Design for manufacture

Design for manufacture

10 Design for manufacture 10.1 Limits and fits and process capabilities 10.1.1 Selected 150 fits - hole basis (see 854500 for more details) Some of t...

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Design for manufacture 10.1 Limits and fits and process capabilities 10.1.1 Selected 150 fits - hole basis (see 854500 for more details) Some of the principal terms relating to limits and fits are given in the figure. Although only cylindrical parts are represented as holes and shafts it must be remembered that the terms should be applied in their broadest sense.

~

~

(I)

;2

s



"i

~

j

HOLE

i ~

J! ~

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c::

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Zero line or line of zero deviation

'0'

For most general applications the hole-basis system of fits is preferred to the shaft-basis system: the size of the hole is fixed as a datum and the shaft size is selected to give the required fit (for the shaft-basis the opposite is the case, as shown in the next figure). The principal reason for preferring the hole-basis system is that the size of holes (particularly small ones less than 20 mm) is usually dependent on the tools used to make them, e.g. drills, reamers, etc., whereas the size of most shafts is achieved by the machine tool used in its making, rather than relying on the cutting tool. Also, small holes are usually checked with standard gauges - the fewer the hole sizes, the fewer the gauges required; shafts are of course measured with adjustable instruments (e.g. micrometers, vernier callipers). 163

Engineering Applications

Hole-Basis System

c

Shaft-Basis System

c

In the British Standard (B84500), shown on p. 165, the whole range of possible fits is divided into three main classes: (a) clearance (b) transition (c) interference. Each main class is then further divided into several types of fit; for example.clearance fits are divided into six types from very fine to very coarse, each type of fit being classified by letter symbols. The wide range of possible fits which the Standard allows may be seen as a strength in that it can cope with virtually every conceivable application. In practice the Standard recommends selected fits to satisfy general requirements (e.g. loose running fits, loose clearance fits, normal running fits, precision sliding fits - similar requirements are identified for transition and interference fits). In using this tabulation of limits and fits, note that the horizontal line marked '0' in the shaft/hole diagram represents the exact (nominal) size of the hole or shaft. Hatched areas above this line represent tolerance zones for sizes greater than the nominal and hatched areas below the line represent tolerance zones for sizes less than nominal size. The selected fits allow for six possible clearance fits, two transition fits and two interference fits. They are based on four different tolerances (H11, H9, H8 and H7) on the nominal hole size which, at its smallest, is never less (defined by the letter H) than the nominal size; the actual allowable variation or tolerance is defined by the accompanying number (11, 9, 8 or 7). The position of the tolerances on the shaft size relative to the nominal size are indicated by the letters c, d, e, f, g, h, k, n, p and s while the extent of the tolerance is represented by the numbers 11,·10, 9, 7 and 6. Thus, for example, Hand h both represent tolerances for holes and shafts respectively which, at their maximum material condition, would give exact features. Similarly, 11 represents a particular tolerance for any particular size: for a 10 mm hole or shaft, 11 represents a tolerance of 0.110 mm. 164

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+ 01 I

K«O

d10

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SELECTED ISO FITS - HOLE BASIS

[«~
f7

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[£££.

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h6

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TRANSITION

1'£££4'»>'1'£££'

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Hole

Key

~

~ Shaft

,

56

,A-~--.-..

IV££A

I

INTERFERENCE

1[£££'

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(Data extracted from BS 4500A - 1985)

I

CLEARANCE

K«<'

e9

s

Diagram to scale for 100mm diameter.

