Production engineering : Jig and tool design

Production Engineering

Jig and Tool Design

. J. Η. JONES

M.B.E., M.I.P.E.

Revised by

H. C. TOWN

C.Eng., F.I.Mech.E., F.I.Prod.E., F.R.S.A.

LONDON

NEWNES-BUTTERWORTHS

T H E BUTTERWORT H GROU P

ENGLAN D

Butterworth & Co (Publishers) Ltd

London: 88 Kingsway, WC2B 6AB

AUSTRALIA

Butterworth & Co (Australia) Ltd

Sydney: 586 Pacific Highway Chatswood, NSW 2067

Melbourne: 343 Little Collins Street, 3000

Brisbane: 240 Queen Street, 4000

CANAD A

Butterworth & Co (Canada) Ltd

Toronto: 14 Curity Avenue, 374

NEW ZEALAND

Butterworth & Co (New Zealand) Ltd

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SOUTH AFRICA

Butterworth & Co (South Africa) (Pty) Ltd

Durban: 152-154 Gale Street

First published in 1940

by George Newnes Ltd

Second edition 1941

Third edition 1941

Fourth edition 1945

Fifth edition 1948

Second impression 1954

Sixth edition 1956

Seventh edition 1963

Second .impression 1964

Eighth edition published in 1972 by

Newnes-Butterworths, an imprint

of the Butterworth Group

© Butterworth & Co. (Publishers) Ltd, 1972

ISBN 0 408 00078 3 Standard

0 408 00079 1 Limp

Filmset by V. Siviter Smith ά Co Ltd, Birmingham

Printed in England by Hazell, Watson & Viney Ltd,

Aylesbury, Bucks

Foreword

When this book was first published in 1940 it was recommended by the

institution of Production Engineers as being of outstanding merit. The

author, Mr. E. J. H. Jones was recognised as being an eminent authority on

the subject of engineering manufacture, this being based and dependent

upon a knowledge of cutting tools, jigs and fixtures.

The reception of the book by the engineering industry and technical

colleges was such that, from the first publication to the present day, seven

editions were produced and some new chapters were added. Nevertheless,

it was realised that, valuable as most of the material still is, for basic principles

change but little, engineering development has proceeded so rapidly that

both designer and manufacturer are faced with problems unknown a few

years ago.

These problems relate to the introduction of new manufacturing processes,

the use of high grade materials for machine construction, and the develop￾ments in cutting tool materials. Of outstanding importance is the possibility

of machine or tool control by compressed air or hydraulic operation to obtain

an increase in productivity with reduced complication.

Thus it was considered that the time had arrived for a major revision of the

book to be undertaken, and I was privileged to be asked to undertake the

work. More than half the book has been replaced to bring the work up to

date, and it is hoped that in the future the book in its new form will prove

as valuable to the engineering industry and educational establishments as

it did at its inception by Mr. Jones.

H. C. TOWN

Preface

This work is intended not only for the experienced jig and tool designer but

also for the student of production engineering and the technical college

lecturer. Those readers already skilled in the science of jig and tool design

will, it is hoped, find much of real value in many of the chapters. The examples

given have been tried out and used successfully on production programmes

and can be relied upon as sound practice in relation to their respective

problems.

There is in every jig, fixture, or tool layout certain essential elements upon

which success or failure depends, and the designer competent to be trusted

with important work is one who understands what the purpose is, and has a

thorough knowledge of the functions they must perform. The designer

today has the advantage of several alternative power systems, so to

mechanical operations descriptions have been added of the modern applica￾tions of pneumatic, hydraulic, and electrical actuation.

The subject of cutting tool materials has been well covered and prominence

given to the science of surface technology and the effects on the economics

of tooling, comparisons being made with multi-tooling operations and tracer

controlled copying systems. To this has been added a section on the

economics of jig and fixture practice. Recent research on surface texture has

focused attention on fine finishing operations, so a comprehensive chapter on

diamond tools has been introduced to give the necessary information on

boring and turning operations.

Much new information has been added to the chapter on inspection and

gauging indicating the use of comparators and measuring machines, for the

increased accuracy now required on many components shows the need for

high precision which is not attainable by the traditional types of limit gauges.

This feature applies on the machine tool itself, and examples are given of

the new features of preset tooling.

The chapter on air or oil operated fixtures contains new examples from

actual practice, some of the pneumatic examples being applicable to holding

small units where the machining time is in seconds, and the rapid insertion

and removal of work is essential. At the other extreme, material on hydraulic

operation shows the advantages of oil clamping on large components, and

what is rarely appreciated, the use of accumulators to simplify the system.

