Computer Numerical Control of Machine Tools

324 Pages·1991·25.229 MB·English

by  G. E. Thyer (Auth.)

Computer Numerical
Control of
Machine Tools
G. E. Thyer
Second edition
^ N E W N ES
Newnes
An imprint of Butterworth-Heinemann Ltd
Linacre House, Jordan Hill, Oxford OX2 8DP
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First published 1988
Second edition 1991
©G. E. Thyer 1988, 1991
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British Library Cataloguing in Publication Data
Thyer, G. E.
Computer numerical control of machine tools.
I. Title
621.9
ISBNO 7506 0119 1
Printed and bound in Great Britain by
Butler & Tanner Ltd, Frome.
Preface
Since this book was first published, computer • Additions have been made to Chapter 4
numerical control of machine tools has con- which it is hoped will provide a better
tinued to develop in a number of directions. understanding of the designation of axes.
These include the introduction of mill-turning • The introduction of in-process measurement
centres and of turning centres with Y axis on cylindrical grinding machines has re-
control, and new developments in FMS cells quired an extension of Chapter 5, which also
and the use of probes. This new edition deals deals with the principles of operation of
with these and also provides more information touch trigger probes.
on CNC grinding machines, turret punch • Chapter 7 has been revised to meet the more
presses and EDM spark erosion. general use of magnetic discs (floppy discs)
The chapter on computer-aided part program- as the storage medium; information on
ming has been extended and completely re- punched tapes is still included.
written, and the updating amendments through- • Chapter 10 has been revised to contain more
out the book include eighty-eight new illustra- detail on the use of touch trigger probes.
tions which it is hoped will be found interesting • The biggest revision is to Chapter 13, which
and informative. has been greatly extended to provide in-
The main changes to the book are the fol- formation on conversational programming
lowing: and computer-aided machining, with exam-
ples.
• Chapter 2 has been extended to provide
information on the newer CNC machines and The opportunity has also been taken to correct a
FMS cells. few errors in the first edition.
• The sections in Chapter 3 dealing with tool I hope that the book will continue to be found
holding and work holding have been greatly useful in this new edition, and I should wel-
broadened and revised. The use of rotary come advice on new topics that might usefully
spindle heads on turning centres, seen to be be included.
of great advantage, has added fuel to the
discussion on the respective merits of lathe
and milling machines. G. E. Thyer
xi
Preface to the first edition
This book looks at computer numerical control setting and operating information on CNC ma-
from the viewpoint of the machine tool user chine tools will probably be more interested in
rather than that of the computer operator or Chapters 7 to 12. The questions at the ends of
programmer. The evolution of numerical con- chapters can be used for self-checking: the
trol is the latest result of a continuous develop- answers are to be found in the text of the chapter
ment in component production since the first concerned. Three useful example programs are
machine tools were made. The application of given in Chapters 11 and 12.
computers provides an extremely useful The book should also be useful for anyone in a
additional tool that allows the production engi- training school involved with numerical con-
neer to use machine tools more economically for trol, and can be used in the retraining of techni-
a very wide range of work. It must be remem- cians and skilled craftsmen or women who are
bered that although the computer is itself very being introduced to NC machines. Similarly,
powerful, the material removal techniques, cut- production managers needing to update their
ting tools and machining operations used on NC knowledge with the requirements for the effi-
machines are essentially no different from those cient operation of CNC machines should find
used on manually ope.rated or fixed cycle (mass this a useful reference work.
production) machines. With the development of Chapters 3 to 6 explain to the machine user
machining and turning centres, the computer the 'why' and 'how' of the use of certain con-
has enabled multiple machining operations to structional and instrumentation features
be carried out simultaneously on small quanti- developed for numerical control; they are not
ties of components. intended to be sufficiently detailed for the
The book is based on notes for lectures given designer of machine systems. Various aspects of
to students ranging from sixth-form pupils to machining technology are introduced to remind
honours degree students. It is intended and readers of the necessary background knowledge,
hoped that students, at whatever level of study, but do not cover all that is required. It is
will be able to obtain from the book the informa- expected that readers will have studied or be
tion on computerized numerical control that concurrently studying manufacturing tech-
they need. Β TEC students at levels NIII and NIV, nology and undergoing practical training. In the
and equally those following degree courses in author's opinion, it is essential for optimum
universities or polytechnics, will find useful utilization of CNC machine tools that everyone
information in every chapter. Students on City & involved, particularly those engaged in part
Guilds courses requiring part programming, programming, should have a thorough under-
xiii
Computer numerical control of machine tools
standing of material removal techniques and their own discretion, based on experience, in
machining technology. When computer sys- devising operating techniques, and thus it has
tems with stored technology become more been necessary to use the words 'generally',
economically viable and widely available, it 'frequently', 'normally' etc' a number of times
will still be necessary for part programmers to in the book when explaining principles and
use their judgement, based upon practical techniques.
knowledge, in selecting methods of work The section on punched tape in Chapter 7 is
holding, speeds, feeds etc. for some com- important; although the use of punched paper
ponents. tape is decreasing, there are still control systems
There are a number of different systems of that use it. Moreover, from an educational
computer-aided programming and graphical viewpoint a punched tape is far more useful
numerical control, and the final chapter can than a floppy disc, as it is possible for students to
only be regarded as a guide to how they are used. decode a section of the tape.
