Metal Machining Theory and Applications Thomas Childs University of Leeds, UK Katsuhiro Maekawa Ibaraki University, Japan Toshiyuki Obikawa Tokyo Institute of Technology, Japan Yasuo Yamane Hiroshima University, Japan A member of the Hodder Headline Group LONDON Copublished in North, Central and South America by John Wiley & Sons Inc. New York-Toronto Childs Prelims 28:3:2000 4:07 pm Page i First published in Great Britain in 2000 by Arnold, a member of the Hodder Headline Group, 338 Euston Road, London NW1 3BH http://www.arnoldpublishers.com Copublished in North, Central and South America by John Wiley & Sons Inc., 605 Third Avenue, New York, NY 10158–0012 © 2000 Thomas Childs, Katsuhiro Maekawa, Toshiyuki Obikawa and Yasuo Yamane All rights reserved. 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Please send your comments to feedback.arnold@hodder.co.uk Childs Prelims 28:3:2000 4:07 pm Page ii Contents Preface vii 1 Introduction 1 1.1 Machine tool technology 3 1.2 Manufacturing systems 15 1.3 Materials technology 19 1.4 Economic optimization of machining 24 1.5 A forward look 32 References 34 2 Chip formation fundamentals 35 2.1 Historical introduction 35 2.2 Chip formation mechanics 37 2.3 Thermal modelling 57 2.4 Friction, lubrication and wear 65 2.5 Summary 79 References 80 3 Work and tool materials 81 3.1 Work material characteristics in machining 82 3.2 Tool materials 97 References 117 4 Tool damage 118 4.1 Tool damage and its classification 118 4.2 Tool life 130 4.3 Summary 134 References 135 5 Experimental methods 136 5.1 Microscopic examination methods 136 5.2 Forces in machining 139 5.3 Temperatures in machining 147 Childs Prelims 28:3:2000 4:07 pm Page iii 5.4 Acoustic emission 155 References 157 6 Advances in mechanics 159 6.1 Introduction 159 6.2 Slip-line field modelling 159 6.3 Introducing variable flow stress behaviour 168 6.4 Non-orthogonal (three-dimensional) machining 177 References 197 7 Finite element methods 199 7.1 Finite element background 199 7.2 Historical developments 204 7.3 The Iterative Convergence Method (ICM) 212 7.4 Material flow stress modelling for finite element analyses 220 References 224 8 Applications of finite element analysis 226 8.1 Simulation of BUE formation 226 8.2 Simulation of unsteady chip formation 234 8.3 Machinability analysis of free cutting steels 240 8.4 Cutting edge design 251 8.5 Summary 262 References 262 9 Process selection, improvement and control 265 9.1 Introduction 265 9.2 Process models 267 9.3 Optimization of machining conditions and expert system applications 283 9.4 Monitoring and improvement of cutting states 305 9.5 Model-based systems for simulation and control of machining processes 317 References 324 Appendices 1 Metals’ plasticity, and its finite element formulation 328 A1.1 Yielding and flow under triaxial stresses: initial concepts 329 A1.2 The special case of perfectly plastic material in plane strain 332 A1.3 Yielding and flow in a triaxial stress state: advanced analysis 340 A1.4 Constitutive equations for numerical modelling 343 A1.5 Finite element formulations 348 References 350 2 Conduction and convection of heat in solids 351 A2.1 The differential equation for heat flow in a solid 351 A2.2 Selected problems, with no convection 353 iv Contents Childs Prelims 28:3:2000 4:07 pm Page iv A2.3 Selected problems, with convection 355 A2.4 Numerical (finite element) methods 357 References 362 3 Contact mechanics and friction 363 A3.1 Introduction 363 A3.2 The normal contact of a single asperity on an elastic foundation 365 A3.3 The normal contact of arrays of asperities on an elastic foundation 368 A3.4 Asperities with traction, on an elastic foundation 369 A3.5 Bulk yielding 371 A3.6 Friction coefficients greater than unity 373 References 374 4 Work material: typical mechanical and thermal behaviours 375 A4.1 Work material: room temperature, low strain rate, strain hardening behaviours 375 A4.2 Work material: thermal properties 376 A4.3 Work material: strain hardening behaviours at high strain rates and temperatures 379 References 381 5 Approximate tool yield and fracture analysis 383 A5.1 Tool yielding 383 A5.2 Tool fracture 385 References 386 6 Tool material properties 387 A6.1 High speed steels 387 A6.2 Cemented carbides and cermets 388 A6.3 Ceramics and superhard materials 393 References 395 7 Fuzzy logic 396 A7.1 Fuzzy sets 396 A7.2 Fuzzy operations 398 References 400 Index 401 Contents v Childs Prelims 28:3:2000 4:07 pm Page v Childs Prelims 28:3:2000 4:07 pm Page vi Preface Improved manufacturing productivity, over the last 50 years, has occurred in the area of machining through developments in the machining process, in machine tool technology