H11

Over To

d10

H9

e9

Tolerance

+160 -130 0 -290

+190 -140 0 -330

+190 -150 0 -340

+220 -170 0 -190

18

30

40

50

65

80

10

18

30

40

50

65

80 100

+220 -180 100 120 0 -400

+160 -120 0 -280

+87 -120 0 -260

+74 -100 0 -220

+62 -80 0 -180

+52 -65 0 -149

+130 -110 0 -240 -40 -92

-32 -75

-25 -61

-20 -50

-14 -39

mm

+87 -72 0 -159

+74 -60 0 -134

+62 -50 0 -112

+52 0

+43 0

o

+43 -50 0 -120

+30 0

+25 0

mm

+110 -95 0 -205

+36 0

-30 -78

-20 -60

mm

+36 0

-80 -170

+30 0

+25 0

mm

-40 -98

+90

10

6

o

+75

6

3

-70 -145

-60 -120

o

+60

3

mm

mm mm mm

-

H9

Tolerance

H8

f7

Tolerance

H7

g6

Tolerance

H7

h6

Tolerance

H7

k6

Tolerance

H7

n6

Tolerance

H7 p6

Tolerance

H7 86

Tolerance

+54 0

+46 0

+39 0

+33 0

+27 0

+22 0

+18 0

+14 0

mm

-36 -71

-30 -60

-25 -50

-20 -41

-16 -34

-13 -28

-10 -22

-6 -16

mm

+35 0

+30 0

+25 0

+21 0

+18 0

+15 0

+12 0

+10 0

mm

-12 -34

-10 -29

-9 -25

-7 -20

-6 -17

-5 -14

-4 -12

-2 -8

mm

+35 0

+30 0

+25 0

+21 0

+18 0

+15 0

+12 0

+10 0

mm

-22 0

-19 0

-16 0

-13 0

-11 0

-5 0

-8 0

-6 0

mm

+35 0

+30 0

+25 0

+21 0

+18 0

+15 0

+12 0

-10 0

mm

+25 +3

+21 +2

+18 +2

+15 +2

+12 +1

+10 +1

+9 +1

+6 0

mm

+35 0

+30 0

+25 0

+21 0

+18 0

+15 0

+12 0

+10 0

mm

+45 +23

+39 +20

+33 +17

+28 +15

+23 +12

+19 +10

+16 +8

+10 +4

mm

+35 0

+30 0

+25 0

+21 0

+18 0

+15 0

+12 0

+10 0

mm

mm

+78 +59

+72 +53

+59 +43

+48 +35

+39 +28

+32 +23

+27 +19

+20 +14

mm

6

3

65

50

40

30

18

10

-

80

65

50

40

30

18

10

6

3

mm mm

Over To

Nominal sizes

+35 +93 80 100 0 +71 +59 +37 +35 +101 100 120 0 +79

+30 0 +51 +32 +30 0

+42 +25 +26 0

+35 +21 +22 0

+29 +18 +18 0

+24 +15 0 +15

+20 +12 +12 0

+12 +10 0 +6

mm

0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001

c11

Tolerance

Nominal sizes

H11

Over To

H9

d10

Tolerance

H9

e9

Tolerance

H8

17

Tolerance

H7 g6

Tolerance

H7

h6

Tolerance

H7

k6

Tolerance

H7

n6

Tolerance

+81 -56 0 -108

+89 -62 0 -119

+97 -68 0 -131

+320 -300 0 -620 +130 -190 +130 -110 0 -400 0 -240 +320 -330 280 315 0 -650

+360 -360 0 -720 +140 -210 +140 -125 0 -440 0 -265 +360 -400 355 400 0 -760

+400 -440 0 -840 +155 -230 +155 -135 0 -480 0 -290 +400 -480 450 500 0 -880

400 450

315 355

250 280

+63 0

+57 0

+52 0

+46 0

-20 -60

-18 -54

-17 -49

-15 -44

+63 0

+57 0

+52 0

+46 0

-40 0

-36 0

-32 0

-29 0

+63 0

+57 0

+52 0

+46 0

+45 +5

+40 +4

+36 +4

+33 +4

+63 0

+57 0

+52 0

+46 0

+40 0

mm

+80 +40

+73 +17

+66 +34

+60 +31

+52 +27

mm

-63 0

+57 0

+52 0

+46 0

+40 0

mm

mm

mm

+40 +117 120 140 0 +92

mm

+202 280 315 +170

+190 250 280 +158

+63 0 +108 +68 +63 0

+292 450 500 +252

+272 400 450 +232

+57 +226 315 355 0 +190 +98 +62 +57 +244 355 400 0 +208

+52 0 +88 +56 +52 0

+46 +169 225 250 0 +140

+79 +46 +159 200 225 0 +130 +50

+68 +40 +125 140 160 0 +100 +43

+290 -280 0 -570

-50 -96

+28 +3

mm

225 250

+72 0

+40 0

mm

+290 -260 +115 -170 +115 -100 0 -355 0 -215 0 -550

-25 0

mm

200 225

+40 0

mm

+46 +151 o +122 180 200

-14 -39

mm

+290 -240 0 -530

+40 0

mm

180 200

-43 -83

mm

+40 +133 160 180 0 +108

+63 0

mm

+250 -230 0 -480

mm

160 180

mm

+250 -210 +100 -145 +100 -84 0 -305 0 -185 0 -460

mm

140 160

mm

+250 -200 0 -450

mm

120 140

