Metal Machining Episode 1 docx

Chapter 1 is an introduction that places the process inits broader context of machine tool technology and manufacturing systems management.Chapter 2 covers the basic mechanical engineeri

Hiroshima University, Japan

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First published in Great Britain in 2000 by Arnold, a member of the Hodder Headline Group,

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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 No part of this publication may be reproduced or transmitted in any form or by any means, electronically or mechanically, including photocopying, recording or any information storage or retrieval system, without either prior permission in writing from the publishers or a licence permitting restricted copying In the United Kingdom such licences are issued by the Copyright Licensing Agency: 90 Tottenham Court Road,

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ISBN 0 340 69159 X ISBN 0 470 39245 2 (Wiley)

1 2 3 4 5 6 7 8 9 10 Commissioning Editor: Matthew Flynn Production Editor: James Rabson Production Controller: Iain McWilliams Cover Design: Mouse Mat Design Typeset in 10/12 pt Times by Cambrian Typesetters, Frimley, Surrey Printed and bound in Great Britain by Redwood Books Ltd.

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3.1 Work material characteristics in machining 82

5.4 Acoustic emission 155

6.3 Introducing variable flow stress behaviour 1686.4 Non-orthogonal (three-dimensional) machining 177

7.3 The Iterative Convergence Method (ICM) 2127.4 Material flow stress modelling for finite element analyses 220

8.2 Simulation of unsteady chip formation 2348.3 Machinability analysis of free cutting steels 240

Appendices

1 Metals’ plasticity, and its finite element formulation 328

A1.1 Yielding and flow under triaxial stresses: initial concepts 329A1.2 The special case of perfectly plastic material in plane strain 332A1.3 Yielding and flow in a triaxial stress state: advanced analysis 340A1.4 Constitutive equations for numerical modelling 343

A2.1 The differential equation for heat flow in a solid 351A2.2 Selected problems, with no convection 353

iv Contents

A2.3 Selected problems, with convection 355

A3.6 Friction coefficients greater than unity 373

4 Work material: typical mechanical and thermal behaviours 375

A4.1 Work material: room temperature, low strain rate, strain hardening

A4.3 Work material: strain hardening behaviours at high strain rates and

Contents v

Improved manufacturing productivity, over the last 50 years, has occurred in the area ofmachining through developments in the machining process, in machine tool technologyand in manufacturing management The subject of this book is the machining processitself, but placed in the wider context of manufacturing productivity It is mainly concernedwith how mechanical and materials engineering science can be applied to understand theprocess 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 morefrom 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 ascomplementing

It is intended that this book will be of interest and helpful to all mechanical, turing and materials engineers whose responsibilities include metal machining matters It

manufac-is, however, written specifically for masters course students Masters courses are a majorfeature of both the American and Japanese University systems, preparing the more abletwenty year olds in those countries for the transition from foundation undergraduatecourses to useful professional careers In the UK, masters courses have not in the past beenpopular, but changes from an elite to a mass higher education system are resulting in anincreasingly important role for taught advanced level and continuing professional devel-opment courses

It is supposed that masters course readers will have encountered basic mechanical andmaterials principles before, but will not have had much experience of their application Afeature 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 tomeet 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 retical detail We hope that a reader who wants to know how much power will be needed

theo-to turn a common engineering alloy, or what cutting speed might be used, or what ial properties might be appropriate for carrying out some reader-specific simulation, willhave a reasonable chance either of finding the information in these pages or of finding ahelpful reference for further searching

mater-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 inits 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 machiningstudies

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 withthe development and application of, finite element methods in machining analysis Chapter

9 is concerned with embedding process understanding into process control and tion tools

optimiza-No book is written without external influences We thank the following for their adviceand 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 closelyconnected with this book, we also especially acknowledge many discussions with, andmuch experimental information given by, Professor T Kitagawa of Kitami Institute ofTechnology, who might almost have been a co-author

We also thank the companies Yasda Precision Tools KK, Okuma Corporation and ToyoAdvanced Technologies for allowing the use of original photographs in Chapter 1, BritishAerospace Airbus for providing the cover photograph, Mr G Dean (Leeds University) fordrafting many of the original line drawings and Mr K Sekiya (Hiroshima University) forcreating some of the figures in Chapter 4 One of us (it is obvious which one) thanks theBritish Council and Monbusho for enabling him to spend a 3 month period in Japan duringthe Summer of 1999: this, with the purchase of a laptop PC with money awarded by theJacob 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 JapanSeptember, 1999

viii Preface

Introduction

Machining (turning, milling, drilling) is the most widespread metal shaping process inmechanical manufacturing industry Worldwide investment in metal-machining machinetools holds steady or continues to increase year by year, the only exception being in theworst of recessions The wealth of nations can be judged by this investment Figure 1.1shows 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 = 109) in thelate 1970s It fell abruptly in the world recession (the oil crisis) of 1981–82 and has nowrecovered 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