and in manufacturing management. The subject of this book is the machining process itself, but placed in the wider context of manufacturing productivity. It is mainly concerned with how mechanical and materials engineering science can be applied to understand the process better and to support future improvements. There have been other books in the English language that share these aims, from a vari- ety of viewpoints. Metal Cutting Principles by M. C. Shaw (1984, Oxford: Clarendon Press) is closest in spirit to the mechanical engineering focus of the present work, but there have been many developments since that was first published. Metal Cutting by E. M. Trent (3rd edn, 1991, Oxford: Butterworth-Heinemann) is another major work, but written more from the point of view of a materials engineer than the current book’s perspective. Fundamentals of Machining and Machine Tools by G. Boothroyd and W. A. Knight (2nd edn, 1989, New York: Marcel Dekker) covers mechanical and production engineering perspectives at a similar level to this book. There is a book in Japanese, Modern Machining Theory by E. Usui (1990, Tokyo: Kyoritu-shuppan), that overlaps some parts of this volume. However, if this book, Metal Machining, can bear comparison with any of these, the present authors will be satisfied. There are also more general introductory texts, such as Manufacturing Technology and Engineering by S. Kalpakjian (3rd edn, 1995, New York: Addison-Wesley) and Introduction to Manufacturing Processes by J. A. Schey (2nd edn, 1987, New York: McGraw-Hill) and narrower more specialist ones such as Mechanics of Machining by P. L. B. Oxley (1989, Chichester: Ellis Horwood) which this text might be regarded as complementing. It is intended that this book will be of interest and helpful to all mechanical, manufac- turing and materials engineers whose responsibilities include metal machining matters. It is, however, written specifically for masters course students. Masters courses are a major feature of both the American and Japanese University systems, preparing the more able twenty year olds in those countries for the transition from foundation undergraduate courses to useful professional careers. In the UK, masters courses have not in the past been popular, but changes from an elite to a mass higher education system are resulting in an increasingly important role for taught advanced level and continuing professional devel- opment courses. Childs Prelims 28:3:2000 4:07 pm Page vii It is supposed that masters course readers will have encountered basic mechanical and materials principles before, but will not have had much experience of their application. A feature of the book is that many of these principles are revised and placed in the machin- ing context, to relate the material to earlier understanding. Appendices are heavily used to meet this objective without interrupting the flow of material too much. It is a belief of the authors that texts should be informative in practical as well as theo- retical detail. We hope that a reader who wants to know how much power will be needed to turn a common engineering alloy, or what cutting speed might be used, or what mater- ial properties might be appropriate for carrying out some reader-specific simulation, will have a reasonable chance either of finding the information in these pages or of finding a helpful reference for further searching. The book is essentially organized in two parts. Chapters 1 to 5 cover basic material. Chapters 6 to 9 are more advanced. Chapter 1 is an introduction that places the process in its broader context of machine tool technology and manufacturing systems management. Chapter 2 covers the basic mechanical engineering of machining: mechanics, heat conduc- tion and tribology (friction, lubrication and wear). Chapters 3 and 4 focus on materials’ performance in machining, Chapter 5 describes experimental methods used in machining studies. The core of the second part is numerical modelling of the machining process. Chapter 6 deals with mechanics developments up to the introduction of, and Chapters 7 and 8 with the development and application of, finite element methods in machining analysis. Chapter 9 is concerned with embedding process understanding into process control and optimiza- tion tools. No book is written without external influences. We thank the following for their advice and help throughout our careers: in the UK, Professors D. Tabor, K. L. Johnson, P. B. Mellor and G . W. Rowe (the last two, sadly, deceased); in Japan, Professors E. Usui, T. Shirakashi and N. Narutaki; and Professor S. Ramalingam in the USA. More closely connected with this book, we also especially acknowledge