Production engineering  jig and tool design

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 com

T H E B U T T E R W O R T H G R O U P

E N G L A N D

Butterworth & Co (Publishers) Ltd

London: 88 Kingsway, WC2B 6AB

A U S T R A L I A

Butterworth & Co (Australia) Ltd

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C A N A D A

Butterworth & Co (Canada) Ltd

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N E W Z E A L A N D

Butterworth & Co (New Zealand) Ltd

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SOUTH AFRICA

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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 T O W N

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 T O W N

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 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

neces-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

Figure 1.2 Milling machine spindle with surface finish assessment

t i e d *

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 M A C H I N E S P I N D L E 0-4% C E N 8 80 mm dia χ 400 mm long

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

7 Hob 7 involute splines 120 2 2 0 1 1 0

8 Drill full length of spindle, deep hole drill 60 24-0 1 2 0

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 6 0

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 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

opera-a given period of time The production engineer copera-an thus opera-ascertopera-ain 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 mercial equipment (see Figure 1.1)

com-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 T h u s :

Time for centre lathe χ χ = turret set-up time + machining time χ χ (Case b) 60x = 45 + 30.x, χ = 1-5

FUNCTION AND ORGANISATION OF THE JIG AND TOOL DEPARTMENT

Figure 1.3 Diagrams showing the economics of tooling

capstan or turret, but the cost of machine setters must be taken into 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

con-a multi-tool lcon-athe with con-a front con-and bcon-ack slide mcon-ay provide the most ical proposition

econom-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 A N D TOOL DEPARTMENT 7

In lower half of Figure 1.4 :

V V

In upper half of Figure 1.4 :

V t = = 7Γ&7 =t a n a 0-577 c1 73 v i* a nd V c = = ^ = 2 V sin a 0-5 1 x

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 m m can be produced by both copy turning and boring Material removal

of 6 m m 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 0 0 7 5 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

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 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

inter-/ /

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 H E J I G A N D T O O L D E S I G N E R V E R S U S T H E

E N G I N E E R I N G D E S I G N E R

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

E C O N O M I C S O F J I G A N D F I X T U R E P R A C T I C E

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

elimina-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 skilled labour When planning a machining operation, consideration should

un-be given to the cost of machining the work with or without the jig N o 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:

Ε = 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 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

grip-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

ECONOMICS OF JIG AND FIXTURE PRACTICE 11

C = cost of special equipment

With fixtures, depreciation is made up of two factors, deterioration and obsolescence As a rule these two do not bear equally In one case deteriora-tion through wear may be the chief factor, but more often obsolescence due

to change in design is responsible The factor which operates the faster should be used In dealing with fixtures, the economic problem centres on the answers to some of the following questions :

(1) How many pieces must be made to pay for a fixture of given estimated cost which will show a given estimated saving in direct labour cost per piece? For instance, how long a run will justify a fixture costing £200 which will save 4p on the direct labour cost of each piece?

(2) How much may a fixture cost which will show an estimated unit saving in direct labour cost on a given number of pieces? For instance, how much can be paid for a fixture to 'break even' on a run of 10000 pieces, if the fixture will save 4p on the direct labour cost of each piece? (3) How long will it take a proposed fixture, under given conditions, to pay for itself, carrying its fixed charges while so doing? For instance, how long will it take a fixture costing £200 to pay for itself if it saves 4p

on the direct labour cost per unit, production being at a given rate?

The questions above assume an even break, but there is also the practical question :

(4) What will be the profit earned by a fixture, of given cost, for an estimated unit saving in direct labour cost and given output? For instance, what will be the profit on a £200 fixture if it will save in direct labour cost 4p each on 10000 pieces?

The questions involve something more than simple arithmetic The credit items for the fixtures depend mainly on the number of pieces machined, but the debit items involve time and the number of set-ups required, i.e whether the pieces are run off continuously or in a number of runs An important time element is that many companies now require that any new equipment shall pay for itself within a certain period Investigations show wide vari-ations in the time required, ranging from one to five years, but the general practice seems to be about two years

Proposed equipment formulae

Let Ν = number of pieces manufactured per year

Debit factors

A = yearly percentage allowance for interest on the initial investment

12 FUNCTION AND ORGANISATION OF THE JIG AND TOOL DEPARTMENT

If the interest may be taken on the depreciating value, this becomes,

under uniform depreciation for η years, Α χ n + 1, the value of

which decreases from A, for one year, and approaches Aß as η

grows large For a life of two years this is 3,4/4, for three years it is 2,4/3 In the following formula either the original cost or the depreci-ating value can be used with equal facility It is suggested, however, that one or the other basis be used uniformly to facilitate com-parisons

Β = yearly percentage allowance for insurance and taxes

C = yearly percentage allowance for upkeep

Ε = yearly cost of power and supplies (When the equipment is new, this

item appears in full When it replaces old methods or equipment, the

difference only is used It is a debit if Ε on the new equipment exceeds that on the old, but a credit if the new Ε is less than the old Ε may

therefore be plus or minus.) If this item is small it may be disregarded / = estimated cost of the equipment or fixture, i.e cost installed and

ready to run, including drawing and tool room time, material and tool room overheads

IjH = yearly percentage allowance for depreciation and obsolescence on

the basis of uniform depreciation, where Η is the number of years

required for amortisation of investment out of earnings

Κ = unamortised value of the equipment displaced, less scrap value

(In the case of fixtures for new work, Κ drops out.)