It is essential that the manual for the particular There has been a tremendous increase in the
machine or system in use is studied to obtain the scope of CNC machine tools over a very short
exact requirements, particularly in part pro- time, and I hope that the book will provide
gramming. The only additional skill required in readers with a firm base on which they can build
using a computer is keyboard skill; the know- a fuller understanding of the subject. In such a
ledge of the various commands will be gained wide coverage it is possible that there may be
with practice, and in many cases prompts points of contention, and if so I will be pleased
appear on the screen. The development of to be notified of any. In addition I would
machines with numerical control capability has welcome advice on topics or chapters that
been the result of the work of different machine would be more helpful if they were further
manufacturers, and consequently there is no developed.
one absolute or definitive machine tool or con-
trol system. The different manufacturers use G. E. Thyer
xiv
Acknowledgements
I am indebted to many persons, firms and Mr A. Stevens of Mills Marketing Services Ltd
organizations for help, advice and photographs Mr K. Rook of Monarch DS&G Ltd
used in the book. Sincere thanks are extended to Mr T. Newman of N. C. Engineering Ltd
the following who have been of particular help Mr D. A. Goldsmith and Mr I. Robotham of
at various times. Pathtrace Engineering Systems Ltd
Mr R. Rushbrooke of Pratt Burnerd International
The Principal and Governors of the South East Ltd
London College for permission to write the Mr R. Barnes of Pullmax Ltd
book using my lecture notes Mr R. Jeffery of Sandvik Coromant UK Ltd
Mr C. Leach of Agie UK Ltd Mr M. Humphrey and Mr P. Williams of
Mr G. Davies of Anilam Electronics Corporation Renishaw Metrology Ltd
Mr B. Hancock of Bridgeport Machines Ltd Stanmatic Precision Ltd, agents for Magnescales
Mr R. Ansell of Cincinnati Milacron UK Mr Rose of L. S. Starrett Ltd
Mr P. Waller of Crawford Collets Ltd Mr R. Eacott of Sumiden Hardmetal UK Ltd
Mr D. Collins of DBC Machine Tools Ltd Trimos-Sylvac Metrology Ltd
Mr L. Mustarde of Denford Machine Tools Ltd Mr M. Mathews of Unimatic Engineers Ltd
Mr Banner of Dicksons (Engineering) Ltd
Mr D. P. Clark of Euchner UK Ltd
Mr D. Averill of Fanuc Europe, UK branch
Mr K. G. Pritchard of W. Frost Engineers I would also like to thank a number of my
(Coleshill) Ltd colleagues, particularly:
Mr D. Stockton of George Kuikka Ltd
Mr W. D. Hogben of Graticules Ltd
Dr J. Liverton of Jones and Shipman pic Mr Tom Tebbutt of the Polytechnic of Central
Mr H. Behrens and Mr N. Prescott of Heidenhain London and Dr Bera of the Polytechnic of the
(GB) Ltd South Bank for reading the first draft of the
Mr R. J. Lacey of Hightech Components Ltd book and making helpful suggestions.
Mr M. Crabtree of J. C. Holt Ltd Mr Stan Brion of SELTEC for the help and
Mr R. Ricketts of Marposs Ltd comments related to the electrical section in
Mr M. Powell of Matchmaker Machines Ltd Chapter 3 and, finally, I thank my son Antony
Mr M. Roberts of Mercury Idéographies for for the comments he made on Appendix C on
photographs of computer systems computers.
xv
Chapter One
Principles of machine tools
1.1 Material cutting techniques
&
The first cutting tool used by mankind is likely
to have been a piece of broken bone or splintered
flint. The deficiencies of these materials as
cutting tools started a search to find better tool w
o1 T
materials and more efficient cutting methods.
AlH °
This search is still going on: a new technique to
cut materials is developed when the existing
methods are not efficient or not quick enough.