many discussions with, and much experimental information given by, Professor T. Kitagawa of Kitami Institute of Technology, who might almost have been a co-author. We also thank the companies Yasda Precision Tools KK, Okuma Corporation and Toyo Advanced Technologies for allowing the use of original photographs in Chapter 1, British Aerospace Airbus for providing the cover photograph, Mr G. Dean (Leeds University) for drafting many of the original line drawings and Mr K. Sekiya (Hiroshima University) for creating some of the figures in Chapter 4. One of us (it is obvious which one) thanks the British Council and Monbusho for enabling him to spend a 3 month period in Japan during the Summer of 1999: this, with the purchase of a laptop PC with money awarded by the Jacob Wallenberg Foundation (Royal Swedish Academy of Engineering Science), resulted in the final manuscript being less late than it otherwise would have been. We must thank the publisher for allowing several deadlines to pass and our wives – Wendy, Yoko, Hiromi and Fukiko – and families for accepting the many working week- ends that were needed to complete this book. Thomas Childs, Katsuhiro Maekawa, Toshiyuki Obikawa, and Yasuo Yamane England and Japan September, 1999 viii Preface Childs Prelims 28:3:2000 4:07 pm Page viii 1 Introduction Machining (turning, milling, drilling) is the most widespread metal shaping process in mechanical manufacturing industry. Worldwide investment in metal-machining machine tools holds steady or continues to increase year by year, the only exception being in the worst of recessions. The wealth of nations can be judged by this investment. Figure 1.1 shows the annual expenditure on machine tools by each of the most successful countries – Germany, Japan and the USA. For each, it was between £1bn and £2bn (bn = 10 9 ) in the late 1970s. It fell abruptly in the world recession (the oil crisis) of 1981–82 and has now recovered to between £2bn and £3bn (all expressed in 1985 prices: £1 was then equivalent to 300¥ or $1.3). Figure 1.1 also shows similar trends (a growth over the last 20 years from Fig. 1.1 International demand for machine tools, 1978–88, £bn at 1985 prices (from European community statistics 1988) and projected at that time to 1995 Childs Part 1 28:3:2000 2:32 pm Page 1 50% to 100% in annual expenditure) for the developed European Community countries. Only in the UK has there been a decline in investment. Over this period, investment in metal machining has remained at about three times the annual investment in metal form- ing machinery. Investment has continued despite perceived threats to machining volume, such as the displacement of metal by plastics products in the consumer goods sector, and material wastefulness in the production of swarf (or chips) that has encouraged near-net (casting and forging) process substitution in the metal products sector. One reason is that metal machining is capable of high precision: part tolerances of 50 mm and surface finishes of 1 mm are readily achievable (Figure 1.2(a)). Another reason is that it is very versatile: complicated free-form shapes with many features, over a large size range, can be made more cheaply, quickly and simply (at least in small numbers) by controlling the path of a standard cutting tool rather than by investing considerable time and cost in making a dedi- cated moulding, forming or die casting tool (besides, machining is needed to make the dies for moulding, forging and die casting processes). One measure of a part’s complexity is the product of the number of its independent dimensions and the precision to which they must be made (Ashby, 1992). Figure 1.2(b) gives limits to the component size (weight units – a cube of steel of side 3 m weighs approximately 2 × 10 5 kg) and complexity of machining and its competitive processes. Complexity is defined by C = n log 2 (l/Dl) (1.1) where n is the number of the dimensions of the part and Dl/l is the average fractional preci- sion with which they are specified. A third reason for the success of metal machining is that the need from competition to increase productivity, to hold market share and to find new markets, has led to large changes in machining practice. The changes have been of three types: advances in machine tools (machine technology), in the organization of machining (manufacturing systems) and in the cutting edges themselves (materials technology). Each new improvement in one area 2 Introduction Fig. 1.2 (a) Typical accuracy and finish and (b) complexity and size achievable by machining, forming and casting processes, after Ashby (1992) Childs Part 1 28:3:2000 2:32 pm Page 2 [...]