry will not thin the chip, form will already be determined even before it meets therefore surface f nish passes are necessary on the chip-breaker, this being the result of its cross-sec- both groove side walls. tion and the natural tendency to bend according to the NB For long-chipping materials the chip-former ‘line of least resistance’. If the chips width is no largerdoes not provide enough resistance to produce chip than its thickness, for example, the resistance to bend- curling, hence, a straight f at chip occurs, that may ing in the segment-stif ened thickness direction is larger than in the width direction. In this case, unless this kind of chip is broken early, by colliding with ei- ther part of the tool, or the workpiece whilst it is still One of the problems with this type of chip-breaking, is the stif and short – called ‘self-breaking’ – a helical chip potential for secondary wear on the insert’s non-cutting zone will be formed. In this case, the barbed, or serrated on the face, caused by the chip helix breaking locally against edge is turned outwards causing additional bending, this face. Such an occurrence happens when the chip helix at- tains such a diameter and pitch that its free-end continually this being introduced by the chip-breaker. For exam- strikes the non-cutting portion of the insert’s edge – termed ple, the helical chip type shown in Fig. 34c (right), ‘chip-hammering’ – causing the edge to be locally weakened becomes dif cult and awkward to control. Tis out- and to subsequently crumble. ward-curving helical chip also has weakened sections NB Chip-hammering can be alleviated by slightly increasing in the serrations between the chip segments, but ap- the helix diameter (i.e. by somewhat modifying the cutting plied loads on it are readily absorbed by the spring ac- data) causing the chip to break against the tool’s f ank – be- tion of the chip. Tis type of chip will break as it hits low the insert’s cutting edge, this being one of the previously employed and favoured chip-breaking mechanisms, as shown in Fig. 27b. . Figure 35. The chip-breaking envelopes related to cutting data and chip-curling behaviour. [Courtesy of Sandvik Coromant] wrap itself around either the tool, or workpiece, but tool/chip interface temperatures. Te negative factors such a geometry is perfect for machining alumin- of such a machining strategy, are that the: ium, or non-ferrous materials. • Part cycle times are increased and as a result, any batch throughput will be lessened, • Radial top rake (illustrated in Fig. • As the cutting edge is in contact for a longer du- ration, more heat will be conducted into the tool, 4 middle and than into the chip, which could have a negative im- to the lef – three grooving insert sizes illustrated). pact of inconsistent workpiece size control, Tis radial top rake is designed to thin the chip. • Due to the lower workpiece surface speed, the ben- Such chip thinning, eliminates the need to under- ef ts of the insert’s coating will be reduced, as such take f nishing passes on the groove’s side walls. Fur- coating technology tends to operate more ef ec- thermore, this type of grooving insert geometry be- • wide shallow grooves tively at higher interface temperatures. ing on-centre, enables axial turning of diameters for , or recesses. Raised bumps on top rake (see Tis sophisticated grooving geometry is utilised for (iii) Increasing the feedrate – by increasing the feed Fig. 27a – lef ). materials where chip control is difcult, as it pro- allows it to engage the chip-former more ef ectively vides an ‘aggressive barrier’ to the curling chip. Te – this being the preferred technique for chip control. A raised bumps force the chip back onto itself, either heavier applied feedrate, produces a chip with a thicker producing a tightly curled watch-spring chip, or cross-section. Further, a thicker chip engages the in- causes the chip to break. sert’s geometry with higher force, creating a greater tendency to break. Hence, by holding a constant work- (ii) Surface speed of the workpiece – in order to ob- piece surface speed, allows the faster feedrate to reduce ing abilities, the chip must be allowed to f ow into the tain full advantage of a grooving insert’s chip-form- cycle times. chip-former. Tis chip-f ow can be achieved by either Tr ansversal, or Fa ce decreasing the workpiece’s surface speed, or increasing Grooving the feed – more will be said on this shortly. Te former Transversal grooving geometry has a curved tear- technique of decreasing the surface speed, allows the shaped blade onto which, the insert is accurately lo- material to move slower across the top rake of the cut- cated and positioned. Te transversal insert follows ting edge and as a result, has greater contact time to the 90? plunged feed into the rotating face of a work- engage the chip-former. Tis slower workpiece speed, piece. Tese tools are categorised as either right-, or has the benef t of increasing tool life, through lower lef -hand, with the style adopted depending upon whether the machine tool’s chuck rotates anti-clock- wise (i.e. using a right-hand tool), or clockwise (i.e. lef -hand). Te minimum radius of curvature for such transversal grooving tooling is normally about 12mm, A groove, or recess, can normally be considered as a straight- walled recessed feature in a workpiece, as illustrated in Fig. with no limit necessary on the maximum radial curva- 40. Typical applications for grooves are to provide thread re- ture that can be machined. For shallow face grooves, lief – usually up to a shoulder – so that a mating nut and its of -the-shelf tooling is available, but for deep angular washer can be accurately seated , or for retaining O-rings. As face grooves they require specialised tools from the the groove is produced in the workpiece, the tool shears away tooling manufacturers. the material in a radial manner, via X-axis tool motion. Te chip formed with insert geometries having a f at top rake, will If a relatively wide face groove requires machining have an identical width as the tool and can be employed to with respect to the insert’s width, then the key to suc- ‘size’ the component’s width feature. However, this chip action cess here, is establishing where in the face to make the – using such a tool geometry, creates high levels of pressure f rst plunge. Tis initial face plunge should be made at the cutting edge as a result of the chip wall friction, which within the range of the tool’s diameter, otherwise the tends to produce a poor machined surface texture on these sidewalls. Grooving with an advanced chip-former insert getool will not have suf cient clearance and will ulti- - ometry, reduces the chip width and provides an ef cient cut- mately break. Successive plunges to enlarge the face ting action, this results in decreasing the cutting edge pressure groove should be made by radially moving the insert somewhat. Chip-formers of er longer tool life and improved 0.9 times the insert’s width, for each additional plunge. sidewall f nishes with better chip control, than those top-rakes Te rotational speed for face grooving is usually about that have not incorporated such insert chip-forming geomet- ric features. 80% of the speed used for parting-of – soon to be mentioned. Feedrates are normally around 50% of For any face grooving of workpiece material that is parting-of values, with the proviso that for material subject to a continuous chip formation, always use which is subject to work-hardening, minimum feeds copious amounts of coolant and at high-pressure – if are necessary. possible, to not only lubricate the cutting zone, but to In transversal grooving operations, a unique chip aid in chip f ushing from this groove. form occurs, because the chip is longer the further away it is from the workpiece’s centre line of rotation. Parting-off Tis results in the chip which no longer f ows in a straight line across the insert’s edge, instead it moves Te parting-of process is normally considered to be a at an angle. Such a naturally curved chip is dif cult to separate machining operation, but it simply consists of exhaust from the face groove, particularly if it is bro- cutting a groove to centre of rotation of the workpiece, ken. Hence, no attempt should be made to break the to release it from the bar stock, or to ‘part-of ’ to a pre- chip. For deep and narrow grooves, the best solution viously formed internal diameter (shown in Fig. 40 for is to retract the tool at short intervals, to check that lef -hand side operations). Essentially in a parting-of the blade shows no signs of rubbing, this is to guard operation, two time-periods are worthy of mention, against any likely breakage that might occur when these are: machining outside the blade’s range. Due to the fact that transversal grooving tooling is susceptible to chat- (i) At separation from the bar stock – a lower spindle ter , any excessive overhang of the tool should be mi- speed than was previously used on the workpiece, will nimised. Te chip should never be allowed to become prevent the ‘released part’ from hitting the machine from the bar stock, avoiding the parting-of tool from entangled within the transversal groove and should be and potentially damaging its surface. Moreover, it al- getting ‘pinched’ between the stock and the soon- ejected speedily, otherwise the tool is likely to break. lows an operator – if present – to hear the change in to-be-released component. Of en, ‘Part-catchers’ are Chatter is a form of self-excited vibration and such vibrations the lower spindle speed tone, as it is about to separate utilised to reduce any surface damage to the falling are due to the interaction of the dynamics of the chip-removal process, together with the structural dynamics of the machine component, once it has been parted -of . tool. Such chatter, tends to be at very high amplitude, which can result in either damage to the machine tool, or lead to pre- NB If the component to be parted-of is held in a mature tool failure. Typically, chatter is initiated by a distur- co- bance in the cutting zone, for several reasons, such as: – Lack of homogeneity – in the workpiece material (i.e. typi- spindle supports the workpiece and axial/sub-spindle, at component release, the additional cally a porous component, such as is found in a Powder ditions, the parting-of operation is virtually identical under these con- Metallurgy compact), to that of found in a grooving cycle. – Workpiece surface condition (i.e. typically a hard on a hot-rolled steel component, utilsing a shallow D ), OC oxide scale (ii) Surface speed reduction – this ef – Workpiece geometry (i.e. if the component shape produces – for example, because of un- curs when the machine’s spindle attains its maximum ectively oc- even depth of casting material being machined, or light cuts either a variation in the D OC speed. For example, on a machine tool having a maxi- on interrupted shapes, such as hexagon, square, or rectan- mum speed of 3,000 rpm, 90 m min– would only be gular bar stock), achievable until the parting diameter has reached – Frictional conditions (i.e. tool/chip about 8.6 mm. When parting to a smaller diameter interface frictional Regenerative chatter is a type of self-excited vibration, result- than 8.6 mm, the surface speed would decrease at a variations, whilst machining). ing from the tool cutting a workpiece surface that has either fxed spindle speed. As the parting diameter reaches signif cant roughness, or more likely the result of surface dis- 5.8 mm the surface speed would be 55 m min– , or 60% turbances from the previous cut. Tese disturbances in the of the ideal, thus signif cantly increasing the chip load- workpiece surface, create f uctuations in the cutting forces, ing as the tool approaches the workpiece’s centreline. with the tool being subjected to vibrations with this process continuously repeating, hence the term ‘regenerative chatter’In order to alleviate. the increasing tool loading, lower- Self-excited vibrations can be alleviated by either increas- ing the feedrate by about 50% until separation is just ing the dynamic stif ness of the system, or by increasing the about to occur, then f nally dropping the surface speed damping. to almost zero at this point, reduces the tendency for a ‘pip’ to be present on the workpiece. On a CNC driven NB Dynamic stif ness can be def ned as the ratio of the am- plitude of the force to the vibrational amplitude. spindle, it is not advisable for parting-of operations, to utilise the ‘canned cycle’ such as the ‘constant surface mal tool pressure. If excessive tool pressure occurs, speed’ this can promote work-hardening of the ‘transient function. surface’ of the workpiece. Tese abilities are impor- NB A more serious parting-of problem has been that tant points when machining relatively low mechanical in order to eliminate the pip formed at the centre of the strength components, which might otherwise buckle ‘released component’, some tools have been ground if machined with negative-style inserts when subse- with the front edge angle of between 3? to 15?. Such quently parted-of . a front edge geometry, can introduce an axial cutting Positive cutting edge parting-of tooling having force component, leading to poor chip control, which chip-formers, are ideal for applications on machine in turn, causes the tool to def ect. Tis parting-of tool tools when either low f xed feedrates are utilised, or def ection, can lead to the component’s face ‘dishing’, if the workpiece material necessitates lower cutting creating a convex surface on one face and a concave speeds. Tis positive-style of parting-of tooling, oper- surface on the other – so this tool grinding strategy ates ef ciently when machining sof er workpiece mate- should be avoided. rials, such as: aluminium-or, cooper-based alloys and many non-metallic materials, typically plastics. Feed- Today, parting-of inserts normally consist of two rates can be very low with these positive-type part- – main types with top rakes that are either of, negative, ing tools, down to 0.0254 mm rev or positive cutting edge chip-forming geometries. Te chip control and consistent tool life. One major diswith exceptional - negative-style of chip-formers are possibly the most advantage of using these positive tooling geometries commonly utilised. Tese inserts have a small nega- for parting-of , is that the tool is much weaker than its tive land at the front edge which increases the insert’s equivalent negative geometry type. strength, giving protection in adverse cutting condi- Te concept of insert self-grip in its respective tool- tions, such as when interrupted cutting is necessary holder, was developed by the cutting tool manufac- during a parting-of operation. Te land width – of en turer Iscar tools in the early 1970’s and has now been termed a ‘T-land’, is relative to the breadth of the part- adopted by many other tooling manufacturers (Fig. 40 ing-of tool. Tis width of the insert’s land has a direct top lef -hand side). Tese ‘self-grip’ tooling designs, correlation to the feedrate and its accompanying chip rely on the rotation of the part and subsequent tool formation. Te feedrate must be adequate to force the pressure to keep the ‘keyed and wedged’ insert seated workpiece material over the land and into the chip- in its respective toolholder pocket. Previously, double- former . ended inserts termed ‘dogbones’, were of en used but Notwithstanding the widespread usage of negative were limited to low DOC’s – due to the length of the parting-of tooling, positive-style insert geometries secondary cutting edge, so have been somewhat over- have some distinct advantages. Te chief one being the shadowed by the ‘self-grip’ varieties of parting-of ability to narrow the chip at light feedrates, with minitooling.- 2.5.5 Chip Morphology ‘Constant surface speed’ CNC capability as its name implies, allows the machine tool to maintain a constant surface speed The Charac terisation of Chip Forms as the diameter is reduced. Te main problem with using (Appendix 2) this ‘canned cycle’ , is that as the maximum spindle speed is In the now withdrawn ISO 3685 Standard on Ma- reached, the chip load will also increase. Tis is not a prob- chinability Testing Assessment, of some interest was lem, so long as the maximum speed has not occurred, such as the fact that this Standard had visually characterised when parting-of a component with a large hole at its centre. Parting-of operations that employ a negative-style insert (i.e. with a land and accompanying chip-former), normally have the feedrate determined in the following manner: by multi- plying the width of the insert by a constant of 0.04. For ex- Transient surfaces are those machined surfaces that will be ample, for a 4 mm wide tool, it is necessary to multiply the removed upon the next revolution of either the: insert’s width of 4 mm by 0.04 to obtain a feedrate of 0.16 mm – – Workpiece (i.e in rotating part operations), or rev . Tis will give a ‘start-point’ for any parting-of opera- – Cutter (i.e. for rotating tooling – drilling, milling, reaming, etc.). tions, although it might be prudent to check this feedrate is valid, from the tooling manufacturer’s recommendations. chip forms under eight headings, with several varia-For every cutting insert geometry, there is a recom- tions appearing in each groups (i.e. see Appendix 2 mended application area – termed its ‘chip-breaking for an extract showing these chip form classif cations). envelope’ (i.e. see footnote 38 below) – with regard to Although in the main, the chip forms were related to its range of feedrates and DOC’s. Within this ‘envelope’, turning, some of these chip morphologies could be ex- chips of acceptable form are produced by the cut- trapolated to other manufacturing processes. Te chip ting insert’s geometry. Conversely, any chips that are type that will be formed when any machining opera- formed outside this ‘envelope’ are not acceptable, be- tion is undertaken is the product of many interrelated cause they are either formed as unbroken strands, or factors, such as: are too thick and over-compressed. When component • Workpiece material characteristics – will the mate- prof ling operations are necessary (Fig. 31a), this nor- rial that forms the chip signif cantly work-harden?, mally involves several machining-related parameters: • Cutting tool geometry – changing, or modifying the variations in DOC’s, together with path vectoring of the cutting insert geometries and its plan approach feeds and as a result of this latter point, changes to the angles will have a major inf uence on the type of resultant chip’s path on the rake face. Tese factors are chip formed, important as they can modify the chip morphology • Temperatures within the cutting zone – if high, or when prof ling operations include: recessed/undercut low temperatures occur as the chip is formed, this shoulders, tapers and partial arcs, facing and sliding will have an impact on the type of chip formed, operations with the same tool, together with many • Machine tool/workpiece/cutting tool set-up – if other combined prof led features. All of these opera- this ‘loop’ is not too rigid, then vibrations are likely tions make signif cant demands on the adaptability of to be present, which will destabilise the cutting the cutting insert’s geometry to ef ciently break the process and af ect the type and formation of chipschip. produced, In general, the cutting insert’s chip formation prin- • Cutting data utilised – by modifying the cutting ciples are concerned with the chip-breaker’s ability to data: feeds and speeds and DOC’s, with the insert ge- create a chip form that is neither not too tight a curl, ometry maintained, this can play a signif cant role nor too open. in the chip formed during machining operations. If chip curling is too tight for the specif c machin- ing application, the likely consequences are for a chip NB Chip formation has become a technology in form creating: its own right, which has shown signif cant devel• - ‘Chip-streaming’ – producing long chip strands opment over the last few decades of machining ap- that are undesirable, wrapping itself around the plications. machined surface of the workpiece with work- hardened swarf and possibly degrading this ma- As has been previously mentioned, chip formation chined surface, or may become entangled around should always be controlled, with the resultant chipsthe various parts of the machine tool, which could formed being broken into suitable shape formation, impede its operation, such as ‘spirals and commas’, as indicated by the re• - Excessive heat generation – this can decrease tool sultant chip morphology shown in Fig. 35a. Uncon- life, or be conducted into the machined part and trolled chip-steaming (i.e. long continuous workpiece consequently may af ect specif c part tolerances for strands), must be avoided, being a signif cant risk-fac- the individual part, or could lead to modif cations tor to both the: machine tool’s operation and its CNC in the statistical variability of a batch of parts, setter/operator alike. Chip-breaking envelopes (see Fig. 34 middle right), are the product of plotting both the feedrate and DOC on two axes, Statistical variability in component production can cause with their relative size and position within the graphical area variations from one part to another, as the standard deviation being signif cantly af ected by the cutting insert’s geometry and mean changes, these important factors will be mentioned – as depicted by the three cutting insert geometric versions shown by types: A, B and C (Fig. 34). later in the text. • Increased built-up edge (BUE) formation – which having a small angle when compared to the cutting 0 through ‘attrition wear’ may cause the risk of pre- edge. Equally, a larger depth means that the nose ra- mature cutting edge failure. dius has somewhat less af ect from its radius and greater inf uence by the entering angle of the cutting When the chip curling is too open, this may result in edge, producing an outward directed spiral. Feedrate the following negative tendencies: also af ects the width of the chip’s cross-section and its • Poor chip control – creating an inef cient chip- ensuing chip f ow. breaking ability by the cutting insert, Chip formation begins by the chip curving, this be- • Chip hammering – breaking down the edge and ing signif cantly af ected by combinations of the cut- causing it to crumble and as a result creating the ting data employed, most notably: feedrate, DOC, rake likelihood of prematurely failing, angle, nose radius dimensions and workpiece condi- • Vibrational tendencies – af ecting both the ma- tion. A relatively ‘square’ cross-sectional chip nor- chined surface texture and shortening tool life. mally indicates that an excessively hard chip compres- sion has occurred, whilst a wide and thin band-like Chip formation and its resultant morphology, is not chip formation is usually indicative of long ribbon-like only af ected by the cutting data selected, but will be chips producing unmanageable swarf. If the chip curve inf uenced by the plan approach (i.e. entering) angle is tight helix, coupled to a thick chip cross-section, of the insert. In most machining operations, they are this means that the length of the chip/tool contact has usually not of the orthogonal, but oblique cutting in- increased, creating higher pressure and deformation. sert orientation, so the af ect is for the entering angle It should be noted that excessive chip cross-sectional to modify the chip formation process. Te insert’s en- thickness, has a debilitating ef ect on any machining tering angle af ects the chip formation by reducing the process. By careful use of CAD techniques coupled chip thickness and having its width increased with a to FEA to construct the insert’s cutting edge, comma- smaller angle. With oblique cutting geometry, the chip shaped chips are the likely product of any machining, formation is both ‘smoother and sof er’ in operation as providing that the appropriate cutting data has been the plan approach angle tends toward say, 10? to 60?, selected. In some machining operations, chip forma- furthermore, the chip f ow direction will also advanta- tion can be superior using a slightly negative insert geously change with the spiral pitch increasing. rake angle, thereby introducing harder chip compres- As the nose radius is changed with dif erent cutting sion and self-breaking of the chip, particularly if utilis- inserts, this has the ef ect of changing both the direc- ing small feeds. Conversely, positive rakes can be give tion and shape of the chips produced. Tis nose radius other important machining advantages, depending geometry is a fundamental aspect in the development which chip form and cutting data would be the most of chips during the machining process – as depicted by advantageous to the part’s ensuing manufacture. Usu- Fig. 35b. Here, an identical nose radius and feedrate is utilised, but the dif erence being the DOC’s, with a shallow DOC in Fig. 35b (lef ), giving rise to a slow chip Chip-f ow is the result of a compound angle between the chip’s helix, whereas in Fig. 35b (right) the DOC is somewhat side- and back-f ow. Te chip’s side-f ow being a measure of the deeper, creating a tighter chip helix which is benef - f ow over the tool face (i.e. for a f at-faced tool), whilst back- cial to enhanced chip-breaking ability. Shallow cutting f ow establishes the amount of chip-streaming into the chip- depths produce ‘comma-shaped’ chip cross-sections, breaker groove. Detailed analysis of chip side-f ow (i.e. via high-speed photography), has indicated that it is inf uenced by a combination of groove dimensions and cutting data. If the feedrate is increased, this results in a higher chip back- f ow angle, promoting chip-streaming into the chip-breaker 0 Attrition wear is an unusual aspect of tool wear, in that it is groove. Te ratio of feed-to-length of restricted contact has the result of high cutting forces, sterile surfaces, together with been shown to be an important parameter in the determina- chip/tool af nity, creating ‘ideal’ conditions for a pressure tion of chip- back-f ow. Typically with low feedrates the cor- welding situation. Hence, the BUE develops, which builds-up responding chip back-f ow is going to be somewhat lessened, rapidly and is the ‘swept away’ by the chip f ow streaming over resulting in poor chip-breaker utilisation. When the restricted the top rake’s surface, taking with it minute atomic surface lay- contact between the chip and the tool is small – due to low ers from the tool’s face. Tis continuous renewal and destruc- feed – the chip-f ow does not fully engage the chip-breaker tion of the BUE, enhances crater wear formation, eventually and will as a result curve upward, with minimal ‘automatic’ leading to premature cutting edge failure. chip-breaking ef ect. ally, for larger feedrates, a positive insert rake angle indicating how to obtain the desired chip-break- might optimise the chip-curving tendency, by not pro- ing control. In the chart shown in Fig. 36a (right), ducing and excessively tight chip helix. Chip curve, its the DOC’s indicate on the associated visual table the resultant chip f ow direction, the chip helix and its ac- expected chip type showing that here types ‘C and companying shape are designed into each cutting edge D’ of er ‘good’ broken chips. Such chip morphology by the tooling manufacturers. Tool companies ensure charts as these from tooling manufacturers, attempt that a controlled chip formation should result if they to inform the user of the anticipated chip-breaking are exploited within the recommended cutting data if their recommendations are followed. Whereas the ranges specif ed. f ow-diagram illustrated in Fig. 36b, indicates that In Fig. 36a (lef ), ef ective chip-breaking decision- ‘good chip control’ improved productivity will result, making recommendations are shown on a f ow-chart, if a manufacturing company adopts the machining . Figure 36. Chip-breaking control and chip morphology and its afect on productivity. [Courtesy of Mitsubishi Carbide] strategy high-lighted to the lef-hand side. On the con- wear of the heel and ultimately this heel becomes trary, ‘poor chip control’ with an attendant decrease in fattened and chip-breaking is severely compromised. productivity will occur, ifthe problems shown to the Conversely, when the cutting data produces a wear right-hand side transpire. zone concentrated at the insert’s edge (Fig. 37b), then Chip morphology can indicate important aspects chip side-fow occurs and poor chip-breaking results, ofthe overall cutting process, from the cutting edge’s together with low tool life. Tis accelerated tool geometry and its design, through to work-hardening wear, resulting from an extended tool/chip contact ability ofthe workpiece. Many other factors concern- region over the primary rake face, promotes a rough ing cutting edge’s mechanical/physical properties can surface texture to the machined part. In the case of be high-lighted, these being important aids in deter- Fig. 37c, these are ideal conditions for optimum chip- mining a material’s machinability – which will be dis- breaking action and a correspondingly excellent and cussed in more depth later in the text. predictable tool life, because the wear zones at both the heel and edge are relatively uniform in nature, illustrating virtually a perfect chip-forming/-breaking 2.5.6 Chip-Breaker Wear action. Higher tool/chip interface temperatures can result Any form oftool failure will depend upon a combi- as the heel wears, forming a crater at the bottom ofthe nation ofdiferent wear criteria, usually with one, or chip-breaker groove. Combination wear – as shown in more wear mechanisms playing a dominant role. Pre- Fig. 37c – generally results in signifcantly improved viously, it was found that the workpiece surface texture tool wear, in conjunction with more predictable tool and the crater index act as appropriate tool failure cri- life. In the photographs ofchip-breaker grooves shown teria, particularly for rough turning operations. More- for an uncoated and coated Cermet cutting insert ma- over, tool life based upon these two factors, approxi- terial in Figs. 38a and b respectively, the relative wear mated the failure curve more exactly than either the patterns can clearly be discerned. In the case ofFig. fank, or crater wear criterion. 38a – the uncoated insert – the predominant wear In cutting tool research activities, it has been found concentration is primarily at the edge, indicating that that when machining with chip-breaker inserts, fank the cutting data had not been optimised. While in the wear (i.e. notably VB) is not the most dominant factor case ofthe coated Cermet insert ofidentical geometry in tool failure. In most cases, the ‘end-point’ ofuse- (Fig. 38b), the wear is uniform across the: edge, groove ful tool life occurs through an alteration ofthe chip- and heel. Tis would seem to suggest that ideal cutting groove parameters, well before high values of fank data had been utilised in its machining operation. In wear have been reached. Te two principal causes of both ofthese cases some fank wear has occurred, but wear failure for chip-breaker inserts are: this would not render the chip-breaking ability when • For recommended cutting data with a specifc in- subsequent machining invalid. sert, the design and positioning ofchip-breakers/ grooves may promote ‘unfavourable’ chip-fow, re- NB A complex matrix occurs (i.e. Fig. 38c) with Cer- sulting in wear in the chip-breaker wall – causing mets, this ‘metallurgy’ can be ‘tailored’ to meet the consequent tool failure, needs of specifc workpiece and machining require- • Alterations in the cutting data, particularly feedrate,ments. afects chip-fow, which in turn, generates various wear patterns at the chip-breaker’s heel and edge (see Fig. 37). 