mm mm mm

mm mm

Over To

s6

H7

H7 p6

Nominal sizes

Tolerance

Tolerance

0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001

c11

Tolerance

Nominal sizes

Engineering Applications

10. 1.2 Ascribing tolerances to dimensions The tolerance on a size can be specified as· a deviation in one direction only from the nominal size (unilateral) or in either direction from the nominal (bilateral), as shown in the figure.

Unilateral

a c ca

as

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High Tolerance Limit

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~ Low Tolerance Limit

Bilateral Unilateral tolerances are arranged so that the nominal size gives the tightest fit, automatically giving a situation in which the nominal size is the 'maximum material condition'. Since this means that further material could be removed without the dimension necessarily going outwith the tolerance, this arrangement generally results in less scrap because the tendency is for manufacture to aim at the nominal size.

10.1.3 Dimensioning Although BS308 Engineering Drawing Practice, Part 2-Dimensioning and Tolerancing of Size and other publications give valuable guidance in correct procedure, dimensioning remains something of an art. Care must be taken if contradictions and unacceptable accumulations of tolerances are to be avoided. For 168

Design for manufacture

example, the following illustrations show how different dimensioning schemes allow different results. In the first instance, applying general tolerances to all three dimensions produces the contradictory result that the overall dimension falls outwith its specified tolerances if the other two dimensions are at their upper limit. Removing one dimension (or designating it as an auxiliary dimension by the use of brackets) avoids the contradiction but components produced to the second scheme will not necessarily fall within the acceptable limits of the third scheme (and vice versa).

I~ I~

50

...

....

I

After Manufacture

.1

100

50.05

....------.1 I

50.05

I

50mm features are machined at the upper limit, therefore overall size 0.05 above the upper limit

(i) General Tolerance ± 0.05

50

4

·1-"'50

1

.1

4

50

.1

I~

50.05

50.05

.1

After Manufacture

14

50mm features are machined at the upper limit, therefore part acceptable

(100)

(ii) General Tolerance ± 0.05 50.05

50 99.95

or

After Manufacture

I~

100

49.95

.1

(iii) General Tolerance ± 0.05

I~

100.05

Right hand feature varies between 49.90 and 50.10, ie effective tolerance ± 0.10

169

Engineering Applications

In applying a dimensioning scheme, as in the next figure, functionally important features of the component should be chosen as datums (although it should be emphasized that a functional datum may not necessarily be an important manufacturing feature): a good understanding of the function of the component is therefore a pre-requisite. The illustrations in the figure identify the functional dimensions of the components which contribute to the end float of a lever on its pivot pin and then show two different dimensioning schemes, only one of which controls the functional dimensions correctly. This scheme allows the tolerances on critical sizes to be 2.5 times as large (on average) as those dimensioned by the other; machining will be easier and, therefore, cheaper.