... machines per unit mass is similar to turning machines All this is developed in Figure 1. 16 Childs Part 1 28:3:2000 2:34 pm Page 13 Machine tool technology 13 Fig 1. 13 A traditional – column and knee – design and (right and below) partly-built and complete views of a modern (bed) design of milling machine In Figures 1. 16(a) and (b) the capacity of a milling machine is measured by its crosstraverse capacity... Figure 1. 3 apply It is now understood that there are intermediate layouts for manufacturing systems, Childs Part 1 28:3:2000 2:34 pm Page 18 18 Introduction Fig 1. 18 Materials transfers in a jobbing shop environment (after Boothroyd and Knight, 19 89) Fig 1. 19 The spectrum of manufacturing systems (after Groover and Zimmers, 19 84) appropriate for different mixes of part variety and quantity (Figure 1. 19)... work diameter (Figure 1. 8) Figures 1. 16(a) and (b) show that torque and power increase as cross-traverse cubed and squared respectively An assumption that machines are designed to accommodate larger diameter cutters in proportion to workpiece size yields the D3 and D2 relations derived in the previous paragraph Childs Part 1 28:3:2000 2:34 pm Page 14 14 Introduction Fig 1. 14 A 5-axis milling machine... turning and milling machines around the 19 80 watershed Pre -1 9 80, the purchase of mechanically controlled machines was holding steady, with roughly twice the investment in turning as in milling machines At the same Childs Part 1 28:3:2000 2:33 pm Page 11 Machine tool technology 11 Fig 1. 11 Annual UK investment in mechanically and CNC controlled turning (•,o) and milling (+,x) machines, from UK government... machine tools and machining centres As with turning machines, there have been two stages of development: a post -1 9 70 stage, which saw the substitution of mechanically controlled machines by their CNC equivalents; and a post -1 9 80 stage, which has, in addition, seen the development of more versatile machining centres Figure 1. 11 compares the annual UK investment in mechanical and CNC turning and milling... in 19 90) Childs Part 1 28:3:2000 2:33 pm Page 10 10 Introduction Fig 1. 10 Mass/power and price/mass relationships for turning machines Here and later in the Chapter, prices and costs have been collected in the UK, during the early 19 90s A decision has been made to leave the information in units of UK£, unadjusted for inflation An approximate conversion to values in the USA may be made at UK 1 = US $1; ... time, and even the poorly productive machines that were then common were idle for up to 50% of the time (Figure 1. 3) Manufacturing technology has, in fact, evolved hand in hand with manufacturing system organization, sometimes one pushing and the other pulling, sometimes vice versa Childs Part 1 28:3:2000 2:34 pm Page 16 16 Introduction Fig 1. 16 (a) Torque and (b) power as a function of cross-traverse... turning in the post -1 9 80 period as shown in Figure 1. 11 Figure 1. 14 shows an example of a new design of machine with a tiltable spindle and interchangeable worktables Figure 1. 15 shows a detail of a tool change magazine As far as process mechanics is concerned, equations (1. 2) for torque and power can be applied to milling if D is interpreted as the diameter of the cutting tool and fdV remains the... Childs Part 1 28:3:2000 2:34 pm Page 19 Materials technology 19 Fig 1. 20 Reduced materials flow through cell-oriented organizations and group technology (after Boothroyd and Knight, 19 89) If part variety reduces, perhaps to the order of hundreds, and batch size increases, again to the order of hundreds, it begins to pay to organize groups or cells of machine tools to reduce materials handling (Figure 1. 20)... more simple traditional machine tools and need to be heavily used to be cost effective The implications of this for the development of metal cutting practice – a trend towards higher speed machining – will be developed in Section 1. 4 Childs Part 1 28:3:2000 2:33 pm Page 5 Machine tool technology 5 Fig 1. 4 A mechanically controlled lathe and (below) partly-built and complete views of a numerically controlled . in machining 82 3.2 Tool materials 97 References 11 7 4 Tool damage 11 8 4 .1 Tool damage and its classification 11 8 4.2 Tool life 13 0 4.3 Summary 13 4 References 13 5 5 Experimental methods 13 6 5 .1. ii Contents Preface vii 1 Introduction 1 1 .1 Machine tool technology 3 1. 2 Manufacturing systems 15 1. 3 Materials technology 19 1. 4 Economic optimization of machining 24 1. 5 A forward look 32 References. Introduction 15 9 6.2 Slip-line field modelling 15 9 6.3 Introducing variable flow stress behaviour 16 8 6.4 Non-orthogonal (three-dimensional) machining 17 7 References 19 7 7 Finite element methods 19 9 7.1