Y = yearly cost of set-up This should include the time required for taking

down apparatus and putting the machine into normal condition In some plants with departments large enough to employ several tool-makers regularly, this time can be included in the departmental overhead, in which case the factor disappears as a separate item

Credit factors

S = yearly saving in direct cost of labour

= TV (old unit labour cost minus the new unit labour cost)

= N x (saving in unit labour cost)

= N s this covers direct unit labour cost only

s = saving in unit labour cost

Τ = yearly saving in labour burden

= S t , where t is the percented used on the labour saved

= N sV (The latest form of the materials-handling formulae breaks this

into T a = burden on labour saved and T b = burden on the equipment

displaced For use with fixtures the latter element may usually be disregarded for simplification.)

U = yearly saving or earning through increased production = saving in

unit cost χ increased yearly production capacity χ the percentage

of that increased capacity which will be utilised χ (1 + t) This cares

for the burden saved, plus cost of extra old equipment which would

be necessary to care for the increase if the improvement was not

adopted (In many cases U may drop out.)

V = yearly net operating profit, in excess of fixed charges

ECONOMICS OF JIG AND FIXTURE PRACTICE 13

Proposed formulae

Equations (1.6), (1.7), (1.8) and (1.9) reflect the essential conditions, and are

easily applied They take into account the number of pieces manufactured, the saving in unit labour cost, the overhead on the labour saved, the cost

and frequency of set-ups, interest on investment, taxes, insurance, upkeep,

and depreciation The equations (1.2), (1.3), (1.4) and (1.5) take into account,

in addition to the foregoing, the value of increased production capacity, cost of supplies and extra power, and interest on equipment displaced, if it is

deemed that conditions require their consideration These may be used for the more elaborate fixtures

In using the formulae, Ν is the number of pieces manufactured in a year,

not per run, except for the case of a single run of less than one year's duration

The items A, Β and C, once settled upon, need little change If a plant has the

practice of requiring new equipment to pay for itself in a definite time Η (say

For an even break the yearly operating savings = total fixed charges

(S+T+U-E)- (yearly cost of set-ups) = I (A + Β + C + IjH) + KA

Since S + Τ + N s + N st = N s (1 + t), then,

N t (l + 0 + U- Ε - Y= I (A + Β + C + IjH) + KA (1.1)

To find the number of pieces required for a given cost I, solving for N

I(A + B+C + Iim + Y- U+E+KA

To find the net operation profit {V) over all fixed charges ( = gross

operating profit, less set-ups and fixed charges)

V = N s (\ + 0 - Y-I(A + B+ C + IjH)+ U-E-KA (1.4)

To find the time Η in years for the fixture to pay for itself, the net profit V

in equation (1.4) = 0 Therefore setting the right hand of equation (1.4)

equal to 0 and solving for H,

N s (l + Þ - Y - I(A + Β + Q + U - Ε - KA (1.5)

In most cases it will be found that U, E, and KA may be neglected, so that

equations (1.2), (1.3), (1.4) and (1.5) may oe written:

14 FUNCTION AND ORGANISATION OF THE JIG AND TOOL DEPARTMENT

two years), the depreciation II H may be added to the other carrying charges, making a single percentage factor for the term {A + Β + C + IjH) which

can be used until the management deem that changed conditions require modification

2

All consumers want quality in the widest sense of the term, that is, fitness for purpose Good design and standards in themselves must lead towards good quality, but a lowering of quality may result from the human factor of care-less or slipshod work This brings in the need for inspection which costs money, and the problem of management is to provide an inspection which will ensure adequate quality and reliability without incurring the cost of an unnecessary elaborate system

Factory inspection department

In the organisation of inspection departments substantial independence of control is essential, while co-operation with other departments must be maintained If the chief inspector is subservient to an official whose pre-occupation is output, obvious difficulties may arise A sound arrangement

is to make the chief inspector responsible to the general manager, for being concerned with production, technical and commercial problems, he is likely to have an objective approach to any problems due to rejected work Inspection should be concerned with everything entering the factory from the raw material to the finished assembly To inspect a finished component only to find that the material was incorrect would obviously not be sound economics Inspection departments are mainly concerned with raw materials and components bought out, manufacturing operations, and tools and gauges The majority of the inspection staff are likely to be engaged in the sphere of manufacturing operations This is especially so in factories engaged on large-scale production, where batteries of automatic machines are used and where delays on the assembly line cannot be tolerated Hard and fast rules as to the type of inspection cannot be laid down owing to the diversity of manufacture in firms making high-grade and expensive com-plicated productions with those producing cheap and simple components