2.6 Multi-FunctionalIn the schematic diagrams shown in Fig. 37, are il- lustrated the concentrated wear zones on the: back Tooling wall (i.e. heel), cutting edge, or on both positions for Te concept of multi-functional tooling was devel- a typical chip-breaker insert. Under the machining oped from the mid-1980’s, when multi-directional conditions for Fig. 37a, the chip-groove utilisation tooling emerged. Tis tooling allowed a series ofop- is very low, with the chip striking the heel directly. erations to be performed by a single tool, rather than Tus, as machining continues, this results in abrasive many, typically allowing: side-turning, profling and . Figure 37. Schematic representations of dif ering chip-breaking insert tool wear mechanisms – due to altera- tions in the cutting data. [Source: Jawahir et al., 1995] . Figure 38. Improved wear resistance obtained with an uncoated and coated cermet, when turning –1 –1 ovako 825B steel, having the following cutting data: Cutting speed 250 m min , feed 0.2 mm rev , DoC1.0 mm and cut dry. [Courtesy of Sandvik Coromant] grooving, enabling the non-productive elementsin tional tools are critically-designed so that for a specif c the machining cycle to be minimised. In the original feedrate, the rate of elastic def ection is both known multi-directional tooling concept, the top rake geom- and is relatively small, being directly related to the ap- etry might include a three-dimensional chip-former, plied axial force, in association with the selected D’s. OC comprising of an elevated central rib, with negative K- At the tool-setting stage of the overall machining cycle, lands on the edges. Such a top rake prof le geometry compensation(s) are undertaken to allow for minute could be utilised for ef cient chip-forming/-breaking changes in the machined diameter, due to the dynamic of the resultant chips. Tis tooling when utilised for elastic behaviour of one of these tools in-cut. For a say, grooving operations, employed a chip-forming specif c multi-functional tool supplied by the tooling geometry – this being extended to the cutting edge, manufacturer, its actual tool compensation factor(s) which both narrowed and curled the emerging chip will be available from the manufacturer’s user-manual to the desired shape, thereby facilitating easy swarf for the product. evacuation. A feature of this cutting insert concept, In-action these multi-functional tools (Fig. 39b), was a form of ef ective chip management, extending can signif cantly reduce the normal tooling inventory, the insert’s life signif cantly, thus equally ensuring that for example, on average such tools can replace three adequate chip-f ow and rapid swarf evacuation would conventional ones, with the twin benef t of a major have taken place. When one of these multi-directional cycle-time reduction (i.e. for the reasons previously tools was required to commence a side-turning opera- mentioned) of between 30 to 60% – depending upon tion, the axial force component caused it to elastically defect at the front region of the machined. Some other important benef ts of using a toolholder. Tis tool def ection enabled an ef cient multi-functional tooling strategy are: acting on the insert the complexity of features on the component being feed motion along the workpiece to take place, be• - Surface quality and accuracy improvements – due cause of the elastic behaviour of the toolholder created to the prof le of the insert’s geometry, any ‘machined a positive plan approach angle in combination with a cusps’ , or feedmarks are reduced, providing excel- front clearance angle – see Fig. 39a and b (i.e. illus- trating in this one of the latest ‘twisted geometry’ insertlent machined surface texture and predictable di - multi-functional tooling geometries). mensional control, • Turret utilisation improved – because fewer tools Any of today’s multi-functional tooling designs are need in the turret pockets, hence ‘sister tooling’ (Figs. 39 and 40), allow a ‘some degree’ of elastic be- can be adopted, thereby further improving any un- haviour in the toolholder, enabling satisfactory tool tended operational performance, • vectoring to occur, either to the right-, or lef -hand Superior chip control – breaks the chips into man- of the part feature being machined. Tese multi-func- ageable swarf, thus minimising ‘birds nests’ and entanglements around components and lessens au- • Improved insert strength tomatic part loading problems,– allows machining at sig - nif cantly greater DOC’s to that of conventional in- Non-productive elements are any activity in the machining cycle that is not ‘adding value’ to the operation, such as: tool- changing either by the tool turret’s rotation, or by manually changing tools, adjusting tool-of sets (i.e. for either: tool wear compensation, or for inputting new tool of sets – into the ma- chine tool’s CNC controller), for component loading/unload- ‘Machined cusps’ the consequence of the insert’s nose geom- ing operations, measuring critical dimensional features – by etry coupled to the feedrate, these being superimposed onto either touch-trigger probes, non-contact measurement, or the machined surface, once the tool has passed over this sur- manual inspection with metrology equipment (i.e. microme- face. ters, vernier calipers, etc.), plus any other additional ‘idle-time’ ‘Birds nests’ are the rotational entanglement and pile-up activities. An Axial force component is the result of engaging the desired of continuous chips at the bottom of both trough and blind feedrate, to produce features, such as: a diameter, taper, pro- holes, this work-hardened swarf can cause avoidable damage f le, wide groove, chamfer, undercut, etc. – either positioned in the machined hole, furthermore, it can present problems in externally/internally for the necessary production of the ma- coolant delivery for additional machining operations that may chined part. be required. . Figure 39. Multi-functional cutting insert geometry for ef cient stock removal and increased part productivity. [Courtesy of Iscar Tools] . Figure 40. Multi-functional tooling for the machining of rotational features such as: turning, grooving and prof ling operations – having excellent chip control. [Courtesy of Sandvik Coromant] . Figure 41. By employing twin turrets on a mill/turn centre, ‘balanced turning’ of the component can remove large volumes of stock at one pass. [Courtesy of DMG (UK) Ltd.] serts, with improved insert security its toolholder References location, • CNC programming simplifed – many tooling man- Journal and Conference Pa pers ufacturer’s have specially-prepared sofware to sig- Boston, O.W. A Research into the Elements of Metal Cutting nifcantly reduce CNC programming input times. Trans ASME 48, 749–848, 1926. NB Tis latter point utilises CNC ‘canned cycles’ to Cocquilhat, M. Experiences sur la Résistance utile Produ- ite dans le Forage Ann. Trav. Publ.en Belgique 10, 199, reduce program lengths. 1851. Doi, S. and Kato, S. Chatter Vibration of Lathe Tools, Trans. In Appendix 3, a guide for ‘Trouble-shooting for turn- of ASME, Vol. 78 (5), 1127–1134, 1956. ing operations’ are listed, with possible causes and rem- Fabry, D. T e Tool Channel. Cutting Tool Eng’g, 58–64, edies to potential production problems. Sept. 2003. In the following chapters, many other important Gadzinski, M. Parting Know-how. Cutting Tool Eng’g, chip-forming production processes will be discussed, 52–57, March 2001. with hole-making techniques such as drilling being Galloway, D.F. Some Experiemnts on the Inf uence of Vari- around 25% of all manufacturing techniques under- ous Factors on Drill Performance. Trans. of ASME, taken by machining-related companies – these and 191–231, Feb., 1957. associated hole-production methods will be reviewed Humphries, J.R. Energing Technologies and Recent Ad- next. vances in Multi-functional Groove/Turn Systems. Int. Conf. on Industrial Tooling, Shirley Press Ltd, 65–85, Time, I. Soprotivlenie Metallov I Dereva Rezaniju St. Peters- Sept. 1979. burg, 1870. Isakov, E. Te Mathematics of Machining. American Ma- Tipnis, V.A. and Joseph, R.A. Testing for Machinability, in: chinist, 37–39, Aug. 1996. Inf uence of Metallurgy on Machinability, ASM Pub. 11–30, 1975. Isakov, E. Reassessing Power Factors. American Machinist, 43–45, Dec. 1996. Tresca, H. Mémoire sur le Rabotage des Métaux Bull. Soc. d’Encourgement pour I’Industrie Jawahir, I.S. Te Tool Restricted Contact Ef ect as a Major Nationale 585–685, 1873. Inf uencing Factor in Chip-breaking: An Experimen- tal Analysis. Annals of the CIRP, Vol. 31 (1), 121–126, Venkatesh, V.C. and Satchidanandam, M. A Discussion on 1988. Tool Life Criteria and Total Failure Causes. Annals of the CIRP, Vol. 29 (1), 19–22, 1980. Jawahir, I.S., Ghosh, R., Fang, X.D. and Li, P.X. An Inves- tigation of the Ef ects of Chip-f ow on Tool Wear in Ma- Watson, D.W. and Murphy, D.C. Te Ef ect of Machining chining with Complex Grooved Tools. WEAR, Vol.184 on Surface Integrity . Metallurgist and Matls, 199–204, (2), 145–154, 1995. April 1979. Kasahara, N., Sato, H. and Tani, Y. Phase Characteristics of Webzell, S. Wiping away Cycle Times. Metalworking Prod., Self-excited Chatter in Cutting. J. of Engg for Ind., ASME Oct. 2003. Pub, Vol. 114, 393–399, Nov. 1992. Kennedy, B. Facing Facts. Cutting Tool Eng’g, 29–37, Feb. Books, booklets and guides 2002. Kennedy, B. Take a Bigger Bite. Cutting Tool Eng’g, 25–29, Armarego, E.J.A. and Brown, R.H. Te Machining of Met- Aug. 2003. als. Prentice-Hall Pub., 1969. King, K. Added Functionality. Cutting Tool Eng’g, 52–55, Boothroyd, G. and Knight, W.A. Fundamentals of Metal Feb. 2005. Machining and Machine Tools. Marcel Dekker (NY), Kondo, Y., Kawano, O. and Sato, H. Behaviour of Self-excited 1989. Chatter due to Multiple Regenerative Ef ect. J. of Engg. Finish Turning – Application Guide. AB Sandvik Coromant For Ind., ASME Pub., Vol. 103 (3), 324–329, 1981. Pub., 1995. Lewis, B. Turn your Wipers on. Cutting Tool Eng’g, 47–51, Hartig, E. Versuche über Leistung und Arbeitsverbrauch der Jan. 2003. Werkzengmaschine. 1873. Mallock, A. Te Action of Cutting Tools. Proc. of Royal Soc. Kaczmarek, J. Principles of Machining by Cutting Abrasion Lond. 33, 127–139, 1881–882. and Erosion. Peter Pregrinus Pub. (Warsaw), 1976. Paterson, H. Strictly Boring. Cutting Tool Engg., 22–30, Modern Metal Cutting – A Practical Handbook. AB Sandvik Oct. 1995. Coromant Pub., 1994. Pekelharing, A.J. Built-up Edge (BUE): is the Mechanism Shaw, M.C. Metal Cutting Principles. Clarendon Press, Ox- Understood? Annals of the CIRP, Vol. 23 (3) 207–211, ford, 1984. 1974. Smith, G.T. Advanced Machining – Te Handbook of Cut- Piispanen, V. Eripanines Teknilliseslä Aikakauslehdeslä 27, ting Technology. IFS/Springer Verlag, 1989. 315, 1937. Smith G.T. CNC Machining Technology. Springer Verlag, Reuleaux, F. Uber den Taylor Whiteschen Werkzengstahl in 1993. Verein zur Beförderung des Gewerbef eisses in Preus- Smith, G.T. Industrial Metrology – Surfaces and Roundness. sen. Sitzzungsberichte, 79, 179, 1900. Springer Verlag, 2002. Smith, G.T. Fundamentals of Chip-breaking for Continuous Cutting Operations. Int. Conf. on Industrial Tooling, Stainless Steel Turning, AB Sandvik Coromant Pub., 1996. Molyneux Press Ltd, 72–82, Sept. 1999. Tlusty, G. Manufacturing Processes and Equipment. Pren- tice Hall, 2000. Teets, B. Facing up to Grooving Problems. Machinery and Prod. Eng’g., 51–52, Oct. 1988. Trent, E.M. Metal Cutting. Oxford: Butterworth Heine- mann (3 rd Ed.), 1991. 3 Drilling and Associated ‘In all things, success depends Technologies upon previous preparation and without such preparation？？there is sure to be failure.’ CONFUCIUS (c550–c487BC ) [Analects] 3.1 Drillingtively conventional chip formation, as shown in the ‘quick-stop’ photomicrograph in Fig. 44b. An oblique Technology 3.1.1 Introduction to thecutting action occurs to the direction of motion, being the result of an o,set of the lips that are parallel to a Twist radial line – ahead of centre – which is approximately Drill’s Development Drilling operations are perhaps the most popular ma- equal to half the drill point’s web thickness and in- chining process being undertaken today, with their creases toward the centre of the drill. origins being traced back to cutting tool develop- responsible for inducing chip ,ow in a direction nor- ments in North America in the 19 century. In 1864 mal to the lips in accordance with Stabler’s Law . th toward the latter part of the American Civil War, Ste- increasing chip ,ow obliquity can be seen in Fig. 45a, ven Morse (i.e. later to design the signi,cant ‘Morse by observing the ,ow lines emanating from the chip’s taper’ – for accurate location of the ‘sleeved drills’ interface along the lips and up the ,ute face. Such into their mating machine tool spindles) founded an oblique cutting action serves to increase the twist the Morse Twist Drill and Machine Company in the drill’s e,ective rake angle geometry. With the advent most ‘North’. Morse then proceeded to develop probablimportant cutting tool advance to date, namely, taining direct threey the of ‘Spherical trigonometric computer so-dimensional calculations – previ- the ubiquitous twist drill. In Fig. 42, several of today’s ously described by Witte (1982) in two-dimensional twist drills are illustrated along with just a small range formulae for cutting edge performance – these calcu- of ‘solid’ contemporary designs. Morse’s originally-de- lations have been enhanced. signed twist drill has changed very little over the lastUnder the chisel point, or web, the material re - 150 years – since its conception. In comparison to the moval mechanism is quite complex. Near the bottom somewhat cruder-designed contemporary drills of that of the ,utes where the radii intersect with the chisel time, Morse stated: ‘ edge, the drill’s clearance surfaces form a cutting rake be drilled, while mine cuts the metal and discharges the surface that is highly negative in nature. As the centre chips and borings without clogging’ . Morse’s statement of the drill is approached, the drill’s action resembles was at best, to some extent optimistic, whereas the that of a ‘blunt wedge-shaped indentor’, as illustrated ‘cold reality’ tells a di,erent story, as a drill’s perfor- in Fig. 45b. An indication of the ine,cient material mance is in,uenced by a considerable number of fac- removal process is evident by the severe workpiece tors, most of which are listed in Fig. 43. deformation occurring under the chisel point, where such deformed products must be ejected by the drill to produce the hole. 3.1.2 Twist Drill Fundamentals wiped into the drill ,ute whereupon they intermingle with the main cutting edge chips. substantiated by force and energy analysis, based on a depicted in Fig. 44a. From this illustration two dis- combination of cutting and extruding behaviour under tinct cutting regions can be established: ,rstly, the the chisel point, where agreement has been con,rmed main cutting edge, or lips; secondly at the intersection with experimental torque and thrust measurements. of the clearance and main cutting edge – termed the chisel edge. In fact for a twist drill, the cutting process has no ‘true’ point, which is one of the major sources can be equated to that of a le for a drilled hole’s dimensional inaccuracy. tool, where the rake and clearance face geometries are identical and the correlation between these two ma- chining processes have been validated in the experi- mental work by Witte in 1982. Both of these regions remove material, with the cutting lips producing ef- Stabler’s Law – for oblique cutting, can be formulated, as be- ,cient material removal, while the chisel edge’s con- low: tribution is both ine,cient and is mainly responsible Chip ,ow (cos η) = cos I (bc/b) for geometric errors in drilling, coupled to high thrust Where: I = inclination of cutting edge, bc = chip ,ow vector, loads. b = direction of cutting vector. tainable accuracy of holes generated whilst drilling is can be seen in Fig. 46, together with associated no- dependent upon grinding the drill to certain limits. menclature for critical features and tolerance bound- Any variations in geometry and dimensions, such as: aries. From the relatively complex geometry and dissimilar lips and angles, chisel point not centralised, dimensional characteristics shown in Fig. 46, the ob- and so on, have a profound e,ect on both the hole di- . Figure 42. A selection of just some of the many ‘solid’ and ‘through-spindle’ drilling varieties and ‘inserted-edge’ insert geometries currently available. [Courtesy of Seco Tools] . Figure 43. The principal technical drill performance criteria and factors associated with drilling operations in this case for ex- ample, on castings . Figure 44. The twist drill geometry and associated chip shearing mechanism. [Source: C.J. Oxford Jr., 1955] . Figure 45. The twist drill shearing and extrusion mechanism at the bottom of a hole. [Source: C.J. Oxford Jr., 1955] . Figure 46. Twist drill geometry mensional accuracy and roundness, with some ‘helical R = ratio of the transverse reaction at the drill point, wandering’ as the drill passes through the workpiece. T = thrust force, Hole accuracy and in particular the ‘bell-mouthing ef- I = system’s ‘moment of inertia’, fect’ , is minimised by previously centre-drilling prior k = ?T/E I. mouthing’ is probably the inconsistency in the drill As suggested above, this ‘axis slope error’ is initto drilling to ‘size’. iated geometry. Such e,ects are exacerbated using Jobber when the chisel edge begins to penetrate the workpiece drills , or even worse, by utilising longer-series drills, and unless the feed is discontinued, or in some man- sult of lessening rigidity promoting some drill bendwhich tend to either slightly ‘unwind’, or bend as a re- - will increase as drill penetration continues.ner the error is corrected, the magnitude of de,ection ing/de,ection. magnitude of de,ection can reach up to 60 μm, under It is worth noting that the rigidity of a tool such exaggerated drilling conditions. as a drill will decrease by the ‘square of the distance’. of considerable research and development for many tion into the workpiece, the progressively larger the years, with some unusual departures from the ‘stan- de,ection and, the further from the ‘true axis of rota- dard’ 118? drill point included angle. Typical of these tion’ will be the subsequent drill’s path. extreme approaches were the so-called ‘Volvo point’, drilled hole slope angle ‘,’ , can be de,ned in the fol- having a negative 185? included angle – primarily lowing manner: utilised to avoid ‘frittering’ of drilled holes, or the Drilled hole slope angle highly positive geometries such as 80? included an- ‘φ’ = 3/2 l × R/T (1 – I/k × tan k l) gle used for drilling some plastics. Not only can the point angle be modi,ed, but the shape and pro,le ofthe chisel point, or web o,ers numerously-ground Where: opportunities for detailed geometric modi,cations, l = length of de,ected tool, with only some of which being shown in Fig. 47. Four of the most commonly-ground drill point geometries being: • Conventional – the ‘original’ Morse geometry, hav- ing a straight chisel edge, with poor self-centring ‘Helical wandering’ is the result of the drill’s geometry be- drilling action (Fig. 46a), ing ‘unbalanced’ , resulting from of di,ering lip lengths, or an • Split-point – there are a range of point-splitting o,set chisel point, causing the drill to ‘spiral-down’ through techniques available to alter the point pro,le, which the workpiece, as it progresses through the part (see Fig. 70). has the e,ect of modifying the chisel point to allow ‘Bell-mouthing’ of the drilled hole is attributable to the chisel a reasonable self-centring action (Fig. 47b), point and is produced by the line-of-contact, as the drill point initially touches the component’s surface, causing it to ‘walk’ until the feed/penetration stabilises itself at the outer corners (i.e. margins) entering the workpiece, whereupon, these mar- gins guide the drill into the part. ‘Bell-mouthing e,ect’ is produced by the drill chisel point’s ‘Frittering’ refers to the break-out at the hole’s edge as the drill eccentric behaviour as it attempts to centralise its rotational motion as it enters, or exit’s the workpiece. exit’s the part, on some brittle materials, such as on several Powder Metallurgy compacts. ‘Jobber drills’ are considered to be ‘standardised drills’ that ‘Web’ refers to the internal core of the drill – which imparts are normally utilised for most drilling general operations, un- mechanical strength to the drill. less otherwise speci,ed. ness the further one gets from the chisel edge (i.e. shown in ‘Rigidity rule’: a drill, reamer, tap, or a milling cutter held in Fig. 47 – in lower diagrams and with cross-sections). Hence, a spindle will have its rigidity decreased by the ‘square of the if the drill is reground many times, the chisel point width will distance’ , namely, if a drill is twice as long it is four times less obviously increase, this necessitates that the chisel point must rigid. be ‘thinned’ , otherwise too high a thrust force occurs and an NB A cantilevered tool such as a boring bar has its rigidity de- ine,cient drilling action will result. ‘Split-point’ ground drills are sometimes referred to as ‘Multi- creased by the ‘cube’ or the distance – meaning that too much facet drills’. tool overhang, will seriously reduce tooling rigidity. . Figure 47. A range of typically ground twist drill points • Web-thinning – as its name implies, the chisel point edges of the drill, increasing both tool life and improv- is web-thinned/notched, by regrinding to reduce ing the hole’s ‘Surface Integrity’ . ing the prof le, giving a partial selfthe width of the chisel point, while slightly modify-centring action- 3.1.3 The Dynamics (Fig. 47c), • of Twist DrillingHelical – the chisel point is ground to an ‘S-shape’, Holes which modif es both the chisel point and its pro- f led shape, improving the drilling performance Introduc tion and self-centring action (Fig.47d). Te term ‘drilling’ refers to all production techniques NB On some drills a sophisticated grinding action for the manufacture of cylindrical holes in workpieces 0 has imparted drills without a chisel point, which sig- using chip-making cutting tools, for shortand-hole nif cantly improves their drill penetration rates into deep-hole drilling operations. Te expression ‘solid the workpiece, but requires a complex drill regrinding drilling’ has been introduced in recent years – which is operation to re-sharpen them when the edge becomes hole-making generation undertaken in a single opera- ‘dulled’. tion, to dif erentiate it from that of the previous tech- niques of either: centre-drilling or, pilot-hole drilling Not only are drills supplied with appropriate point (i.e, see Fig. 50b) prior to drilling to size. Drill technol- geometries, but for twist drills the twin spiral f utes ogy includes a range of specialised hole-making tool- of the drill can also be specif ed – from the tooling ing, including: twist drills, solid drills, counter-boring manufacturer, as this gives the drill its ‘equivalent of and trepanning tools and deep-hole drills the rake angle’ as found on a single-point turning tool. instance, mention will be made of twist drilling opera- On conventional jobber drills, the normal f ute angle tions, then a review of these other drilling production . In the f rst is 29? – giving a relatively ‘slow’ helix (Fig. 47a) and in methods will occur. the past, typically being utilised for drilling most plain carbon steel grades. Conversely, a drill with a ‘quick’ Tw ist helix angle (Fig. 47d), might be employed to drill sof Drills materials such as certain plastics. Brittle materials on Twist drilling operations have been carried out for the other hand, which might be utilised typically when around 150 years, with a twist drill imparting ‘bal- drilling Cartridge Brass (i.e. 70Cu 30Zn composition), anced cutting conditions’ , assuming that the drill’s require a zero, or slightly negative helix. geometry is symmetrical. It has been suggested that the work of drilling may be considered as two single- NB It is possible to temporarily modify the drill’s he- point lathe tools engaged in an internal straight turn- lix angle by re-grinding, termed ‘drill dubbing’, which ing operation. A twist drill produces both torque and refers to lightly ‘f ash-grinding’ the f utes at the lips to thrust as it rotates and is fed into the workpiece. Te decrease the ef ective f ute helix angle. main contribution to torque is through the lips, with a small amount of torque being generated by the chisel Te main strength of a drill is via its web, or its cross- point as the drill rotates against the resistance of the section which can be changed and as a result, will modify the f ute’s geometric prof le (i.e. see Fig. 48). In general, drill cross-section are classif ed in three ‘Surface Integrity’ has been coined to describe the ‘altered ma- groups, namely: terial zone’ (AMZ), for localised sub-surface layers that dif er • Axe-shaped – having well-def ned margins (Fig. 48 from those of the bulk material – considerably more will be –top), said on this subject in Chapter 7. • Rounded heel – with increased web, but small mar- 0 ‘Short-hole drilling’ operations cover depth-to-hole-diam- gins (Fig. 48 – middle), eter-ratios of up to 6D (i.e. for diameters up to 30 mm), whilst • Rhombic – incorporating a large web, with wide larger drilled holes are limited to depths of 2.5D. margins (Fig. 48 – bottom). Where: D = nominal drill diameter. ‘Deep-hole drilling’ and ‘Gun-drilling’ operations are virtu- NB Some twist drills feature oil/coolant holes to allow ally the same, with the term Deep-hole drilling being the pre- cutting f uid to reach right down toward the cutting ferred term in this text. . Figure 48. Symmetrical twist drill cross-sectional pro,les *After: Spur and Masuha, 1981, workpiece (see Fig. 49). Te thrust force (Fig. 49) is one of the cutting resistances in a drilling operation, the result of the selected penetration rate (i.e. feed), in contributions to drill resistance are from the: combination with the bulk hardness of the workpiece• Lips – equal lip lengths and angles are important and its work-hardening ability and the ef ciency of the for a ‘balanced cutting action’, this being consid- coolant supply – if any – to the cutting edges (i.e lips). ered an ef cient cutting process, Te resolution of the cutting resistance into their vari-• Chisel edge – is highly negatively skewed and as it ous components when twist drilling, is shown in Fig. acts like a ‘blunt wedge-shaped indentor’, extrud- 49 at a mid-point along the lips. Te thrust force is just ing the workpiece material from this vicinity, . Figure 49. The balanced cutting forces resulting from drilling holes utilising twist drill geometries. [After: Kaczmarek, 1976] • Land, or margin – via a rubbing, or frictional ac- A = 2Az = szd = sd/2 = 2hzb (mm ). tion. Conversely, in the case of ‘Pilot’ hole drilling (Fig. NB Te latter two are relatively inef cient processes, 50b), the undeformed chip elements are identical to moreover, the resistance components of the lips and ‘Solid’ drilling, but for the exception of the DOC, which chisel edge are the product of resistance of the unde- can be expressed in the following manner: formed chip to plastic strain, in combination with re- sistance due to external friction. Te land resistancea = d-do/2 (mm). occurs from the friction (i.e. rubbing) against the side of the drill’s hole. Where: d = diameter of f nal hole (mm), When symmetrical twist drilling (illustrated in Fig. do = diameter of primary hole (mm). 50), the undeformed chip can be characterised by its: • Cutting depth (a) – where ‘a’ = d/2, with ‘d’ being Tus, for example in the case of ‘Pilot’ hole drilling, the drill’s diameter (mm), the total cross-sectional area of the undeformed chip, • Feedrate (s) – this being the distance the cutting will be: edge moves in the drilling axis direction during one revolution. Normally two rigidly joined cutA- = 2Az = sz(d – do) = s(d – do)/2 = 2hzb (mm ). ting edges are cutting at any instant, each one in its travel corresponds to feed ‘sz’, which removes an Te calculation of cutting forces in ‘Solid’ hole drilling undeformed chip whose size – in the direction of (Fig. 50a), can be found from the general formulae for the drilling axis is: ‘s ’ and ‘s ’ respectively (i.e see axial force (F) and torque (M), in the following man- Fig. 50a) and, as most drills are symmetrical in de- ner, respectively: sign, then: bF uF F = CF d s KH (kg) s = s1 = s2 = s/2 (mm), bM uM KH M = CM d s (kg mm) • Undeformed chip thickness (hz) – to be removed by each of the drill’s cutting lips, which can be deterWhere:- mined from the following relationship: CF and CM = constants (i.e. derived from Kacz- marek‘s f ndings), hz = sz sinθ (mm). d = nominal drill diameter (mm), bF and bM = exponents characterising the inf u- ence of the drill diameter, z = h = h . s = feed rate (mm rev ), NB With a symmetrical drill, then: H uF and uM – • Undeformed chip thickness (b) – can be found from = exponents characterising inf uence of the following relationship: KH feedrate, = workpiece material’s correction co- ef cient (i.e. concerning mechanical b = a 1/sinθ = d/2sinθ (mm). properties). , It follows from these expressions, that the trans- verse cross-sectional area of the undeformed chip at each of the twist drill’s cutting lips, can be shown by CF is derived from experimental data, typically: Carbon steel the following relationship: (construction) 84.7, Grey CI 60.5, Malleable CI 52.5, Bronze (medium hardness) 31.5 – with HSS drills, ranging from ,10 Az = szd/2 = sd/4 = hzb (mm ). to 60 mm. CM is derived from experimental data, typically: Carbon steel Hence, the total transverse cross-sectional area when (construction) 33.8, Grey CI 23.3, Malleable CI 20.3, Bronze drilling of the undeformed chip will be: (medium hardness) 12.2 – with HSS drills, ranging from ,10 to 60 mm. . Figure 50. Drilling a hole with/without a ‘pilot’ hole and the cutting, rubbing and extrusion mechanism. [After: Kaczmarek, 1976] Conversely, for ‘Pilot’ hole drilling (Fig. 50b), these edges are normally def ected away from increases in mathematical formulae are modif ed in the following the load. manner: A common form of failure of twist drills in opera- tion is from shattering, with such catastrophic failure bF eF uF F = CF d a s KH (kg) being related to the dynamic nature of twist drilling. By way of illustration, a ,4.5 mm long-series twist drill bM eM uM KH M = CM d a s (kg mm) is capable of withstanding a torque of approximately 6 Nm before it catastrophically fails. Normally, the Where: torque for most drilling operations is around 1 Nm. a = DOC (mm), eF and eM = exponents indicating the DOC’s inf u- Te mperatures in Tw ist ence. Drilling Te accumulation of heat in the vicinity of cutting is Tese axial force and torque formulae derived in the an important factor in the cutting process, with much work by Kaczmarek, are concerned with ‘so-called’ av- of the mechanical energy necessary for machining be- erage twist drilling values. Tese ‘averages’ are related ing converted into heat, then conducted into the chip, to drill diameters between 15 to 35 mm, having feed workpiece and tool (Fig. 51). Te consequential ther- – ranges in the vicinity of 0.2 to 0.4 mm rev. Terefore, mal phenomena are important, as they can af ect the: the entire axial force (F) and torque (M), comprises of• Mode of deformation – elastic/plastic behaviour of contributions of the lips, land and chisel point, in the the chip, • following manner: Machined surface – for metals the ultimate metal- • Axial force (F) – lips (50%), land (10%) and chisel lurgical state of the material, • Tool wear rate – which depends upon a number point (40%), • Torque (M) – lips (80%), land (12%) and chisel of criteria, such as the tool’s coating, cutting data employed, work-hardening ability of the workpiece point (8%). and coolant delivery and its ef ciency. NB Tese contributing factors to axial force and torque are for drill depths that do not exceed 2.5d.It is imperative to comprehend the factors that control both heat generation and its dissipation, together with If the drilling force is signif cantly increased , then this the tool and work’s temperature distribution in and has the ef ect of distorting the drill shaf . Such distor- near the cutting zone. tion, causes the drill’s cutting edge to move forwardA drilling operation can be considered as a complex into the workpiece material, in this manner it jointly machining process, with specif c and unique charac- increases the DOC and the drilling force. Correspond- teristics, not least of which, are the production of chips ingly, if the drilling force is reduced, the twist drill will when drilling. Tese chips are in continuous contact recover its shape, with the cutting edge moving back with the drill f utes and the generated hole’s surface. from the workpiece, thus reducing both the DandOC Hence, any minute changes in the drill’s geometry, can cutting force. Tis stretching and compression of the cause enormous modif cations to the either the drill’s drill’s shaf – somewhat like a spring – is unique to wear rate and its predicted life. Heat generated whilst twist drilling, being an unstable element in the cut- drilling will be transformed by a range of ‘states’, in- ting process. By way of comparison, most cutting toolcluding: • Conduction – through the chips, workpiece and drill, • Convection and radiation – via the ‘air-spaces’ in the hole as the drill penetrates deeper into the workpiece. ‘Lengthening e,ect’ is associated with the twist drill’s shaf being twisted by the application of torque, with elastically Te drilling temperature during a prolonged operation springs-back