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 inmetal 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 thedisplacement of metal by plastics products in the consumer goods sector, and materialwastefulness in the production of swarf (or chips) that has encouraged near-net (castingand forging) process substitution in the metal products sector One reason is that metalmachining is capable of high precision: part tolerances of 50mm 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 mademore cheaply, quickly and simply (at least in small numbers) by controlling the path of astandard 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 diesfor moulding, forging and die casting processes)

One measure of a part’s complexity is the product of the number of its independentdimensions 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 weighsapproximately 2 × 105kg) and complexity of machining and its competitive processes.Complexity is defined by

C = n log2(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 toincrease productivity, to hold market share and to find new markets, has led to largechanges in machining practice The changes have been of three types: advances in machinetools (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)

throws pressure on to another It is worthwhile briefly to review the evolution of thesechanges, from the introduction of numerical controlled machine tools in the late 1950s

to the present day, in order to place in its wider context the special content of this book (the consideration of the chip forming process itself), which is at the heart ofmachining

1.1 Machine tool technology

In the early 1970s a number of surveys were carried out on the productivity of machineshops in the UK, Europe and the USA (Figure 1.3) As far as the machine tools wereconcerned it was found that they were actually productive, removing metal, for only 10 to20% of the time: different surveys, however, gave different values For 40 to 60% of thetime the machine tools were in use but not productively: i.e they were being set up formanufacture, or being loaded and unloaded, or during manufacture tools were beingmoved and positioned for cutting but they were not removing metal For 20 to 50% of thetime they were totally unused – idle

As far as work in progress was concerned, batches of components typically spent from

70 to 95% of their time inactive on the shop floor So overwhelming was the clutter ofpartly finished work that a component requiring several different operations for its comple-tion, on different machine tools, might find these carried out at the rate of only one a week.From 10 to 20% of their time components were being positioned for machining and foronly from 1 to 5% of the time was metal actually being removed

From the late 1960s to the early 1970s both forms of waste – the active, non-productiveand the idle times – began significantly to be attacked, the former mainly by developingmachine tool technology and the latter by new forms of manufacturing organization

Machine tool technology 3

Fig 1.3 Idle and active times in batch manufacturing, from surveys circa 1970

1.1.1 Machine tool technology – mainly turning machines

From 1970 onwards, machine tools of new design started to be introduced in significantnumbers into manufacturing industry, with the effect of greatly reducing the times for toolpositioning and movement between cuts These new, computer numerical control (CNC),designs stemmed directly from the development of numerically controlled (NC) machinetools in the 1950s In traditional, mechanically controlled machine tools, for example thelathe in Figure 1.4, the coordination needed between the main rotary cutting motion of theworkpiece and the feed motions of the tool is obtained by driving all motions from a singlemotor The feed motions are obtained from the main motion via a gear box and a slenderfeed rod (or lead screw for thread cutting) With the exception of machines known as copy-ing machines (which derive their feed motion by following a copy of a shape to be made)only simple feed motions are obtainable: on a lathe, for example, these are in the axial andradial directions – to machine a radius on a lathe requires the use of a form tool In addi-tion, the large amount of backlash in the mechanical chain requires time and a skilled oper-ator to set the tool at the right starting point for a particular cut

In a CNC machine tool, all the motions are mechanically separate, each driven by itsown motor (Figure 1.4) and each coordinated electronically (by computer) with the others.Not only are much more complicated feed motions possible, for example a combinedradial and axial feed to create a radius or to take the shortest path between two points atdifferent axial and radial positions, but the requirement of coordination has led to thedevelopment of much more precise, backlash-free ball-screw feed drives This precisenumerical control of feed motions, with the ability also to drive the tools quickly betweencuts, together with other reductions in set-up times (to be considered in Section 1.2), hasapproximately halved machine tool non-productive cycle time, relative to its pre-1970levels

This halving of time is indicated in Figure 1.5(a) (Figure 1.5(b) is considered in Section1.1.2) A further halving of non-productive cycle time has been possible from about 1980onwards, with the spread throughout all manufacturing industry of new types of machine

tools that have become called turning centres (related to lathes) and machining centres