Methods of truing grinding wheels has been extended to include surface

grinding, and means for generating spherical surfaces have also been

described. Much new information is given on boring operations and diamond

compared with carbide tools. Examples are given to show the means to

eliminate vibration by corrective design. Also included for the first time is the

operation of honing with information on the new process of diamond honing.

As a contrast to the economic advantages of large scale production, the

problem of small batch manufacture is discussed in a new section on Group

Technology and the cell system of workshop layout of machines in the plant.

H. C. TOWN

1

Function and Organisation of the

Jig and Tool Department

It is not intended to explain all the functions of the departments relative to

engineering organisation except in so far as the jig and tool department

operates in collaboration. Such reference is, however, briefly necessary in

order that the position occupied by the department responsible for jigs and

tools is appreciated. The extent of the organisation necessary will vary in

proportion to the size of the works in which it is installed. In a very small

undertaking it is possible to visualise one man performing all the duties of the

tool department. The following, however, is a brief survey of the organisation

generally adopted.

When the management of a concern decides which type of mechanism

or assembly is to be manufactured, the decision, if not made in conjunction

with the chief engineer, is conveyed to him. It then becomes his responsibility

to provide the designs and carry out what experimental work may be neces￾sary. His arrangement drawings are then handed over to the chief draughts￾man, who distributes certain units among his staff, whose duty it is to make

detailed drawings of each individual piece, on which should be all the infor￾mation required by the factory to produce the piece, including the whole of

the dimensions, particulars of material and heat treatment, also including

the limits to which certain parts are to be made and the finish required.

Surface technology

It is difficult in practice to divorce surface finish from geometrical accuracy,

for most problems involving consideration of fine surfaces are also concerned

with problems of wear, i.e. with one surface moving on another. In such

cases the surface finish and geometrical accuracy are inseparable, for ex￾ample, it would be useless to make a cylinder bore perfectly smooth, if the

errors in roundness and parallelism made it impossible for the piston rings

to seal the bore. In general, it can be stated that the more accurate a tool does

its work, the better the surface finish.

1

2 FUNCTION AND ORGANISATION OF THE JIG AND TOOL DEPARTMENT

Numerical assessment

Most surfaces are irregular, and since it is undesirable to rate the surface on

the basis of the highest peaks and lowest valleys, some method of averaging

becomes necessary. The British standard of using the micro-inch as the unit

of measurement is now replaced by the micrometre, the centre line average

height (CLA) method being used for the assessment of surface texture. Thus a

figure of 100 micro-inches now becomes 2-5 micrometres, and the table gives

Figure 1.1 Chart showing surface finish values

7 SPLINES X Y < ^

Figure 1.2 Milling machine spindle with surface finish assessment

tied *

FUNCTION AND ORGANISATION OF THE JIG AND TOOL DEPARTMENT 3

Operation layout

The work of deciding upon the type and sequence of the operations on a

given component is the responsibility of the planning department whose

members must have an intimate knowledge of the machines and tools

available. Thus, given a drawing such as Figure 1.2, but fully dimensioned

with limits indicated in addition to the surface finish symbols shown, an

operation sheet can be prepared on the lines indicated in Table 1.1

Table 1.1

MILLING MACHINE SPINDLE 0-4% C EN 8 80 mm dia 400 mm long

Set-up Time Standard

Operation sequence time allowed time

( min ) ( min ) ( min )

1 Saw to length 30 4-3 2-2

2 Face ends and centre 45 7-5 3-7

3 Copy turn full length, using 'Kosta' driver 30 1 4 0 7 0

4 Grind spline section X to size 30 7-8 4 0

5 Rough grind bearing diameters Y 30 9 0 4-5

6 Grind flange 15 3-3 1-6

7 Hob 7 involute splines 120 2 2 0 1 10

8 Drill full length of spindle, deep hole drill 60 24-0 1 20

9 Copy bore front taper hole 30 6 0 3 0

10 Bore hole in end for draw bolt, and chamfer 45 7-0 3-5

11 Mill slot in end of flange 90 8-5 4-3

12 Drill and tap holes in flange 45 2 6 0 1 60

13 Induction harden taper bore and front face 60 14-4 7-2

14 Finish grind taper bore 30 25-0 12-5

15 Using taper plug, finish grind bearing diameters Y 60 12-8 6-4

16 Grind end face and flange diameter 15 4 0 2 0

17 Thread roll diameters Ζ 30 8-0 4 0

The sheet may also indicate which machines must be used for each opera￾tion, and also what fixtures, tools, or gauges are required, so that work can be

scheduled and any particular machine's committment can be determined for

a given period of time. The production engineer can thus ascertain whether

plant will be available.