upon release of the drilling torque. Not only can approximate steady-state conditions, with the heat will the twist drill ‘spring’ , but it can also ‘bend’ due to the generated whilst cutting when employing a new drill increased thrust loads produced by high penetration rates. . Figure 51. The drilling process and the asociated zones of heat generation whilst hole-making. [After: Trigger and Chao, 1951] is associated with two distinct regions (i.e. see Fig. 51 to reduce machining temperatures signif cantly and – Section on X-X) at the: aid drill penetration rates, while increasing tool life. • Primary shear zone – where plas- Coolant holes through-the-nose are not restricted to tic deformation occurs, this being the ma- twist drills, as Spade-and Gun -drills , together with jor source of heat generation, Indexable drills also of en incorporate this coolant de- • Secondary shear zone – from within the tool/chip livery feature, to remove heat and lubricate the cutting interface, where pronounced friction takes place. edges. NB Te drill clearance surface temperature, is sig- nif cantly af ected by the rake face interface tem- 3.1.4 Indexableperature. Drills Indexable drills have some signif cant advantages over Te bulk rise in the drill’s temperature is multifarious, their twist drilling counterparts (i.e. a range of both due to the necessity to consider a range of factors, in- indexable and twist drills are depicted in Fig.52a). cluding: heat f ow distribution, the geometric shape of Tese indexable drills – allowing the cutting inserts to the conducting bodies, together with any variation of be changed (see Fig. 52c), permit faster cutting speeds thermal properties of both the drill and workpiece ma- and enable a wider range of workpiece materials to be terials with temperature changes. Te generated heat successfully drilled than when utilising conventional distribution when drilling depends upon the thermal twist drills. Normally, indexable drills are limited to properties of the tool, workpiece and chip. Terefore, shorter hole depths of around ‘4D’, than equivalent the thermal difusivity (K/ρc), will determine the rate diameter twist drills. at which heat transfers through the material, while Indexable drills must be set up with care and in the also controlling the penetration depth of the surface correct relationship to the machine tool’s headstock/ temperature. While the absorption coef cient (Kρc), spindle, ensuring that both the drill’s and the spindle’s determines the quantity of heat being absorbed by a centrelines are coincident, otherwise over-, or under- given mass of material. Drilling temperatures vary sized holes may be produced (see Fig. 53a – top). Yet considerably in the research work undertaken over another problem that needs to be addressed when the years, being heavily inf uenced by a wide range of employing these indexable drills, is termed ‘radial cutting-related parameters, making it extremely dif runout’ - , which af ects the inserts centre height and cult to obtain meaningful comparisons of local tem- should be limited to <0.127 mm. One advantage of be- peratures in a real-time drilling operation. For exam- ing able to manipulate the indexable drill’s axis, is that ple, the scatter of ‘bulk’ temperature values for say, a it can be used to adjust the drilled hole’s diameter, by ,6mm twist drill, can vary between approximately 200 parallel adjustment of the drill’s and spindle’s respec- to 380?C, under steady-state drilling conditions, for tive centrelines – this being very useful for controlling comparable workpiece materials, making it very dif- f cult to obtain meaningful drill life comparisons. Coolant delivery is imperative when drilling and to this end, through-the-nose coolant operation enables Spade drills are twist drills (i.e. bodies are normally manu- the lubrication and cooling of the drill’s point (Fig. 52c factured from either 1018, or 1020 low-carbon steel) with a – illustrating the coolant holes behind the lips). Tis standard blade being inserted at the drill’s point, enabling ef cient technique of ensuring that the coolant gets to holes to be generated up to 8D deep. Blades are usually coated the action of drilling, gives better chip control, helping micrograin HSS with high-cobalt content, or coated cemented carbides, ranging from stub drills to extra-long lengths, with either straight, or spiral f utes. Gun-, or Deep-hole drills will be mentioned in some detail later in this chapter, but they allow considerable length-to-diameter ratios to be drilled in workpieces, necessitating high-pressure Twist drill interface temperatures have been reported to be coolant delivery, with ef cient chip-f ushing capabilities. over 870?C in the workpiece’s ‘plasticity region’ , which some- Radial runout refers to misalignment in the radial direction, what contradicts the ‘bulk’ temperatures, although in mitiga- which should be minimised, as it alters the position of the tion, it should be said that these very high temperatures at the drill’s cutting inserts. interface at somewhat localised. . Figure 52. Short hole drilling. [Courtesy of Sandvik Coromant] drilled hole tolerances. On turning centres this can be moment to the resultant cutting force FA is found. readily achieved by modifying the CNC cutting pro- Te bending strength attained in this manner can be gram, to of set the drill with respect to the machine’s greatly increased by employing round prof led chip centreline. Moreover, for turning operations employ- f utes. Tis rounded chip f ute cross-section, does not ing drilled features, then the indexable insert’s top sur- signif cantly weaken the drill’s body and provides op- faces must remain parallel to the X-axis of the machine timum chip-f ow – even when drilling long-chipping tool. Te arrangement of the inner and outer cutting workpieces. A taper of the tool holder behind the in- edges of an indexable insert drill relative to each other, sert seats, prevents a ‘squeezing’ of the chips between together with the drill’s position to the axis of rotation the drill and the drill-hole wall. are vital for perfect drilling operations(i.e. see Fig. Due to the design of the indexable drill, the two 53a). Te cutting inserts positions by possible X-axis cutting inserts are subjected to very dissimilar stresses adjustments, are critical for the: smooth running, re- when drilling. For example, the indexable drill (Fig. sultant cutting forces and, will inf uence the drilled 53a – bottom), has the outer insert being subjected hole’s alignment. Preferably, the cutting edges are ar- to greater stress than its inner counterpart, typically ranged in such a manner that the inner- (SBI) and having both thermal and abrasive stresses, while the outer-inserts (SBA) have identical cutting widths (Fig. inner insert must have high toughness characteris- 53a – bottom lef ). When new insert cutting edges are tics. Some cutting tool manufacturers recommend so- utilised, this results in a balance of the cutting forces in called ‘mixed-tipping’ of inserts, where a toughened the Y-axis, guaranteeing drilled holes of accurate size grade is used for the inner insert and a wear-resistant 0 and surface texture without ‘retraction striae’ . When grade for the outer insert. However, some discretion selecting the appropriate adjustment angles (χi) and should be used when utilising indexable drills with (χA), the lines of force via feed forces (Fa and Fi) will co- mixed cutting inserts, so perhaps reference back to incide with the drill’s axis in the centre of the clamping the tooling manufacturer may be advisable if produc- shaf (Fig. 53a – top). Hence, the clamping shaf must tion quantities are suf cient in order to optimise this only transmit torque resulting from the cutting forces potential ‘mixed grade strategy’. Typically, by exploit- and the bending moment of the resultant cutting force ing ‘mixed-tipping’, for example when machining which will be present. Typically, the outer insert’s re- free-cutting steel grades cutting speeds of up to 400 m sultant cutting force FA (i.e. Fig. 53b) is comprised ofmin are possible, whereas when drilling low -silicon the following forces: – • Remaining cutting force (?Fc) – generated through be achieved with tool lives of up to 45 min of cuttingaluminium grades cutting speeds of 600 m min can greater wear rates at the periphery of the outer cut- time per edge. – ting insert, Several design factors will inf uence an indexable • Passive force (Fp) – generated by the corner radius drill’s performance, these include: of the outer cutting insert. • Sintered cutting insert chip-breakers – these will improve chip control and enable high penetration With indexable drills, the chip f ute is selected so that rates to be utilised, the drill’s prof le from the tip up to the chip f ute, has• Advanced ,ute design – allowing deeper chip gul- its runout twisted by between 65? to 85?. In the vicin- lets, thus minimising chip-jamming tendencies, • ity of the chip f ute the runout (i.e. the longest ‘lever Faster-, slower-, or straight-,uted designs – with arm’ of the force), is where the maximum resistance wider f ute prof les reduce chip-binding and degra- dation of the drilled hole surface, whilst also im- proving penetration rates, • Cutting insert shape – utilising square (i.e. having 4 cutting edges), rectangular (i.e. with two edges), Some tooling manufacturers recommend that an indexable or triangular inserts (i.e. having three edges) – the drill’s inner insert is positioned slightly below the spindle’s cen- treline, as this allows a small core of uncut material to pass over the top surface of the insert and break of – being carried away with the rest of the chips. 0 ‘Retraction striae’ refers to the ‘trail-lines’ resulting from the ‘Mixed-tipping’ , refers to having dissimilar grades of inserts outer insert’s gouging, or ploughing the previously drilled sur- for the outer and inner cutting edges, as they fulf l dif erent face as it is withdrawn from the hole. mechanical working criteria whilst drilling. . Figure 53. Indexable insert drills – insert position and ,ute geometry. *Courtesy of Kennametal Hertel, latter being the most popular version for general- be up to 3.8 mm, or 7.6 mm on a diameter , which purpose drilling operations. in reality, amounts to a ‘f ne-boring’ operation, giv- NB Double-sided cutting inserts are available, but are mainly used for milling operations. On machining centres this of set is somewhat less, with the max- One major advantage that indexable drills have over imum radial of set being approximately 1 mm, or 2 mm on dia- twist drills, is that they can be ofset to produce dif er- meter. Of note, is that when an indexable drill is of set, then the ent hole diameters. Tis of set for turning centres can maximum feedrate should be no greater than 0.15 mm rev– . ing considerable scope in diametric sizing of holes. 3.1.5 Counter-Boring/Trepanning Tese drills should be selected for the minimum overhang and need to have both the drill’s and ma- A counter-boring operation is an of en utilised tech- chine tool’s centrelines parallel to within 0.076 mm, nique for the enlargement of previously manufac- or better. tured holes, normally to provide them with accurate By utilising an indexable drill on a turning opera- dimensions and/or improved surface f nish (i.e. see tion, they can be benef cial when attempting to start Fig. 55a). Counter-bores are also produced to register drilling angled, or uneven workpiece surfaces, as illus- a larger-faced shouldered shaf , or to sink a precision trated by some of the surface faces depicted in Fig. 54. cap-head bolt below the clamped part’s surface. In this Indexable drills, employed for such non-f at workpiece latter case, of en the previously-drilled hole is used faces, obviates the need for either a previous counter to align the bolt’s axis, by using a counter-boring tool boring, or spotfacing operation. with a ‘pilot-bush’ of the same drilled diameter to act as Te major advantage of indexable drills over either a guide to allow machining to a correct counter-bored their HSS, or carbide twist drill counterparts, is theirdepth. ability to run at much higher rates. For example and, Counter-boring heads are employed to open-out by way of simplistic comparison, if a ,18 mm hole has existing holes (Fig. 55a). Te heads were in the past, to be drilled in free-cutting stainless steel, then an: of en of a single insert design, but latterly, they are – • HSS twist drill – can be run at 16.8 m minwith produced with multiple inserts – particularly when the – a 0.2 mm rev feed would produce 280 rpm at working clearance is such, that it cannot cope with by – 57 mm min , the single insert variety. Te multi-insert counter-bor- • – Solid carbide twist drill – can be run at 61 m min ing heads, can have their inserts f nely diametrically- – feed would produce 1,019 rpm adjusted by means of precision wedge and radial ad- with a 0.1 mm rev – at 103 mm min , justing screws. • – Indexable drill – can be run at 170 m minwith A trepanning operation is undertaken in one op- – feed would produce 2,852 rpm at eration, but instead of machining the complete hole, a 0.76 mm rev – 217 mm min . only part of the hole is cut, leaving a core (Fig. 55b). For large workpiece dimension machining, a trepan- – NB Te indexable drill can be run up to 0.8 mm revning operation uses less power and axial pressure than and if one were to attempt to utilise a twist drill with other equivalent manufacturing processes. However, the cutting data shown above, then it would either one major problem occurs with the trepanning opera- burn-out, or catastrophically fail in the endeavour. tion, which is that the core produced as the trepanning tool penetrates into the workpiece, becomes quite dif- An important safety note when using these indexable f cult to handle (i.e. see Fig. 55b -lower diagram). drills in turning operations, is that the high penetra- Trepanning heads are normally utilised on: tion rates, coupled to the exit of a through hole in the• Large workpiece diameters – greater than 120 mm, part, creates a slug which is thrown clear from either Limited machine tool power – alternatively, if it is • the chuck, or from the rear of the bore of the machine’s not prudent to switch to another machine tool and/ headstock at tremendous force. If the machine tool is or lose part orientation and register , not guarded appropriately, it could prove to be hazard• - ous to any operator in the local vicinity. utilised as precision material stock.Core utilisation – large cores can be usefully Axis o,set/misalignment : for example, if there is a 1: mis - Part orientation and register , refers to the initial setup, where alignment between these centrelines, then the centrelines will once the workpiece has been clamped and partially machined, it cannot be satisfactorily removed then reset without a loss of be 0.43 mm apart at 25 mm from the point at which the cen- trelines cross at the tool’s tip. both accuracy and precision. . Figure 54. A wide range of drilling/boring operations can be undertaken using indexable insert drills. [Courtesy of Seco Tools] . Figure 55. Counterboring and trepanning. [Courtesy of Sandvik Coromant] 3.1.6 Special-Purpose, or Customisedchamfering and counter-boring. In this case, not only does the usage of special-purpose tooling here seem the obvious solution, as it combines these individual Drilling and Multi-Spindle operations in one, it has the advantage of meeting all Drilling Most of today’s Special-purpose, or Customised Drills three of the production criteria listed above, with the and Multi-spindle drills are normally designed and added advantages of both using fewer tools and utilis- manufactured to meet the following criteria: ing less space in the tool magazine. Some special-pur- • Long production runs – these enable the extra pose tools are very complex in their design and quite cost of this purpose-designed and built tooling to sophisticated in operation, but their supplementary amortised over the cost of the production period, cost more than outweighs this by the production gains • Short cycle times – when time is the ‘essence of of ered by their consequent implementation. importance’ in the production of the component, Multi-spindle drilling tooling is ideal when a series but it is not necessarily related to the overall quan- of hole patterns are required on a component, such as tity of the production run, for specif c conf gurations of: pitch circle diameters, • Tooling accuracy is re,ected in the part’s manufac- hole grid patterns, line of holes, or a combination of ture – if for example the precision part must have these (i.e. see examples of specif c patterns in Fig. 57 all its component features in accurate alignment, – top right). Hole pitch circle diameters can easily be or in a specif c relationship to a particular datum: accommodated, for large and small pitch diameters on face, plane or point. the same tooling, Likewise, hole line and grid patterns can be quite diverse, within the diametral area of the Special-purpose, or Customised tooling is normally ‘cluster plate’ (i.e. see Fig. 57 – top lef ). required if one, or several of the criteria mentioned Multi-spindle drilling heads utilise a main drive above are to be met. To have a tooling manufacturer gear which is engaged with an idler and then onto the design special-purpose tooling to meet the production drill spindle gear, this being attached to the individual demands of manufacture, is not undertaken lightly, as drill (i.e. see Fig. 57 – exploded view of a typical sys- for complex tooling, its: design, build and prove-out, tem). T e cluster plate orientates the individual drill prior to use, could prove to be expensive. However, spindles and their rotational speeds can be margin- many companies resort to this type of custom-built ally increased, or decreased, by changing the driver- tooling, because it is the only way that the product can to-driven gear ratio, moreover, their rotational direc- be manufactured economically. Of en, multiple fea- tion can be changed by the introduction of another tures are incorporated into just one tool, typically for idler into the gear train. T erefore, if additional idlers hole- and post-hole making operations, such as those are present, to change the drill’s rotation, then the an depicted in Fig. 56a. A relatively simple example of appropriate lef -hand drill would be required here. this special-purpose tooling is illustrated in Fig. 56b, By purposefully modifying each drill’s rotational di- where three production operations for the manufac- rection, this has the advantage of minimising overall ture of the female part of the pull-stud mechanism for torque ef ects on the multi-spindle drilling head, al- a milling cutter toolholder is depicted, namely, drilling, lowing a large number of drills to be utilised for one particular operation (i.e. see Fig. 57 – lower lef -hand photograph). An important point in utilising multi- spindle drilling heads, is presetting their respective drill lengths, so that they engage with the workpiece’s Long production runs usually refers to some form of continu- surface at the correct height. ous production , or large batch sizes, of , 5,000 components, By the correct production application of both Spe- to make the cost of the Special-purpose tooling viable. cial-purpose and Multi-spindle drilling tooling, then Amortisation, refers to the ‘pay-back’ of the tooling over the ‘life’ of the production of the parts produced. Short cycle times, are considered to be the quickest time that the part can be produced, under ‘standard’ machining conditions. Datum – the term refers to origin of the measurements for Multi-spindle tools, refer to more than one individual tool ro- the particular component feature, which could be from a face, tating in its respective toolholder, enabling several holes to be plane, or point. manufactured in just one operation. . Figure 56. Special-purpose multi-functional tooling can be designed and manufactured to machine many part features simultaneously . Figure 57. The application of multi-spindle drilling heads to increase productive throughput signifcant economic savings can be made and their major problems of utilising Deep-hole drilling, is chip initial capital outlay will have been worthwhile. Howdisposal, as the deeper the hole is drilled, the further - ever, such complex and expensive tooling used inapthe chip must- travel from the cutting edge to the hole’s propriately can be counter-productive, so consider- exit. Tis chip evacuation distance, can increase the able thought and care should be made into any future probability ofchip-jamming, or binding in the fute implementation ofthese tools. as the chip attempts to exit the deep-drilled hole. Notwithstanding the problems associated with BUE, which hinders the tool’s ability to break chips. Coolant 3.1.7 Deep-Hole Drilling/control and its operational usage is important in any the Deep-hole drilling technique, as one ofits main functions is – apart from lubricating/cooling the cut- Gun-Drilling ting edge and chip fushing – is to restrict frictional ef- Deep-Hole Drilling – an fects between the: drill, chip and hole wall. Moreover, Introduc tion Deep-hole drilling can be characterised by, high mate- if friction builds-up due to poor coolant delivery, this rial removal rates, having excellent: hole straightness,can result in higher torsional efects, which may cause dimensional tolerances and machined surface texture.the drill to snap. Deep-hole drilling applications are utilised across di- verse industrial applications, including: aerospace, Gun- nuclear power, oil and gas, as well as for steel and Drills chemical processing industries. Tese industries place Gun-drills (i.e. see Fig. 58a), are normally utilised to a high demand on all aspects of drilled hole quality machine small, straight diameters to high tolerances and reliability, with components being very expensive, and having excellent fnishes in a single operation. any failures will have severe economic consequences.Drilled hole sizes can range from as small as ,1.5 mm Te name Deep-hole drilling implies the machin- to ,75 mm in a single pass, with depths equating to ing of holes with a relatively long hole depth to its 100 times the tool’s diameter . Te ‘drilling system’ is diameter. Typically at the lower length-to-diameter a highly developed and ef cient technique for produc- ratios they can be as short as x5 the diameter, con- ing deep holes in wide variety ofworkpiece materials, versely at the other end of the scale, ‘ratios’ of > x100 ranging from: plastics, fbreglass, to high-strength ma- the diameter can be successfully generated, with close teria ls such as Inconel. Tis tooling usually consists of tolerances and a surface texture approaching 0.1 µm either a cemented carbide, or cemented carbide-tipped (Ra). Tere are a considerable number ofdeep-drill- drill head ftted to a tube-shaped shank . Te former ing production techniques, with each one having an solid carbide drill head version allows the tooling to appropriate usage for a particular hole generation be reground as necessary, while the latter version is method. A typical deep-drilling tooling assembly normally employed for larger diameter hole drilling is essentially ‘self-piloting’ , in that the cutting forces operations. Te drill head has two distinct designs, generated are balanced, not with respect to the cut- either having a ‘kidney-shaped’, or a cylindrical hole ting edges – as is the situation with Twist drills – but present, for the delivery of cutting fuid, which pro - invariably, by pads that are situated at 90? and 180? to vides: that ofthe cutting edge. Tese pads rub against the • Flow of cutting f uid – to create the maximum fow bore’s surface being generated and therefore support rate and chip-fushing, the head, while burnishing the surface. Te machine tools enabling these deep-drilled holes to be generated 0 can be expensive, along with the appropriate tooling, ‘Special-purpose’ Gun-drills can be produced to generate but the production costs can be dramatically reduced, drilled holes up to ,150 mm having 200:1 length-to-diameter by employing such a machining strategy. One ofthe ratios, at penetration rates of better than equivalent diameter Twist drills. Gun-drills would as a rule, have their drill head’s brazed – via silver soldering – onto a tube-shaped shank, these in turn, are 0 Burnishing will improve the surface fnish and dimensional ac- also brazed onto a ‘driver’ (i.e. ofvarious designs) ofthe re- curacy, by plastically deforming the machined surface layers – quired length for the successful drilling of long slender holes cusps – without removing any additional workpiece material. in the workpiece. . Figure 58. Deep-hole drilling operations, such as: (a) gundrilling, (b) double tube ejector drilling and (c) single tube ejector drill- ing. [Courtesy of Sandvik Coromant] • Minimal fuid-fow disturbance – giving consis- ure mode. Yet another Gun-drill failure situation may tent/regular fow-rate to drill-head, arise if there is excessive clearance between the drill • Minimum offuid-turbulence – allowing chips to bush and the drill’s tip. Under these circumstances, the be easily evacuated from the cutting region. Gun-drill’s edge cuts a signifcant volume ofworkpiece material and, as this edge is not designed to cut – hav- Typically, cemented carbide heads, have an external V- ing a zero clearance angle (i.e. created by the circular shaped chip-fute which extends along the shank, the margin at this edge) – the excessive cutting forces angle ofthis chip-fute has been experimentally-deter- cause the edge to prematurely fracture. mined to be 110?, providing the following advantages: Ifinsuf cient coolant fow occurs, this is also a typ- • Optimum fute cross-section – allowing the most ical factor in subsequent Gun-drill failure. T is lack of rapid cutting fuid return and chip transportation, coolant causes the chips to pack in the V-fute, forming • Facilitates an extra support pad – this is necessary a plug, which then creates excessive torque in the Gun- when drilling through crossing holes, drill and, this plug allows the tip to separate away from • Provides optimal torsional strength – important the shank. Occasionally, end-users blame the Gun-drill for workpieces having very long length-to-diameter tooling manufacturer for poor brazing, ifthe tool’s tip ratios, separates from the shank. However, when analysis of • Facilitates tool clamping – enabling the tool to be the brazed fractured surfaces occurs, invariably, small held in a three-jaw chuck for convenient regrinding carbide particles are adhered to the shank, this being on a suitable cutter-grinder. evidence ofthe fact that the braze was stronger than the tip, clearly demonstrating that the brazing was not at fault. Gun-Drill Fa In many circumstances, the Gun-drill tool manu- ilure One ofthe main reasons for Gun-drills to fail in op- facturer is blamed by the customer for its failure dur- eration, is through an excessive misalignment of the ing machining, but when investigated, it is usually drill bushing and this will be in relation to the drill’s premature failure being the result of a poor tooling rotational axis (i.e. see Fig. 58a). T is type of align- installation and operation. One ofthe major causes of ment failure mode is termed a ‘balk-crash’ – caus- Gun-drill failure, is via the coolant distribution sys- ing the tool to fracture into numerous pieces . Ifthe tem, where inconsistent delivery ofthe fuid can either drill is rotated rather than the workpiece, the stress is ‘starve’ the Gun-drill’s cutting edge, or ‘over-food’ the re-applied to difering portions of the tip and, at the system. One ofthe major factors contributing to this weakest point, namely the drill’s corner, the tip will over-/under-supply of coolant delivery, is due to the most likely fracture in this region. A potential failure fact that in the main, coolant pressure is being moni- rigidity decreased with increased length . T e shank rate. Ifholes are Gunmode is related to the Gun-drill’s length, which has its tored, rather than the measurement of coolant fow-drilled <,4 mm, then high-pres-- ofa longer Gun-drill will not transmit a large amount sure coolant fow-rate to the point is essential, but in ofbending force to the cutting tip – when misaligned many cases ofcoolant systems ftted to ‘standard’ ma- – however, the tip does not fracture, but instead, any chines, they are ofrelatively low-pressure delivery. Re- axis misalignment causes the shank to fex with each cently, one machine tool manufacturer, has designed revolution, a situation that is ideal for a fatigue fail- and developed a coolant intensifer pump coupled to a special high-pressure union, which gives variable pump pressures ofover 200 bar, with special-purpose couplings to overcome the problems of poor coolant ‘Balk-failure’ of Gun-drills is the result of the ‘brittle’ carbide fow-rates to the cutting vicinity. tip being unable to withstand the bending stresses created by its unintentional axis misalignment. Gun-drill ‘rigidity rule’: as the drill’s length increases, its ri- 3.1.8 Double-Tube Ejector/Single- gidity decreases by the ‘cube’ ofthe distance. For example, if Tube Systemtwo identical Gun-drill diameters are employed for drilling Drills the same workpiece material, then if one drill is twice as long Double-tube Ejector drills (i.e. ofen just termed ‘Ejec- as the other, then its rigidity will 8 times less rigid than its counterpart (i.e. namely: 2). tor Drills’), are designed around a twin tube system (i.e. see Fig. 58b – for the schematic and inset a photo tating workpiece and a stationary tool, with any centre of the drill head). Here, the self-contained system (i.e. divergence resulting in bell-mouthing at the hole’s en- not requiring specif c sealing arrangements), of the trance and a wavy hole surface. Once the support pads cutting f uid, is externally pumped along the space be- in the drill head have moved x5 their length down the tween the inner and outer tubes. Te major portion of drilled hole, then any further waviness is negligible, as the cutting f uid is fed forward to the drill head, while they begin to press down on the hole’s curvature. Many the remainder is forced through a groove in the rear deep-drilled hole prof le and tolerance abnormalities section of the inner tube. A ‘negative pressure’ occurs result from centre divergence , which needs special at- in the front portion of the inner tube, which causes the tention to minimise such ef ects. cutting f uid at the drill head to be sucked out through Single-tube [Ejector] System drills (i.e. commonly the inner tube along with the chips. As is the case for referred to and abbreviated as simply ‘SST’) are sche- Gun-drilling coolant supply, it must be of suf cient matically depicted in Fig. 58c. With this SSTtooling pressure and volume, to overcome any likelihood of assembly, the cutting f uid is pumped under pressure ‘starvation’. between the drill and the hole wall (i.e. normally this Te ‘ejector head’ of the drill comprises of: a con- width of space is approximately 1 mm) and it exits nector, outer and inner tubes, a collet and sealing with chips through the inside of the drill tube (Fig. sleeve, together with a drill head. Disposable heads 58c). Te quantity of cutting f uid passing through the with cemented carbide tips are utilised for diameters drill is twice as great and with higher pressure, than ranging from 18.5 to 65 mm, normally supplied with for an equivalent ‘Ejector’ tooling assembly. Hence, the two types of cutting edge geometries, with the carbide SSTset-up provides improved chip-breaking and mi- cutting tips precisely located on either side of the drill nimises any potential chip-jamming, even when vary- head. Te asymmetric design of these ‘Ejector Drills’ ing chip lengths occur. has support pads provided, to absorb the radial cut- Te drill head arrangement of cutting inserts will ting forces and guide while supporting the tool as it vary from two, three, or more, depending on the drill’s penetrates into the workpiece. At the commencement diameter, usually made of cemented carbide, of en as of the deep-drilling operation, the drill bushing’s main brazed over-lapping tips, although disposable index- function (i.e. shown in Fig. 58b), is to guide and sup- able pocketed inserts with chip-breakers are of en port the drill at initial workpiece entry and until drill utilised for larger diameter holes. SSTtools can be used penetration allows the support pads to bear on the to drill small diameter holes, ranging from ,12.5 mm partially-drilled hole surface and thereupon remain- upward, with 100:1 depth-to-diameter ratios. Te SST ing in contact throughout the drilling operation. tooling system copes with dif cult-to-machine work- Whilst deep-hole drilling, the drill and workpiece piece materials, such as Monel, Inconel and Hastel- centrelines must not deviate by > 0.02 mm, so any sub- loy and other ‘exotic materials’. In actual production sequent drill bush wear needs to be carefully moni- machining trials, it has been found that SSTtools can tored and controlled. It is usual practice to have a ro- produce deep-drilled holes up to 15 times faster than is achievable by conventional Gun-drilling. Tis high production output level gives an 80% improvement in machining rates for this SST Deep-drilled hole production output and, it has been shown in several Asymmetric Drill Head design, refers to the fact that the instances, to give a ‘Return on Investment’ (ROI) cutting inserts are not only radially, but are angularly of set. Terefore, they normally require two support pads to counter- in act and sustain the radial cutting forces generated while dril- about 6 months. ling deep holes. By locating the cutting inserts on both sides of the drill head, the greater percentage of radial forces are negated at these pads. Drill bushing tolerances between the drill and bush for both Return on Investment (ROI), for Deep-hole drilling operati- the ‘Ejector’ and Single-tube Systems, require a f t of ISO G6/ ons (i.e. in % terms), is given (i.e. in simplistic terms) by the h6, equating to a minimum play of 0.006 mm. Tis drill bush following formula: is usually manufactured from a hardened material (i.e. 60 to 62 HRC) such as cemented carbide, as it has a longer service Totalconversioncost life, with bush wear normally limited to 0.03 mm. %ROI = Costofa -to- productivitygain 3.1.9 Deep-Hole Drilling –hole drilling power. Terefore, in order to estimate the machine tool’s power requirement (i.e. ‘P’ in kW ), an allowance must be made for any power losses in the Cutting Forces and In Deep-hole drilling operations, the underlying the- machine tool. Hence, the gross power required can be Power ory for the calculation of cutting forces and for torque established by dividing the Deep-hole drilling power are similar to that utilised for ‘conventional’ drilling (i.e. Pc + Pµ), by the machine tool’s ef ciency ‘η’. Tis operations. Te major dif erence between the hole ef ciency indicates what percentage of the power sup- production calculations for Deep-hole drilling to that plied by the machine tool, that can be utilised, while of ‘conventional hole-making’ techniques, lies in the Deep-hole drilling. fact that support pads create a sizeable level of fric- tional forces, that cannot be ignored. Tese increased Power (kW): frictional ef ect contributions – by the pads – to the c overall Deep-hole drilling cutting forces and torquec µ p , (P +P )= kc,ap , f ,v ( . ?a ,D) values are somewhat dif cult to precisely establish, however, an approximate formulae can be used to estiWhere:- mate them, as follows: Pc + Pµ = Power contributions of: cutting and friction respectively (kW), – vc Feed force (N): = Cutting speed (m min ). Fp + Fpµ = 0.65 × kc × ap × f × sinκr ,P = Pc + Pµ/η Where: Fp = Feed force, or drilling pressure (N), Where: Fpµ = Force and Frictional ef ects (N), η = Machine tool ef ciency. – kc = Specif c cutting force (N mm ), ap = Depth of cut (mm), f 3.2 Boring Tool– = Feed per revolution (mm rev ), Technology – sinκ r = Entering angle (?). Torque, or Moment (Nm): Introduction c p Te technology of boring has shown some important c µ k ,a , f ,D( advances in recent years, from advanced chip-break- p,D) M +M = . ?a Where: ing control tooling (i.e. see Fig. 59, this photograph Mc illustrates just some of the boring cutting insert ge- Mµ ometries that can be utilised), through to the ‘active = Torque cutting (Nm), – kc ), suppression of chatter’ – more will be mentioned on = Torque and Frictional ef ects (Nm), ap = Specif c cutting force (N mm f the topic and reasons why chatter occurs and its sup- = Depth of cut (mm), D pression later in the text. Probably the most popular – type of boring tooling is of the cantilever type (Fig. = Feed per revolution (mm rev ), Relatively high speeds are utilised for Deep-hole Drill59), although the popularity of either - ‘twin-bore-’ , or ing operations, in order to achieve satisfactory chip- = Hole diameter (mm). breaking, this necessitates having a machine tool with a reasonable power availability. ‘Chatter’ , is one of the two basic types of vibration (i.e. Te underpinning theory for calculating the power namely, ‘forced’ and ‘self-excited’) that may be present dur- ing machining. In the main, chatter is a form of self-excita- requirements, corresponds with that of ‘conventional’ tion vibration.