Functional dimensions governing the end float of the trunion bearing are marked (NB This does not mean all marked dimensions are the same size).

D

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a

Actual end float determined by the addition of four tolerances

Actual end float determined by addition of tolerances on all ten dimensions, ie allowable tolerance on end float shared out between ten machining operations. POOR!! Actual end float determined by addition of tolerances on only four dimensions (marked '0), ie shared by four machining operations. BEST!! (for end float)

170

Design for manufacture

The next figure shows that there is a definite relationship between dimensional tolerance and surface finish tolerance. That is, for any particular process, a certain dimensional tolerance will be accompanied by a corresponding level of surface roughness. However the converse is unlikely to be true.

50 (N12)

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25 (N11)

0)

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~/

0

a:

12.5 (N10)

/

6.3 (N9) 3.2 (NS)

Surface

~Zft//// /

1.6 (N7)

Roughness Ra (Jim) 0.8

/1'i1/!~/~1t/

(N6)

i ~cf~ / /11

0.4 (N5) 0.2 (N4)

/

0.1 (N3) ..c:

0.05 (N2)

(5 0

E

en

0.025 (N1)

I 0.005 0.01

0.05

0.1

0.5

1.0

2.0

Tolerance Range (mm)

171

Engineering Applications

The following figure shows how the surface roughness is dependent on the manufacturing process used. Using this, the preferred process for producing a specific roughness can be found. However, should the need arise to use a non-preferred, though probably more expensive, process then the appropriate roughness values are also given.

Process

\1

Flame cutting

Roughness Values (J.Lm R2 ) 50

~ 7

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6.3

7

~7

3.2 ~7

1.6 V

0.8 "'7

lfi!il

Planing/Shaping

~7

0.05 0.025 0.0125 ~7

V

~7

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Chemical Machining Electro-discharge machining Milling

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Broaching

::::::,;::::::,::::::::"I!, "I;" "\"

Reaming

I,

Boring, turning

I:::':::::::'::::::::::::::: ':::::,::;::::::::::::::::::

Barrel finishing

... :::::::,:::::::,:::::: 1'::::,:;:::::::,:::::::1::: :';::::::,:::::;:,:::::> j~~~ljjj~j~~~~jj j1j~~~~~j~ij~~jj~jj 1~j~~~~jj~i~j1j~j jt~j~~j~jj~jjjjl~~1~ ',,,""'1. 1:::;<::::',"::::::':::::. ,:::::::::::::::::::'::::::: ijjjl~1~j~j~jjjj~1 ij~ j1j~1~jI1jl1~ lj r::::::::':::::::::::::::':: .:::::::::::':':::::::::::' "

',:I ,: : ":1 ~ ~ 1~1 1 1~1~1j1~1 ~ j~ 1~j1~ ,:::::::::::::::..:::>,:: :>..:::: .:::,:;:::::,:::::::::::::' jj~~1~~jj~jij~~~ ~>,: : : : : : ,: : : :,: ;::::,::::;',:'<::::::::: :::::1:::::>::::::::,::: j~~~~ll~~~~~ ~jit~~~~il~~~ l~~~~~~j~~lj~~~j~ jji~i~J~~~~jj~ :;::::,:::::::,::::::';:::: ,:::::::::::::::1:::::::1;' ;:::1:::::::,::::::;,:::;::: ~~~j~~j~~jj~~ ~ij~~j~~~1~ ~j~lj~j~1j~~~~~: :::::::;:::::::,::><: ::,::;::::1:::::::,:::::::' "1.",::'<:::'

Electrolytic grinding

'10,

Roller burnishing Grinding Honing

'I"

