computer-aided design volume 26 issue 7 1994

7/24/2019 Computer-Aided Design Volume 26 Issue 7 1994

1/16

Product design for

manufacture and assembly

G eoffrey Boothroyd

Design i s the f i rs t s tep in manufa ctur ing , and i t is where m ost

of the im porta n t decis ions are m ade that af fect the f inal cost

of a p roduct . S ince 1980 , analysis techniques have been m ade

avai lab le which can gu ide designers towards products which

are easy to manu facture a nd assemble. The avai lab i l i ty of these

techniques has created a revolu t ion in manufactur ing indust ry ,

especial ly in the USA, lead ing to reduced product cost , bet ter

qual i ty , shor ter t ime to ma rket , low er inventory , few suppliers ,

an d m an y o th e r im p ro v em en t s .

The paper f i rs t s t resses the importance of tak ing carefu l

acco u n t o f m an u fac tu r i n g an d assem b ly p ro b l em s i n t h e ea r l y

stages of pro du ct design. Then, using a case study, the

p h i l o so p h y o f t h e Des ig n fo r Man u fac tu re an d Assem b ly

(DFMA) methodology and i t s appl icat ion are explained . The

his tor ical development of design-for-assembly and design-for-

manufactur ing techniques in Japan , Europe and the USA is

presente d. A review of publishe d case histories em phasiz es the

enormous advantages to be gained by adopt ing th is relat ively

n ew ap p ro ach as t h e m ajo r t o o l in co n cu r ren t an d s im u l t an eo u s

engineering. Finally, a discussion of the various roa dblo cks

affecting D FM A imp leme ntat ion i s fo l lowed by a d iscussion of

curren t deve lopme nts , which include produ ct design for

disassembly, service and recycling.

Keyw ords : as sembly, m anufacture

h a s t h e m o s t p r o d u c t i v e p l a n t s . H o w e v e r , i t w a s f o u n d

t h a t a u t o m a t i o n c o u l d o n l y a c c o u n t f o r o n e - t h i r d o f t h e

t o t a l d i ff e r e nc e i n p r o d u c t i v i t y b e t w e e n p l a n t s , a n d t h a t ,

a t a n y l e ve l o f a u t o m a t i o n , t h e d i ff e re n c e b e t w e e n t h e

m o s t a n d l e a s t e f fi ci e nt p l a n t s i s e n o r m o u s . F o r e x a m p l e ,

t h e l e a s t - a u t o m a t e d J a p a n e s e d o m e s t i c p l a n t ( i n w h i c h

3 4 % o f a ll s te p s a r e a c c o m p l i s h e d a u t o m a t i c a l l y ) , w h i c h

i s a l s o t h e m o s t e f f i c ie n t p l a n t i n t h e w o r l d , n e e d s o n e - h a l f

o f t h e h u m a n e f fo r t o f o n e c o m p a r a b l y a u t o m a t e d

E u r o p e a n p l a n t , a n d o n e - t h i r d o f t h e e ff o r t o f a n o t h e r .

T h e E u r o p e a n p l a n t t h a t i s t h e m o s t a u t o m a t e d i n t h e

w o r l d ( in w h i c h 4 8 % o f a ll a s s e m b l y s t ep s a r e a u t o m a t e d )

r e q u i re s 7 0 % m o r e e f f o r t t o p e r f o r m a s t a n d a r d s e t o f

a s s e m b l y t a s k s o n a s t a n d a r d c a r t h a n d o e s t h e m o s t

e f f i c i e n t p l a n t , w h i c h i s o n l y 3 4 % a u t o m a t e d .

T h e q u e s t i o n i s th a t o f w h e t h e r m a n u f a c t u r a b i l i t y a n d

e a s e o f a s s e m b l y a re m o r e i m p o r t a n t t h a n a u t o m a t i o n

i n im p r o v i n g p r o d u c t i v i t y . T h e a u t h o r s o f t h e s t u d y

c o n c l u d e t h a t n o i m p r o v e m e n t s in o p e r a t i o n c a n m a k e

a p l a n t f u l ly c o m p e t i t i v e i f t h e p r o d u c t d e s i g n i s d e fe c t i v e.