‘*It is,？ due to the interaction of the dynamics drilling operations. However, the friction forces that are of the chip-removal process and the structural dynamics of the present, due to the employment of support pads, gives machine tool. Te excited vibrations are usually very high in rise to a torque contribution (Mµ), which in turn pro- amplitude and cause damage to the machine tool, as well as duces an associated contribution ‘Pµ’ to the total Deep- lead to premature tool failure’. [Af er: Kalpakjian, 1984]. ‘tri-bore-heads’ , with ‘micro-bore adjustment’ of the ei-hole’s contour, but generates its own path and will ther the individual inserts, or having a simultaneous therefore eliminate drill-induced hole errors by the adjustment of all of the actual cutting inserts, is be- subsequent machining operation (i.e. see the sche- coming quite common of late. matic representation shown in Fig. 60), Boring operations invariably utilise cantilevered• Improvement of surface texture – the boring tool (i.e. overhung) tooling, these in turn are somewhat can impart a high quality machined surface texture less rigid than tooling used for turning operations. to the enlarged bored hole. Boring, in a similar manner to Deep-hole drilling and Gun-drilling operations, has its rigidity decreased by NB In this latter case, boring operations to previ- the ‘cube’ of the distance (i.e. its overhang), as the fol- ously drilled, or to any cored holes in castings, can be lowing equation predicts: adjusted to give exactly the desired machined surface texture to the f nal hole’s dimensions, by careful ad- , o justment of the tool’s feedrate and the selection of an L M . Mb t π ,EI appropriate boring tool cutting insert geometry. f = ( + ) Where: fo = normal force acting on the ‘free end’ of the can- tilever (i.e boring tool overhang), *EI = f exural stif ness (i.e. I = cross-sectional moment 3.2.1 Single-Point Boring Tooling of Inertia) (Nm ), Mt = boring bar mass (kg), ‘Traditional’ boring bars were manufactured as solid L = length of cantilever (mm), one-piece tools, where the cutting edge was ground Mb = Modulus elasticity of the boring barof to the desired geometry by the skilled setter/operator, – (N mm ). which meant that their useful life was to some extent * E, relates to the boring bar’s ‘Young’s modulus’. restricted. Later boring bar versions, utilised indexable cutting inserts, or replaceable heads (Fig. 61). Boring Boring a hole will achieve several distinct production bars having replaceable heads are versatile, with the criteria: same bar allowing dif erent cutting head designs and Enlargement of holes – a boring operation can en- cutting inserts (Fig. 61a). Here, the insert is rigidly • large either a single, or multiple series of diameters, clamped to the tool post, with replaceable ‘modular to be either concentric to its outside diameter (i.e. tooling’ heads with the necessary mechanical coupling O.D.), or machined eccentric (i.e. of set) to the to be utilised (i.e. Fig 61b), of ering ‘qualif ed tooling’ O.D., dimensions. 0 • Correction of hole abnormalities – the boring process does not follow the previously produced necessitating correction by a boring operation. Tis ‘correc- ‘Eccentric machining’ of the bore of a component with respect tion’ is necessary, because the drill’s centreline follows the to its O.D., was in the past accurately achieved by ‘Button-bo- path indicated, ‘visiting’ the four quadrant points as it spirally ring’ – using ‘Toolmaker’s buttons’ (i.e. accurately ground and progresses through the part. Hence, hole eccentricity along hardened buttons of ‘known diameter’) that were precisely of - with harmonic departures from roundness can be excessive, set using gauge blocks (i.e ‘Slip-gauges’). Tis technique might if the drill’s lip lengths and drill point angles are of -centre. still be employed in some Toolrooms, but normally today, on Te cross-hatched circular regions represent the excess stock CNC-controlled slideways, a simple ‘CNC of set’ will achieve material to be removed by the boring bar, where it corrects 0 the desired amount of bored eccentricity. these hole form errors, while machined surface texture is also Correction of hole abnormalities , as Fig. 60 schematically il- considerably improved. ‘Qualif ed Tooling’ , refers to setting the tool’s of sets, with all lustrates, how boring can correct for ‘helical wandering’ of the the known dimensional data for that tool, allowing for ease of drill as it had previously progressed through the workpiece. tool presetting and ef cient tool-changing – more will be said Te drill’s helical progression would cause undesirable hole on this subject later in the text. eccentricity, resulting from minute variations in its geometry, . Figure 59. A selection of some tooling that can be employed for boring-out internal rotational features. [Courtesy of Seco Tools] In the case of the boring bar’s mechanical interface four clamping screws provide an accurate and secure (i.e. coupling) example shown in Fig. 61a- top, the ser- f tment for the replaceable head, with internal tension rated V-grooves across the interface along with the adjustment via the interior mechanism illustrated. . Figure 60. The harmonic and geometric corrections by a boring operation, to correct the previous helical drift, resulting from the drill’s path through the workpiece . Figure 61. Interchangeable cutting heads for boring bars utilised in machining internal features. [Courtesy of Sandvik Coromant] Possibly a more adaptable modular system to the ‘ser- 3.2.2 Boring Bar Selection Toolholders, Inserts and Cuttingrated and clamped’ version, is illustrated in Fig. 61b, of: where the cutting head is held in place by a single rear- mounted bolt and grub screws around the periphery Parameters of the clamped portion of the boring bar securely lock the replaceable head in-situ, enabling the cutting head Boring Bar To olholder – to be speedily replaced. Some of these boring bar’s Decisions have a dovetail slide mechanical interface, with the Whatever the material chosen for the boring bar, its is dovetail coupling providing radial adjustment of the always preferable to use a cylindrical shank whenever cutting insert’s edge. Tis ‘universal system’ (Fig. 61b), possible, as it of ers greater general cross-sectional ri- is normally used for larger bored diameters, that would gidity, to other boring bar geometric cross-sections. range from 80 to 300 mm. Furthermore, it is possible Once the bar cross-section has been selected, the next to add spacers/shims to precisely control the boring decision to be taken concerns the tool’s lead angle. bars overall length, this is particularly important when Usually the f rst choice for lead angle would be a 0? medium-to-long production batches are necessary, in lead, as the radial cutting forces are minimised, with order to minimise cycle time and its non-productive the resultant forces being directed axially along the setting-up times. bar, toward the tool’s clamping point – which is ideal. In Fig. 62a and b, are illustrated single-point inter- If, a 45? lead angle is selected, then the cutting forces changeable boring insert tooling, with Fig. 62a giving are split between the axial and radial directions. Tis typical length-to-diameter (i.e. L/D) ratios for actual latter radial cutting force, can increase the probabil- boring and clamping lengths. Te amount of boring ity of increased bar def ection and be a source for un- bar-overhang will determine from what type of ma- wanted vibrational ef ects. terial the boring bar will be manufactured. Te most common tool shank materials are alloy steel, or ce- NB For more information concerning boring bar se- mented carbide, for L/D ratios of <4:1, with the for- lection, see Appendix 1b, for the ISO ‘code key’ for mer tool material in the main, being used here. For ‘solid’ boring bars. L/D ratios of between 4: to 7:1, steel boring bars do not have adequate static, or dynamic stif ness, so in this case cemented carbide is preferred. One limitation of Inser t Selec tion – utilising cemented carbide tool shanks, is its greater Decisions brittleness when compared to steel, so careful tool Apart form the boring bar’s lead angle, an insert’s ge- design is necessary to minimise this problem. ‘Com- ometry will also af ect vibration during machining. pound’ boring bar tool shanks have been exploited Te two main types of insert inclination (i.e. rake) an- to reduce both problems associated with either steel, gles are either positive, or negative – referring to their or cemented carbide tools. A successful compound angular position in the bar’s pockets. It is well known, tool used in cutting trials by the author, featured a ce- that a positive insert shears workpiece material more mented carbide core surrounded by alloy steel, which readily than a negative style insert, as a result, the proved to be quite ef cient in damping performance positive insert will generate a lower tangential cutting and machining characteristics. Fig. 62b, illustrates the force. Tis positive rake angle, is at the expense of de- internal mechanism of the boring bar, for potential creased f ank clearance and, if too small, the insert’s ‘bar-tuning/damping’ – to reduce vibrational inf u- f ank will rub against the workpiece creating friction, ences whilst machining. Here, the mechanism consists causing potential vibrations to occur. of a heavy slug of metal, held at each end by rubber Assuming that the insert’s edge strength will be grommets, in a chamber f lled with silicon oil. Tere- adequate for the machining application, then when fore, as the boring operation commences the slug vi- selecting an insert for boring, selection of a positive brates at a dif erent frequency to the steel bar, which geometry with a small amount of edge preparation, counteracts the vibration, rather than intensifying vi- having a suitable coating (i.e. PVD, rather than CVD), brational ef ects. Such ‘damped’ boring bars, have been is a good start point. Furthermore, the choice of a pe- utilised with large overhangs, of between 10: to 14:1 ripherally-ground insert having a sharper cutting edge L/D ratios. More information on ‘damping ef ects will in comparison to that of a directly-pressed and sin- be mentioned in Section 3.2.4. tered insert, is to be recommended. Te insert’s substrate – if cemented carbide – re- tial edge-chipping condition more readily, then, if this quires some thought, as if it is too hard, this type of proves successful, a harder grade may be selected. insert may chip via the ef ects of machining vibrations, this is particularly so, if the tool geometry has an ex- Cutting Parameters – Decisions tra-positive and sharp insert cutting edge. It might be more prudent to initially choose a medium-hard ce- Two complementary cutting parameters are the insert’s mented carbide grade, as it tends to cope with a poten- nose radius and the inf uence it has on the DOC. For . Figure 62. Interchangeable cutting heads for machining internal features. [Courtesy of Sandvik Coromant] example, when a fnish boring operation is required, all ofthe boring bar’s periphery in the toolpost, allow- then it is recommended that both a small nose radius ing much greater tool rigidity and cutting stability, al- and DOC is used. Tis smaller boring insert nose ra- leviating many ofthe potential problematic in-service dius, minimises contact between the workpiece and machining conditions. insert, resulting in lower tangential and radial cutting forces. For fne-boring applications, a good start point is to choose an insert with a 0.4 mm nose radius, with 3.2.3 Multiple-Boring Tools a 0.5 mm DOC. It should be noted that the DOC ought to be larger than the nose radius, this is because ifit was Twin cutting insert tooling, usually consists of a cy- the other way around, cutting forces would be directed lindrical shank with slides mounted at the front (Fig. in a radial direction – increasing potential vibrational/ 63a), or a U-shaped bar with cartridges (Fig. 63b). Te bar-bending (i.e. push-of ) problems. slides and cartridges can be radially adjusted, allow- Feedrates should be identical regardless of tool’s ing for a range ofvarious bored diameters to be ma- overhang, as any feed selection is normally based upon chined. Normally, such tooling has a 7 mm maximum the insert’s chip-breaking capabilities. Avoidance of cutting depth recommended – for both edges simul- very high feedrates when rough boring is necessary, as taneously in-cut. With Twin-edged boring tools the it can signifcantly increase the tangential cutting force cartridges can be so arranged, that ‘Step-boring’ can component. For fnish boring operations, it is normally be utilised. the workpiece’s surface texture requirement that dic- When large diameter component features require tates the maximum feedrate that can be utilised. More a boring operation, then the ‘Divided-version boring’ will be mentioned on the machined cusp height’s efect tooling can be exploited, but diametral accuracy is not on surface texture, this being created by the remnants as good as for some ofthe other types ofboring tool ofthe partial nose arc (i.e. radius) ofthe cutting insert designs. An advantage ofthe Divided-version’ boring and the periodic nature ofthe selected feedrate on the tools, is the fact that a large diameter range can be cov- bored workpiece’s surface, later on in the relevant sec- ered, with this single tool. If a ‘Universal fne-boring’ tion in the book. tool is utilised (Fig. 63b), either internal (Fig. 63b-top), A mistake ofen made by setters/machinists in or external machining (Fig. 63b – bottom), can be un- order to attempt to minimise vibrational tendencies, is dertaken. In this case, the fne-bore cartridges (1) are to reduce the rpm. Tis strategy will not only decrease mounted on a radially-moveable slide (2), which is productivity, but the lower rotational speed can lead mounted on a bar (3). In the latter case ofexternal com- to BUE formation, which in turn, modifes the insert’s ponent feature boring, there is a physical limit to the cutting geometry and could change the cutting force minimum diameter that can be machined – this being directions. Instead of rpm reductions, modifcation controlled by the bar’s actual size. (i.e. Here, it should be of other cutting data variables is suggested, in order said that this particular tooling ‘setup’ can be thought to improve these adverse vibrational/chatter efects. of as virtually a Trepanning operation with a boring Sometimes even increasing the rotational speed, can tool). Moreover, with this external fnishing operation, eliminate unwanted chatter. the spindle must rotate in a lef-hand rotation. Although it is not a specifc cutting performance Tri-bore tooling ofen having individual micro-bore parameter, an ofen disregarded measure is that of cartridge adjustment (i.e. not shown), as its name im- boring bar tool clamping. In many circumstances, cy- plies, uses three cutting inserts equally-spaced at 120? lindrical boring bars are simply clamped with several apart. Tis boring tool arrangement ofcutting inserts, setscrews, this is a poor choice ofclamping method, as ofers very high quality bored diametral accuracy and at best, setscrews only contact about 10% ofthe boring bar. Conversely, a split-tool block, clamps along almost ‘Step-boring’ , refers to using special shims with one of the cutting inserts axially situated a little way in front of the other, while at the same time, the cartridges are radially adjusted en- ‘Tool push-of ’ – ofen termed ‘spring-cuts’ , are the result of abling the front insert to cut a slightly smaller diameter to that tool defection, particularly when light cuts are used. To mini- of the rear one. It should be noted that when ‘Step-boring’ , the maximum DOC is normally 14 mm, with an associated feedrate mise the ‘push-of ’ , very rigid workpiece-machine-tool setup – OC with a smaller nose radius to that of the D is recommended. of 0.2 mm rev . . Figure 63. Twin-edged boring tooling. [Courtesy of Sandvik Coromant] precision to the machined hole, but such tooling can a boring bar’s overhang increasing under standardised be somewhat more costly than when utilising a single- machining conditions, the amplitude will also increase. insert tool. However, if the boring bar was dampened in some way, perhaps by utilising a ‘shock-absorber ef ect’, ma- chining could be undertaken at longer overhangs. Tis 3.2.4 Boring Bar‘damping ef ect’ is indicated by the highly centralised amplitude of oscillatory movements quickly reducing Damping For boring bars that have an L/D ratio of <5:1, then with time, indicating a high level of dynamic stif ness, relatively stable cutting conditions with controllable this being crucial for long L/D ratios. Obviously, the vibrational inf uences can be tolerated. However, if L/D boring bar’s cutting edge def ection at its tool tip, is ratios utilised are larger than this limiting value, then directly related to the amount of bar overhang, this de- potentially disastrous vibrational tendencies could oc- f ection being the result of the applied cutting forces. cur, leading to a variety of unwanted machining and Te magnitude of a boring bar’s def ection being de- workpiece characteristics, these include: pendent upon: bar composition, diameter, overhang • Limited tool life – caused by forced and self-excited and the extent and magnitude of tangential and radial vibrations, restricting both cutting ef ciency and cutting forces. Te rigidly clamped and cantilevered tool life, boring bar’s ‘free-end’ will def ect/deform by forces • Unacceptable machined surface texture – vibra- acting upon it and, some idea of the magnitude of this tions in the form of workpiece surface chatter, can def ection can be gleaned by the simple application of be the cause for component rejection, ‘mechanics of materials’, using the following formula: • Substandard machined roundness – vibration/ chatter ef ects creating high-frequency harmonic (mm) ,L ef ects on the roundness prof le. ,= F ,E,I Where: Stif ness can be expressed in terms of either static, or = Boring bar def ection (mm), dynamic stif ness. Static stif ness of a bar is its ability to = Cutting force (N), ? resist a bending force in a static condition, conversely, = Boring bar overhang (mm), – dynamic stif ness is the bar’s ability to withstand os- = Bar material’s coef cient of elasticity (N mm ), F cillating forces (i.e. vibrations). Dynamic stifness is an *I = Moment of Inertia (mm ). L essential property for a boring bar, as it is a measure of * For a boring bar of circular cross-section, the Mo- E its capacity to dampen the vibrations occurring during ment of inertia will be: machining, being greatly dependent of its overhang. As one would expect in testing for dynamic stif ness, with I = /64π × D (mm ). For example, assuming that if a ,25 mm steel boring bar has an L/D overhang of 4:1, with an applied cut- ting force of 100 kP, then the magnitude of bar def ec- ‘Harmonics’ – on a machined component are the product tion, using the above formula, would be: of complex interactions, including method of manufacture: component geometry, cutting data utilised, any vibrational ?L = D = 0.083 mm. inf uences encountered and material composition and its manufacture (e.g. Powder Metallurgy parts can vary in both porosity and density throughout the part, which may af ect, or If the overhang of this boring bar was now increased locally destabilised the cutting edge). to L/D ratios of 7:1 and 10:1, respectively, this would produce tool tip def ections of: NB Harmonics on the machined workpiece, can be thought of as a uniform waveform (i.e. sinewave) that is superim- posed onto the part’s surface. Te part’s low frequency harmo?-L = D = 0.444 mm nicof en has higher frequency harmonics superimposed onto ?L = 0D = 1.293 mm. the roundness. For example, a 15 undulation per revolution (upr) harmonic, could have a 500 upr harmonic superimposed Hence, these def ection values emphasise the impor- onto it, requiring suitable a Roundness Testing Machine with tance of reducing overhang as it increases by approxi- Gaussian f lters to separate out the respective harmonic con- mately ‘cube’ of the distance. Moreover, def ection can ditions – for metrological inspection and further analysis. also be reduced by utilising a diferent boring bar ma- method ofachieving this bar damping has already been terial, as this will improve its coef cient ofelasticity mentioned in Section 3.2.1, with the relationships . be- In boring-out roughing operations, any vibrations tween the size ofthe bar’s body, suspension, viscosity present are only a problem ifthey lead to insert dam- of the liquid media, being carefully designed by the age. For fnish-boring operations, vibrational condi- tooling manufacturer. During the boring operation, tions that may occur could be the diference between the vibrations set the body in oscillation. Hence, the success and failure for the fnished machined part. body and the liquid alternate, taking each others place So, the boring bar’s ability to dampen any vibrational in the space within the actual boring bar. A pattern is source becomes imperative, once a fne-boring opera- established during boring, where the oscillations ofthe tion is necessary. Vibrations can occur in any number body are not in harmony with the vibrations resulting of ways that could afect the boring operation, from from machining. Tis out-of-harmony, means that the the constructional elements of the machine tool, vibrations are virtually neutralised – to an acceptable through to slideways, or their recirculating ball bear- level – via the kinetic energy being transformed by the ings, etc.. Hence, the joints in a machine tool can be ‘system damping’. Any vibrations present during bor- regarded as a complicated dynamic system, with any ing, are relative to the amount ofbar overhang, there- slideway motion ofvibrating contact faces, necessitat- fore on longer boring bar lengths, they are normally ing lubricating oil to not only reduce any stiction and ftted with some means ofadjustment, so that they can frictional efects, but to help dampen these structural be ‘tuned’ to the frequency occurring within its range. elements. Machine tool builders are acutely aware that Te simplest manner of achieving adjustment, is by certain machine tool materials ‘damp’ more readily a rotation ofa lockable set screw, which when either than others. Cast iron and in particular ‘Granitan’ (i.e. tightened, or slackened, afects the suspension ofthe a product ofcrushed granite and epoxy resin), can pre- body in the liquid, thus ‘tuning’ the boring bar to the dominantly act as built-in dampening media for any actual machining conditions present. vibrational sources present. Te main source for any vibrations in boring, results from the long overhangs, necessary to machine the hole depth of the compo- 3.2.5 ‘Active-suppression’ ofnent’s feature. Terefore, the magnitude ofvibrations in the overall system result from the dampening capa- Vibrationbilities ofthe actual boring bar. As has been stated at the beginning ofSection 3.2.4, if s boring bars have an L/D ratio >5:1, then vibrational ef- fects may result in tool chatter. It has been observed in Tu ned Boring experimental work, that the boring bar’s tip produces Bars A boring bar that has been ‘tuned’, has the ability to a vibration motion that follows an elliptical path in the dampen any generated vibrations between the work- plane normal to the longitudinal axis ofthe bar. Te piece and the cutting edge while machining. Te ratio ofthe amplitude ofvibration along the major and ‘dampening efect’ is achieved through a vibration ab- minor axes varies with cutting conditions, further- sorbing device (i.e see Figs. 61a and 62b), this has the more, the inclination ofthese axes to the ‘radial line’ consequence ofincreasing the bar’s dynamic stifness, ofthe tool also varies. Ofsignifcance, is the fact that giving it the ability to withstand oscillating forces. Te the build-up ofchatter will begin almost immediately, even before one revolution of the workpiece has oc- curred. Tis build-up continues almost evenly until some limiting amplitude occurs, which suggests that the well-known ‘Orthogonal mode coupling’ is pres- Coefcient ofelasticity, for a steel boring bar composition, ent, further, with the phase diference between the vi- E = 21 × 10 (N mm), conversely, using a cemented carbide brations causing an elliptical tool tip path, the vibra- material for an identical boring bar, E = 63 × 10 (N mm ), giv- – ing three times greater stifness, allowing much greater boring tional energy is fed into the tool-workpiece system, – bar overhangs. promoting self-excitation. As has been suggested, the dynamic stability ofthe NB In reality, the boring bar’s defection will be higher than boring bar is ofprime importance, with the onset of the values given in these examples, as the formula is based self-excited chatter, being governed by the ‘Multiple upon the assumption that the bar is absolutely rigidly clamped, regenerative efect’, which is a function ofthe so-called which is impossible to achieve. ‘space phase’. Tis ‘space phase’ condition, is the phase the ‘damping’ and vibrational forces are controllable. of vibration around respective turns of work, f uctu- Tis type of ‘active’ boring bar arrangement, achieves ating between 90? and 180? and is equal to the phase directional damping characteristics via its ‘dampers’, between the inner and outer modulation. Moreover, here they control two ‘degrees of freedom’ via the ‘Re- it has been shown that by modifying the workpiece’s generative feedback loop’, which diminishes oscillatory consequently inf uences the ‘time phase’, leading torotational speed, this disturbs the ‘space phase’ and, motion (i.e. harmonics), by careful control of enelosses. rgy a reduction in self-excited chatter. It has been practi- In recent years with the advent of artif cial intelli- cally demonstrated that by modifying the peripheral gence (AI) applications to major industrial engineering speed of the workpiece, this technique is only partially problems, and more specif cally, in the performance successful in alleviating chatter. More success can be and robustness of certain types of ‘Neural networks’ , made by utilising damped boring bars, such as the the goal of obtaining some form of real-time monitor- ‘Lanchester’ type , with dynamic vibration absorbers ing and control in the machining process is now closer (DVA’s), to really suppress vibrational inf uences dur- to reality. Tese AI systems have been successfully ing the boring process. utilised for applied research applications to tool wear Some progress has been made on the development monitoring in turning tool operations – af er suitable of DVA techniques, but the potential ‘step-change’ ‘training’ of a pre-selected neural network architecture. will occur in vibrational suppression for boring bars, Tese ‘networks’ could be successfully applied to bor- when the improvement of production versions of ‘ac- ing bar vibrational monitoring and control situations. tive’ dampers for such tooling becomes a reality. Just More detailed information will be said on how, where such a potential ‘active’ boring bar is shown schemati- and when Neural network decision-making and, why cally in Fig. 64. Invariably, the boring bar has a supply these cutting tool monitoring applications should be the cutting edge’s position by monitoring of energy to it – via an external source, that controls utilised in the production environment, later in thethe feedbacktext. of the relative displacement of tool’s edge with respect to the workpiece. In later research work by Matsubara et al. (1987), chatter suppression was analysed for the 3.2.6 Hard-part Machining, Using Boring Bars boring bar using ‘feed-forward’ control of the cutting force. Further, the cutting edge was positioned in re- sponse to this force, with these type of ‘active’ control Although ‘hard-part’ turning has been utilised for systems being known as: ‘Cutting edge positional con- some considerable time, with the advent of polycrys- trol systems’. talline cubic boron nitride (PCBN) tooling, etc., it has Typical of a vibrational control approach is illus- seen little in the way of exploitation for boring opera- trated by the ‘active’ boring bar already mentioned and tions, to date. One of the major reasons for this lack depicted in Fig. 64, where the forces are damped in re- of tooling application, is because most hardened parts sponse to the vibrational velocity of the cutting edge, are in the region of hardness values ranging from 42 which has been termed a: ‘Vibrational velocity control to 66 HRC. Such high component hardness, requires system’. In this damping technique, the boring bar sup- considerable shearing capability by the tooling to suc- pression is by a series of piezo-electric elements that cessfully machine the excess stock from the workpiece. act as ‘active dampers’. Such a ‘damper’ responds to Generally, the robust nature of toolholding for turning onset of chatter vibration (i.e. the high-energy com- ponents). Moreover, the damping force achieves opti- mal phase dif erence, since the phases between both ‘Degrees of Freedom’ , the ‘free-body kinematics’ , exhibit 6 de- grees of translatory (i.e. linear) motions in space, these are: back- ward/forward, upward/downward and lef ward/rightward. ‘Lanchester boring bars’ , normally utilise an internal metal NB Of some interest but in the main, to machine tool build- slug which is usually surrounded by some form of: liquid/f uid ers for the purposes of volumetric calibration, are the rotary medium, DVA’s, or more primitively, sprung-loaded and as motions of: yaw, pitch and roll, giving 18 degrees of freedom, such, the slug is free to move out-of-phase with the cutting together with the 3 squareness errors, totalling 21 possible de- conditions, dictated by the boring bar’s applied cutting forces, grees of freedom. thereby the onset of chatter will be potentially ‘cancelled out’. . Figure 64. An ‘active’ boring bar and their capacity to suppress vibrational efects on boring holes [After. Mat- subara; Yamamoto and Mizumoto; 1987] tools with their modest overhangs, does not present in•- Small chip cross sections – these exert high pres- surmountable difculties during machining, however sure near the insert’s cutting edge, ofen necessitat- for the much longer overhangs associated with boring ing an edge preparation to the insert’s corner, • operations (i.e. see Figs. 62a and 65a), then the cutting Greater tool wear rates – ofen more rapid cutting forces generally dictate, short L/D ratios of<5:1 and edge wear, or the tendency to catastrophic break- relatively large and robust boring bars (Fig. 65b). down ofthe insert, • Tere are considerable difculties to be over- Workpiece stresses during cutting – these stresses come when any form of hard-part machining is are released during machining and may present required – particularly for boring operations, when localised geometric variations to the fnal shape of the components have been either case- or through- the part, hardened, these are: • Poor homogeneity in the workpiece material • High temperatures in the cutting zone – necessitat- – hardness variations across and through the part ing high temperature resistant and thermally-sta- (e.g. diferential case hardened depths), can lead to bility ofcutting insert materials, signifcant and variable cutting force loadings on • Cutting force magnitudes are both higher and more the boring insert, • variable – robust cutting edge geometry is neces- Insuf cient stability – ifthe ‘machine-tool-work- piece loop’ is not sufciently robust, then due to the sary to withstand these increased shearing/cutting greater cutting forces when hard-part machining, force demands on the insert, . Figure 65. Boring bar operational limitations and hard part boring at relatively high speed. [Cour- tesy of Sandvik Coromant] . Table 5: IT values related to the basic tolerance for various diameter ranges IT5 0.6 0.8 1.3 2.2 3.2 4.5 0.9 1.2 1.9 3.1 4.6 IT6 6.4 1.4 IT7 1.9 3.1 5.0 7.4 10.1 2.0 15.8 IT8 2.9 4.7 7.8 11.5 3.6 18.3 25.2 IT9 7.5 9.4 12.4 5.7 20.0 29.8 40.5 IT10 7.6 12.1 IT11 8.6 19.1 31.4 46.2 63.8 11.8 [Source: Sandvik Coromant (1995)] this creates potential tool defection which could graph depicts defections ‘?’ (i.e. both the tangential become a major problem. ‘?T’ and radial defection ‘?R’), as a function of the cutting depth ‘aP’. Due to the fact that the tangential defection (?T) linearly increases with increasing DOC Boring Bar Deflec (aP), it is usually recommended that machining passes tion When any boring operations take place, even with a are divided into a number of cuts when close toler- very rigid tool mounting and a small boring bar over- ances are needed (i.e. in the region ofIT7 ) – see Table hang, some vibration and tool tip defection will in- 549 for an abridged version of the ITtolerances, with evitably occur, this is exacerbated by machining hard- *Rmax values in µm. parts. Te former problem ofvibration has previously Te magnitude of radial defection as a function been mentioned and methods of minimising it are of the cutting depth, is also infuenced by the ratio possible. However, tool defections are more dif cult, between the insert’s nose radius and the DOC (aP), to- if not impossible to completely eliminate, with these gether with the boring insert’s entering angle. In some longer cantilevered tools. Ofnote regarding overhang- cases, a boring bar is situated slightly above the work- ing tool defections, are that a tool tip defects in two piece centreline, so that when it enters the cut at full directions (i.e. see Fig. 66a), these are: depth it will have tangentially-defected to the actual • Radial defection (?T) – afects the machined (i.e. bored) diameter, • Tangential defection (?R) – causes the tip to move ‘IT’ (i.e. in units of ,m) – represents the average value of the downward for the centreline. basic tolerance for the ‘diameter range’ in question. Hence, it will vary according to the choice of diameter range selected. In each ofthese tool tip defections, both the size and Tese values are related to surface texture expression of: direction of the cutting forces are infuenced by the *Rmax (,m), which is: Te maximum individual peak-to-val- chip thickness and insert geometry selected (i.e. illus- ley height. Te Rmax values (i.e. in Table 5) can be calculated trated in Fig. 66b). Te radial defection will be equal from the ITvalue, using the following equation, rather than to the diference between the diameter which was orig- the conventional equation: Rmax = (fn /rε) 125 inally set and the actual bored diameter, this can be this equation tends to give excessively high surface texture va - lues, thus more practical values related to IT are to be found easily found by the simple expedient ofmeasuring it, from: then adjustment can be made for this apparent defec- IT tion. Te tangential defection of the boring bar’s tip Rmax , n , can be established by either ‘direct’, or ‘indirect’ met- = ,IT (,m) Where: n = Te number of IT’s. rological techniques at the tool’s tip. In Fig. 66a, the . Figure 66. Hard-part boring, can create excessive boring bar defections and potential vibrational problems – ifnot carefully controlled. [Courtesy of Sandvik Coromant] workpiece’s centreline. Boring bar overhang is not a 0 problem when ‘Line-boring’ as the tool is supported at both ends, or in the case of the novel ‘Telescopic line- . boring tooling’Te chip area (i.e. illustrated in Fig. 66b – right), has an ef ect on the load on the insert’s cutting edge, par- ticularly when hard-part boring, although with small chip areas, this may not create a vibration problem, unless high friction is present between the insert and workpiece. However, the cutting forces substantially in- crease if a large chip area is utilised, necessitating some means ‘damping stability’ to the boring tool. 3.3 Reaming Technology – Introduction Te reamer is the most commonly utilised tool for the production of accurate and precise holes, having high surface quality being true to form and tolerance. Ma- chine reamers can have either a single-blade design (Figs. 67 and 68), or are produced with a multiple series of cutting edges – of constant diameter (Fig. 69) or, ta- . Figure 67. A sample of indexable insert reamer technology pered (Fig.73b) across a diverse range of diameters and – lengths. Te surface texture quality obtainable by ream- for solid and f oating reamer applications. [Courtesy of Seco Tools] ing ranges from approximately ‘Ra’ 0.2 to 6.5 µm, ac- cording to recommendations of DIN 4766. Normally, 0 ‘Line-boring’ , as its name implies is utilised for boring part’s with concentric and of en varying diameters throughout the reamed f nishes of about Ra 0.5 µm can be regarded as overall component’s length. Normally, a ‘Line-boring tool’ is satisfactory. In general, reaming achieves tolerances of supported by a steady with suitable bushing and a mating ex- IT7, but if the reamer has been carefully ground, it can tension bar, some distance from the cutting edge and its re- achieve tolerances of IT6, or even to IT5. spective rotating toolholder. Tis additional support enabling long bored features to be precisely machined to the part’s cen- treline in-situ. ‘Telescopic line-boring tool’ , One major machine tool builder in association with a tooling manufacturer, produced a rather novel and clever ‘Telescopic line boring tool‘, for the machining Arithmetic roughness ‘Ra’ parameter – it is the arithmetic of quite long crankshaf bearing housings on both automo- tive engine blocks and bored cam-seatings for cylinder heads. mean of the absolute ordinate values Z(x) within the sampling Tis uniquely-designed ‘Telescopic line-boring tool’ , machined length. It is the most frequently quoted international surface the f rst bore, then continued to extend (i.e. telescopically texture (i.e. amplitude) parameter, expressed in the following manner: feed-forward), whilst supporting its progress by mating with each automotive-machined bore, as it progressed through the lr large automotive component, thereby supporting the machin- Ra= lr,Z(x),dx , ing operation throughout its boring cycle, then retracting on NB In the past and specif cally in the USA, its equivalent completion, allowing the tool to be held in the machine tool’s term was known as the ‘Arithmetic Average’ , denoted by sym- magazine, allowing/facilitating an ef cient and speedy multi- bols: ‘AA’. ple in-line boring operation to be executed. . Figure 68. Single-blade reamers ofer superior hole geometry over conventional reamers. *Courtesy of Shefcut Tool & Eng’g Ltd., Prior to beginning the reaming process, holes LH spiral reamers ar e employed. T e chip direction is have to be either pre-drilled, or holes cored-drilled. always in the feed direction and, for this reason, the Due to the nature of the role of the burnishing pads on spiral f ute geometry is virtually exclusively used for the hole’s machined and highly-compressed surface in through hole reaming operations. Gun-drilling operations, it is not particularly suitable for reaming. Machine reamers can be divided into several cat- 3.3.1 Reaming – of Hole’s Roundnessegories, these are: multi-point reamers with either Correction a straight, or Morse taper shank, these reamers are Profiles usually either manufactured from: HSS, Tungsten Machine Reaming Tungsten carbide (solid) reamers can be run at 10% In the ‘classical’ reaming operation, it is centre-drilled, carbide (Solid), or with carbide tips. Typically, the higher feedrates, to their HSS equivalents and can then the hole is through-drilled possibly producing ream workpiece materials up to a tensile strength of a variety of hole form harmonic out-of-roundness – 1200 N mm . errors present (i.e. see Fig. 70 ‘polar plots’ – bottom Machine reamers are available with: straight f utes, lef ), including ‘bell-mouthing’at the entry and exit lef -hand (LH) spirals, or 45? LH ‘quick’ spirals this lat- of through drilled holes. Not only is there a possibil- ter reamer version is of en termed a ‘Roughing reamer’ ity of ‘bell-mouthing’, but a serious likelihood of the and is of en used for ‘long-chipping’ workpiece mate- drill following a helical path through the part, this is rials. Reamers with straight f utes are usually utilised termed: ‘helical-wandering’ (i.e. see ‘Footnote No. 3’, to ream blind holes, but with the absence of chip space for an explanation of this drilling condition). By a fol- at the bottom, this means that swarf must be evacuated lowing boring operation, this will correct for any prof le by the f utes. For virtually all other machining tasks, errors, while improving both the part’s overall out-of- such as holes with keyways, or intersecting holes, etc.,roundness as exhibited by the ‘polar plots’ (ie. as il- lustrated in Fig. 70 middle-lef ), but the hole’s ‘cylin- dricity’ . Finally, the machine reamer is used to fulf l several functions: improve both the harmonic out-of- ‘Hand-reamers’ , are available for the reaming both cylindrical and tapered holes. NB A basic rule to be observed when hand-reaming, is to ‘Bell-mouthing’ , is the result of the unsupported drill (i.e. only turn the tool in the cutting direction and, never reverse the margins as yet, not in contact with the drilled hole’s side walls), producing the so-called ‘bell-mouth prof le’ , upon hole it (e.g. T is is the standard practice in cutting a thread with entry. At exit, if the drill is allowed to feed too far past the un- hand taps), as the reamer’s cutting edges will immediately be- derside of the hole, the drill has a ‘whipping-tendency’ , which come blunt. ‘Core-drilling’ , this is normally undertaken with a multi- could introduce a smaller ‘bell-mouthing ef ect’ beneath the f uted drill, as the hole already exists in the cast component part’s lower face. ‘Out-of-roundness’ , was a term previously utilised, but today, and in the main, the drill cuts on its periphery, so needs more the term used has been changed to: ‘Departures from round- cutting edges in contact with the cored hole. Coring is result ness’ , moreover, the term ‘polar plot’ has also been super- of employing a core, prior to casting and it stays in the cavity seded by the term ‘displayed prof le’ , however, in the current as the molten metal is gently poured to cast the part (i.e. cores context the former terms will be used. are normally made from an appropriate sand and binder, or ‘Cylindricity’ , is the term def ned as: ‘Two, or more roundness another suitable material, that can be removed at the ‘fettling planes used to produce a cylinder where the radial dif erences stage’ – leaving the hole), hence, its name: cored hole. ‘Morse taper’ , was developed in the USA in the mid-to-late are at a minimum’. 1800’s by Steven Morse (i.e famed for his design and develop- NB A more easily-understood appreciation of what ‘cylindric- ment of the original geometry for the Twist drill). T e Morse ity’ is, can hopefully be gained by the following ‘working ex- taper is a ‘ self-holding taper’ , which can be suitable sleeved ei- planation’: If a perfectly f at plate is inclined at a shallow angle ther upward, or downward in ‘ioned diameter ’ to f t the inter- and, a parallel cylindrical component is rolled down this plate, nal taper for the machine tool’s spindle/tailstock, requiring a then if it is ‘truly round’ as it rolls there should be no discern- ‘drif ’ to separate the matching tapers upon completion of the ible radial/longitudinal motion apparent. In other words, the work. T e Morse taper’s included angle varies marginally, de- component is a truly round cylinder, which can be equated to pending upon its Number (i.e ranging from 0 to 6). Typically, a hole, or indeed, to a turned, or ground diameter. a ‘No. 1’ is: 2? 58? 54? ?, with a ‘No. 6’ being: 2? 59? 12? ?. roundness (Fig. 70 top-lef) and surface texture, while hole depths, here the ‘plots’ are shown near the top, ‘sizing’ the hole’s diameter. in the middle and close to the bottom ofthe drilled To further emphasise the point that drilling does hole. Correction ofthese roundness and diametrical not produce a consistent harmonic out-of-roundness, errors by machine reaming is not always the case, here nor even a straight hole, Fig 71a, illustrates how the (i.e. shown in Fig. 71b), ifthe reamer is either not set ‘polar plots’ are fundamentally modifed at diferent up correctly, or is slightly axially bent, in this case a . Figure 69. Types of solid reamer and their associated geometry. [Courtesy of Guhring Ltd.] . Figure 70. The exaggerated hole errors caused by an incorrect drill point geometry and the manufacturing techniques for its subsequent correction . Figure 71. Reaming can correct an assymetrically drilled hole – when correctly adjusted large harmonic variation in the ‘plots’ is depicted, as is• Cylinder head tappet rail drill -reaming – in a sin- the case when a ‘Floating reamer’ with roller drive hasgle operation, • been used inappropriately. Cylinder head valve seats and guides – machining both features, in the parent bore and fnish machin- Floating ing, Reaming Solid machine reamers can be ‘foated-down’ a pre- drilled hole, to produce a much straighter reamed hole, than would otherwise be the case. When ‘foat- ing’ reamers within their specially-located toolhold- ers, two techniques are used to ‘foat reamers’ (i.e see Fig. 72), these are: 1. Radial play – where the machine reamer has lim- ited movement laterally with respect to the princi- pal axis, 2. Composed radial and pendulum play – this has both radial play, together with a degree oflimited angular movement (i.e. this motion is similar to that ofa Grandfather clock’s timing mechanism, via its pendulum motion). NB Tis latter ‘foating’ technique has the potential for a combination ofboth radial and pendulum motions to the machine reamer. Tese unrestrained kinematic motions gives it free motion without lateral and angu- lar constraint, to simply follow the ‘line ofleast resis- tance’ along the spindle axis, as the reamer progres- sively feeds down through the predrilled workpiece. 3.3.2 Radially-Adjustable Machine Reamers Special-purpose machine drill/reamers (Fig. 74a) are ofen utilised in high-volume production envi- ronments such as in the automotive sector, for util- ity engines which can account for >55,000 complex- reaming operations per week. Conversely, for defence vehicle engines the production volumes are quite low, accounting for <300 operations per month. Typical operations on such automotive components, using a machining centre include the reaming of: . Figure 72. Solid machine reamers can be ‘foated-down’ pre-drilled hole, by two distinct ‘foating techniques’: (I) radial a ‘Floated-reaming’ , relates to the reamer’s ability to have some degree of lateral compliance, namely limited motion, allowing play, (II) composed radial pendulum play. [Courtesy of Guhring Ltd.] it some ‘play’ to follow the hole’s path, but still correcting for any previous ‘helical wandering’ by the drill. • Engine block and crank bores and cheek faces – Case-Study of Engine Block Bore fnish machining, with this latter feature requiring Features controlled ‘radial infeed’ ofthe cutting/reaming in- In this novel, but interesting automotive ‘case-study’, sert. all of the challenges facing such special-purpose reamers are present. Here, the machining application NB Te special-purpose ‘radial-infeed’ tooling neces- consisted of the following: a six-cylinder diesel cast sary for the satisfactory machining ofthe cheek faces iron engine block for an armoured personnel car- ofthis latter low-volume production engine block, will rier, reaming at 70 m min , requiring a bore straight- now be briefy discussed. ness of 0.02 mm/m, tolerance on the bore diameter – of 0.025 mm, with >0.003 mm tolerance between the individual journals. Te solution to this demanding industrial problem, was the machining with two tools and three operations ofthe crank bore and the genera- tion oftwo cheek faces – this latter operation was nec- essary to minimise the ftted crankshaf’s end foat. Tis particular special-purpose reamer had a ra- dial feed-out/retract cutting insert requirement for the fnal-machining of the cheek faces. Terefore, the base-tool holder contained a thrust and feed-out mechanism, in addition to the whole tooling assem- bly ‘running-true’, so that it could be ‘datum-out’ and precisely and axially-set with respect to its potential engine block machining features. Te radial mecha- nism would incorporate an actuator shaf mechanism which can be pulled-/pushed-back, thereby resulting in either a radial infeed, or retraction, respectively, of the cutting insert. Tis bi-directional control of the feed-out/-in of the radial mechanism is achieved in conjunction with the CNC feed spindle ofthe machin- ing centre. In general, these special purpose reamers, have two guide pads and a blade (Fig. 74a) with the reaming blade set with a back-taper, producing the well-known characteristic ‘saw-toothed prof le’ to the reamed sur- face (Fig. 74b – right). Such reamed surface texture to- pography has been highly magnifed in the schematic diagram (Fig. 74b – right) and, requires very high vertical magnifcation of the surface topography (i.e. x50,000) to see any trace profle details at all! Te posi- tion ofthe cemented carbide guide pads, with respect to the blade is critical to the reamer’s performance, as is the residual stifness ofthe whole cantilevered tool- ing assembly. For many automotive industrial reaming applica- tions, the components are ofen cast from high-silicon aluminium materials, as the addition of the element silicon, creates a micro-grained and harder cast struc- ture, than would otherwise be the case. However, the disadvantage from a machining viewpoint, is that the . Figure 73. Reamers in action, reaming automotive resultant cast matrix is highly abrasive to the cutting parts. edge. Under these circumstances, the reamer’s blade *Courtesy of Shefcut Tool & Eng’g Ltd., . Figure 74. High-performance reamers, having the ability for radial infeed (i.e. ‘feed-out inserts’) – when f tted. [Courtesy of Cogs- dill Tool & Eng’g Ltd., is ofen produced from an abrasive-resistant mate•- Countersinking a countersunk-headed screw rial such as PCD, in order to maintain and extend the – for ‘fush-ftting’ to the surface (Fig. 75a), • tool’s life and holding a good cutting edge over many Short tapers – can be adequately machined on a machined parts. component, • Providing a lead – for a soon-to-be-tapped hole, • Deburring operation – on a previously drilled 3.3.3 Reaming –hole. Problems Countersinks are available with a range of included and Their Remedies For any resultant reamed surface, its form, accuracy taper angles and come in a variety of dimensional and surface quality are tremendously improved by sizes, the most popular being either: 60?, 90?, or 120?, dividing the machining process into either, roughing, or indeed ‘specials’ can be ground to suit any angular or fnishing reaming operations. Low cutting speed and diametral workpiece features, ofvarying lengths. together with high feedrates, in association with good Countersinks are available from simply HSS, through lubrication agents ofering adequate cooling poten- to a coated cemented carbide matrix. tial, provide the basis for optimum reaming practice. While, observing these ‘rules’, improves both the Counter-Boring reamed surface quality and its individual tolerance. It is worth restating, that a reamer only follows the pre- Counter-bored tooling (Fig. 75b) is available as either drilled hole, consequently it cannot correct for any a solid tool, or is designed to be modular in construc- previous alignment errors that might be present (i.e. tion. T is latter modular counter-boring tooling, ofers see the schematic diagram in Fig. 70). Although er- a range offexibility to machine a wide assortment of rors between the spindle’s axis and the axis ofthe pre- component features, by simply changing the ‘pilot‘, or drilled hole, can be adjusted with the aid of‘foating cutting element’s diameter. T e ‘pilot’ as its name im- reamer’ toolholders (Fig. 72). In Table 6, the following plies, follows a pre-drilled hole and guides the counter- fault-fnding chart may be useful in tracing the pos- bored cutting element enabling it to remain concentric sible causes ofsome common reaming problems. with the hole’s axis. T is is important for any cap-head bolts that require to be recessed either fush to a part’s surface, or sunk below its outer face. Counter-boring is also employed to machined a clearance face in the 3.4 Other Hole-female part feature allowing for a stepped bar to have a fush face to locate against, or simply to provide clear- Modification ance for such a workpiece feature. Again, as with most ofthese tool materials, they are produced from HSS, Processes Once the hole has either been: cast, core-drilled, or through to coated cemented carbides. drilled into solid workpiece material, it ofen requires a further post hole-making operation to complete the Spot-Facing job, for example, a tapping operation. T ere are a num- ber ofthese pre- and post-drilling hole operations that Spot-facing tooling is normally utilised to produce a require specifc tooling to fnish of the hole-making consistent and uniform seating on for example, a cast, activities. T e most popular ofthese are briefy men- or forged component, allowing a washer, or bolt-head tioned below, but this is by no means an exhaustive to be fush across its contact face. Spot-faced tools account ofthe many ofen hybrid operations that are (Fig. 75c), are available as either a solid, or modu- available to the potential designer, or machinist. lar constructional design – the latter version, giving greater fexibility across a wider range offeatures to that of the former counterparts. Materials for these Countersinks tools are similar to those mentioned for other post- T ere are several reasons why a countersink tool might drilling tooling, namely, HSS through to coated ce- be employed when machining features on a compo- mented carbides. nent, ranging from: . Table 6: Potential reaming problems and their possible causes, with some remedies Holes to large i) Concentricity error of either: machine spindle, toolholder, or tool. (ii) Damaged f t between tool and toolholder (i.e. taper, chuck, or collet). (iii) Bevel lead on tooling incorrect. (iv) Cutting speed, or feedrate too high. (v) If problem is the result of workpiece material, eliminate it by using a weaker coolant medium (i.e. by increasing its cooling potential, sacrif cing some of the lubricating abilities). Hole too small i) Tool tolerance incorrect. (ii) Ductile material that contracts after reaming – possibly eliminated by using a quick spiral reamer. (iii) Excessive heating during the reaming process: perhaps by the hole expanding, then subsequently contracting. (iv) Reamer blunt. (v) Cutting speed, or feedrate too low. (vi) Insuf cient stock left on for reaming: tool seizes in the hole. (vii) In most cases, eliminate problems using a more concentrated soluble oil mixture (e.g. 1:15 to 1:10, alternatively use cutting oil). Conical, non-circular and (i) Machine spindle not concentric. other hole malfunctions (ii) Bevel lead not correct. (iii) Axis of pre-drilled hole and reamer not in alignment – eliminate by using a ‘f oating’ toolholder. i) Reamer blunt. Unsatisfactory surface texture of hole (ii) BUE on edges, caused by ‘cold welding’, eliminate by using high concentration coolant, possibly cut- ting oil, or by a reduction in reamer’s land width – to almost zero. (iii) Cutting speed too high, feedrate too low. (iv) Stock removal allowance too small – caused by the pre-drilled hole being too large. (v) Incorrect bevel length. Reamer seizes and breaks (i) Reamer blunt. (ii) Too high a cutting data employed (i.e. speed and/or feed). (iii) Pre-drilled hole too small. (iv) Poor coolant mixture – lubrication too dilute. (vii) Reamer geometry requires modif cation. [Courtesy of Guhring Ltd] . Figure 75. Some alternative hole modifcation machining tooling. [Courtesy of Guhring Ltd.] Grif ths B.J and Grieve, R.J. Te Role of the Burnishing Pad Back Spot-Fa in the Mechanics of Deep Drilling Processes. Int. J. Prod. cing Res., Vol. 23 (4), 195–205, 1985. Back Spot-faced tools (Fig. 75d), are usually employed in f ush-facing an internal hole’s face on either a cast- Colvin, K. Farewell to BUE *In drilling,. Cutting Tool Eng’g, 44–47, Feb. 2001. ing, forging, or wrought stock. Te Back Spot-facing operation, enables a bolt-head, or nut and its washer Comstock, T.R. Chatter Suppression by Controlled Mechani- cal Impedence. PhD Tesis, Dept. of Mech. Eng’g, Univ. to be accurately seated. In some instances, it is possible of Cincinnati (Ohio), USA, June 1968. to generate, the back-face, rather than to form it, via Deren, M. Check the Index. Cutting Tool Eng’g, 51–55, specially-modif ed tools that are fed to the other side Sept. 2002. of the part, then circular interpolation techniques are Fiesselmann, F. and Dietz, G. Pointing Towards Drilling used to create the required back-face. Rates. Modern Machine Shop, 1–7, June 1982. Fitzgerald, G.W. A Comparative Study of the Lanchester NB With most of these post-drilling operations, the Damper and the Segmented Slug Damper in Boring Bar Applications. Proc. of ASME, No. 81–DET–90. cutting data is restricted and calculated to the outer di- ametral dimension of the part feature to be machined. Frade, T. Twist Drill Shape-up. Cutting Tool Eng’g, Vol. 42 Solid post-drilling tooling can usually be operated at (2), 39–44, Feb. 1990. higher cutting data to that of their modular tooling Galloway, D.F. Some Experiemnts on the Inf uence of Vari- counterparts. ous Factors on Drill Performance. Trans. of ASME, 191– 231, Feb., 1957. Habeck, A. Steadying the [Boring] Bar. Cutting Tool Eng’g, 46–50, Dec. 2000. References Haggerty, W.A. Ef ect of Point Geometry and Dimensional Symmetry on Drill Performance. Int. J. Mach. Tool Des. Journal and Conference Pa pers Res., Vol. 1, 41–58, 1961. Hall, J. Boring Tools get ？ Interesting. Cutting Tool Eng’g, Agapiou, J.S. and DeVries, M.F. On the Determination of 34–37, Dec. 2004. Termal Phenomena during Drilling – Part I. Analytical Hanson, S. Cutting the Hard Stuf Right. Manuf. Eng’g., Models of Twist Drill Temperature Distributions. Int. J. 179–187, May 2005. Mach. Tools Manufact., Vol. 30 (2), 203–215, 1990. Inada, S. et al. On a Method to Prevent Chatter in Boring Agapiou, J.S. and DeVries, M.F. On the Determination of Operations. Bull. of the JSME, Vol. 17 (108), 835–840, Termal Phenomena during Drilling – Part II. Compari- June 1974. son of Experimental and Analytical Twist Drill Tempera- Inamura, T. and Stat, T. Stability Analysis of Cutting under ture Distributions. Int. J. Mach. Tools Manufact., Vol. 30 Varying Spindle Speed. J. of JSPE, Vol. 43 (1), 80–85, (2), 217–226, 1990. 1977. Anderson, P. Good points [Drilling geometries]. Cutting Javed, M.A, Littlefair, G. and Smith, G.T. Tool Wear Moni- Tool Eng’g, Vol. 45 (6), 50–56, 1993. toring for Turning Centres. Proc. of LAMDAMAP Int. Astakhov, V. Gundrilling Know-how. Cutting Tool Eng’g, Conf., Computational Mechanics Pub., 251–259, 1995. 34–38, Dec. 2001. Johnson, B. Reaming Technology. Industrial Tooling Int. Atabey, F. Lazoglu, I. and Alintas, Y. Mechanics of Boring Conf., Southampton (UK), Molyneux Press, 132–152, Processes – Part I. Int. J. Mach. Tools and Manufact., Sept. 2001. 463–476, Vol. 43, 2003. Johnson, B. Reaming Application for the Automotive Indus- Atabey, F. Lazoglu, I. and Alintas, Y. Mechanics of Boring try. Industrial Tooling Int. Conf., Southampton (UK), Processes – Part II – Multi-insert Boring Heads. Int. J. Test Valley Pub., 288–304, Sept. 2003. Mach. Tools and Manufact., 477–484, Vol. 43, 2003. Kahng, C.H. and Ham, I. Ef ect of Metallurgical Properties Benedict, B.W. and Lukens, W.P. An Investigation of Twist on Drill Life. In: Inf uence of Metallurgy on Hole Mak- Drills; Part 1. Bull. Univ. o Illinois Eng’g Exp. Station, ing Operations, ASM Pub. (Ohio), 182–204, 1978. No. 103, 1917. Kashara, N., Sato, H. and Fani, Y. Phase Characteristics Boston, O.W. and Gilbert, W.W. Te Torque and Trust of Self-excited Chatter in Cutting. J for Eng’g for Ind., of Small Drills Operating in Various Metals. Trans. of ASME Pub., Vol. 114, 393–399, Nov. 1992. ASME, Vol. 58 (2), 1936. Klein, R.R. and Nachtigal, C.L. A Teoretical Basis for Ac- Smith, G.T. Investigating the Machining Performance of tive Control of Boring Bar Operations. Trans. of ASME: Damped and Undamped Boring Bars. Proc. of LAM- J. Dynamic Systems Measurement and Control, Vol. 97, DAMAP Int. Conf., Computational Mechanics Pub., June 1975. 