(developed from milling machines) These new tools, first developed in the 1960s for massproduction industry, individually can carry out operations that previously would haverequired several machine tools For example, it is possible on a traditional lathe to present

a variety of tools to the workpiece by mounting the tools on a turret In a new turningcentre, some of the tools may be power driven and the main power drive, usually used torotate the workpiece in turning operations, may be used as a feed drive to enable millingand drilling as well as turning to be carried out on the one machine

Figure 1.6 is an example of a keyway being milled in a flanged hollow shaft Pitchcircle holes previously drilled in the flange can also be seen This part would have requiredthree traditional machines for its manufacture: a lathe, a milling and a drilling machine,with three loadings and unloadings and three set-ups It is the possibility of reducing load-ings and set-ups that has led to the further halving of cycle times – although this figure is

an average Individual time savings increase with part complexity and the number of ups that can be eliminated Centres are also much more expensive than more simple tradi-tional machine tools and need to be heavily used to be cost effective The implications ofthis for the development of metal cutting practice – a trend towards higher speed machin-ing – will be developed in Section 1.4

set-4 Introduction

Machine tool technology 5

Fig 1.4 A mechanically controlled lathe and (below) partly-built and complete views of a numerically controlled

machine with individual feed drive motors

The increased versatility of machine tools (based on turning operations as an example)has been briefly considered: the freedom given by CNC to create more complicated feedmotions, both by path and speed control; and the evolution of multi-function machine tools(centres) The cost penalty has just been mentioned As part of the continuing scene settingfor the conditions in which metal cutting is carried out, which will be combined withsystems and materials technology considerations in Section 1.4, some broad machine toolmechanical design and cost considerations will now be introduced – still in the context ofturning.

Figure 1.7 sketches a turning operation, in which, in one revolution of the bar, the tool

moves an axial distance f (the feed distance) to reduce the bar radius by an amount d (the depth of cut) The figure also shows the cutting force Fcacting on the tool, the diameter D

at which the cutting is taking place and both the angular speed W at which the bar rotates

and the consequent linear speed V (in later chapters this will be called Uwork) at the

diam-eter D Material is removed, in the form of chips, at the rate fdV (More detail of cutting

terminology is given in Chapter 2)

6 Introduction

Fig 1.5 Reductions from the levels shown in Figure 1.3 of (a) machine tool non-productive time and (b) work in

progress idle time, due to better technology and organization

Fig 1.6 A flanged shaft turned, drilled and milled in one set-up on a turning centre

The torque T and power P that the main drive motor must generate to support this

turn-ing operation is, by elementary mechanics

T = Fc(D/2) ≡ (Fc*fd)(D/2) (1.2a)

P = FcV ≡ (Fc*fd)V or Fc*(fdV) (1.2b)

A new quantity Fc*has been introduced It is the cutting force per unit area of removedmaterial Called the specific cutting force, it depends to a first approximation mainly on

the material being cut Equation (1.2a) indicates that, for a constant area of cut fd, a

turn-ing machine should be fitted with a motor with a torque capacity proportional to the largestdiameter being cut It is shown later that for any combination of work and tool there is a

preferred linear cutting speed V Equation (1.2b) suggests that for a constant area of cut

the required motor power should be independent of diameter cut Observing what motors,with their torque and power capacities, are fitted to production machine tools can giveinsight into what duties the machine tools are expected to perform; and what forces thecutting tools are expected to withstand This is considered next

Machine tool manufacturers’ catalogues show that turning machines are fitted withmotors the torques and powers of which increase, respectively, with the square of andlinearly with, the maximum work diameter A typical catalogue specifies, among otherthings, the main motor power, the maximum rev/min at which the work rotates and themaximum diameter of work for which the machine is designed Figure 1.8(a) plots the

torque at maximum rev/min, obtained from P = WT, against maximum design diameter,

both on a log scale, for a range of mechanically controlled and CNC centre lathes andchucking turning centres (as illustrated in Figures 1.4 and 1.6 respectively) Apart fromtwo sets of data marked ‘t’, which are for lathes described as for training and which might

be expected to be underdesigned relative to machines for production use, both the ical and CNC classes of machine show the same squared power law dependence of torque

mechan-on maximum work diameter

It seems that machines are designed to support larger areas of cut, fd, the larger the work diameter D Not only are larger diameter workpieces stiffer and able to support larger

forces (and hence areas of cut), but usually they require more material to be removed fromthem A larger area of cut enables the time for machining to be kept within bounds A

Machine tool technology 7

Fig 1.7 The turning process – not to scale