The first and simplest cutting action would
have been the splitting or dividing of materials
using a chopping action with a thin wedge- or
knife-shaped tool, as shown in Figure 1.1a. With
this type of action and tool it is difficult to
produce components to precise sizes or accurate (a)
Tool /
shapes. The need arose for a cutting action
È I*- Controlled
where control could be maintained over the / / / movement
amount of material being removed. The shaving
action shown in Figure 1.1b was developed,
which makes possible the removal of thin sec-
tions of the material from selective areas requir-
ing change of shape or size. With the shaving
action the depth of cut can be accurately con-
trolled.
L
The accuracy of the work produced by the
/ ^ ~ \ ?/ / ^ ^^ Controlled
cutting technique is entirely dependent on the / ~r -ΓΤ T~ depth of cut
relative movements of the work and the tool. It
required the development of machines where
(b)
the positional relationship and movements of
the work and the tool could be controlled Figure 1.1(a) Dividing cutting action; (b) shaving
accurately for components to be produced to the cutting action
1
Computer numerical control of machine tools
desired size and shape. The invention of the enhanced mechanical and physical characteris-
bow drill over six thousand years ago and a form tics have created problems in the machining of
of lathe some three to four thousand years later these materials using the known machine tools,
are evidence of the start of the development of and alternative cutting techniques have had to
machines for cutting material. The cutting tools be developed. This progress has continued and
used on these machines were made of copper or has resulted in the development of many
more probably bronze. different types of machine tools.
It was not until about 1772 when Wilkinson
developed his boring machine that machine
1.2 Material removal techniques on
tools appeared which were capable of machin-
machine tools
ing relatively large workpieces. Wilkinson's
machine was powered by a water wheel and was Machine tools now use a variety of cutting tools
used to bore the cylinders of the first steam manufactured from different materials, and
engines. The cutting tools used were made of have a range of material removal techniques
high-carbon steel suitably heat treated. In about such as:
1792 Maudsley developed a screw cutting lathe
(a) Cutting with a single-point metal tool
with a slide rest, which was more efficient than
(b) Cutting with multipoint metal tools
any previous machine. Throughout the Indus-
(c) Cutting with abrasives
trial Revolution there was a great increase in the
(d) Electrochemical machining
development of different types of machine tools.
(e) Electric-discharge machining
From the middle to the end of the nineteenth
(f) Ultrasonic impact machining
century came a whole range of new types of
(g) Laser, plasma and electron beam machin-
machine tools such as capstan lathes and turret
ing.
lathes and cam-controlled machines with mech-
anical systems for automatic control. These Each of these techniques has particular applica-
machines were the first machines capable of the tions for which it is the most suitable. The use of
high rates of production required for the mass numerical control increases the efficiency of the
production of components. functioning of most of the machine tools using
The machine operating systems that have these methods. The material removal tech-
now been developed produce parts more niques (d) to (g) can only be carried out on
cheaply and thus generally more quickly than machine tools with special features or facilities.
ever before. This has been achieved with ma- The actual cutting process is exactly the same
chine operating systems that have become more on a machine tool that uses numerical control as
automatic in operation. The machine tools have on machine tools that use a different control
control systems of varying complexity using system but the same material removal tech-
mechancial, electromechanical, electrical or nique. This is because numerical control is not a
fluid power. Numerical control is the most machine tool, but is a technique of controlling
recently evolved control system. On a numeri- the operation of the machine. To improve the
cally controlled machine tool, information on efficiency of operation of a machine, informa-
the shape of the component and on operating tion obtained from the actual cutting process
parameters such as the feed rate and the tools to may be fed into the control unit where it can be
be used is input to the machine in number form used to control the rate of movement and other
- hence the term 'numerical'. factors. However, for some machines the actual
Technological progress in many branches of cutting process may not be directly linked with
engineering has resulted in the introduction of the numerical control system.
materials specially developed to withstand in- Numerical control ensures that the amount,
creased functional conditions such as high direction and rate of movement of the tables on
operating temperatures and high stresses. These which the tool or work is mounted are accurate,
2
Principles of machine tools
repeatable and at the desired rate. In addition, It will be found that the time required to
numerical control is used for producing parts position a tool manually is inversely propor-
more quickly by controlling other machine tional to the degree of accuracy required. The
operating features, such as: more accurate the work has to be, the longer is
the time required to position the tool. It is in the
(a) Selecting the cutting tool
speed of controlling, repeating and maintaining
(b) Turning the cutting fluid on and off
the work/tool positional relationship that
(c) Selecting and controlling the spindle speed.
numerical control has the greatest influence and
It is extremely important that machine tools can be used to the greatest advantage.
should not be considerated merely as chip- or
1.5 Kinematic principles of operation of
swarf-making machines. The criterion is the
machine tools
number of components produced, not the
weight of chips resulting from the process. The geometric form of the workpiece is depen-
Numerical control can be used for a wide dent on the relative movement of the work
range of other applications which require con- and/or tool during machining. There are three
trol of the operation of equipment, such as: different kinematic principles which are used
by machine tools to produce the required geo-
(a) Press tool work on turret presses
metric forms: generation, copying and forming.