509–523, 1997. Larsson, C. Optimizing Deep-hole Drilling. Cutting Tool Stoddard, B.C. Te Replacements [Replaceable drill points]. Eng’g, 32–38, Feb. 1998. Cutting Tool Eng’g., 34–39, Jan. 2007. Lewis, B. Turn your Wipers on. Cutting Tool Eng’g, 47–51, Takemura, T., Kitamura, T. and Hoshi, T. Active Suppression Jan. 2003. of Chatter by Programmed Variation of Spindle Load. J. of JSPE, Vol. 41 (5), 489–498, 1975. Matsubara, T., Yamamoto, H. and Mizumoto, H. Chatter Suppression by using Piezoelectric Active Damper. Dept. Trigger, K.J. and Chao, B.T. An Analytical Evaluation of of Mech. Eng’g, Report, Tottori Univ. (Japan), 79–83, Metal Cutting Temperatures. Trans. of ASME, Vol. 73, 1987. 57, 1951. McColl, M. and Leadbetter, R. CAD of Multifacet Drills. Trmal, G.J. and Wyatt, J.E. Model for Predicting the Torque th Proc. of 13 NARMRC (SME), 490–495, 1984. and Trust and its Application in Monitoring Drilling Operations. Industrial Tooling Int. Conf., Southampton New, R.W. and Au, Y.H.J. Chatter-proof Overhung Bor- (UK), Molyneux Press, 201–210, Sept. 2001. ing Bars Stability Criteria and Design Procedure for a New Type of Damped Boring Bar. Proc. of ASME, No. Valikhani, M. and Chandrashekhar, S. An Experimental 79–WA/DE–3. Investigation into the Comparison of the Performance Characteristic of TiN and ZrN Coatings on Split Point Ng, K.W. and New, R.W. Prof le Boring Operations – Tests Drills using the Static and Stochastic Models of the Force on Damped Bars using Known Vibratory Forces, Trans. System as a Signature. Int. J. Adv. Manuf. Tech., Vol. 2 of Int. J. Prod. Res., Vol 14 (2), 149–169, 1976. (1), 75–106, 1987. Noaker, P.M. Drilling with a Twist, Manufact. Eng’g., 47– Watson, A.R. Drilling Model for Cutting Lip and Chisel Edge 51, Jan. 1990. and Comparison of Experimental and Predicted Results – Oxford Jr., C.J. On the Drilling of Metals: 1 – Basic Mechan- 4; Drilling Tests to Determine the Chisel Edge Contribu- ics of the Process. Trans. of ASME, Feb. 1955. tion to Torque and Trust. Int. J. Mach. Tool Des. Res., Oxford Jr., C.J. Fundamentals of Drilling, Tapping and Vol. 25 (4), 393–404, 1985. Reaming. In: Inf uence of Metallurgy in Hole-making Webb, P.M. Dynamics of the Twist Drilling Process. Int. J. Operations, ASM Pub., 1–18, 1978. Prod. Res. Vol. 31 (4), 823–828, 1993. Patterson, H. Strictly Boring. Cutting Tool Eng’g, 22–30, Webb, P.M. Te Tree-dimensional Problem of Twist Drill- Vol. 47 (7), 1995. ing. Int. J. Prod. Res. Vol. 31 (5), 1247–1254, 1993. Ramakrishna Rao, P.K. and Shunmugam, M.S. Accuracy Williams, R.A and Gibson, A.V. A Survey of Fundamental and Surface Finish in BTA Drilling. Int. J. Prod. Res., Aspects of the Drilling Process. Tech. Paper 19, Proc. of Vol. 25 (1), 31–44, 1987. CSIRO (Melbourne), 1964. Rivin, E.I. A Chatter-resistant Cantilever Boring Bar. Int. Wilshire, B. Getting Peak Performance from Indexable Conf. Wayne State Univ. 403–407, Oct. 1986. Drills. Cutting Tool Eng’g, 30–40, Feb. 1999. Skuma, K. Taguchi, K. and Katsuki, A. Study on Deep-hole Witte, L. Cutting Forces in Drilling Operations: Boring by BTA System Solid Boring Tool – Behaviour ofPart One . Modern Twist Drilling Tech. Vol. 2 (3) 8–11, Tool and Its Ef ects on Prof le of Machined Hole. Bull. Ja-1982. pan Soc. Prec. Eng’g. Vol. 14 (3), 143–148, Sept. 1980. Witte, L. Cutting Forces in Drilling Operations: Salama, A.s. and Elsawy, A.H. Te Dynamic Geometry of a Twist Drill Point. Int. Conf. AMPT’93, Dublin City Part Two. Modern Twist Drilling Tech. Vol. 2 (3) 12–15, Univ. Pub., Vol. 1, 41–50, Aug. 1993. 1982. Schlesinger, G. Te Cutting Angle of Twist Drills. T e Engr., Vol. 166 (4), 138, Dec. 1938. Books, Booklets and Guides Scott, N. Balanced Boring. Cutting Tool Eng’g, 62–66, April 1993. A Guide to Surface Texture Parameters. Taylor Hobson Pre- Shaw, M.C. and Oxford Jr., C.J. On the Drilling of Metals: cision Booklet No. 800–305 4K CP1299 (English). 2 – the Torque and Trust in Drilling. Trans. of ASME, Boothroyd, G. and Knight, W.A. Fundamentals of Metal Vol. 79, 139–148, June 1957. Machining and Machine Tools. Marcel Dekker (NY), Simpson, G., Krenzer, U. and Gsänger D. Capacity of Drill- 1989. ing Tools with Indexable Inserts, Industrial Tooling Int. Boring with Tuned Bars, Sandvik Booklet No. HV-5300:008 Conf., Southampton (UK), Shirley Press, 33–43, Sept. ENG, 1983. 1997. Burrows, L. and Hancox, D. Craf Engineering Data Book. Modern Metal Cutting – Part 9: Drilling Tools. AB Sandvik Stanley Tornes Pub., 1978. Coromant, 1981. th Precision Cutting Tools. Guhring Pub. 8 Modern Metal Cutting – A Practical Handbook. AB Sandvik Ed., 2002. Coromant Pub., 1994. Infuence ofMetallurgy on Hole Making Operations. ASM Shaw, M.C. Metal Cutting Principles. Clarendon Press, Ox- Pub: Materials/Metalworking Technology Series, 1978. ford, 1984. Kaczmarek, J. Principles ofMachining by Cutting Abrasion Smith, G.T. Advanced Machining – T e Handbook ofCut- and Erosion. Peter Pregrinus Pub. (Warsaw), 1976. ting Technology. IFS/Springer Verlag, 1989. Kalpakjian, S. Manufacturing Processes for Engineering Ma- Smith, G.T. Industrial Metrology – Surfaces and Roundness. terials. Addison-Wesley Longman, Inc., 1997. Springer Verlag, 2002. Leach, R. T e Measurement ofSurface Texture using Stylus Instruments – Measurement Good Practice Guide: No. Stainless Steel Turning, AB Sandvik Coromant Pub., 1996. 37. NPL Pub. 2001. Tlusty, G. Manufacturing Processes and Equipment. Pren- Modern Metal Cutting – Part 8: Drilling T eory. AB Sand- tice Hall, 2000. vik Coromant, 1980. 4 Milling Cutters and Associated Technologies ‘Mit der Dummheit kämpfen Götter selbst vergebens.’ TRANSLATION: ‘Against stupidity the Gods themselves struggle in vain.’ FRIEDRICH von SCHILLER (1759 – 1805) [D ie Jungfrau von Orleans, III.vi] milling operations can be utilised for multi-axis free- 4.1 Milling – anform ‘sculpturing’ of complex-curved workpieces, us- ing equipment such as 5-, or 6-axis machining centres, Introduction At its most basic level, a milling operation involves a or even robots with ‘slaved’ (i.e. ,tted) spindles, all co-ordinated linear, or multiple-axis feeding motion these machines being equipped with specially-ground of the multi-edged cutter as it rotates across and into milling cutters – necessary for ‘double-curvature’ ma- the workpiece. Milling cutters are usually ,tted into a chining operations. A milling cutter’s design and its tools.driven rotating machine spindle for a range of machine and sizes (i.e. see Fig. 76). Each of the individual ‘mills’respective cutting edges come in a vast range of shapes and include: milling machines, machining centres ,cutting edges – as the cutter rotates and is fed into the mill/turn centres, plano -mills, etc. In special cases, workpiece, will mill a cert ain amount of material from the part. Milling operations are an e,cient way of re- moving either excess stock from previously fabricated parts, or by machining from wrought material. ‘Fly-cut milling’ , is the exception here, as it is considered a production of a milled surface, o,ers a machined sur - milling operation that will normally utilise only one cutting face texture that is consistently good, having accurate edge, it is similar in design and operation to that of a two-cut- and repeatable dimensions, o,ering great ,exibility ting edged trepanning tool. A ‘,y-cutter’ is usually employed in terms of the geometric types and shapes for these for the machining of large diameter circular features, where milled components. either the hole is the required component feature, or the cir- A milling operation is an ‘intermittent cutting ac- cular blank is the necessary item from for example, a large tion’, where each individual cutting insert continu- wrought plate/sheet. ously enters and exit’s the cut, unlike turning, which is ‘Machining centres’ , are milling machines equipped with an automatic tool changer – for fast and e,cient ‘tool changing’ basically a continuous machining operation, once the (i.e. to reduce down-time to a minimum), having either a cut has been engaged. It follows that with each cutting vertical, or horizontal spindle con,guration, although multi- tooth impacting onto the work’s surface (‘intermittent cutting’), its operation will be a,ected by: the cutter’s centres have what is known as: ‘orthogonal axes’ – having each spindle machines can also be used. Predominantly, machining inherent robustness, the machine tool’s condition and axis at 90? in relation to each other. A ‘basic’ machining centre has three orthogonal axes (i.e. X, Y and Z), but can have rota-the spindle power availability. tional axis (i.e. A, B and C) incorporated onto them, or built great in,uence on the cutter’s ability to e,ciently ma- into the structure of the machine tool. Some of these machine chine the desired component features. Milling opera- tools, may have 6-axes (i.e. 3 linear and 3 rotational), or more, tions can vary considerably and can be performed by necessary for any complex free-form sculptured machining a wide variety of machine tool con,gurations, with a work. diverse range of tooling (Fig. 76) and across a large ar- ‘Mill/turn centres’ , as their name implies, can turn parts and, ray of workpiece materials, shapes and geometries. with controlled rotational axes, coupled with driven/live spin- At an early point once the engineering drawing(s) dles, they can accurately and e,ciently generate: prismatic features, faces, splines, keyways, cams, etc., onto the work- have been designed and produced and, prior to ma- piece at one setting – termed ‘One-hit machining’. In order to chining the part, an in-depth study of the type of cuts increase the versatility of such machine tools, these mill/turn to be made to produce the desired component features, centres can also be ,tted with ‘co-axial spindles’ , where two either out of: wrought, cast, forged, or extruded mate- spindles are coincident with each other (i.e. their respective spindle centrelines are in-line, but opposing each other), al- rials should be instigated. O initial stage in lowing for example, twin tool turrets to work simultaneously machining assessment, the ‘study’ should address new on two parts. approaches to machine the desired part geometries, distinctly di,erent part geometries/dimensions and, this level by attempting to manufacture the part in the shortest of complex and sophisticated costly plant, requires quite sig- possible cycle-time ni,cant linear and rotational CNC axes programming capa- bilities. co-axial spindles, o,er two machines in one, but on a quite . Moreover, when considering the small shop-,oor ‘footprint’ – an important and perhaps vital bene,t when ,oor-space is at a premium. Cycle-times for the manufacture of parts, should include the ‘Plano-mills’ , have the ability to generate large ,at surfaces productive operations (i.e. all machining times) and non- with their traditional range of single-point planning tools, but productive elements (i.e. including: tooling and workpiece with an additional milling capability of having one, or more set-ups, tool-changing operations and any necessary form of milling machine spindles ,tted, for large-part surface milling workpiece measurement), in producing the overall completed operations to be conducted on the large part. component. . Figure 76. Just a small selection of the vast range of milling cutters avail- able for both machining and mill/turn centres. [Courtesy of Seco Tools] individual part feature to be machined, judge whether ity of the production process including the attainable just one cut, or several passes are the best machining milled surface texture, together with rigidity/instabil- strategy for its subsequent production. In scheduling ity of the overall process for the selected machine. particular components for milling operations, the se- latter factor of milling stability, via the rigidity of the lection of which CNC type of machine tool con,gu- machine-tool-workpiece loop, will dictate not only the ration should be of prime importance, for example a anticipated milling cutter’s tool life, but a,ects the to- milling machine, or machining centre equipped with tal performance of the overall production process with either a: vertical, or horizontal spindle, or a univer- large-scale rami,cations for potential component part sal mill, or even a large gantry mill. Once the CNCeconomics. machine tool has been selected, other secondary, but perhaps nonetheless important factors should be ad- 4.1.1 Basic Milling Operations dressed, such as the potential accuracy and repeatabil- Regardless of the type of milling cutter selected, a ma- chining operation utilises one, or more of the follow- ing production milling techniques, with any variations Scheduling of parts , is based upon a range of crucial produc- tion decisions. Typically the ,nal decision may be due to a in methods being related to feed directions in relation number of interrelated factors: when the parts are needed, to the tool’s rotational axis. the quantity of parts in the batch, their geometry and size, erations are: the availability of the correct machine tool, any potential • Face milling (Fig. 77a, b and c – depicts some typi- cycle-time reductions when utilising a speci,c machine tool. cal machined features produced by facemilling). Other important machining economic factors may need to be A facemilling operation is a combined cutting ac- addressed such as: workholding methods, cutting tool moni- tion by the inserts, in the main on the tool’s periph- toring, automated part loading/unloading, etc. In many large- scale volume production environments, ‘line-balance’ deci- ery and, to a lesser extent by insert edges on the sions that may have to be made could arise. Particularly when cutter’s face. In facemilling, the cutter rotates at 90? a diverse sequence of operations on component features that to that of the direction of radial feed against the must be machined, these being part of a series of operations workpiece. Facemilling has a DOC in an axial direc- across several machine tools. tion, which is determined by how deep the periph- mounted. eral cutting insert’s edges cut, with the insert’s faces axis, this axis being parallel to the tangential feed- on the edge of the cutter generating the ,nished ing direction. Peripheral milling has a DOC in a workpiece surface. radial direction that will determine how deep the • Peripheral milling – radial (Fig. 78) – utilises pe- cutter’s diameter will penetrate into the workpiece. two peripheral milling strategies that can ripherally-located cutting edges that are situated in be used with these horizontally-mounted cutters, a milling cutter body which is horizontally spindle- . Figure 77. Just a few of the ma- chined features that can be produced by a range of face milling cutters. [Courtesy of Sandvik Coromant] these are either ‘Up-cut’ (Fig. 78 top-right denoted ‘U’), or ‘Down-cut’ (Fig. 78 top-lef denoted ‘D’) milling operations – more will be said on this topic shortly. • Peripheral milling – tangential (Fig. 79) – allows the cutter to not only face mill, but has the capabil- ity to work along a third direction feed – axially (i.e. downward into the part’s surface – Fig. 79 – top). Essentially, this milling operation is a form ofdrill- ing, being performed by the cutting edges on the cutter’s face, ofen termed ‘slot-drilling’ . Tis tech- nique allows the cutter – perhaps ground with radi- used indexable cutting inserts (i.e. see Fig. 79b), to machine open and closed pockets, or slots, enabling the cutter’s peripheral edges to complete a range of cutter-paths to open up the a rectangular, or irregu- lar ‘pocketed features’ in the workpiece (Fig. 79a). Up- Cut and Down- Cut Milling Here (Fig. 78), the workpiece is fed into the horizon- tal-mounted peripheral milling cutter, which has its rotation either clockwise (i.e. termed ‘down-cut mill- ‘Up-cut’ and ‘Down-cut milling’ , these two peripheral milling techniques are sometimes referred to as either: ‘Conventional milling’ , or ‘Climb-milling’ operations, respectively. ‘Slot-drilling’ , normally utilises two cutting edges on the cutter’s face. One cutting edge being longer that the other – crossing the cutter’s centreline, thus as it rotates, its dissimi- lar length of cutting edges will sweep across the total area of cut. Tis action, allows the cutter to be plunged-down into the workpiece’s surface and then feed along – to produce a slot – hence the name: ‘slot-drill’. NB If the slot-drill has its cutting edges rounded (i.e. radiused), a ‘Ball-nosed slot-drill’ will result, and such a cutter geometry can produce a range of‘blended/curved’ workpiece features, allowing complex-curved profles (i.e ‘sculpture-milling’) op- erations to be undertaken. In some cases, the curvature of the radius is modifed (i.e. in the case of an ‘APT tool’ – meaning: automated-programmed tool) geometry is ground, to mini- . Figure 78. Peripheral milling (radial), can be undertaken by mise step-over/cusp height efects, when (post) fnishing-of either: up- or down-cut milling operations operations are more speedily rendered on complex-curved workpiece features. For example, when completing the high- NB These di,erent milling cutter rotational directions, quality fnishing operations on moulds and dies. impart ‘Pocketed features’ (Fig. 79a), these operations can be gener-totally dissimilar resultant force vectors whilst machining, play - ing a signi,cant role in the attendant: power consumption fac- ated by feeding the cutter to a pre-determined series ofsuc- tors, resultant machined workpiece residual stresses and sur- cessive depths these being consecutively opened-up by a range face inegrity present, together with geometric shapes/types of tool-path strategies. Typical of the techniques for such pocketing cutter path control, is to employ either ‘Lace-’ , or of workpieces that can be succesfully machined. [Courtesy of ‘Non-lace cuts’ – more will be said on this later. Sandvik Coromant] ing’ – Fig. 78 top-lef ‘D’), or anti-clockwise (‘up-cut In such circumstances ofperipheral milling concern- milling’ Fig. 78 top-right ‘U’). Hence, the workpiece is ing the milled chip’s area, the chip thickness begins fed either with, or against the milling cutter’s rotation to decrease from the initiation ofthe cut, efectively direction, which determines the nature ofthe begin- reaching zero on completion ofthe insert’s peripheral ning, or completion ofthe cut. rotation, prior to the adjacent cutting insert continu- In ‘down-cut milling’ (Fig. 78 top-lef), it can be ing this trend in chip development. Furthermore, as seen that the workpiece direction offeed is the same each milling cutter insert enters the cut with a large as that ofthe cutter’s rotation, in the vicinity ofthe cut. chip thickness, this avoids any potential rubbing and . Figure 79. Axial feed milling using either: solid carbide end mills, slot drills, or a ball-nosed milling cutter NB These type of milling cutters can be widely utilised across a vast range op po- tential workpiece geometrically-shaped features. [Courtesy of Sandvik Coromant] the likelihood of workpiece surface burnishing, mini- component, the relationship of these ‘Up- and Down- mising both the probability of temperature increases cut milling’ factors being schematically illustrated in and work-hardening tendencies. As a cautionary note, Fig. 78 – bottom diagrams. Tis resultant force is a although this is a very ef ective and ef cient way of combination of the tangential and radial cutting forces. peripheral cutting, if any ‘back-lash’ is present, then Te resultant cutting force will dif er signif cantly in the cutter will attempt to ‘snatch’, or at worst, ‘ride- its vectored angle, depending upon the cutter position 0 over’ the workpiece’s surface, as it is pulled-into the relative to that of the workpiece’s. Tese vectored angles cut. Tis possible ‘snatching-efect’ , being created by (Fig. 78 – bottom diagrams), will become larger with the resultant cutting force tending toward a back-ward increased DOC – for ‘Up-cut milling’, the net ef ect be- direction (Fig. 78 middle-lef ). Tis adverse action, re- ing that the milling process needs more spindle power. sulting from presence of ‘back-lash’ could even cause In ‘Down-cut milling’, due to the fact that the resultant cutter, or spindle damage, together with part scrap- force is in the same direction to that of the feed, it will page, if it is not minimised/eliminated. signif cantly reduce the feed power requirement. Yet In ‘Up-cut milling’ (Fig. 78 top-right), the work- another advantage of using a ‘Down-cut milling’ strat- piece feed direction opposes that of the milling cutter egy, is that there will be no reverse change of direction, rotation, within the cutting vicinity. Terefore, the so any workpiece clamping is simplif ed. chip thickness commences at zero and increases as it ap- proaches the exit point, at the end of the cut. Due to 4.1.2 Milling Cutter Geometry – Inser t the fact that at the initiation of the cutting sequence Axial and Radial Rake Angles the cutting edge has no ef ective forces acting on it, so it must be forced into the cut and during this time, some ef ects of rubbing/burnishing create excessive lo- With any machining operation, the combination of calised friction, which in turn, results in an increased the rake and clearance angles determines the cutting temperature. Here, contact with the workpiece mate- edge’s wedge angle, this greatly inf uences the insert’s rial from the previous insert can work-harden the sur- strength. Of en, cemented carbide cutting inserts have face. Yet another disadvantage of utilising ‘Up-cut mill- a small negative primary land present, this helps to ing’, is that the resultant force (Fig. 78 middle-right) avoid fracture during the intermittent cutting action can attempt to lif the workpiece on the machine tool’s associated with a milling operation. A helix angle is table, therefore it must be securely clamped/f xtured present on many milling cutters , be it a: side-and- into place. Te machine tool’s spindle power will depend on the either the feed force (Vf), or the resultant force 0 ‘Back-lash’ , is a problem concerning slideway ‘f oat’ (i.e. slight, but unwanted lateral uncontrolled back-and-forward motion) in the machine tool’s leadscrew, or ballscrew. On Helix angles, can vary considerably, the helices being selected conventional milling machines (i.e. with no CNC-controlled for the workpiece material to be cut. For example, when mill- axes), these are normally equipped with an Acme thread (i.e. ing aluminium, a quick helix is necessary to take advantage of having a truncated Vee-form thread of 29? included angle), they require a ‘Back-lash eliminator’ to be f tted, which when the low shear characteristics of the material, conversely, when rotated/tightened on the split-nut assembly surrounding the milling grey cast iron a slow helix is preferable, as this mate- rial is somewhat brittle in nature. In many instances when ma- Acme thread which is mechanically-connected to the work- piece’s table, reduces any back-lash present – thereby allowing chining the so-called ‘sticky’ materials such most aluminium down-cut milling to be successfully performed. grades, etc., then the milling cutter is designed to have a larger ‘chip-gusset’/‘chip-pocket’ (i.e. clearance space for the chip). NB Ballscrews, are usually f tted to CNC-controlled machine Tis larger ‘chip-pocket’ may take the form of alternating the tools, as they can be pre-loaded by the machine tool builder, bias of the helices for the cutting edges on say, a peripheral thus minimising any potential back-lash problems. It is nor- milling cutter, having a positive helix on one tooth, with the mally desirable to use down-cut milling techniques in CNC adjacent tooth being of negative helix – typically a milling machining, as it is more ef cient cutting technique, in com- cutter of this design is the so-called: ‘staggered-toothed side- parison to that of up-cut milling. and-face cutter’. face cutter, face-mill, end mill, slot -drill, etc. Te clamping method, then the double-positive insert helix angle brings the cutting edge progressively intogeometry is once again, the most suitable milling cut, resulting in ‘quieter running’, but instead of an geometry to use. orthogonal cutting action (i.e. with two component• Double-negative cutting edges (Fig. 80B – shown forces – tangential and radial – acting on the cutting here with either a round insert – bottom-lef , or edge), an oblique cutting action occurs (i.e having an square insert – top-right) – in this case, the radial additional axial force component present). Tis axial and axial angles are both negative. When a double- force component resulting from the geometry of the negative insert is used, the required clearance is ob- helix, has a tendency to either ‘pull’ the cutter out of tained by tilting the insert. Tis ‘tilt’ of the insert, the spindle, or push it towards it, depending upon has the added economical benef t of allowing both whether it is of a lef - or right-hand helix. sides of the insert to be utilised, enabling the avail- For all of the designs for Endmills and Slotdrills ability of more cutting edges coupled to stronger currently available, there are basically three types ofedges, whe n compared to the former insert geom- axial and rake angled cutting edge geometries, theses etry. Tis double-negative milling cutter insert ge- are: ometry, is most suitable for machining conditions • Double-positive cutting edges (i.e shown in Fig. involving heavy impact stresses, associated with workpieces produced from: hard steels, certain 80C) – normally employs a single-sided insert, cast iron matrices and on some of their ‘chilled-sur- as this geometry allows a relatively ‘free’ cutting/ faces’ shearing action. Here, the positive axial and rake mally using the ultra-hard PCBN-grades of inserts. geometries, produce low cutting forces, owing to , or ‘induction-hardened surfaces’ – nor- With these ‘Double-negative’ insert cutting geom- the reduced chip thickness and a shorter length of etries, the demands on spindle power requirement contact at the chip/tool interface. As a result, less and its stability are considerable, owing to the large spindle power is necessary, enabling a lower insert cutting forces and chip thickness factors associated strength requirement enabling high-shear cutting with this type of geometry. availability, when compared to either of the fol- • Positive/negative cutting edges (Fig. 80A – illus- lowing insert geometries. Te manner in which trating a square insert – top-lef , or an inclined the chip formation occurs is benef cial, in that spi- and chamfered square insert – middle-lef ). For ex- ral chips are formed, which can easily be broken ample, in the case of Fig.79a (top-lef ), the insert and exhausted from their respective chip pockets. When milling ductile materials such as grades of: aluminium, steel, as well as some stainless and heat- resistant steels, where there is a tendency to form ‘Chilled cast iron surfaces’ , are used in order to produce a BUE, the double-positive geometry is the only suc- hard-wearing surface, this being necessary for example on the cessful solution to machining such workpiece ma- Vee-and-f at slideways’ for cast lathe beds. Here, a ‘chill’ (i.e. terials. If the workpiece has a tendency to be some- normally a metallic interface) is for example strategically-po- what unstable, perhaps due to either its fragility, or sitioned in the cavity, prior to pouring the liquid melt. Tis ‘metallic chill’ acts as a ‘heat-sink’ to quickly allow solidif ca- tion at the liquid/wall interface, producing a high tempera- ture/cooling gradient and subsequent crystalline growth of ‘Endmills’ , can not only have the cutting edges designed with small grains, that have become both locally hard and wear re- a helix angle, but for the ‘solid’ end-milling varieties, they may sistant, but are surrounded by a graphite-based matrix that of- have either their cutting edge land lengths interrupted by a fers excellent ‘damping’ qualities to the overall cast structure. ‘Induction-hardened surfaces’ , are usually obtained by a trav- groove this feature being a long-lead spiral groove – to act as elling electric resistance induction heating/cooling unit. Tis a form of chip-breaker (i.e. they are of en called ‘Rippa-cut- equipment, once it has locally heated the surface, will be im- ters’ – utilised for high stock removal rates), for long-chipping materials. For some cutter designs, such as: ‘Roller-, or Slab- mediately quenched*, as the apparatus slowly moves along the mills’ , they can have cutting inserts staggered along the tool’s required surface to be heat-treated, imparting a surface-hard- periphery covering the length of the body, to disrupt the long- ened layer to the cast iron.*Tis induction-hardened surface chipping swarf (i.e. see Fig. 76). For Endmills versions of this now consists (i.e. metallurgically) of a very hard ‘white-iron’ , inserted-toothed design, they are of en termed ‘Porcupine appearance – af er suitable metallographical preparation cutters’ (i.e. an example of a ball-nosed version, is depicted in – with a white surface layer, once it has been suitably acid- Fig. 79b). etched. . Figure 80. The rake and clearance angles for various types of face-milling cutter insert geometries. [Courtesy of Sandvik Coro- mant] geometry comprises of a combined positive axial of ers good chip formation, with the added bonus angle and a negative radial angle. Although the ba-of directing the chips away from the cutter body . sic form of the insert may have a negative geometry, For any general-purpose milling applications, the the edge on its end face must be positive in order to cutters having positive/negative cutting insert ge- give a positive axial rake. Te spindle power require- ometries are usually ideal. ments for thiscombined geometry are acompromise between the lower demands of the double-positive insert geometry and the higher ones associated with that of its double-negative counterparts. With ‘Chip-evacuation/-exhaust’ , are terms that are readily used to explain how the chips are removed from the milling cutter’s this positive/negative milling insert geometry, high body. It is a very important consideration, as any chip-jam- feeds per tooth combined with large DOC’s can be ming tendencies must be avoided at all cost, as the cutter, achieved, because the negative radial rake provides workpiece, or both, can be severely-damaged if this potential high insert strength, whilst the positive axial rake and avoidable problem arises. 