""h,'·

!i~~~~1ii~i~1~! i!i1!~!i!i~ !~ ~1 I~ :::::::::::.::1:::::::,:::: ';:::::::,:::'::::::::::;::: ,:::,;::::::,:::::::,::::::; f::::::,:::::;::::::::::,::' ~ii~1~~j~!i!i 1j~~1~~~~~~~~i~i 1~~~~~~~~i1i~i1~ :::::::::::::;'::::::::::::' ::::::::::::,;::;:::,;:::;, [,;::::::.:::::::::,:::::;" ::<:::>::::::,::::::: 1~1~1~ ~ l~i~ ~ ~ ij~i~ij~ i~1i~ i~ ~ :::;:';::::::,:::::::';:::;: I,;::;::,::::;::;:::::::,:::

Polishing

.:;;::,:::::::,::;::::<::

Lapping Superfinishing

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Sand casting

;::::::::::,:::::::::::::::: th,

Hot rolling

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Forging

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~ ~~~~I~~~~~j ~1t~i1~~~~~~ ::::::::::::::,:::::::.::::' ~ tt! jI

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Permanent mould casting

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1:::::::::::::<::::' 1'\, "\\ ::::::

1~ii~j~~111~~i~~~ ,:::::::<::::::::::::';' 'h,

~: ~: : : :':::::::::::;:::' ~~~~~l~~ ~j~~~~~~~1~~~i~ "I:,;::::::':::::::,::::::,

Investment casting Extruding

'\:::::::1:::::::::::::::::: 1::::::::::::<:::":::::: /,

,

Cold rolling, drawing

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Die casting

6

50 Typical for process

(from BS 1134: Part 2, 1990)

172

0.1

:::::::,:::,

Drilling

I~rt}~~~~~jl

0.2 "'7

'It I

Sawing

KEY

0.4 "'7

~l~~~~~~ ::::;::::;::::::::>:::::: I:;:::::,:::' <:: ~~i\~~~~~~~ ~~~~i~~~~~l~j~~j~ :::::::,:::::::1::::::;,::: ::11:::::::;: ~~~~Iii~~j~ 1~~~t.iili~~~j111 1~j~~~~ili11~~11~ ::::::::::::::::::::<::: '1'1; .:1';1'::::'11::::':::::> ~~llili~~~~~~ ~1~~~1~~i~ljj ~r.~~~~i1~~~~ 1jj~~11~~i1j~~~l1 ~~~j~~~11~m1~~j [:::,:::::::,::::::::::::::: I'ii; tri1!f

Snagging

12.5

25

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6.

6.

25

12.5

6.

6.3

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6

3.2

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0.8

6.

0.4

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1',.'::::<:::"<::'<;1 Process applied under special conditions, 'II.,

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less frequent application

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0.1

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0.05 0.025 0.0125

Design for manufacture

The final figure in this section gives a relative comparison of processes for specific tolerance ranges.

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35

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0.15

0.20 0.23

0.18

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0.05

0.10 0.13

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(0.001" - a "thOU,")

Tolerance

(from BS PD6470, 1981) 173

Engineering Applications

10.2 Design details for casting 10.2. 1 Introduction

Casting processes allow metal to be positioned according to the load to be carried by the components, thus making for more uniform stress distribution and good material utilization. The compressive strengths of casting alloys are considerably higher than their tensile strengths since, when in tension, discontinuities in the grain structure, such as graphite flakes in grey cast iron, become stress-raising features. Castings should therefore be designed keeping the material in compression where possible, as shown in the figure. When bending is unavoidable the tension side can be reinforced using additional metal. Ribs on the tension side may help: their thickness should be 0.8-0.9 times the wall thickness and their depth 1.1-1.2 times the wall thickness.

Load

X Poor Design

x

Load

X Good Design

174

Force

Force

Poor Design

Good Design

Design for manufacture

Different casting sections are preferred for different load conditions. For transverse bending, asymmetric

'I' sections give best use of material; for torsion, closed circular sections are best. For combined bending

and twisting asymmetric closed sections are the optimum compromise.