16 INSPECTION AND GAUGING

Accuracy of tools

The quality of work produced in a factory greatly depends on the accuracy and suitability of the tools, jigs and gauges used This is particularly the case where vast quantities of goods are being mass produced In these circumstances 'limit gauging' is widely applied and GO and NO GO gauges,

as for examples of the types shown in Figures 2.1 and 2.2, may be provided

Figure 2.1 Internal-limit plug gauge

for use by operators as well as inspectors in checking the dimensions of almost every component produced It is essential that the gauges are regularly inspected in service, so that when they become worn to such an extent as to

be no longer serviceable they can be scrapped A tool inspector is sometimes stationed in the tool stores for this purpose

The inspection equipment available to the tool inspector must be capable

of much greater precision than that used for normal inspection in the shop This is because the manufacturing tolerance on gauges must be only a small fraction, usually about 10%, of the tolerance provided on the com-ponent; the widening of gauge tolerances has the effect of reducing the working tolerance available to the operator Suppose a plug gauge is re-quired for gauging a hole on which a working tolerance of 0Ό125 mm is allowed, then the tolerance available to the gauge maker on each limit of size would be only 0Ό01 25 mm Obviously such instruments as hand micro-meters are not suitable for this purpose, and in modern tool-inspection and standards departments a wide range of precision instruments such as length standards, comparators and measuring machines are used Some of these are capable of detecting errors as small as a few micrometres

work-bad work can be detected before much of it has been produced and rejects can often be rectified in the machine shop before the set-up is broken down An exception here is that of work being turned from the bar in a lathe, unless the work is inspected before being parted-off

While an inspector must bear responsibility for the work which he approves, the production department and the machine-setter must be held partly responsible for good standards and low scrap percentages Inspectors and section leaders who are engaged on a line or patrol system, have a good opportunity to help the production departments by keeping foremen and operators advised on those particular dimensions which experience has shown to be especially important The importance of materials inspection increases with greater use of automatic machines of high output, because serious losses from faulty material can arise so quickly

INSPECTION AND GAUGING 17

Figure 2.2 (a) flat type, (b) reference plug, (c) limit reference gauge

Limits and fits

In order to allow for unavoidable imperfections in manufacture it is necessary

to establish 'tolerances' This is the amount of the difference from a required dimension laid down in order that unavoidable faults in workmanship can be tolerated The upper and lower limits of size are known as 'limits' A 'fit' is obtained by varying the dimension of hole and shaft so that the appropriate amount of clearance or interference is obtained This difference in dimensions between the hole and shaft is known as 'allowance', see Figure 2.3

Most existing systems of limits and fits are on the hole basis with unilateral tolerances The hole-base system is one in which holes are produced to a standard size, the clearance or interference required being obtained by vary-ing the size of the shaft to suit The reason for this preference for a hole basis rather than a shaft basis, is that most holes are produced to size by means of tools of fixed dimensions such as reamers, whereas the shaft can easily be varied in size when finished on a grinding machine

18 INSPECTION AN D GAUGING

In large-scale work, it is as easy to vary the size of a bore as it is to vary the size of a shaft In these circumstances a shaft-basis system may be preferred Generally speaking the hole basis is used, although in some systems pro-vision is made for the use of either (see Figures 2.4 and 2.5) A limit system is

- H HOLE LIMIT

SHAFT

LIMIT-L LIMIT-LIMIT - H (*HOLIMIT-LE TOLIMIT-L

SHAFT

Figure 2.3 Terms used with limits and fits

said to be unilateral when the lower limit of the hole is equal to the basic size of the hole (on the hole-basis system), while a limit system is said to

be bilateral when the limits for the basic member are disposed one above and one below the basic size for that number The unilateral type is nowadays preferred because it is slightly simpler

Fit diagrams

A diagram of fits will usually convey more than separate tolerance-zone diagrams for holes and shafts The examples shown in Figure 2.6 are (a) for clearance fits, (b) for interference fits, and (c) for transition fits It is notice-able in example (c) that the possible variations in conditions of fit are in fact wider than might be thought from a casual examination of the more con-ventional separate-tolerance diagrams While limit-gauging systems play an essential part in the development of quantity production, their limitations and inherent weaknesses are becoming increasingly apparent The use of the traditional types of limit gauge such as plug, ring and gap gauges often fails

to supply the needs of precision in design which is becoming increasingly necessary In practice some degree of skilled fitting to grade and refine the drawing fits is still required in high-quality manufacture