H o w e v e r , t h e y f ai l t o m a k e a d i r e c t c o n n e c t i o n b e t w e e n

p r o d u c t d e s i g n a n d p r o d u c t i v i t y , a n d a n a t t e m p t i s m a d e

i n t h is p a p e r t o s h o w t h a t t h e r e i s n o w o v e r w h e l m i n g

e v i d e n c e t o s u p p o r t t h e v ie w t h a t p r o d u c t d e s i g n f o r

m a n u f a c t u r e a n d a s s e m b l y c a n b e t h e k e y t o h i gh

p r o d u c t i v i t y i n a l l m a n u f a c t u r i n g i n d u s t r i e s .

I t h a s b e e n e s t i m a t e d t h a t , i n t h e U S A , m a n u f a c t u r i n g

c o n t r i b u t e s a b o u t 2 3 % o f t h e g r o s s n a t i o n a l p r o d u c t ,

a n d , m o r e i m p o r t a n t l y , a b o u t 7 0 % o f a l l w e a l t h -

p r o d u c i n g a c t i v i t i e s . T h o s e w h o c o m p l a c e n t l y s a y t h a t

t h e U S A i s c h a n g i n g t o a s e rv i c e e c o n o m y m a y e v e n t u a l l y

f in d t h a t t h e y n o l o n g e r h a v e t h e m e a n s t o p u r c h a s e t h e s e

s e r v i c e s . T h e U S A h a s b e e n l o s i n g $ 3 4 0 M p e r d a y t o i t s

f o r e ig n c o m p e t i t o r s , a n d t h e n a t i o n a l d e b t i s n o w a r o u n d

$ 4 0 0 0 O0 0 M

C o m p e t i t i v e n e s s h a s b e e n l o s t i n m a n y a r e a s , m o s t

n o t a b l y , a s c a n b e s e e n i n F ig u r e 1 , i n a u t o m o b i l e

m a n u f a c t u r e . I n 1 9 9 0, th e r e s u l t s o f a $ 5 M w o r l d w i d e

s t u d y o f t h is i n d u s t r y w a s p u b l i s h e d 1. T h e s t u d y

a t t e m p t e d t o e x p l a i n t h e w i d e v a r i a t i o n s i n a u t o m o b i l e

a s s e m b l y - p l a n t p r o d u c t i v i t y t h r o u g h o u t t h e w o r l d .

F i g u r e 2

( w h i c h is ta k e n f r o m t h i s s t ud y ) s h o w s t h a t J a p a n

University of Rho de Island, K ingston, RI 02881, USA

Pape r received." 15 Sep temb er 1993

(60)

A i r c r a ft & O t h e r T r a n s p o rt E q u i p m e n t

Pr o f . , Sc ien t i f i c & Con t r o l Instruments

Syn the t i c Res ins , Ru bbe r & P las t i cs

Medicinals Pharmaceuticals

O r g a n i c C h e m i c a l s & Products

(1.9)

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Textiles, Yarns & Fibers

Paper Paper Board

Other Electrical Machinery & Par ts

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Nonmetallic Mineral Manufacturers

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Apparel Accessories

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r r r r

10 20 30 40 50 60

Figure i Breakdow n of US trade balance

[Values in $ 1 0 9 . ]

0 0 1 0 - 4 4 8 5 / 9 4 / 0 7 / 0 5 0 5 - 1 6 @ 1 9 9 4 B u t t e r w o r t h - H e i n e m a n n L t d

C o m p u t e r - A i d e d D e s i g n V o l u m e 2 6 N u m b e r 7 J u l y 1 9 94

5 0 5


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P r o d u c t d e s i g n f o r m a n u f a c t u r e a n d a s s e m b l y : G B o o t h r o y c l