Best for Bending

Best for Torsion

Best for Combined Bending and Torsion (All three sections have the same cross-sectional area and same height)

10.2.2 General design rules 1 Aim for uniform grain size and structure without residual stress This is done by designing for a uniform rate of cooling - keep the sections as uniform as possible and blend the changes in section lengthwise; do not allow 'step' changes in features.

T

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f--------

L

.1

) a - . - __

L - (T -t)

---. tt

X4

Also reduce heavy sections at junctions of sections by keeping fillet radii small (abount O.5t) and by staggering them if possible.

-,

radius about O.5t

t

175

Engineering Applications

2 Avoid metallurgical defects (a) Avoid sharp corners with their associated plane of weakness where columnar crystals inhibit each others' development (outside corner) or grow away from each other (inside corner), as shown in the following figure. The extent of formation of the columnar crystals is primarily a function of the type of metal used, the solidification rate and the shape of the casting.

Typical casting grain structure

Columnar crystal growth

At sharp corners, hot spots during casting can cause local weaknesses and possible cracking

Rounded corner forms give better columnar crystal formation

176

Design for manufacture

(b) Reduce residual stresses and the risk of hot tears by designing for flexibility. Try to design to minimize cooling stresses as shown in the next figure.

Rim susceptible to hot tearing or residual compressive stresses. The spokes may be susceptible to tearing or residual tensile stresses.

Spokes susceptible to tearing or residual compressive stresses; the rim may be susceptible to residual tensile stresses and flattening.

This wheel has a balanced section thickness which gives uniform cooling.

3 Design for ease of moulding (a) Try to reduce the number of external and internal cores.

No Cores Required

Internal Core Required

(b) Avoid large areas of thin sectioned castings where surface to volume ratio is high and premature solidification is likely. 177

Engineering Applications

4 Design for ease of machining

(a) Minimize the areas to be machined. (b) Arrange for all surfaces to be at the same height.

-,

Motor

/

Pump

Base Plate ORIGINAL DESIGN OF BASE PLATE

IMPROVED DESIGN 10.3 Design details for machining Careful attention to detail can reduce the cost a component contributes to an assembly by reducing costs in a number of areas; some of these are described below.

10.3.1 Cost of material The cost of material can be reduced by designing to utilize standard bar sizes held in stock by the company. Choosing a free-machining grade of the required specification may increase the material costs slightly but can show a substantial reduction in machining costs later on. 178

Design for manufacture

10.3.2 Cost of machining The cost of machining can be reduced by designing with the following aims in mind: (a) Design so that the component can be machined on as few machines as possible with a minimum of setups. (b) Minimize the area to be machined by designing castings, forgings and fabrications in such a way that only those areas which need to be machined are left standing proud, as shown in the figure. This can be especially worthwhile when tight tolerances on dimensions and surface finish have to be achieved.

Plain Bearing

Improved Design (c) Allow high metal removal rates by avoiding features which cannot be machined by standard coated carbide cutting tool. These can remove material 3-5 times faster than high-speed steel tools which are used for non-standard shapes. (d) Avoid long slender sections, thin walls, deep bores and other features which give rise to undue deflections of the workpiece or the tool; the component may consequently have to be machined at reduced feed rates. 179

Engineering Applications

10.3.3 Cost of assembly The cost of assembly can be reduced by selecting ball races and other pressed on/in parts so that they only have to be pressed along the shaft or into the bore in which they will ultimately fit. Adequate leads on the shafts and housings can also help to reduce assembly time.

Leads denoted

byD

180

Design for manufacture

10.3.4 Particular details for machining (a) Slender cylindrical components (length: diameter ratio> 3) will have to be supported by a centre at the tailstock end when being turned. Thus internal features cannot be machined at the same set-up. (b) Tools such as twist drills and slot drills are only effective when their length: diameter ratio is less than 5 or 6, so avoid features which are inaccessible to standard tools.