The errors due to variation in feel can be quite large, as for example, when gap gauges (Figure 2.7) are used for checking a shaft of large diameter, 'springing' of the gauge being almost inevitable to a certain extent Limit gauges, especially gap gauges, are in many cases being superseded by indica-ting gauges or comparators, while the feel of the operator is entirely elimin-ated for the measurement of fine tolerances Direct measuring, as distinct from gauging apparatus, is essential where statistical quality control is applied Despite this, limit gauges, together with direct-measuring equip-ment, will no doubt continue to be used owing to simplicity and speed of operation

Design of limit gauges

INSPECTION AND GAUGING 19

Just as tolerance is provided on a component to allow for unavoidable manufacturing errors, so must tolerance be placed on gauges It is essential that the tolerances be as small as possible, because they have the effect either

of reducing the tolerance available to the production operator or of allowing components to be accepted which are strictly outside the drawing limits These 'gaugemaking tolerances' are normally held to about 10% of the tolerance allowed on the component, and is covered by BS 969 The dis-position of gauge tolerances is important If it is desired that all components

Figure 2.5 Limit and fits with shaft basis

acceptable to a gauge are strictly to drawing specification, it is necessary to place the gauge tolerances immediately inside the component tolerance zone If on the other hand it is desired that the gauge must not reject work which is strictly correct to drawing specification then it becomes necessary

to dispose the gauge tolerances immediately outside the tolerance zone for the component

To avoid the necessity of two sets of gauges, workshop and inspection, the latest edition of the British Standard allows only one class of gauge tolerance, known as a General grade This is essentially a compromise between the workshop and inspection class of tolerance The GO gauge tolerance is placed

immediately inside the component tolerance—this tends to give a minute allowance for wear of the gauge—while the NO GO gauge-tolerance is placed

immediately outside the component tolerance zone, being indicated in Figure 2.6b

20 INSPECTION AND GAUGING

(a) (b) (c)

Figure 2.6 Diagram of fits

Wear allowances

To increase the working life of GO gauges, a small allowance for wear can

be made Such allowances tend to reduce the working tolerance for the component and this tends to increase the cost and reduce the speed of pro-duction It is sound practice to make such wear allowances only where economy on gauges, due to their longer life, more than compensates for the loss of working tolerance on the component This may be the case in mass production and particularly so where the component tolerance is fairly large BS 69 provides for a small wear allowance where the component tolerance exceeds 0 0 8 6 mm

Taylor's principle

This is an important principle relating to the design of limit gauges It states

that a GO gauge should be of the full-form type, measuring as many of the

maximum metal limits as it is convenient to gauge in one operation, while

separate NO GO gauges should be used to check each individual NO GO

dimension The maximum metal limit for a hole is the low limit and for a shaft it is the high limit To conform to this principle a hole should be gauged

as follows :

The GO gauge should be a cylindrical plug, theoretically of the same length

as the hole This ensures that no part of the hole is undersize The NO GO

gauge should be of bar form with more or less rounded ends This can be used in various positions to make sure that no part of the hole is oversize Conversely, a shaft should be checked for the following points

The GO gauge should be of full form (a cylindrical ring), while a gap or snap type of gauge should be used for the NO GO dimensions, which again can

be tried in various positions In practice, gauge design is usually a promise between theoretical principles on the one hand and practical

com-considerations on the other For example, NO GO gauges for small holes are

invariably made of cylindrical form Where such gauges are used there is a slight chance that incorrect oval holes might be accepted as correct by a

gauge having GO and NO GO ends of full form As there are practical

diffi-INSPECTION AND GAUGING 21

Principles of precision measurement

N o great mathematical ability is required for the solution of most measuring problems which confront the inspector, but some knowledge of trigonometry

is required In addition there are several scientific principles which he ought

to know, and some of these will be discussed The principle of alignment requires that the line or axis of measurement should coincide with the line of the scale or other dimensional reference In some measuring instruments the distance to be measured is traversed by a slide or other movable member, the displacement being determined by a micrometer screw Should the guideway

Figure 2.7 Adjustable external-limit gauge

culties in making and using small pin gauges, in say a hole of 6 m m diameter, then it is necessary to depart from the Taylor principle It must be emphasised that no degree of accuracy can be assured where components are checked using limit gauges which do not satisfy this principle