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100 [ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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JINA (5) USINA (14) US&J/E (9) E/E (13) NIC (11)

Pa r en t Loca t ion /P lan t Loca t ion { Sam p le S ize }

F i g u r e

2 W o r l d w i d e a u t o m o b i l e a s s e m b l y - p h m t p r .~ d u c ti v i b , (~,tdtma

producers in 198~))

[ A d a p t e d f r o m R e f e r e n c e I . l ' h e v o l u m e p r o d u c e r s i n c lu d e t h e L i b b l ~

th ree ' , F ia t . PS A, Renau l t an d Volk s wag en in E urope . and a l l o f the

J a p a n e s e c o m p a n i e s . J ,' J : J a p a n e s e - o w n e d p l a n t s i n J a p a n , J N A

J a p a n e s e - o w n e d p l a n t s i n N o r t h A m e r i c a . i n c l u d i n g j o i n t - y e n ( m e

p lan t s w i th Am er ican f i rm s , US / NA : l, J S -owned p la n t s in N or (h

A m e r i c a , U S & J / E : U S - a n d J a p a n e s e - o w n e d p l a n t s in E u r o p e , 1 t :

E u r o p e a n - o w n e d p l a n t s i n E u r o p e , N I C : p l a n t s m n e w l y i n du s t r ia l i z in F

c o u n t r i e s : M e x i c o , B ra z i l, T a i w a n a n d K o r e a . S o u r c e : I n t e r n a t i o n a l

M o t o r V e h i c l e P r o g r a m S t u d y . ]

D E S IG N F O R M A N U F A C T U R E A N D

A S S E M B L Y

I n th e co n tex t o f th i s p ap er , d e s ig n i s th e f i r s t s t ep m

m a n u f a c t u r e , a n d i t i s a n a c t i v i t y t h a t s t a r t s w i t h s k e t c h e s

o f p a r t s a n d a s s e m b l i e s, a n d p r o g r e s s e s t o t h e d r a w i n g

b o a r d o r CA r) w o r k s t a t i o n , w h e r e a s s e m b l y d r a w i n g s a n d

d e t a i le d p a r t d r a w i n g s a r e p r o d u c e d . T h e s e d r a w i n g s a r c

t h e n p a s s e d t o t h e m a n u f a c t u r i n g a n d a s s e m b l y e n g in e e r s.

w h o s e f u n c t i o n i s t o o p t i m i z e t h e p r o c e s s e s u s e d t o ,

p r o d u c e t h e f i n a l p r o d u c t . F r e q u e n t l y , i t i s a t t h i s

s t a g e t h a t m a n u f a c t u r i n g a n d a s s e m b l y p r o b l e m s a r c

e n c o u n t e r e d a n d r e q u e s t s a r e m a d e f o r d e s i g n c h a n g e s .

S o m e t i m e s , t h e s e d e s i g n c h a n g e s r e s u l t i n c o n s i d e r a b l e

d e l a y s i n t h e f i n a l p r o d u c t r e l e a s e . I n a d d i t i o n , t h e l a t e /

i n t h e p r o d u c t d e s i g n a n d d e v e l o p m e n t c y c l e t h e c h a n g e s

o c c u r , t h e m o r e e x p e n s i v e t h e y b e c o m e . T h e r e f o r e . n o t

o n l y i s i t i m p o r t a n t t o t a k e m a n u f a c t u r e a n d a s s e m b l y

i n t o a c c o u n t d u r i n g p r o d u c t d e s i g n , b u t a l s o t h e s e

c o n s i d e r a t i o n s m u s t o c c u r a s e a r l y a s p o s s i b l e i n t h e

d es ig n cy c le .