Drill chuck fouling work piece

(c) Grinding wheels quickly develop comer radii and therefore cannot grind sharp internal corners such as when a diameter runs up to a shoulder on a shaft. Undercutting is a solution which may introduce a stress-raising feature; it is better therefore to chamfer or counter-bore the mating part.

Grinding Wheel

'Run Out' Radius for Grinding

\

r

--"---1-

Rounded internal corner

181

Engineering Applications

(d) When screw threads run up to a shoulder, the last two or three pitches will not be fully formed. Undercutting is a solution but, as previously, counter-boring the mating part is usually the better course of action. The following figure shows both solutions. Profile of undercuts

p = thread pitch

d = thread diameter g = thread core diameter r= 0.5 x P

f=

3x p

Example for an M16 thread

~ ~(3P)

-1

2

x 45

0

(p)

Undercut and chamfer with external screw thread adjacent to a shoulder

Countersink: large enough to clear shaft maximum corner fillet radius -----

Alternative to an undercut on the external screwthread

(e) Avoid drilling blind holes for tapping and reaming where possible. They cannot be fully tapped or reamed, they are time-consuming and the risk of tool breakage is increased. (f) Drills should enter and break through normal to the surfaces involved to avoid the risk of tool breakage. 182

Design for manufacture

(g) Standard slot drills and end mills do not have comer radii so the bottom internal comers of workpieces should be sharp; other internal comers should have as large radii as possible to allow the maximum diameter of cutter to be used. This is illustrated in the next figure.

Sharp corners

Area Clearance Recess

Radius as big a possible

10.4 Design details for welding The cost of welding can be reduced and the effectiveness of weldments increased if the simple rules outlined below are understood and applied wherever possible.

10.4.1 Minimum weld preparations Chamfering of edges, whether by grinding, machining or flame-cutting, is time-consuming and adds to the overall cost of weldments. The examples illustrated below show how weld preparations can often be avoided. Note, however, that not only does the appearance of the finished job change but also the loading on the weld. (a)

Butt weld - weld preparation on both parts

Lap weld - no weld preparation - no extra weld material

(b)

Assume no access for welding from the left.

Comer weld - no weld preparation.

Fillet weld - no weld preparation

Weld preparation required. (c)

183

Engineering Applications

If access is possible from both sides then weld preparation on both sides (right-hand diagram) reduces the amount of metal to be removed by 50%; consequently the weld requires only 50% of the weld material to be deposited compared with single-sided preparation. This is an important consideration for plate thicknesses above 5 mm.

10.4.2 Design for location

Load

Load

(a)

Weld carries load.

(b)

Load 2

Weld for location only

Load

Load

Load

2

2

2

Load Load applied 10 round bar, weld carries load

184

Load Weld for location only

Design for manufacture

10.4.3 Positioning load-carrying welds for maximum effectiveness

o

o

Load

Load

DESIGN 'B'

DESIGN 'A'

Note:

1) For design 'A' the right hand weld is for location only. 2) For equal strength Design 'B' requires three times as much weld as Design 'A'.

10.4.4 Allow for weld shrinkages The weld metal deposited shrinks on solidification and cooling to ambient temperature. This can lead to distortion, residual stresses and cracking if not taken into consideration.

AFTER

BEFORE (i)

)

v

< )

(ii) Clamping Force

(iii)

Clamping Force

cr

The welding set-up illustrated in (ii) above allows shrinkage to pull the job straight. The flexibility of the set-up eliminates the risk of cracks arising from hot tears and minimizes residual stresses. If the parts to be joined are rigidly clamped during welding, distortion is eliminated but there is a risk of weld cracking and a certainty of residual stresses which will reduce the load-carrying capability of the finished job. Post-weld stress-relieving heat treatment is sometimes necessary. 185