22 INSPECTION AND GAUGING

upon which the traversing member slides be slightly bent and the measuring

pointer of the instrument be displaced any considerable distance from the

axis of the scale, then measuring errors are introduced

Considering Figure 2.8 a formula can be derived from which the error in

measurement can be obtained Applying the theorem of intersecting chords

This assumes that the error in the guideway takes the form of a circular

arc In the formula h represents the maximum departure from straightness,

M the length of traverse and L the horizontal displacement between the

measuring and scale axes

Figure 2.8 Guideway showing alignment principle

Suppose the guideway of a measuring machine departs from true

straight-ness by 0 0 2 5 mm at the centre of a traverse of 250 mm What measuring error

would be introduced if the measuring and axis scale are horizontally displaced

An example of this sort occurs in the use of a vernier caliper where the

scale is displaced some distance from the ends of the measuring jaws

INSPECTION AND GAUGING 23

With measuring instruments it is sometimes required that two parts are located in relation to one another so that there is no play between them and

it is possible to bring them together again in exactly the same relative positions The constraints should just be sufficient in number to achieve that object The forces acting on the body are then equally definite and no difficulties will arise due to small alterations in position of the locating points should distortion take place

Figure 2.9 shows a method of location which satisfies the principle Constraint is required in all directions but that of sliding It is the design

^ ^ Π Ξ PLUGS

Figure 2.9 The principle of minimum constraint

used for locating the floating micrometer upon its base in the screw thread effective-diameter measuring machine to N P L design Figure 2.10 shows the error that can be introduced when a gauge with sharp-pointed ends is measured by a micrometer in a length comparator, if the gauge is slightly misaligned relative to the machine centres It will be seen that the length

Figure 2.10 Errors in gauge measurement

actually measured (X) is slightly shorter than the true gauge length (G) The difference between G and X depends upon the cosine of the small angle

δθ and for small angles the value of the cosine is very nearly unity Thus for

small angles cosine errors of this kind are negligible It is interesting to note that if flat-ended standards are used a sine error can be introduced, but

if the gauges have spherical ends of radius equal to half the length of the gauge then there will be no error of misalignment

Principle of minimum constraint

24 INSPECTION AND GAUGING

Slip gauges

These gauges,.shown in Figure 2.11, are supplied in sets which enable various combinations of sizes to be built-up with great accuracy A useful property of slip gauges is the 'wringing' effect obtained when such gauges are placed There is a British standard specification for slip gauges BS 888, in which three

Figure 2.11 Set of standard slip gauges (by courtesy of Coventry Gauge and Tool Co Ltd)

grades of accuracy are listed These are calibration, inspection and workshop grades The blocks can be used for direct measurement, e.g the width of a slot In conjunction with a pair of precision rollers they can provide a precise method of measuring the diameter of a hole (see Figure 2.12) with an accuracy

Figure 2.12 Blocks and rollers measuring bore

of determination as small as 0-01 mm, although experience is required before the right 'feel' is obtained The range of work that can be measured by using slip blocks and length bars is considerably extended by the use of slip and length gauge accessories which are available

mm with the dial graduated in 0 0 2 5 , 00075 or 0 0 0 2 5 mm A high tivity giving apparently high precision is not necessarily advantageous, as

sensi-high magnifications are usually obtained only by further gearing in the ment, this leading to an increase in the frictional force which has to be over-come when the instrument is used

instru-A useful application is to combine the indicator with a magnetic stand, applications of use can then include machine tool alignment testing, checking the concentricity of circular parts, and for height and depth comparisons

The basic function of a comparator is to indicate the small difference in size between the standard and the work by a highly magnified reading on a scale An inspection and tool room mechanical comparator may have a magnification of about 1 000 to 1, while a slip gauge comparator may be as high as 50000 to 1, but only small differences in length can be measured In general, the magnification should only be high enough for the work in hand, for excessive magnification means an unnecessary restricted range of measurement

The optical lever

The magnification obtainable from mechanical indicating systems is limited

by considerations of the elasticity of the members of the mechanism, friction

at the pivots and space occupied by the parts For instruments of high sensitivity an alternative method is to use an optical-lever system in which use is made of a beam of light reflected from a mirror, the reflected ray taking the place of the mechanical pointer Consider the theoretical diagram

Figure 2.13 Mechanism of dial indicator

26 INSPECTION AND GAUGING

of Figure 2.14 An incident ray of light falls on a mirror Α - A and is reflected

from the normal at an angle Θ When the mirror is tilted through a small angle δθ to take up the position B - B , the reflected ray moves through an

A Β

Figure 2.14 Theoretical diagram of measuring by optics

angle 2 δθ, so that the angle between the incident ray and the reflected ray

is then 2(0 + δθ) This doubling effect is made use of in the magnification

system of an optical lever N o w consider the system in Figure 2.15 In practice the mirror is tilted by a movable plunger which is attached to the

Figure 2.15 Principle of the optical lever system

stylus of the instrument at a distance d from the fixed point or fulcrum The movement of the plunger through a distance h results in the movement of a spot of light through a distance X on the screen If the distance between the movable point and the fixed point is d and the distance between the mirror