In the heat treatment of components it is advantageous to use induction

hardening as against carburising and the necessity of protecting parts to be

drilled. In the component shown the induction hardening process causes no

a representative selection of degrees of surface finish obtainable by com￾mercial equipment (see Figure 1.1).

There are new surface roughness symbols for use on drawings, and Figure

1.2 shows a milling machine spindle with the type of symbols to be used, the

symbol including a number indicating the number of micrometres. The

number indicates the CLA required, and for normal machining, say drilling

or turning to be followed by grinding, the symbol itself is sufficient to indicate

this, the number being restricted to diameters or faces where special accuracy

is required.

4 FUNCTION AND ORGANISATION OF THE JIG AND TOOL DEPARTMEN1

difficulties with the drilled holes, while the operations of tracer-controlled

copying and thread rolling are effective in reducing the operation time.

The economics of tooling

The amount of money spent on tool equipment depends on the number of

parts required, or the possibilities of repeat orders. Considering the pump

plunger shown in Figure 1.3a, this shows the tool layout to produce the

plunger in small quantities on a standard lathe. Eleven operations are re￾quired for completion, necessitating the use of three tools in the compound

rest and four in the tailstock spindle. The various parts of the plunger re￾quiring machining are numbered with the same figures as the tools perform￾ing the operations, these being in the following sequence, (a) Turn diameter 7

full length, (b) Turn diameter 4. (c) Square out 5, 6, and face end 8. (d) Cut

shoulders 1, 2, 3. (e) Centre and recess end of bore from tailstock. (f) Drill

main bore 10. (g) Drill small bore 11 using extension socket, (h) Ream main

bore, (j) Cut off to length using tool 8.

Using the same tools, but now on a capstan lathe, the set-up is that shown

in diagram (b), use being made of the square and hexagon turrets. The main

feature is the saving in time by every tool being in a permanent position as

against the re-setting required in case (a). In addition, stops are set to limit

the tool traverses, so that depth measurement is not required.

If the plunger is required in large quantities, a more elaborate set-up is

used as shown in diagram (c). The main difference from (b) is that tool 7 is

taken from the square turret and used in conjunction with the drill 10, so that

turning and drilling proceed together. A comparison of the three methods

shows :

Case (a). Machining time, including trial cuts, moving tools and tailstock,

60 min per piece or 600 min for 10 components.

Case (b). Changing tools 15 min, adjusting tools to size 17 min, setting

stops 13 min. Total 45 min. Machining time 25-J- min χ 10 pieces =

255 min. Full total time 300 min.

Case (c). This set-up is for a total of 40 pieces, the machining time being

19 χ 40 = 760 min. Adding 180 min for setting-up gives (760 + 180) ^

40 = 23^ min each. Thus the respective times per piece are 60, 30, and

23^ min.

It is obvious there is much to be gained by special tooling for large batches,

but for a small number of parts, savings may be reduced by the setting-up

time. A simple formula for checking is one in which χ is the number of pieces

on which production times of centre and turret lathes are equal. Thus:

Time for centre lathe χ χ = turret set-up time + machining time χ χ

(Case b) 60x = 45 + 30.x, χ = 1-5

(Casec) 60x = 180 + 19x, χ = 4-4

Automatic lathes

The question as to when to introduce automatics instead of turret lathes is

only partly affected by the number of parts required. The time per piece will

be less over a large batch, say 1 000, on an automatic than if produced on a

FUNCTION AND ORGANISATION OF THE JIG AND TOOL DEPARTMENT

I

(a) 1,2,3 4, 7 8

1,2,3 (b)

] Ό 0 = d

(c)

Figure 1.3 Diagrams showing the economics of tooling

capstan or turret, but the cost of machine setters must be taken into con￾sideration and the number of machines one setter can keep in operation may

influence the final cost. The initial cost of an automatic is greater than centre

and turret lathes, and in the matter of production of multi-diameter shafts

a multi-tool lathe with a front and back slide may provide the most econom￾ical proposition.

i \ 11,10,4,5,6^ 7.8, 9

5

6 FUNCTION AND ORGANISATION OF THE JIG AND TOOL DEPARTMENT

Tracer controlled copy turning

Previous comments on the economies of tooling have been in relation to

multiple tool operation, but a complete departure from this system is by the

use of a single tool to produce complicated shapes in either turning or boring

operations. Copy turning or boring is being employed on an increasing scale,

so that it is not too much to claim that the process must rank as one of the

greatest advances in the history of cutting metals.