Th is i s sh o w n q u a l i t a t iv e ly b y th e c h ar t in / - 7.qw ' c 3 .

w h ich sh o w s th a t ex t r a t ime sp en t ea r ly in th e d es ig t~

p r o c e s s i s m o r e t h a n c o m p e n s a t e d f o r b y s a v i n g s i n t i m e

w h e n p r o t o t y p i n g t a k e s p l a c e . T h u s . i n a d d i t i o n l ~ ,

r e d u c i n g p r o d u c t c o s t s , th e a p p l i c a t i o n o f t h e D e s i g n fo~

M a n u f a c t u r e a n d A s s e m b ly ( D F M A ) m e t h o d o l o g y *

s h o r t e n s t h e t i m e t a k e n t o b r i n g t h e p r o d u c t t o m a r k e t .

A s a n e x a m p l e , t h e I n g e r s o l l R a n d C o m p a n y r e p o r t e d :

t h a t t h e a p p l i c a t i o n o f D F M A r e d u c e d p r o d u c l

d e v e l o p m e n t t i m e f r o m t w o y e a r s t o o n e . I n a d d i t i o n .

t h e d e si g n t e a m r e d u c e d t h e n u m b e r o f p a r ts i n a p o r t a b l e

c o m p r e s s o r r a d i a t o r a n d o i l - c o o l e r a s s e m b l y f ro m 8 0

t o 2 9, d e c r e a s e d t h e n u m b e r o f f a s t e n e r s f r o m 3 8 t o 2 0 .

t r i m m e d t h e n u m b e r o f a ss e m b l y o p e r a t i o n s f r o m 1 59 to

4 0 , a n d r e d u c e d t h e a s s e m b l y t i m e f r o m 1 8 . 5 m i n t o 6 . 5

m i n . D e v e l o p m e n t s t a r t e d i n J u n e 1 9 8 9 , a n d t h e n e w

d e s i g n w e n t i n t o f u l l p r o d u c t i o n i n F e b r u a r y 1 9 9 0 .

T r a d i t i o n a l l y , t h e a t t i t u d e o f d e si g n e r s h a s b e e n " w e

d e s i g n i t , y o u b u i l d i t ' . T h i s h a s n o w b e e n t e r m e d t h e

' o v e r - t h e - w a l l a p p r o a c h ' , i n w h i c h t h e d e s i g n e r t h r o w s

* ' D E M A ' i s a t r a d e m a r k o f B o o t h r o y d D e w h u r s t I n c ., U S A .

c o n c e p t u a l

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91l W Bi.~ Beaxc~ Roa d. " lro, . . MI 48084 S ' ~

t h e d e s i g n s o v e r a w a l l t o t h e m a m t l a c t u r i n g e n g i n e e r >

w h o t h e n h a v e t o d e a l w i t h t h e ' , a r i o u s m a n u f a c t u r i n g

p r o b l e m s a r i s i n g b e c a u s e t h e y w e r e n o t i n v o l v e d i n t h e

d es ig n e f f o r t ( s ee Fi.qurc 4). O n e m e a n s o f o v e rc o m i n g

t h is p r o b l e m i s t o c o n s u l t t h e m a m , f a c t u r i n g e n g i n e e r s

a t t h e d e s i g n s t a g e . T h e r e s u l t i n g t e a m w o r k a v o i d s

m a n y o f th e p r o b l e m s t h a t a r is e . H o w e v e r . t h es e t e a m s .

n o w c a l l e d s i m u l t a n e o u s - e n g i n e e r i n g o r c o n c u r r e n t -

e n g i n e e r i n g t e a m s , r e q u i r e a n a l y s i s t o o l s t o h e l p t h e m

s t u d y p r o p o s e d d e s i g n s a n d e v a l u a t e t h e m f r o m t h e p o i n t

o f v i e w o f m a n u f a c t u r i n g a n d a s s e m b l y d i f f i c u lt y a n d c o s L

H O W D F M A W O R K S

B y w a y o f e x a m p l e ,

k~.qurc

.5 s h o w s t h e r e q u i r e m e n t s o f

a m o t o r - d r i v e a s s e m b l y t h a t m u s t b e d e s i g n e d t o s e n s e