Figure 2.16 Principle ofSolex pneumatic comparator

accurate and speedy, and in another system air from the jets passes over two platinum-wire coils which form two of the arms of a Wheatstone bridge The air passing over the coils cools them to a temperature which varies with the velocity of the air flow As the resistance of the wire varies with tempera-ture, the amount by which the bridge circuit is out of balance will depend upon the relative velocities of the control and gauging jets Thus, the vari-ations in measurement can be indicated on a microammeter scale calibrated

in linear units

Electrical comparators

Electrical principles are used in comparators of high sensitivity, an example being shown in Figure 2.17 which depicts the Electro-limit head The measuring plunger, when raised, moves the iron armature which is supported

by a flexible spring-steel strip Movement of the armature changes the characteristics of the magnetic fields associated with the coils wound on the pole-pieces shown The coils are arranged to form two arms of an a.c bridge, the change in reactance in the coils leading to a current change in the bridge which is indicated on a microammeter calibrated to read in linear units

In the Sigma signal comparator components can be classified as correct, oversize or undersize according to which coloured light is illuminated, the colours showing amber, green, or red The basic principle relies upon the

jets on the measuring plug are so proportioned relative to the control jet that partial closure of these jets causes the pressure in the second chamber to vary accordingly This in turn varies the height of the water in the mano-meter tube, the height being read off on a calibrated scale placed at the side

of the tube The fit of the plug when placed in the bore of a component determines the air flow, which in turn determines the pressure drop across the jets

The sizes of jets and plugs are related to the size of the bore and the ance allowed on the component, so that the system is more applicable to quantity production than to general inspection work The method is very

toler-28 INSPECTION AND GAUGING

V

Figure 2.17 The Electro-limit head

arrangement of wiring so that green light operates only when the end of a lever is between two stops, the red and amber lights being shown when contact is made with right or left hand stops The stops can be adjusted to suit the tolerance imposed on the dimension under inspection

Measuring machines

These differ from comparators in that they carry their own standards of reference, usually in the form of a scale, and can measure any length within their range The Zeiss measuring machine has a range of 100 mm, the standard of reference being a glass scale, Figure 2.18 The main head attached

Ο M

Figure 2.18 Zeiss measuring machine

to the supporting column carries a microscope M for viewing the graduated scale, the scale itself being situated centrally within a steel cylinder, which runs vertically between ball-race guides in the head The optical indicating unit is shown at Ο and glass plugs at G There is a prism at Ρ and condensing lenses at L, while the scale is indicated at S Measuring is between the anvils

A The work table can be adjusted to a zero setting, after which the machine may be used for direct measurements up to 100 mm

COMPARATORS 29

Angular measurements

The protractor is the most commonly used instrument for measurement of angles, the better class of instrument incorporating a vernier scale by which angles can be determined within about 10 minutes of arc

The sine bar

Errors in a good quality sine bar correspond to about 5 seconds of arc in the measured angle Normally, a sensitive indicator is used to set one face of the gauge being checked in a truly horizontal position As the inaccuracies in the sine bar and slip blocks are very slight the accuracy of the actual measure-ment using a sine bar is mainly dependent on the accuracy of the level setting

As shown in Figure 2.19 as the accuracy of a sine bar tends to fall off as the

Figure 2.19 Method of setting sine bar

angle is increased beyond 45°, it is best to avoid setting the bar to angles greater than this It is usually a simple matter to set-up the sine bar against a surface which is square to the surface plate, in which case the bar is set to the complement of the angle actually being measured

Rollers and slip blocks

Sets of precision rollers accurate to within 0 0 0 2 mm are available, which

in conjunction with a set of slip blocks provide a versatile method of ing angles Figure 2.20 shows how pairs of rollers can be set to obtain measurement of a taper in different transverse planes The measurements over the rollers are made by micrometer, and if extreme accuracy is required

measur-These instruments may be used as comparators, in which case accurate zero setting is unnecessary The advantage is then that the wide range, which

is of the order of 1 000 times as great as that of a normal comparator of similar accuracy Much of the work done on measuring machines does not require extreme accuracy, but the long direct-measuring range is a great convenience and speeds up many measuring operations For internal checking, special hanger brackets are fitted to the anvils

30 INSPECTION AND GAUGING

Figure 2.20 Checking taper plug using blocks and rollers

the micrometer should be used as a comparator with slip gauges as the standard

Where minimum diameter = d, Maximum diameter = Z),

θ M - m

tan ^ — ^ryj— (from which angle θ may be found)

Minor diameter d = m — 2R(l + tan j + sec -= )

Major diameter D = M - 2R{I + sec | j + 2(L - H V ^ ^

Another example is shown in Figure 2.21 This is a component which has a hole bored at an angle through its vertical face It is necessary to check

accurately the height X of the hole from the horizontal face and also the

R) tan θ

Figure 2.21 Measurement of a hole at an angle

angle Θ A close fitting bar is pushed into the hole and two rollers of equal

diameter are placed on it, the rollers being separated by means of a slip block Then