4.1.3 Milling Cutter – Approach Angles ample, inserts having an approach angle of 90?, are termed ‘Square-shoulder cutters’ (i.e. depicted in Fig. Although cutting insert axial and radial rake angles 83a – lef ), as their name implies, they are normally are important to correctly select, probably of similar utilised when machining up to a shoulder, or perhaps importance is the milling cutter’s approach, or enter- a stepped-feature on the workpiece. Tere are some ing angle (i.e described and illustrated in Fig. 81). problems associated with 90? approach angled milling Te insert’s inclination can vary by pre-selecting the cutters, their limitations are: most suitable one for the workpiece to be milled. Of• - Chip thickness is at a maximum – for a given ten a compromise has to be made when selecting the feedrate, resulting in high loads on the cutting in- cutting insert’s inclination/approach angle. For ex- serts, . Figure 81 . Face milling cutter insert approach angles. [Courtesy of Stellram] . Figure 82. Face-milling cutters, having inclined approach angles, with either high-shear (i.e. tangentially-mounted) inserts, or cutter insert density variations. [Courtesy of Ingersoll, Courtesy of Sendrik Coromant] 4.1.4 Face -Milling Engagement – • Chip fow may be hampered – the vectored angle Angles and Inser t Density for exhausting chips may be compromised, • High radial force component – i.e. in relation to the axial force, produces unfavourable loads on the Fa ce-Milling spindle, creating vibration tendencies, hence the Engagement feeds must be restricted, In any milling machining operation involving the po- • Positive geometry triangular inserts should not be sitioning ofthe cutter in relation to a workpiece, some used – these weaken insert corners, whereas rhom- thought should be given to not only the cutter’s: diam- boid-shaped insert geometries, or similar, ofer eter; number ofteeth, or cutting inserts; width ofthe much stronger insert cutting edges. workpiece; but also how the resultant cutting force(s) might infuence the overall efectiveness of the pro- For general-purpose milling operations, intermedi- duction process. Tis latter point, not only infuences ate approach angles such as inserts having an 75? ap- method ofcomponent clamping, dictating: how, where proach are common (i.e see Fig. 82a), as they provide and what will be the optimum method of‘location and good edge strength in combination with a favourablerestraint’ ofthe workpiece, but on exit from cut, the relationship between insert size and cutting depth. sudden disengagement and release of cutting forces Moreover, if these 75: approach-angled inserts are will potentially create not only a exit-burr on ductile tangentially-mounted (i.e. as depicted in Fig. 82a), the materials, or frittered edge on a brittle material. Te edges are even stronger because ofthe ‘body’ ofthe in- cutter’s exit can infuence the type of stress induced sert is more fully-supported, than is generally the case into the workpiece surface – a compressive stress be- for most ofthe radially-mounted variants. Equally, an ing preferred (more will be said on this topic later, in insert with an 45? approach angle, will spread the load the section dealing with ‘Machined Surface Integrity’). over a longer cutting edge (Fig. 83b). Tis 45: insert In most milling operations the term: ‘engagement’ con- approach geometry, provides good chip-fow for long- cerns the relationship ofcutter-to-workpiece position- chipping materials, with a low radial force component ing and, in virtually all face-milling operations, one in comparison to that ofthe axial force, what is more a tries to prevent at the exit ofthe cut, the chip being at strong insert edge allows higher feedrates to be utilised its thickest, as this is an unfavourable machining strat- (Fig 83a). egy. Te objective when milling, is to always try to get When roughing-cuts are necessary, or difcult-to- the thinnest possible chip at exit from the cut. Some machine workpiece materials must be milled requir- ofthese engagement positioning relationships are de- ing strong insert edges, then a round insert might be picted in Fig. 84, indicating where the most favourable the answer. In general, round inserts have a positive cutter/workpiece relationships are present. Also in geometry with no sharp edges and as a result, ofer Fig. 84, are depicted some unfavourable engagements very strong cutting edges and chip-loads are relatively that should be avoided, this may be possible by either evenly distributed along the rounded contact region changing the milling cutter’s diameter, or its tool path (Fig. 83b – right). Furthermore, a round insert usu- ifpossible, to avoid such engagements. ally has a positive insert geometry, which can then be Te milled cut length is infuenced by the position turned in its seating to simply provide additional cut- ofthe cutter with respect to the workpiece, with tool ting edges. life being related to each cutting insert’s amount of time engaged in the actual cut. For example, in Fig. 84g, a cutter has been positioned centrally over the workpiece, this produces the shortest possible time ‘Square-shoulder cutters’ , can produce a high radial force component, which means that feedrates must be limited, as ‘Location and restraint’ , are important factors when work- they may cause ‘edge frittering’ (i.e. see Fig. 81 – bottom lef, holding. A component needs to be not only accurately located illustrating an edge break-out condition), this unacceptable with respect to either a ‘datum’ , or held on a ‘grid-plate’ in a machining condition is particularly prevalent on brittle-types known relationship to that of the cutter’s position, but it must ofworkpiece materials, such as: (most) Brasses, (many) Cast also be properly restrained – to prevent any, compliance of irons, (some) Powder Metallurgy compacts, together with its ‘degrees offreedom’ while it is clamped during machining. (many) non-metallic materials – Plastics, Perspex, Tufnol, Hence the term: ‘Location and restraint’ . Carbon-fbre, etc. in-cut, conversely, in Fig. 84h the cutter has been ture edge breakdown. However, with of -centre mill- moved just of-centre, causing a longer arc of cut for ing (i.e. Fig. 84h), this machining strategy introduces each insert, which is likely to reduce the tool’s ‘cutting a constant force direction, moreover, as the cutter is life’ somewhat, but this is only part of the problem of positioned not quite centrally over the workpiece, this of -centre cutting. Returning to the cutter positioned central region produces the largest average chip thick- centrally (i.e. Fig. 84g), here the direction of the radial ness. Just to complicate matters still further, if the cut- component cutting forces will fuctuate, with respect ter is positioned even further of -centre, this will allow to the cutting edges start and f nish cutting, which even more inserts to be simultaneously brought into may create potential vibrational problems, or prema- cut (i.e. shown as ‘α’, in Fig. 85). Tere are of en many . Figure 83. The importance of cutting insert approach angle inclination on the resultant chip shape. [Sources: Fig. a: Tooling University, 2003; Fig. b: Heuwinkel & Richter, 2005] milling strategy decisions and frequently some com- Milling Cutter Densit y timum cutter/workpiece engagement for a particular In any face-milling operations the number of inserts promises that must be made, in order to obtain the op- machining situation. in cut (i.e. see Fig. 85), is a function of the quantity of . Figure 84. Face-milling cutter positioning over the workpiece – indicating favourable/unfavourable cut- ter and workpiece placement – together with other important factors. [Courtesy of Sandvik Coromant] . Table 7: The ratio of cut width-to-diameter ( W/D). Number of inserts in-cut Zc W/D: 0.88 0.80 0.75 0.67 0.56 0.38 0.33 0.19 0.125 Zc: 0.38Z 0.35Z 0.33Z 0.30Z 0.27Z 0.21Z 0.20Z 0.14Z 0.14Z [Source: Isakov – Kennametal Inc./pub. in American Machinist, 1996] inserts around the cutter’s periphery (Z) and the en- Te values of ‘Zc’ can be obtained from the Table 7, for gagement angle (α). An expression for these milling various ‘W/D’ ratios, given below: cutter inserts and the cutter diameter’s relationship is Te face-milling cutter density must be such that derived [Source: Isakov – Kennametal Inc. and pub- it allows the chip to correctly form and exhaust from lished in American Machinist 1996)] from Fig. 85, as the cut. If inadequate chip space is provided, this will follows: result in the chip remaining in the chip gullet. Tis lodged chip is then carried around and merging with α = 90? + α the succeeding chip and as a result, welding itself to it, potentially causing cutting edge breakage and possi- bly damage to the workpiece. In any cutter/workpiece . D D OA D engagement, it is necessary to provide a cutting edge AB = W? . D = (W? . D) = W?D sinα = density with at least one insert in-cut at all times. Fail- ure to achieve this cutter density could result in severe D edge hammering, leading to one, or more of the fol- W?D α =arcsin lowing conditions: chipped cutting edges; a damaged cutter; or excessive machine tool wear. c For ‘coarse-pitched’ milling cutters, having between Z( ,+ arcsin, W?D,D) Z = 1-to-1.5 inserts per 25 mm of diameter, this will allow Where: for larger chip gullet spaces and as such, can be recom- Zmended to be used on either: sof workpiece materials c (i.e. see the following footnote ) that produce continuous chips; or, for wide cuts with D = Cutter diameter (mm), W = Radial width of cut (mm), a long insert engagement. Conversely, ‘fne-pitched’ α = Engagement angle (?), milling cutters, with approximately 4-to-5 inserts per α = Angle between cutter centreline and cutter ra- 25 mm of diameter, are normally utilised where lack of dius to the peripheral point of either exit, or entry (?). insert engagement is a problem. Tese milling cutters having ‘fne-pitches’ , will allow at least one insert to Tis above formula, can be simplif ed to the following be in-cut at all times, even when machining very thin relationship: workpiece cross-sectional areas. Tese high-insert den- sity milling cutters, are usually recommended when Zc = Zα/360 machining high-temperature exotic alloys, or hard steels – where light chip loads are taken. As a result of Tis engagement angle is dependent upon the radial the smaller chips, less chip gullet space is necessary, al- width of the cut (W) and the face-milling cutter’s di- lowing more inserts around the cutter’s periphery. ameter (D). Terefore, if the radial width of the cut equals the cutter diameter (W/D = 1.0) and, the en- 4.1.5 Peripheral Milling Cutter gagement angle is 180?, then: Approach Angles – Their Affect on Chip Thickness Zc = 180Z/360? = 0.5Z As has been previously discussed, a multi-point tool such as a milling cutter, will cut intermittently, as its cutting edges repeatedly enter and exit the workpiece’s ‘Zc’ represents the number of inserts in-cut, which can be arc of cut (i.e. engagement). It was suggested in the found for any cut width (W), by applying the above formula, previous section, that at least one, but preferably two, derived from the schematic diagram, illustrated in Fig. 85. . Figure 85. Typical facemilling cutters and their inserts, with a schematic representation of a milling cutter engagement angle and the number of inserts in-cut. [Source: Isakov/American Machinist, 1996] cutting edges should be in-cut at all times. Tooling decreases. T is cutting strategy ‘boost’ in the ef ective manufacturers design and test their cutters during feed per tooth, provides the twin benef ts of longer engagement, carefully determining both the feed and tool life, with shorter cycle-times. speed of a milling operation, ensuring that the cutting For any operation in milling involving a ‘chip-thin- forces are ef ectively balanced-out around all of the ning exercise’, of paramount importance is the cutter’s teeth. T e machining objective here, is to discover the approach/inclination angle (χ). T erefore, as the ap- optimal chip thickness. As has been previously shown, proach angle (χ) become more inclined from say, 90? milling cutter f utes can be either helical, or straight, to 45?, the chip thickness, its ‘h-value’ decreases (i.e. as with the replaceable cutting inserts being located and schematically-demonstrated in Fig. 83b). T e optimal 0 secured by either a wedge, or screw clamp. With each chip thickness for a given set of cutting data, can be adjacent cutting edge around the cutter’s periphery be- entered into a machine tool’s CNC program, by utilis- ing referred to as its pitch. During a face-milling oper- ing the following formula: ation, a chip is formed at the two cutting edges, where- upon, it slides up the tooth face and into the f ute, fz = hm/sinχ striking the f llet, or rounded corner of this f ute. T e approach angle is a key milling geometry fac- T e chip thickness (fz) is always constant, regardless of tor, being formed the tool’s axis and by the peripheral the approach angle inclination, be it operated at 90?, or edges of either a solid cutter, or its cutting inserts . T is down to 30?, or indeed, at a f atter approach (see Fig. approach angle describes how far the top of the insert 83b). T e exception to this ‘chip thickness rule’ be- inclines away, from that of being parallel to the cutter’s ing when utilising a round, or button-type insert (Fig. axis (i.e. as shown in Fig. 83a). In most general milling 83b-right), as it does not have either a top geometry, or operations, the ‘approach’ ranges from 0? for creating an edge chamfer, thereby creating the strongest type of square shoulders (Fig. 83a-lef ), to 45? in f nish-mill- cutting edge. Round inserts without the straight cut- ing (Fig. 83a-right). Usually, milling cutter approach ting edges associated with other milling inserts, cre- angles ranging between 15? to 45? are the norm, with ate chips that increase in thickness as the DOC becomes a 15? approach enabling deeper cuts to be taken. As deeper. Hence, for round inserts, the average chip the approach angle of the cutting edge inclination in- thickness (i.e. its ‘hm’ – value ), relates to the thickness creases, the chip becomes both longer and thinner for of cut this being based upon the insert’s radial engage- the same DOC, or ‘ae’ (Fig. 83b), with the load being ment of the workpiece via the milling cutter’s diam- spread over longer edge length – resulting in smoother eter. If a comparison is made between a round milling cutting. Larger insert inclination, enable higher fee- insert to that of an insert with a 90? approach (i.e. Fig. drates to be employed, although it must be empha- 83b-right and Fig. 83b-lef , respectively), an identical sised, with shallow cuts (Fig. 83a-bottom). volume of chips will be removed for both at a set feed Taking a dif erent milling operational premise, the objective when ‘rough-milling’, is to remove the maxi- mum workpiece stock in the shortest possible time. T e material removal rate being limited by the ‘avail- 0 ‘h-values’ , for a material group are represented as a range, able’ spindle power, although this condition can be op- with a lower number being the starting value. For example, if utilising a machining centre with a 35 kW spindle power avail- timised by ‘radial chip-thinning’ . T e chip thickness ability for the milling of non-ferrous, or aluminium alloys, ‘h-value’ the , or chip thickness ranges between 0.050 mm to is based upon the calculated feed per tooth (fz) and it 0.076 mm. Alternatively, using this same machine tool to mill, diminishes as the radial width decreases and in reality, either: stainless steels, titanium alloys, or heat-resistant super- creating a lighter actual ‘fz’. T is ‘lessening ef ect’ of the alloys, the ‘h-values’ will range from 0.076 mm to 0.152 mm, whereas, for: plain carbon steels, cast-/nodular-cast irons the chip thickness, causes the cutting edges to rub, rather range will be between 0.152 mm to 0.254 mm. than cut the workpiece material, as a result, the feed per tooth (fz) should be increased as the radial depth NB Do not attempt to mill thicker chips than is recommended in the literature, as this action could result in over-loading the cutting inserts and breaking their edges. ‘Radial chip-thinning’ , is the ef ect of taking a radial D OC (a e) ‘hm value-ranges’ for various workpiece materials are identical of less than 25% of the milling cutter’s diameter. to those ‘h-value ranges’ previously mentioned. per tooth and DOC. Although, if the DOC is half that of feedrate to increase the chip thickness to its required the round insert’s inscribed circle, this round geom- level, when the DOC is less than the round insert’s ra- etry creates chips that are 30% thinner to that of the dius. It can be said that although the chip created by 90? approach inserts. Tis reduction in chip thickness the round insert geometry has an almost identical chip is the result of the round insert having a longer cutting thickness to that of a 90? approach angled insert, the edge, which engages radially with the workpiece (Fig. button-style insert geometry removes workpiece ma- 83b-right). Alternatively, if chip volume for both the terial at a considerably faster stock removal rate. Te round and 90? approach inserts were identical, then only ‘down-side’ to that of utilising button-style milling the chip length generated by the round insert is ap- inserts, is the spindle power requirement is greater. proximately 50% longer and it is much thinner than With increasing insert inclination of the approach its counterpart. Moreover, with the same feed for the angles the chip thins, causing the cutting forces to be round insert, but the DOC is reduced so that it is 25% re-directed. By way of an illustration of this ef ect, if a of it’s inscribed circle, the chip thickness produced is 45? insert approach is used, the axial force component now 50% less for an identical chip volume. Hence, to will be identical to that of the radial force. Tis radial achieve the desired productivity benef ts that will ac- force component, tends to make the tool def ect and crue from utilising a ‘chip-thinning strategy’, the DOC may generate chatter, conversely, the axial component needs to be <25% of the round insert’s inscribed cir- force is toward the direction of the spindle thereby re- cle. As the DOC has now become more shallow using ducing the potential risk of its damage via vibrational a round insert, the chip has now been ‘thinned’, so in ef ects. Terefore, if the insert inclination is such that order to compensate for this loss of stock removal, the the approach angle is almost f at, this has the advan- feedrate needs to be increased. Terefore, as the round tage of the axial force component being in the spindle’s insert’s DOC becomes more shallow the approach angle direction, this will minimise the likelihood of tool de- f attens-out to almost ‘inf nite length’. So, when the f ection. Tus the primary objective here, is to remove average chip thickness and approach angle variables workpiece stock at high rates and speedily, keeping the are entered into the formula for ‘feed per tooth’ in the approach angle low, with a light DOC, in this manner most up to 100% greater.CNC program they can be signif cantly higher – al‘f y’!- allowing chips to be thin and as a result the cutter will In order to establish either round, or button-style geometries for their ‘ef ective approach angles’, the fol- 4.1.6 Spindle Camber/Tilt – lowing formula has been derived: when Face -Milling ‘Ef ective approach angle’ Tan χ = ae/(ICef/2)χ On some conventional and CNC milling machines, Where: the spindle can be tilted slightly , this small inclina- χ = Approach angle (?), tion in the direction of the feed, ensures that the cut- ae = DOC (mm), ter does not lie completely f at to the workpiece’s sur- ICef = Inscribed circle – ‘ef ective’ (mm). face. Tis small spindle tilting technique avoids the so-called ‘re-cutting ef ect’ that normally if present If a 90? approach angle is used, the rate of advance when utilising a large face-milling cutter. In reality, the per tooth equals the chip thickness. When there is a spindle camber is very slight and generally amounting decrease in the approach angle inclination, the chip volume stays the same, but the length of cutting edge engagement with the workpiece will increase. Tis re- On many machining centres it is not always possible to tilt the sults in a chip which is both smaller and longer than spindle and, in such situations, back-, recutting is an unavoid- that programmed, hence it is necessary to raise the able milling surface texture condition. ‘Re-cutting efect’ , this is the product of the cutting inserts on the ‘back-edge’ of a large face-milling cutter scoring the recently machined surface, thereby af ecting and slightly ‘Feed per tooth’ (fz), calculations will af ect the chip-loading degrading the milled surface texture, while simultaneously for the milling cutter and be inf uenced by the spindle power avoiding additional f ank wear on the inserts – prolonging the availability – see previous equation for this relationship. cutter’s life. to between 0.1 to 0.3 mm over a length of 1,000 mm. Fig. 86. Of en, when it is not possible to slightly tilt When this is converted to angular measurements, this the spindle, back-cutting problems can arise through equates to a value of between 20 to 60 seconds of arc such factors as spindle, or workpiece def ections. Tis respectivly – as shown in the exaggerated diagram in problem can be minimised by: . Figure 86. The infuence of spindle camber (tilt) on the milled workpiece surface. [Courtesy of Kennametal Hertel] • Improvements in workpiece support – ensuring the 25 µm over a workpiece width of100 mm – this being both packing and clamping are sufcient to support within the accepted tolerances for many commercial the component, situations. However, for the generation ofhigh-pre- • Modifying the cutter to a positive geometry – this cision milled surfaces, this minute level ofconcavity has the efect ofreducing any back-cutting milling would not be tolerated. In fact, when a milled surface by the cutting inserts, is metrologically inspected at high levels ofmagnifca- • Reduction in cutting forces – though: feedrate re- tion, the ‘surface topography’consists ofboth form ductions, lighter DOC’s/cut widths, or by increasing and surface variations. Tese form and surface varia- the cutting speed, tions are related directly to either the cutter-spindle ac- • Modifcations to approach angles – this will have curacy, or the axial displacement ofthe cutting inserts, the efect ofreducing the axial force component, with the distance between wave crests (i.e. sometimes • Reducing spindle overhang – will decrease cutter termed the asperities – high points – on the machined defection, cusps) frequently coinciding with the feed per tooth. It • Inspection ofcutter milling mounting – this will is possible to establish the reasons why a surface might ensure that any burrs, debris, or misalignments are deviate from the ‘true’ plane, with some ofthe possible minimised. factors being caused by: • Machine tool condition – possibly resulting from Ifthe spindle is slightly cambered when face-mill- the fact that the spindle bearings are in poor con- ing, a plain workpiece surface will not normally be dition, the slideways have appreciable ‘back-lash’ produced. Under these conditions ofa slight camber, present, or poor ‘damping’ generating vibrational the machined surface is normally concave, due to the tendencies – showing-up as ‘chatter-marks’ on the angular tilt ofthe milling cutter (i.e. shown in Fig. 86 milled surface, top/middle schematic diagrams). Te surface concav•- Workpiece clamping/stability – ifthe workpiece is ity generated by this camber, depends upon the rela- not sufciently and correctly clamped, then it could tionship between the: cutter’s diameter; width ofthe fex, or move on the fxture/pallet/table, whilst be- workpiece surface being cut; together with the D. OC ing machined, creating unwanted surface devia- Te milled workpiece concavity ‘f’, can be calculated tions/fuctuations, • using the well-established Kirchner–Schulz formula, Axial insert displacement – possibly created by as follows: cutting inserts not precisely located in their respec- tive pockets, or resulting from movement during milling – due possibly to inadequate locking ofthe e insert in-situ during pre-setting, e q [D , ?(D , ?e , ) ] Milled concavity f = Where: f = Milled concavity (mm), ‘Surface topography’ , is a term that is ofen used to describe q = 1000tanθ where θ is the spindle camber (?), the form, waviness and surface texture fuctuations from the De = Efective diameter ofthe cutting circle (mm), ‘true’ plane. A machined surface may exhibit some, or all of e = Width ofworkpiece surface being milled (mm). these variables, together with its ‘lay’. NB Form errors are long-frequency components of a surface, Alternatively, a reasonable estimate ofthe milled con- with waviness being medium-frequency components, while cavity ‘f’ can be obtained from the graph in Fig. 86 surface texture is normally associated with short-frequency (bottom), that illustrates the variation in the concave components. Depending upon the relative size ofthe work- shape, for a variety ofspindle cambers and face-mill- piece, these variables are superimposed onto each other, but ing diameters. Tese concave surface modifcations each one can be ‘fltered-out’ by suitable magnifcation on produced by the spindle camber are never large devia- a Surface Texture Machine – for future analysis. Te ‘lay’ is tions from the ‘true’ plane surface. For example, even the direction of the dominant surface pattern, created by the under the extreme conditions ofemploying a relatively passage of the cutter over the surface. When assessing a ma- small diameter milling cutter of: ,100 mm; together chined surface with a anisotropic lay condition – this being a with a large spindle camber ‘q’ value of0.05 mm, the surface that has a signifcant lay (i.e. clearly visible machining marks), it is normal procedure to assess the surface’s condi- deviation in milled surface concavity only amounts to tion at 90? to the lay. • Insert wear uneven – possibly resulting from inad- If the pocket is of non-uniform dimensions, then equate pre-setting of the cutting inserts – allowing perhaps a ‘lace’, or ‘non-lace’ cutter path clearance some to ‘stand-proud’ of the rest and as a conse- technique might be the preferred option, when having quence, being subjected to higher wear than the to machine these type of component features. others, • Insert shape irregularities – possibly the result of Milling Closed-Angle poorly manufactured cutting insert geometries, Fa ces creating dif ering heights once secured and accu- For the machining of so-called ‘closed angle features’ rately positioned in their respective milling insert such as a re-entrant pocket , or ‘dovetail’ , these latter seatings, • Irregular chip-fow – possibly the result of either the In Fig. 87bi, the pocket has a land (i.e. to impart adfeatures are typically utilised for drop-forging inserts. - insert chip-breakers operating inconsistently, or the ditional mechanical strength to the corner), this land workpiece material having matrix inconsistencies.which would run around the base of the enclosed pocket, requiring a 5-axis machining centre to com- plete the milling operation. In Fig. 87bii, a normal end mill cannot remove the excess material lef in the base of 4.2 Pocketing, Closed-the re-entrant angle, necessitating either a ball-nosed, or tapered ball-nosed cutter to reach in and mill the Angle desired feature (Fig. 87biii – in this case, the illustra- tion shows a tapered ball-nosed milling cutter). Faces, Thin-Walled and Thin-Based Milling Pocket ‘Lace’ , or ‘non-lace’ cutter path, a ‘lace-cut’ is where the cutter Milling Strategies clears (i.e. machines) an area with cutter paths that step-over In particular and in the aerospace industries, alumin- at regular pre-def ned intervals, normally used when a sur- ium machining from: wrought, extruded stock and face has regular dimensions, such as a square, or rectangular forged parts is a regular practice and, to a lesser extent, feature. Conversely, a ‘non-lace’ cut is normally reserved for this also occurs in many precision machining environ- the machining of irregular surfaces with the tool paths being ments. Of en both for shallow and deep pockets and non-linear in their step-over paths, for example, when milling a triangular-shaped pocket/feature, or similar. for ribs, it is necessary to relieve weight at critical sec- tions on components. NB With the advent of sophisticated Computer-aided Manu- One of the oldest established techniques for achiev- facture (CAM) programming capabilities, much of the auto- mated generation of cutter paths, decision-making is under- ing pocket features, is to drill a hole at the centre of the taken by the sof ware, to optimise area clearances for these pocket to a pre-set depth. Ten change tools and plac- component features. ing the milling cutter in this hole clear-out the pocket, ‘Re-entrant pocket’ , is one where the base of the pocketed fea- repeating this cycle until the pocket is ‘roughed-out’. ture is somewhat larger than its top, meaning that the pocket Perhaps changing cutters and taking f nishing cuts to faces slope inward. complete the feature (Fig. 87ai). Rather than simply ‘Dovetails’ , are normally open at both ends allowing the male plunging to depth with the cutter, ramping-down into dovetail on the part to be held and its tapered key to easily pockets is an ef ective way of reaching the ‘f rst-level’ inserted, positioned and locked in-situ. Usually, such features for the pocket’s area clearance. Both ramping and are generated and formed on machine tools such as: Plano- ‘double-ramping’ (i.e. this latter technique is particu- mills, Shapers, etc., but where such equipment is not available, then a ‘closed-angle milling’ operation is necessary. larly ef cient for smaller pocket dimensions), are ways 5-axis Machining Centre , normally has 3 linear (i.e. X, Y and of removing stock via a ‘diagonal plunge’, while tak- Z) and 2 rotary (i.e. A and B) axes. Te relationship of the ing the milling cutter to its required depth (Fig. 87aii). rotary axis will depend upon the machine tool’s conf gura- Tis technique is an ef cient machining strategy for tion, but they allow axis of a milling cutter into an otherwise the milling of square and rectangular pockets – for closed-feature, negating the possibility of any cutter/spindle high stock removal. fouling on the workpiece. . Figure 87. Pocket and closed-angle feature milling. [Courtesy of Sandvik Coromant] one side of the wall in non-overlapping passes, fol- Milling Thin-Wa lowed by a repetition on the remaining side – as lls For the machining of thin-walls (Fig. 88), such as depicted in Fig. 88a. In all cases of thin-walled ma- when milling rib-sections on aerospace components, chining a ‘fnishing allowance’ is lef on both sides the machining strategy will vary, depending upon the and the base for subsequent machining, respective height and wall thickness. In every case of• Height-to-thickness ratios of <30:1 – there are two thin-walled machining, the number of passes will be basic milling techniques that are usually employed, determined by the component’s wall dimensions and these are: • axial depth of cut, in the following manner: ‘Waterline milling’ (Fig. 88b-lef ) – this is where • Height-to-thickness ratios of <15:1 – then possibly either side of the thin-wall feature is milled to pre- the most favoured milling strategy is to machine determined depths, in non-overlapping passes, . Figure 88. Thin-walled machining strategies. [Courtesy of Sandvik Coromant] • ‘Step-support milling’ (Fig. 88b – right) – this tech- 4.3 Rotary and Frustum- nique utilises a similar approach to the previous method, but in this case, there is an overlap between Based Millingpasses on opposite sides of the wall. Tis strategy gives more support at the vicinity where machin- Cutters – ing occurs and the cutting forces are less likely to distort the wall as it height increases. Design and Operation Rotating Inser t Fa ce- NB For very large height-to-thickness ratios of Mills >30:1, an alternative milling strategy, is to alter- One of the novel face-milling cutters which is cur- natively mill either side of the wall – approaching rently available includes rotating round inserts that the desired wall thickness in stages in a so-called: are self-propelled as they cut (Fig. 90a), promoted by 0 ‘Christmas tree routine’ (i.e. not shown), so that the chip-f ow over the insert’s face. It has been claimed the thinner sections are always supported by thicker by the tooling manufacturers of these interchangeable sections below them. Tis method is then repeated rotating insert cutters, that their unique cutting ac- wall.as the step -wise milling operation moves down the tion provides greater cutting ef ciency and is less destructive to the inserts, than the conventional ‘locked’ - milling inserts. Te term that is used for this rotating cutting action is ‘roll shearing’ . Te rotation of the inserts continually introduces a ‘fresh’ cutting edge to Milling Thin the workpiece, this, it is claimed, minimises any heat Bases Unsupported thin-base features, such as the one il- build-up in the cutting zone, with much of the heat lustrated in Fig. 89, are dif cult to produce once the being transferred to the milled chips. Any remaining previous side has been machined, because of the lack heat being easily dissipated along the entire length of of support, particularly at the base’s central region. these round inserts (i.e. the insert circumference has One milling approach in the production of this unsup- an ef ective total cutting edge length of approximately ported thin-base, is to ‘helically mill’ the feature (i.e. 85 mm). Tis rotating insert has an almost inf nite ef- shown in cross-section in the small inset diagram in fective cutting edge length, enabling around a 10-to-1 Fig. 89). Tis usually necessitates milling at the cen- improvement of insert life. Due to the increased tool tre of the base region, spiralling-down to the required life, less down-time for changing cutting edges is re- depth, then milling outward in a ‘f attened helical quired, thereby improving cutting ef ciency and im- manner’ from that point (Fig. 89 – main illustration pacting on actual overall cycle-times, because faster and plan view). Occasionally, one of the faces has al- cutting speeds can be utilise. ready been machined and under these conditions it It has further been claimed by the tooling manu- must be ensured that the cutter’s f ank makes minimal facturer, that with the very high cutting speeds the lo- contact with this face, for this operation it is usual to calised heat is of benef t, as the heat within the cutting employ tooling with the minimum number of f utes.zone is concentrated in the chip and not in the work- Sometimes a component to be thin-based milled, piece, or the insert. Tis local heating of the metallic has a hole at its base’s centre, in such a situation it is workpiece, allows it to reach its plastic deformation prudent to leave a support leg in place when milling stage, causing the chip to f ow freely away from the the f rst side. Ten machine the second side, f nally re- moving (i.e. milling) this support leg af er both sides have been completed, thereby minimising