The main advantage is the simplicity of machining with a single-point tool,

and producing contours which can normally only be obtained by elaborate

form tools or multiple tool set-ups on an expensive and complicated machine.

One minor limitation, however, is the angular presentation of the tool which

introduces difficulties when, say, machining both sides of a flange, or

producing square shoulders on a shaft with decreasing diameters. This

difficulty is easily overcome by a second setting, or, because copy turning is

generally performed from a rear tool, by using a tool or tools in the front rest.

Angular tool presentation

Figure 1.4 shows that with the copy slide set at an angle of 30° to the vertical,

and with the traverse operating in the direction indicated, shoulders up to 90°

Figure 1.4 Angular tool presentation of copy turning

can be produced, but falling shoulders are limited to an angle of 30°. This

angular setting is more advantageous than with a slide set at right angles to

the work axis, for the turning of shoulders is then limited to 60° in either

direction. Thus it necessitates disengaging the longitudinal feed in order to

produce a square shoulder, but if set at 30° the relationship of the two

movements is

movement of ram _ 2

movement of saddle Τ

thus if the ram retracts twice as fast as the saddle traverse a square shoulder

will be produced.

The effect of the angle of entry can be seen from: Let Vt

= speed of

longitudinal feed, Vf

= speed of transverse feed, and Vc = speed of cutting

tool slide, then if a = 30°,

FUNCTION AND ORGANISATION OF THE JIG AND TOOL DEPARTMENT 7

In lower half of Figure 1.4 :

V V

1

sin a 0-5

1

In upper half of Figure 1.4 :

V

t = = 7Γ&7 =

173 vi* and Vc

= = ^ = 2 Vx

. tan a 0-577

c sin a 0-5

1

Examples of copy turning

Figure 1.5 shows a test piece to indicate some of the contours that can be

produced on bar material using a cylindrical template and with the workpiece

D C Β A

Figure 1.5 Test piece demonstrating possibilities of copying

mounted between centres using a 'Kosta' driver and a pressure gauge on the

tailstock centre. The sections A, B, and C are of 10, 15 and 20° respectively,

followed by a fine pitch broach section, this leading to falling and rising

tapers of 30°. Thence by a parallel portion to a Morse taper section D. The

spindle speed used for the operation on a 'Harrison' lathe is 2000 rev/min

giving a cutting speed of 110 m/min. Copy turning is proving its value in

machining some of the newer materials, and Figure 1.6 shows how an in￾tricate section of an alumina ceramic cone with a wall thickness of only

Figure 1.6 Ceramic component copy turned and bored

0-8 mm can be produced by both copy turning and boring. Material removal

of 6 mm on each face is required, and while the external profile is not difficult

to produce, the internal machining to leave an even wall thickness requires

a copying system of high accuracy and a machine free from vibration.

A rigid boring bar is required with the end cut away so as to produce a

rounded end in a very limited space. Again the cutting speed is 2 000 rev/min

with a feed rate of 0075 mm/rev, giving a floor to floor time of 6 min.

8 FUNCTION AND ORGANISATION OF THE JIG AND TOOL DEPARTMENT

Economy of copy turning

There are two aspects of the problem. (1) The number of parts required, and

(2) whether the batch will be re-occurring at intervals. Considering the spindle,

Figure 1.7 in which the small end requires bevels and recesses for thread

rolling. There is considerable metal to be removed and in comparison with

\ 70 • X V//////V////A/ -/////Δ \* 280 Η

Figure 1.7 Spindle used in output tests

producing the work on a centre lathe, the graph, Figure 1.8, from the inter￾secting point X between curves (a) and (b) shows that even after only three

components, the advantage of copy turning begins to be indicated while the

/

/

/

/

</ /

/J /

if A y

1 >

/ /

κ

-fx

//

15

PARTS

Figure 1.8 Chart showing production results

rapid divergence of the curves show the similar increase in production of the

non-recurring batch. In all cases the curves (b) and (c) follow parallel paths

after the setting-up portion, and are impressive enough to indicate the advan￾tages of copy turning on parts of no great complexity, and are even more

pronounced on components with difficult angular contours or curves.