3

The suitability of a material as a cutting medium is decided upon by its ability to withstand the heat, pressure and abrasion to which all cutting tools are subjected during the machining operation Heat is generated by friction causing the cutting edge to attain temperatures as high as 600°C, and to withstand these high temperatures the material must possess a high 'red hardness' value This term is defined as the measure of the hardness of a material at elevated temperatures As the chips leave the work they move across the top of the tool and tend to wear the tool away The 'red hardness'

of the tool also offers resistance to trrs abrasive action, while pressure which

is caused by the chip bearing down on the tool is resisted by the toughness of the material Toughness being the opposite to brittleness

Carbon tool steels

Up to the beginning of this century carbon tool steel was the only cutting medium in general use Although it has for most purposes now been replaced

by other cutting materials, it is still used for the manufacture of hand tools and woodworking cutters Intricate form tools which cannot be easily ground

Table 3.1

1-4 Files 1-3 Turning tools

115 Drills, reamers, small taps

1 0 Large taps, reamers, wood working tools 0-7-0-8 Cold chisels, press tools

after hardening are often made from this material as very little surface carburisation takes place in the furnace, thus the tool will maintain its original size after heat treatment The steels used today differ from the original carbon steels in that they usually contain some alloying element such as chromium, or tungsten, which improves their cutting qualities

de-31

Cutting-tool Materials

32 CUTTING-TOOL MATERIALS

The hardness of these steels is determined by their carbon content which usually ranges from 0-7 to 1 4 % , thus gaining for them the name of high carbon steels All impurities such as sulphur, silicon and phosphorus are kept as low as possible because of their injurious effect The steels are hardened by quenching at temperatures between 750 and 800°C in water or oil, the quenching temperatures varying slightly with the carbon content Compared with other steels their 'red hardness' figures are low and their toughness high Table 3.1 gives the carbon content of tool steels for various classes of work

If the carbon content is above 1Τ 5 % it is almost impossible to weld these steels, but they are very easily forged and can be heat treated many times

High-speed steel

High-speed steel containing 14, 18, and 2 2 % tungsten, with small inclusions

of the other alloying metals, is used for almost all types of cutting tools, including the taps, reamers, and broaches mentioned above, and is the steel which has superseded the carbon tool steel because of the much faster speed at which it will cut satisfactorily; softening of the tool does not occur at temperatures below 660°C Care should be taken in the selection of high-speed steel for particular tools—for form tools a steel should be chosen which distorts as little as possible in hardening, so that the amount of stoning

or grinding required for correcting is kept to a minimum

F o r m tools, because of the length of cut, have often to withstand heavy load; a steel, therefore, of too brittle or 'short' a character is to be avoided

It is false economy to use a poor-quality steel on what is an expensive tool to produce

For plain turning tools, unless cuts are very heavy, cheaper steel may be used, the amount of distortion in hardening not being of importance; neither does 'shortness' matter All that is required is that the tool is hard enough to make frequent grinding unnecessary On very tough materials, in order to make the tool more resistant to abrasion and raise the temperature

at which the edge breaks down, the addition of cobalt is required often to the extent of 15%

Tungsten carbide

In order that the properties and performance of these materials can be appreciated, it is necessary that some knowledge of their composition and manufacture is acquired Methods of producing this material vary, but gener-ally pure tungsten carbide is pulverised to a grain size of 0-002 mm This flour is then thoroughly mixed with a cemented matrix of metallic cobalt, the whole then being shaken through gauze sieves

Specific quantities, in accordance with the shapes required, are then placed

in dies, the punch member of which transmits heavy hydraulic pressure to the compound

The material is now hard enough to be handled and removed from the die and subjected to a sintering process in a hydrogen atmosphere in care-fully controlled electric furnaces Following this process, the pieces are cut

CUTTING-TOOL MATERIALS 33

Figure 3.1 Cemented carbide tools

on heavy cutting, for tools to be designed as (a) and (b) rather than as (c), which should only be used for light turning and facing, when a better finish

is required

and ground to the required shape before the final sintering, which follows along the lines of the previous one, except that it is more prolonged To prevent cracking, the cooling process is a slow one and is arranged in the furnace so that the pieces when exposed to the atmosphere coincide approxi-mately to the room temperature The hardness of the material is now little less than that of a diamond, and can only be formed by specially prepared grinding wheels