any base de- viation due to the presence of the cutting forces whilst ‘Roll shearing’ , is a combination of the rotating action of the milling the feature. round cutting inserts, in combination with the angled axis which slices, or shears through the workpiece. Rotating insert cutting speeds – with these rotary insert cut- ters has been increased dramatically, when compared to the more conventional ‘locked insert’ face-mills. For example, when the silicon nitride cutting inserts are face-milling cast Christmas tree routine’ , is so-called, because as it is being 0 ‘ iron components 1,000 m min is possible, conversely, when step-wise milled and progressively develops, the silhouette re- – min have been successfully employed. sembles the prof le of the Christmas tree – hence its name. face-milling aluminium workpieces cutting speeds of 2,300 m – . Figure 89. A strategy for the milling of thin-bases. [Courtesy of Sandvik Coromant] . Figure 90. A range of rotary milling cutters. [Courtesy of Rotary Technologies Corp.] milled surface. Tis localised plasticity allows the en- ergy to be maximised and the cutting ef ciency to be increased. Moreover, lower workpiece heat, results in less component distortion. Yet another benef t of this ‘roll shearing’ action, is that when conventional cutters are used the tangential force component is high and it is one of the primary causes for spindle bearing wear, because of the side load it imparts into the spindle’s bearings. Due to the rotary motion of these inserts, they minimise tangential forces and as such, reduce side loads on the machine’s spindle bearings. Frustum-Based Fa ce- Mills When compared to some other milling cutter insert geometries, the round inserts have two advantages: • Inherent strength – no sharp edges, minimising potential points of weakness in the geometry, im- parting high shock resistance and fracture tough- ness. Hence, ‘frustums-based’ face mills have up to 10 times longer tool life, in comparison to conven- tional milling insert geometries, • More cutting edges – they can be turned and locked in their seatings, creating approximately twice as many cutting edges per insert – giving up to 24 in- dexes per insert – when compared to conventional milling inserts. NB Like conventional insert geometries, normal round inserts of er the user two choices, whether to choose a high ef ciency positive insert, or longer insert life using a negative geometry round milling insert. Te frustum-shaped (round) face-milling insert (Fig. 91), has a cutting edge which is reinforced by addi- tional mass while at the same time of ering a 60? posi- tive shearing action (Fig. 91a). Tis frustum-designed insert geometry, eliminates angles and straight lines, allowing high stock removal rates to be utilised. Typi- cally, these frustum-based insert designs, when mill- ing grey cast iron can use peripheral speeds of >700 m – – min at feedrates of 6.4 m min , whereas, for alu- minium milling, the surface speed can be increased to – – 1,650 m min , but with a feedrate of >8 m min . . Figure 91. A frustum-based milling cutter. [Courtesy of Rotary Technologies Corp.] . Figure 92. A range of special tools (i.e. customised), catering for specif c company production needs. [Courtesy of Ingersoll] 4.4 Customised4.5 Mill/Turn Milling Operations On many hybrid machine tools today, the traditional operations associated with one particular type ofma- Cutter Tooling Custom-built tooling is as its name implies, ofers chine tool, are now being produced on others. Take for quite considerably diverse tool designs (i.e. see Fig. example a turning centre, in the past it would simply 92 for just ‘snap-shot’ ofa small range ofthese types have been employed in the production ofworkpieces oftools). Some ofthis customised tooling can be rela- with rotational features. Now, with the addition of a tively simple, perhaps just manufactured to mill only turret equipped with live/driven tooling (i.e. rotating one particular feature, while others are very complex spindles in some, or all of the turret’s pockets), it is and sophisticated in both their design and operation. possible to lock the headstock spindle, mill a feature: Of this latter type are the numerically-controlled, or fat, keyway, gear tooth, or spline, then angular index ‘feed-out’ facing and boring heads (not shown). Tese the spindle and repeat, until all ofthe so-called fats programmable heads allow the machining offeatures – ofen known as ‘prismatic features’ – are completed. such as large bores with intricate profles, typically: Moreover, it is possible to purchase a ‘mill/turn centre’ multiple diameters, grooves, tapers and even threads – with ‘full’ C-axis headstock spindle control giving it on a range ofprismatic parts. Until such heads became the capability to generate contoured surfaces, or faces available, these workpiece features would have required on the previously turned part (i.e. see Fig. 93). Tis the knowledge by either a CNC programmer, or more diversity in the machining operations that can be un- likely they would have been ‘routed’ to a conventional dertaken by simply one machine tool, means that the jig-boring machine for a highly-skilled technician so-called ‘one-hit machining’ operations are possible, known as a jig-borer to complete the complex machin- thereby reducing the risk ofloosing the accurate da- ing task. Tese numerically-controlled heads have tum initially set when the part was turned, so increas- a programmable U-axis tool-slide that can be co-or- ing production consistency due to its more repeatable dinated to that of the Z-axis, enabling it to produce machining precision and accuracy. tapers and contoured bores, or even outside diameter Some companies in England and elsewhere, are features. Once the head is located in the spindle, its now producing rotational features on prismatic parts, powered tool slide via a compact auxiliary d.c. servo- these being produced on machining centres. Tis un- drive motor (i.e. being a closed-loop system with feed- usual reversal technique is achieved by ftting turning back – to monitor its relative position at all times), will tools in a suitable fxture on the machine tool’s bed control the radial motion as it rotates down a bore, or and rotating the part held in a appropriate manner in around the outside diameter ofa component. the machine’s spindle. Moreover, it is now possible to Tooling can be designed to create virtually any not only purchase a machine tool that can: turn, mill, component feature on a workpiece and, with the bore, thread, but it can even cylindrically grind as well CAD/CAM sofware available today, tooling designers – truly showing the diversity over a range ofmachin- have a vast array ofcomputing power to allow them ing operational processes. to ef ciently produce customised tooling within very For further information on the possible problems short lead-times. However, a word of caution here, that may be encountered in milling operations and the these customised tools are not inexpensive and should anticipated solutions, these are given in the Trouble- only be purchased ifthe alternative tooling approach shooting Guide for Milling Operations , in Appendix 6. is such, that cycle-times are otherwise lengthy, or there is simply no other technique that will enable these part features to be produced at economic cost. ‘One-hit machining’ , refers machining parts from wrought stock, etc., in one complete operation. . Figure 93. Driven/live tooling: milling a spiral groove (top) and face-contouring (bottom) under ‘full’ C-axis control, on a mill/turn centre. [Courtesy of DMG (UK) Ltd.] References Smith, G.T. and Booth, S. T e Manufacture [Milling] of Propellers using T ree and Five Axis Machining Cen- Journal and Conference Pa pers tres. Proc. of LAMDAMAP Int. Conf., Computational Mechanics Pub., 307–316, 1993. Stabler, G.V. T e Fundamental Geometry of Cutting Tools, Chaplin, R. Tool Geometry: the Two R’s [Milling Geometry]. Proc. IMechE, Vol/165, 14–26, 1951. Cutting Tool Eng’g., 52–55, Dec. 1999. Tlusty, J. and Masood, Z. Chipping and Breakage of Carbide Crockford, G. Application of Modern Tooling in Machining Tools, ASME J. of Eng’g for Ind., Vol. 100, 403–412, Nov. Centres, Int. Conf. on Industrial Tooling, Shirley Press, 1980. 95–100, Sept. 1997. Yamane, Y. and Narutaki, N. T e Ef ect of Atmosphere on Deren, M. A Tale of Two Metals [Milling Stainless Steel]. Tool Failure in Face Milling – (1 Cutting Tool Eng’g., 48–56, May 2001. st Deren, M. Deep Impact [Milling Deep Pockets]. Cutting Report). J. Jap. Soc. Tool Eng’g., 40–45, Oct. 2001. Prec. Eng’rs. Vol. 49 (8), 521–527, 1983. Eacott, R. Cutting Geometries for Milling and a New Ap- Books, Booklets and Guides proach to Chip Control. Int. Conf. on Industrial Tooling, Molyneux Press, 52–61, Sept. 1999. Application Guide – Aerospace Frames [Milling], Sandvik Ekback, M. Cut Longer and Better [Milling Cutter Applica- Coromant Pub. No. C-2920:17 ENG/01, 2003. tions]. American Machinist, 51–54, Nov. 1996. Application Guide – Die and Mould Making, Sandvik Coro- Godden, T. Tools for Multi-axis Machining. Int. Conf. on mant Pub. No. C-1120:2 ENG, 2000. Industrial Tooling, Test Valley Group, 255–266, Sept. Application Guide – Face milling, Sandvik Coromant Pub. 2003. No. C-1120:1 ENG, 1998. Heuwinkel, M. and Richter, A. Let’s Talk Radial [Approach Application Guide – Side and Facemilling, Sandvik Coro- Angles – Milling,. Cutting Tool Eng’g., 64–70, May mant Pub. No. C-1129:022 ENG, 1996. 2005. Boothroyd, G. and Knight, W.A. Fundamentals of Machin- Heuwinkel, M. As Easy as X, Y, Z [Helical Interpolation ing and Machine Tools. Marcel Dekkar (NY), 1989. – Milling,. Cutting Tool Eng’g., 58–62, Aug. 2006. Childs. T.H.C., Maekawa, K. Obikawa, T. and Yamane, Y. Isakov, E. T e Mathematics of Machining. American Ma- Metal Machining – T eory and Applications. Arnold chinist, 37–39, Aug. 1996. Pub., 2000. Isakov, E. Reassessing Power Factors [Milling]. American Ingersol Pub. Fine Tuning [Milling Cutter Rigidity], 14–15, Machinist, 43–45, Dec. 1996. T e Cutting Edge, No.3, 1987. Isakov, E. Power Equations. Cutting Tool Eng’g., 66–71, Ingersol Pub. Fine Tuning [Milling Optimisation], 14–15, May 2001. T e Cutting Edge, No.2, 1988. Ingersol Pub. Fine Tuning [Milling Cutter Insert Density], Kennedy, B. Facing Facts [Face Milling – Automotive 14–15, T e Cutting Edge, No.1, 1989. Parts]. Cutting Tool Eng’g., 29–37, Feb. 2002. Ingersol Pub. Fine Tuning [Milling Insert Edge Prep.], 14– Kennedy, B. Wall Smart [T in-wall Milling]. Cutting Tool 15, T e Cutting Edge, No.1, 1992. Eng’g., 26–33, Feb. 2007. Kalpakjian, S. Manufacturing Engineering and Technology. Kline, W.A., DeVor, R.E. and Lindberg, J.R. T e Prediction Addison Wesley, 1989. of Cutting Forces in End Milling with Application to Cor- rd Machining Data Handbook. 3Ed., Machinability Data nering Cuts. Int. J. Mach. Tool Des. Res., Vol. 22, 7–22, Center, MetCut Pub. (Cincinnati), 1980. 1982. Metals Handbook – Vol. 16 – Machining. ASM Int. Matls. McNamara, D. Getting Down [Plunge Milling]. Cutting Park (Ohio), 1989. Tool Eng’g., 44–47, Oct. 2003. Modern Metal Cutting – Part 6: Milling T eory. AB Sandvik Richter, A. T e Right Angle *‘Angled’ Milling Heads,. Cut- Coromant Pub., 1980. ting Tool Eng’g., 48–53, July 2004. Modern Metal Cutting – Part 7: Milling Tools. AB Sandvik Rowe, J. Right Face *Face Milling,. Cutting Tool Eng’g., Coromant Pub., 1980. 56–61, Sept. 2001. Sandvik Coromant. Geometries at Work [Milling Cutters]. Cutting Tool Eng’g., 56–63, May 2003. Modern Metal Cutting – A Practical Handbook. AB Sand- Smith, G.T. CNC Machining Technology. Springer Verlag, vik Coromant Pub., 1994. 1993. Oxley, P.L.B. Mechanics of Machining. Ellis Horwood Pub., Tlusty, G. Manufacturing Processes and Equipment. Pren- 1989. tice Hall, 2000. Shaw, M.C. Metal Cutting Principles. Oxford Univ. Press, Usui, E. Modern Machining T eory [i.e. In Japanese]. Kyo- 1989. ritu-shuppan (Tokyo), 1990. Society of Manufacturing Eng’rs., Tool and Manufacturing Zorev, N.N. Metal Cutting Mechanics. Pergamon Press th Engineers Handbook – Vol. 1 – Machining Ed., SME . 4 (Oxford), 1966. Pub., Dearborn, Mich., 1983. Smith, G.T. Advanced Machining – T e Handbook of Cut- ting Technology, IFS/Springer Verlag, 1989. 5 Threading Technologies ‘But I grow old always learning many things.’ SOLON (640 – 558 BC)  *Plutarch: Solon, xxxi, chapter. Referring  to Fig. 95, the angle enclosed  by  5.1 Threadsthe thread ,anks is termed the included thread angle  (β – as illustrated in Fig. 95 – middle right).   thread  form is uniformly spaced along an ‘imaginary  cylin- An Introduc der’ , its nominal size being referred to as the major  tion  originator  of the  ,rst  thread  was  Archimedes  diameter (d).   e,ective pitch diameter (d ) is the  (287–212 BC), although the ,rst modern-day  thread  diameter of a theoretical co-axial cylinder whose outer  can be credited to the Engineer and inventor Joseph  surface would pass through a plane where the width of  Whitworth  in 1841,  where  he developed  the Stan-  the pitch (p) is  the groove, is half the pitch.  dards for today’s screw thread systems. Whitworth’s  normally associated with this ‘e,ective’ diameter (i.e.  55? included  angled  V-form thread,  became  widely  see Fig. 95 – middle right).   minor diameter (d ),  established  enabling  thread-locking  and  unlocking  is the diameter of another co-axial cylinder the outer  precision  parts and of sub-assemblies  – paving  the  surface of which would touch the smallest diameter.  way to the build-up of precise and accurate modern-  clearance is normally achieved via truncating  day equipment  and instruments.  Standardisation  of  the thread at its crest, or root – depending upon where  Imperial thread forms in the USA, Canada, UK, and  the truncation is applied.  elsewhere,  allowed for the interchangeablity  of parts   are the main screw thread factors that con- to become a reality. Around this time, both in France  tribute to a V-form thread, which has similar geom- and Germany metric threads were in use, but it took  etry and terminology for its mating nut – for a thread  until 1957 before both the common 60? included an- having single-start.  gled ISO M-thread and Uni,ed thread pro,les to be- come widely accepted and established (Fig. 95). Along  with these and other various V-form threads that have  5.2 Hand and Machinebeen developed (Fig. 95i), they include quick-release  threads such as the Buttress thread: this being a modi- Taps ,ed form of square thread, along with the 29? included  Hand Ta angled truncated Acme form which is a hybrid of a V- ps form and Square thread. Tapered: gas, pipe and petro- Most ‘solid’  taps come in a variety of shapes and sizes  leum-type threads, were developed to give a mechani- cal sealing  of the ,uid, or gas medium,  with many  three: taper, plug and bottoming (Fig. 96). (Fig. 94), with hand taps normally  found in sets of   pro- in useother types, including multi throughout the world. -start threads that are now  cess of tapping a hole ,rstly requires that a speci,c sized diameter hole is drilled in the workpiece, this is - V-form screw threads  are based  upon a triangle  termed its  ‘tapping size’ .   taper tap along with its  (Fig. 95 – top diagram), which has a truncated crest and  wrench are employed in producing the tapped thread.  root, with the root either having a ,at (as depicted), or  a more likely, a radius  – depending upon the speci,- di,erent diameters,  it follows that they would have    ‘Solid taps’ , are as their name implies, but it is possible to use  cation. If screw threads have an identical pitch, but  dissimilar lead angles. Usually, threads have just one  ese ‘collapsing taps’ have their cutting ele- ‘collapsible taps’.  start, where the pitch and the lead are identical – more  ments automatically inwardly collapsing when the thread is  completed – allowing withdrawal of the tap – without having  will be mentioned on multi-start threads later in this  to unscrew it, moreover, these ‘collapsible taps’ can be self-set- ting ready for the next hole to be tapped.   are ‘sized-re- stricted’ by their major diameter.   ‘Tapping size’ , refers to the diameter of hole to be drilled that  will produce su,cient thread depth for the threaded section    ‘Root radius’ , is usually a stronger thread form, as it is less  to be inserted and screwed down, for a particular engineering  prone to any form of shear-type failure mode in-service. application. For example, the alpha-numeric notation: M6x1,    Pitch, refers to the  spacing, or  distance between any  two cor- refers to a metric V-form screw thread of  ,6 mm with a pitch  responding points on  adjacent threads, normally taken at the  of 1mm. It is not necessary to state whether the thread is le  thread’s e,ective pitch diameter. or right-handed, as the convention is it will be a right-handed  single-start thread. In this case, for an M6x1 thread, the tap- NB  e reciprocal of this pitch, is the threads per inch (i.e.  ping size can be obtained from the tables, as having a drill size  for Imperial units). of ,5 mm.  . Figure 94. A range of hand and machine taps and a die for the production of precision threads. [Courtesy of Guhring] . Figure 95. Basic V-form thread nomenclature. [Courtesy of Sandvik Coromant] . Figure 96. Hand taps and tapping nomenclature. [Courtesy of TRW-Green,eld Tap and Die, Tis taper tap has a large length of taper – hence its  ‘galling ’, or tap-breakage problems in-situ could arise.  name, to lead the tap with progressively deeper cuts as  Once the taper tap has been through a ‘running hole’ ,  it is rotated into the workpiece. As the taper tap enters  it is of en only just necessary to ‘size’ the hole with the  the previously tapping sized drilled hole, care should  bottoming tap. However, if a ‘blind’/non-through hole,  be taken to ensure it remains normal to the work sur- face, otherwise and angled hole will result. As the ta- per tap is rotated, af er each ? turn, it is counter-ro- tated by about a ? turn to break the chips, otherwise    ‘Galling’ , is when the tap, or indeed any cutter becomes  clogged with the remnants of workpiece material, which will  impair its ef ciency, or at worse, cause it to break in the par- tially tapped hole.  is to be tapped to depth , then it may be necessary to  ting direction, or feed direction and are particularly  utilise all three taps in the set, as each successive tap  useful for tapping through-holes.  Whereas, taps with  once rotated to depth, it will have less lead (i.e. taper)  straight futes (Fig. 97bii) in conjunction with a long  on the tapped hole, creating a stronger thread – up to  chamfer lead, can also give good tapping results. For  the thread’s maximum shear strength.  blind-holes, right-handed f utes, or straight f uted taps  Very large diameter  hand taps, require a certain  having shorter chamfer lead lengths give acceptable  level  of skill  in ensuring  that not only the tapped  tapping  results.  Tese  right-hand  futed taps, allow  hole is normal to the surface, but a considerable level  chip-fow in the backward direction – up the futes.  of physical strength is necessary  to tap such a hole!  Te chamfer lead length is such, that it allows return  Curved surfaces are more dif cult to tap, particularly  movement of chips, but they will not jam and are reli- concave ones, as it is ofen dif cult to keep the hand  ably sheared of .  tap normal to the surface With concave surfaces any  When tapping aluminium, grey cast iron, or certain  rotational motion of the tap wrench may be somewhat  brass alloys, the tap should have a short lead length –  restricted, without a suitable extension chuck/bar – as- regardless of whether the hole is ‘blind’ , or ‘through- suming workpiece access conditions allow.  running’. If, when tapping these workpiece materials,  For manual tapping operations, it is of en useful to  a long chamfer lead length was utilised, the tap would  utilise  ‘Tapping chucks’. Tese chucks have a rotational  behave like a  ‘Core-drill’ with chip-breaker  grooves.  drive, coupled to a sprung-loaded Z-axis. Te tapping  Tis ef ect would create ‘drilling’ a tapping-sized hole  chuck is positioned over the pre-drilled hole and man- to the major diameter – instead of actually cutting the  ually-fed down into the hole. Once the tap has engaged  required thread. with the hole, it is pulled and simultaneously ‘f oated  On some machining and turning centres, it is pos- down’ the hole being tapped – giving excellent tapped  sible to  ‘solid tap’ the workpiece, using CNC sof ware  hole accuracy. At ‘bottoming-out’  the tapping chuck  developed just for this task. A ‘solid tapping’ operation  automatically  reverses its direction and ‘drives’ itself  requires that the rotation of the spindle and the Z- out of the hole – while the machine’s spindle continues  axis control are fully  synchronised, otherwise tapping  to rotate in the tapping direction. errors would arise. It is possible to calculate the time  required for a tapping operation (Degamo, et al. 2003  – modif ed for metric units), using the following equa- Machine Ta tion: ps Machine  taps (Fig. 97) are utilised  across a diverse  range of machine tools and special-purpose  tapping  πDL n equipment.  Tey can have a variety of fute helices,  Tm n L R , V ranging from quick-to-straight  futes (Fig. 97a), de- =L ,N= +A +A pending upon the composition of the workpiece ma- Where: terial to be tapped. When tapping, all machining  is   Tm = Cutting time (min.), undertaken  by the cutting teeth and the chamfer. In    L = Depth of tapped hole, or Length of cut (mm), – general, the form and length of this chamfer will de-   n = Feedrate (mm min ), pend upon what type of hole is to be tapped. T apping    N = Spindle (rpm), – ‘through-holes’  is not too dif cult, but ‘blind-holes’    V = Cutting speed (m min ), can present  a problem,  associated  with  the evacu-  AL = Allowance to start the tap (min), ation of swarf in the reverse direction to that of the   AR = Allowance to withdraw the tap (min). feed. Tap fute spirals that are lef-handed and those  with spiral points (Fig. 97bi), remove chips in the cut- , To convert to inches, substitute 12 for the 1000 con- stant in the equation and modify the metric units to  inches.   ‘Tapping depth’ , is an ofen misleading term, as in many situ- ations  holes are tapped too deeply, as its is only necessary to  have a full thread form for 1.5D,, as this is where the maxi- mum thread shear strength occurs, which in turn, is related  to the shear strength of the workpiece material.,D = thread’s  major diameter. . Figure 97. Machine taps: with and without futes. [Courtesy of Guhring] . Figure 98. Fluteles tapping and tool geometry. [Courtesy of Guhring] In Appendix 7, some tapping problems are given, with  5.3 Flutelesspossible causes and solutions that may be of use in  identifying  any potential remedial machining  action  Taps Fluteless  taps (Fig. 98a), do not have cutting  edges  to be taken. (Fig. 98ai) and produce the desired thread geometry  by a ‘rolling action’ of the workpiece material. Treads  produced  by futeless  taps are much  stronger  than  5.4 Threadingtheir equivalent  machined  taps (Fig. 98b). Te bulk  workpiece material approximately follows the thread’s  Dies contour, thereby imparting additional shear strength  On shaf s, having either straight and tapered external  to each thread. Oil grooves are usually incorporated  threads these can be manually cut, up to a realistic max- into the taps periphery, to facilitate workpiece mate- imum ,40 mm, with threading dies. In essence, these  rial movement and to reduce tap wear rates. Like the  threading dies can be considered as analogous to hard- conventional  machine  taps  (Fig.  97),  futeless  taps  ened threaded nuts with multiple cutting edges (Fig.  have a lead to the tap’s edge – termed a ‘forming lead’  99a). Te cutting edges on the front die face are usually  (Fig. 98b – lef ), as opposed to a conventional machine  bevelled, or have a spiral lead to assist in starting the  tap which has a ‘chamfer lead’ (Fig. 98b-right) which  thread on the workpiece. Likewise, it is normal to add  forms part of the cutting action. Terefore, the chi- a reasonable chamfer to the bar’s end to be threaded, as  pless tap in operation  (Fig. 98bi), plastically  moves  this also helps to gently introduce the thread to depth,  workpiece material from the pre-drilled hole into the  as the stock and die are manually-rotated  down its  spaces between the tap’s f anks and in so doing, locally  length. As is the case for tapping, it is normal practice  work-hardening this material to a limited depth in the  to ‘back-of ’ the ‘stock’s’ rotation about every ? of a  workpiece’s substrate.  turn by approximately ? of a turn, to facilitate chip- Several factors need to be considered prior to util- breaking. As a result of these ‘leads’ on both the shaf   ising f uteless taps on engineering components, these  and die, a few threads on the bar’s end will not be to  are: full thread depth. Care must be taken when initially  • Over-sized diameter of pre-drilled hole – if the  starting to cut the thread, as if it is not square to the  hole is too large, then insuf cient workpiece mate- bar’s axis, then a  ‘drunken thread’ will result. Previ- rial will be available to fully form the rolled thread, ously, most dies were manufactured from high carbon  • Undersized diameter of pre-drilled hole –  too  steel and, due to their size, their  ‘ruling section’  and its  small a hole will be likely to cause the chipless tap  to jam – as it attempts to roll the thread, possibly  leading to tap breakage, NB  Terefore, precise control over the diameter of  the pre-drilled hole is imperative. • Workpiece material’s characteristics – both  the  bulk hardness and more importantly, its mechanical    ‘Drunken threads’ , are the result of variations in the helix  working ability and as a result of this action its lo- angle and its associated pitch dif ering in uniformity on each  cal hardening, are important factors when ‘rolling’  side of the thread’s diameter. Hence, a ‘true’ mating nut, would  a thread form.  ‘wobble’ somewhat as it is rotated down such poorly manufac- tured threaded shaf  – hence, its name:  ‘drunken thread’. NB A ‘start-point’ for the size of pre-drilling diam-   ‘Ruling section’ , this term relates to the cross-sectional area  that can normally be hardened, being signif cantly infuenced  eter can be obtained from the tooling suppliers. Of- by the component’s geometry which afects its  ‘critical cool- ten some form of experimentation  is necessary in  – order to obtain the optimum diameter, as this pre- ing velocity’ (i.e. usually around 1,000?C sec ) when being  drilled diameter will vary according to the work- quenched. Tis quenching rate is necessary if the part’s metal- piece material’s previous processing route. lurgical structure is to fully transform into a martensitic state,  prior to subsequent tempering. . Figure 99. Die geometries and their nomenclature. [Courtesy of Guhring] associated  ‘mass efect ’ meant that the dies could be  die -head cutting elements to be preset to take frstly  through-hardened – which gave them an overall ‘bulk  a roughing cut, followed by fnishing cut/chasing  of  hardness’ of greater than HSS. Today, basically  dies  the threads down the bar. At the end of the threaded  are either manufactured from micro-grained HSS, or  section,  these  self-opening  dies  will  automatically  coated cemented carbide.  open and can then be speedily  withdrawn  from the  Solid dies (Fig. 99a), do not have any means to com- threaded portion of the bar. T ese self-opening  dies  pensate for die wear, whereas, their split-die nut coun- can be set to give the correct amount of tolerance, con- terparts (Fig. 99b-lef ), can be manually-adjusted. T is  trolling the ‘play’ on the thread. Moreover, it is pos- adjustment of the die is achieved by turning a centrally  sible to f t dif erent thread sizes and forms into the die  mounted grub-screw  in the stock body, which along  head, for more universal threading applications. Both  with the fxing screws can be made to open, or close  the radial and tangential  threading  elements,  create  on the shaf  to be threaded. In this manner, achieving  less tool fank contact and frictional  rubbing on the  the correct thread tolerance, or ‘play’ for the desired  cut thread.  ftment of its associated  mating nut. It is also usual  practice, to use a suitable die lubricant, to facilitate in  the thread’s production while improving surface f nish  5.5 Thread Turningand prolonging the die’s life – as excessive friction oc- curs during this type of threading process.  – T e major  disadvantage  of using  the  solid-type  threading dies is that they either have to be unscrewed  Introduction from the threaded  workpiece,  or rewound from the  On conventional engine-/centre-lathes, a single-point  thread,  using  up unproductive  time  elements,  this  thread cutting tool (Fig. 100), has a synchronised and  being particularly  important for large batch runs, or  combined linear and rotary kinematic motion for its  in a continuous  production  environment.  Self-open- ing dies  (i.e.  not depicted)  have been  utilised  for  many years on: capstan and turret lathes, single- and  0 multi-spindle automatics and so on, for cutting exter- nal threads.  Several  types of self-opening  die heads  are available, ranging from: radial, tangential, or cir- cular arrangement  of the multi-point  cutting inserts  and thread chasers. In most cases, it is usual for the    ‘Mass efect’ , is related to the component’s ‘ruling section’. For  example, if the part has a  large cross-sectional area, when it is  quenched from the hardening temperature zone (i.e. this can  be found from its associated  thermal equilibrium diagram –  for the present), it will  notexceed the  ‘critical cooling velocity’ and only a partialmartensitic state occurs. T is is because the  quench media used could  not suf ciently drastically reduce  the part’s temperature with an incomplete atomic transforma- tion occurring and in so doing, the heat-treated component  will retain some austenite in the matrix. For this reason, large  holes (e.g. designed into in through-hardened Sine-bars) are  ofen strategically designed in these larger component regions.  Moreover, in many cases the larger component cross-sections  are reduced, so that the  ‘mass efect’ does  not occur – apart  from the obvious factor of relieving weight, etc.  0  Self-opening dies, are ofen termed ‘Tread chasing die heads’ ,  whereas in reality a thread is only ‘chased’ once the main  . Figure 100. External and internal threading tool holders and thread form has initially been cut. T us chasing is employed  in-serts. [Courtesy of Seco Tools] to give the required f t and fnish to the fnal thread form. threading insert. Tis insert is connected to the lead- and moved  back  to its start  point,  then  fed more  screw (i.e normally having a very accurately-hardened  deeply  beginning  another  threading  pass down the  and ground Acme form) which is precisely synchro- same helical groove, this process being repeated until  nised to that of the headstock’s rotation. On a CNC  full thread depth/prof le is accomplished. In order to  turning centre, or similar, this linear motion is reli- obtain a consistent thread pitch on the workpiece, the  ant on the precision and accuracy of the recirculating  feedrate along the threaded portion must exactly co- ballscrew coupled to the programmed  cutterpath. In  incide. Te thread form is dependent upon the pro- this manner, the threading insert being rigidly held in  fled geometry of the thread cutting insert. In order to  either the tool post, or turret, generates a spiral groove  achieve the required fnal thread profle, the feedrate  which when at full depth creates a screwthread of the  must be considerably larger than is normally utilised  desired pitch and helix angle. During successive tra- for conventional turning operations.  verse feeding passes (i.e. to prescribed depths) along  Any V-form thread point angle geometry, is not  the workpiece the thread is cut. A typical thread is  an ideal edge shape for the production of machined  routinely produced on CNC turning centres, using its  threads if the insert is fed in normal to the workpiece’s  fxed/canned  cycles (i.e. ‘bespoke  sofware’). During  axis of rotation (i.e. radial/plunge-fed).  Chip control  these automated threading  passes, the tool precisely  here will be compromised, as each f ank of the V-form  traverses down the bar’s length, is rapidly withdrawn  thread gets successively deeper. Tis narrowing of the  . Figure 101. Screwcutting tech- niques on turning centres and suggest- ed methods for improved chip control. [Courtesy of Sandvik Coromant]