It is conceded that on simple shafts, the short traverse of a set-up of

multiple tools may seem advantageous, but dimensional errors can develop

in inaccurate tool setting, or uneven wear amongst the various tools. A

further factor is the work deflection caused by the cutting pressure and this

may necessitate the fitting of steadies and thus increase the setting-up time.

For these reasons alone, a simple copy lathe may be preferable on first costs

THE JIG AND TOOL DESIGNER VERSUS THE ENGINEERING DESIGNER 9

alone, and may well score on the time taken for machining, for the initial

tool setting and subsequent grinding and re-setting of several tools is

expensive when compared with a single tool required for copy machining.

T HE JIG AN D TOOL DESIGNER VERSUS TH E

ENGINEERIN G DESIGNER

It may be thought that with the passing of time, design errors would tend to

diminish, but many designers have little experience in production of parts,

and while serious errors may not be frequent, it is often possible to improve

the design of a component and thereby cheapen its manufacture. The details,

Figure 1.9 Design faults and re-design of components

Figure 1.9, give some indications from actual practice of faults and their

corrections which have aided production.

Diagram (a) shows the threaded end of a casting. The original drawing

showed the thread touching the face. This operation would require special

extended dies, and should not be considered. A recess should be provided

at X, and the shank end bevelled to assist starting the cut. In setting out the

tool layout, say, on a turret lathe, extra tools must always be provided for

such apparently minor operations as well as for bevelling sharp corners.

These operations should never be left for the operator to use hand tools, or

be expected to be done in the fitting shop.

Diagram (b) shows the axle pin. For retaining the pin in its bearing, a

circlip is fitted in one end. If this is duplicated at the other end, the pin is of

one diameter and can be made from bright steel without any machining

apart from the grooves, (c) is the pneumatic cylinder. There are three

difficulties indicated on the left hand view at X. One is the square corner, and

10 FUNCTION AND ORGANISATION OF THE JIG AND TOOL DEPARTMENT

ECONOMICS O F JIG AN D FIXTURE PRACTICE

A primary function of jigs and fixtures is that of reducing cost by the elimina￾tion of hand methods of location or marking out. Also of cardinal importance

is the assurance of interchangeability of the machined parts, and the fact that

a jig or fixture will generally enable high-grade work to be performed by un￾skilled labour. When planning a machining operation, consideration should

be given to the cost of machining the work with or without the jig. No hard

and fast rules can be laid down, because the greater accuracy obtained by the

use of the jig alone may be sufficient to warrant its use, but an approximation

can be obtained from the following:

= cost of machining without special equipment.

S = cost of machining with special equipment.

another the flat face at the bottom of the bore. The third is the angular hole,

almost impossible to drill. The solution is shown at the right hand, the corner

being radiused, and a dimple cast in the bottom to avoid facing to the centre,

and a notch cast on one of the flat faces, to facilitate starting the drilled hole.

Diagram (d) shows the detail of a large lathe spindle. Originally made in

one piece from a forging, the operation of drilling the bore from the solid

took many hours. It was found possible to buy the main length as a tube and

simply weld the flange on one end ready for the external machining. This

feature brings the warning never to make an operation list or estimate a

machining time from a drawing of a component to be supplied as a forging.

In the diagram the flange would appear to have two bosses suitable for grip￾ping in a chuck. As supplied, however, the outline is that shown in chain lines

where only one boss exists, and that with a taper edge. Therefore, insist in

seeing the actual forging if at all possible.

Diagram (e) shows the support for a welded gear box with the distance X

required to be fairly accurate. As first made with the box and support integral,

some difficulties arose in handling the box for milling the base and drilling

the holes. Building the support up from standard tubes and plate solved the

difficulty and allowed adjustment for the height X. Diagram ( 0 shows the

column of a boring and turning mill with two bosses to carry a shaft for an

elevating motion. Handling a large casting and boring the bosses 2 m apart

proved a difficult task. Obviously, in such cases the bearings should be

loose brackets so that facings can be machined on the same machine as the

slideways and base. Aligning the shaft and brackets is thereby much simpli￾fied, (g) shows a bracket to be machined on the spigot. The bracket was im￾possible to hold in a normal chuck, but by the addition of a small boss, shown

in chain lines, the operation can be performed on a centre lathe forming its

own dragger. (h) shows how a tee-slotted machine table with a coolant

trough was re-designed for production. In the left hand view machining is

difficult, but as shown on the right, a clear run-out for the cutting tools is

feasible, (j) shows a deep bore terminating in a small hole. This required the

use of a long small drill soon broken. The solution is to bore the large hole

straight through and fit a drilled plug for the small hole as shown by the

chain lines.