Whilst this material is hard and strong in compression, its tensile strength

in relation to high-speed steel is only about 50 % It is not strong enough to take the loads likely to be imposed upon it, and is therefore used as tips to shanks of other and stronger material It will, however, be apparent that it can be used with advantage on those materials which quickly break down the cutting edge of high-speed steel—such as cast-iron, Bakelite, fibre, etc For the shanks, carbon steel, the carbon content of which should not be over 0-5 %, giving a tensile strength of around 45 720 kg, has been found the most suitable If a higher percentage of carbon was permitted, difficulty would be experienced when brazing the tip, as at a temperature necessary for this operation the shank would tend to temper, which condition would not be satisfactory for secure brazing It is advisable also that the back and also the base of the tip are nicely fitted to the shank, as the success of this material depends on the support it obtains from the shank, coupled with the rigidity of the machine and work-holding devices The shank sizes should always be as large as possible Examples of good practice are shown in Figure 3.1 Because of the physical properties of the material, effort should be made to reduce chip pressure to a minimum, and it is preferable, particularly

34 CUTTING-TOOL MATERIALS

Tantalum tungsten carbide

The addition of tantalum to tungsten carbide has resulted in a material less prone to the 'building up' experienced with tungsten carbide when machining steel It has, however, one disadvantage, in that because of its lower affinity for steel, greater difficulty is experienced in brazing the tips on to the shanks, and great rigidity is required in machine fixtures and tool-holders for its successful application; nevertheless, given the right conditions, it can be used successfully on steel

Molybdenum-titanium carbon alloy

This, one of the latest of the cemented carbides to be offered as a cutting tool material, is manufactured in much the same way as the tungsten carbides, but carbides of molybdenum and titanium are employed instead of tungsten It is claimed that it can be used successfully for cutting all forms of material, in-cluding non-ferrous metals and non-metallic substances as well as cast-iron and steel It is capable of producing a much better finish on the work than the other carbides, but its application appears to be limited to light cuts at very

Figure 3.2 Spring steel chip breaker

high speed, and certainly shows up to advantage for this class of work, particularly on high-tensile steels, where its resistance to abrasion or wear

is remarkable, the same superiority not being apparent on softer steels The same difficulty with brazing occurs as is the case with the tantalum carbide ; also great care has to be taken when grinding, if cracking is to be avoided Because of the speed with which the chip leaves the cutting edge of the tool, there is a likelihood of it being dangerous to the operator, or particularly troublesome if it does not break, but comes away in a continuous ribbon When this occurs it is advisable to fit a chip breaker, an example of which is

Tungsten carbide has, however, an affinity for steel, with the result that when cutting, particles of the steel build up and weld themselves on to the top

of the tool just behind the cutting edge, so that when the particles break away, pieces of the tungsten carbide are carried with them, causing the tool in time

to break down completely or to require regrinding It is, therefore, not entirely satisfactory for machining steels

CERAMIC CUTTING TOOLS 35

Cobalt-chromium-tungsten alloy (stellite)

This material is the oldest among this range and is a cast substance Its ability

to resist abrasion is greater than high-speed steel and it can be used at higher speeds Its tensile strength being greater than the tungsten carbides, it can be used in certain cases without the backing provided by the tool shank It has not, however, the tensile strength of high-speed steel, and for normal machin-ing operations it is best used in the tipped form

This material occupies an intermediate position between high-speed steel and the cemented carbides, and is less costly than the latter It can be used to advantage on cast-iron and non-ferrous metals, but it is not so successful when machining high-tensile steels A favourite and successful application is for inserted blade-milling cutters for machining cast-iron in particular, the blades being solid pieces

C E R A M I C C U T T I N G TOOLS

It is difficult to visualise the use of ceramics for cutting metals Nevertheless, ceramic tools are used with considerable success for machining practically all metals, including the 'difficult' titanium and vanadium This cutting material was first developed around the nineteen-thirties for machining abrasive materials such as carbon, graphite, plastics, fibre, asbestos, etc., which are difficult to machine with ordinary metal-turning tools However,

by slightly altering the composition and manufacturing methods a vastly improved material has now been obtained

Hardness

One such ceramic has a hardness approaching that of diamond and comprises

9 5 % pure aluminium oxide plus silica and certain refractory oxides: no binding medium is used, the ingredients being formed into a homogenous mass by sintering at high temperature Although it is brittle enough to be shattered with a hammer, it has the remarkably high compressive strength

of 150 k g / m m2 It is not affected by vibration in the same manner as other ceramics, as may be seen from the fact that the tools can be used on machines that are not bolted to the floor

The cutting tips

The tools are made in the form of moulded tips or inserts which are clamped

to tool holders: the arrangement is generally such that a negative cutting

shown in Figure 3.2 The top leaf is made from spring steel and hardened, and the chip, after leaving the cutting edge, is guided into a short spiral which will quickly break into short lengths It is possible in some cases, where there is sufficient tip available, to slightly lip the cutting edge to effect this result, but

it is not to be generally recommended on these expensive materials