(b) Flame cutting on oxyacetylene machines
(c) Control of plotters
Generation
(d) Measurement and inspection equipment
Under this principle, the work shape is pro-
(e) Control of robots.
duced as a result of the combined movements of
the tool and/or work. The movements required
1.3 Function of machine tools are those which produce the parameters of the
workpiece, which are generally circles of revo-
The function of all machine tools regardless of
lution and straight lines. Practically all com-
the technique being used to remove material is
ponents are composed of combinations of these
to produce components of specified dimen-
two geometric shapes. It is convenient that only
sional size, geometric form and desired surface
these shapes are used because, except when
texture which are not economically obtainable
numerical techniques are used, it is difficult to
by other processes.
control accurately the simultaneous movement
of the tool or work in more than two directions.
The movements are controlled by the stan-
1.4 Size and form
dard constructional features of the machine.
The similarity of size of components produced Chapter 3 gives details of movement actuating
is dependent on the repeatability and constancy mechanisms and slideways.
of the work/tool positional relationship during
the final cut. The accuracy or precision to which Generation of cylinders
components are made is mainly dependent on A cylinder is composed of a circle and a straight
the minimum depth of cut efficiently obtain- line. There are four techniques of producing
able: if it is possible to remove a layer of material cylindrical forms, as shown in Figures 1.2a-d.
0.001 mm thick, then corrections of that value In Figure 1.2a the tool rotates and moves
can be made to the size of the workpiece. In axially. This technique is suitable when the tool
order to produce cylinders on a lathe to a is small relative to the work; drilling is the most
diameter tolerance of 0.01 mm it is necessary to common example. Vertical boring machines
reposition the tool and maintain its position also use this technique. The fact that the drill
relative to the axis of rotation of the work within and boring tool produce internal cylinders does
0.005 mm for the duration of the operation. not change the principle involved.
3
Computer numerical control of machine tools
^ ^ ^^
Constant relationship
Varying relationship
Figure 1.3 Generation of forms requiring
(c) ^ ^ ^^ synchronized movements
Figure 1.2 Methods of generating cylinders
movements changes for different parts of the
In Figure 1.2b the work rotates and moves curve. At the start of the curve in a given time the
axially. This technique is suitable when the axial movement has to be much greater than the
work is small relative to the tool; cylindrical radial movement, but at the end of the curve in
grinding is an example of this technique. Sliding the same time the radial movement has to be
head automatics also use this technique for considerably faster than the axial movement.
small-diameter work. With the development of numerical control it is
In Figure 1.2c the work rotates and the tool relatively easy to control the actual relationship
moves axially. Turning on lathes is an example of the tool and work movements, with the result
of this technique. that designers are able to specify complex
In Figure 1.2d the tool rotates and the work shapes that can be machined comparatively
moves axially. This technique is suitable when easily.
the cylindrical form is required on a workpiece
which would be difficult to rotate, such as the Copying
bearing housings on a lathe headstock. Horizon- Under this principle, the required work shapes
tal boring machines can use this technique. are again produced as the result of the
In these four methods it is comparatively easy movements of the tool and/or work. However,
to control the rotational movements using spin- the movements are now dependent on the
dles rotating in bearings. The translational passage of a stylus or tracer over a pattern which
straight line movements are achieved using is the shape of the work.
carriages guided by slideways. The roundness of Copying has been very effectively used for
the forms produced will be dependent on the work which has a complex profile consisting of
precision of the rotation of the spindles in the many changes in form or non-circular curves,
bearings. The parallelism of the cylinders will such as multidiameter cylinders or cavities for
be dependent on the accuracy of the alignment moulds and dies. It is now more economical to
of the slideways with the axis of rotation of the use numerical control for this type of work. The
main spindle. shape of the work produced is dependent on the
movements, which are controlled by the
numerical information input to the control unit
To generate forms other than parallel cylinders of the machine. There is no need to produce
it is necessary to synchronize axial and trans- templates or patterns: when a firm has copying
verse movements, as shown in Figure 1.3. To machine tools, numerically controlled ma-
produce the taper section of the component chines can produce the templates required more
there has to be a constant uniform relationship economically than by traditional methods.
between the axial and the radial movements of
the tool. To produce the curved section the Forming
relationship between the axial and radial With this principle, the shape of the tool is the
4