5 0 6 C o m p u t e r - A i d e d D e s i g n V o l u m e 2 6 N u m b e r 7 J u l y 1 9 9 4


3/16

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Configuration of required motor-drive assemb ly

C O V E R S C R E W ( 4 ) E N D P L A T E

0 . 1 2 d i a x 0 3 I c S l e e l . p a i n t e d

/ / 4 5 x 2 2 5 ~ 1 3

B U S H ( 2 ) P L A S T I C B U S H

i p e w e e m e a - - i

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S E T S C a E W

E N D p L A T E S C R E W [ 2 1 ~ - ~ ' ~

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Figure 6 Init ial design of motor-drive assembly

a n d c o n t r o l i t s p o s i t i o n o n t w o s t e e l g u i de r a il s . T h e m o t o r

m u s t b e f u l ly e n c l o s e d f o r a e s t h e ti c r e a s o n s , a n d h a v e a

r e m o v a b l e c o v e r f o r a c c e s s s o t h a t t h e p o s i t i o n s e n s o r

c a n b e a d j u s t e d . T h e p r i n c i p a l r e q u i r e m e n t s a r e a r i g i d

b a s e t h a t i s d e s i g n e d t o s l i d e u p a n d d o w n t h e g u i d e r a i l s ,

a n d t h a t s u p p o r t s t h e m o t o r a n d s e n s or . T h e m o t o r a n d

s e n s o r h a v e w i r e s t h a t c o n n e c t t h e m t o a p o w e r s u p p l y

a n d a c o n t r o l u n i t , r e s p e c t i v e l y .

A p r o p o s e d s o l u t i o n i s s h o w n i n

Figure 6.

T h e b a s e i s

p r o v i d e d w i t h t w o b u s h e s t o p r o v i d e s u i t a b l e fr i c ti o n a n d

w e a r c h a r a c t e r is t ic s . T h e m o t o r i s s e c u r e d t o t h e b a s e

w i t h t w o m o t o r s c r e w s , a n d a h o l e i n t h e b a s e a c c e p t s

t h e c y l i n d r i c a l s e n s o r , w h i c h i s h e l d i n p l a c e w i t h a s e t

s c r e w . T o p r o v i d e t h e r e q u i r e d c o v e r s , a n e n d p l a t e i s

s e c u r e d b y t w o e n d - p l a t e s c r e w s t o t w o s t a n d o ff s , w h ic h

a r e , in t u r n , s c r e w e d i n t o t h e b a s e . T h i s e n d p l a t e i s f i t te d

w i t h a p la s t i c b u s h t h r o u g h w h i c h t h e c o n n e c t i n g w i re s

p a s s . F i n a l l y , a b o x - s h a p e d c o v e r s l i d e s o v e r t h e w h o l e

a s s e m b l y f r o m b e l o w t h e b a s e , a n d is h e ld i n p l a c e b y

f o u r c o v e r s c re w s , t w o p a s s i n g i n t o t h e b a s e , a n d t w o

i n t o t h e e n d c o v e r .

T w o s u b a s s e m b l i e s a r e r e q u ir e d , t h e m o t o r a n d t h e

s e n s o r , a n d , i n t h i s i n i ti a l d e s i g n , th e r e a r e e i g h t a d d i t i o n a l

m a i n p a r t s , a n d n i n e s c re w s , m a k i n g a t o t a l o f 1 9 i t e m s

t o b e a s s e m b l e d .

W h e n D F M A b e g a n t o b e t a k e n s e r i o u s ly i n t h e e a r l y

1 9 8 0 s , a n d t h e c o n s e q u e n t b e n e f i t s w e r e a p p r e c i a t e d , i t

b e c a m e a p p a r e n t t h a t t h e g r e a te s t i m p r o v e m e n t s a r o s e

f r o m s i m p l i fi c a t io n o f t he p r o d u c t b y r e d u c t i o n o f t h e

n u m b e r o f s e p a r a t e p a r t s . T o g i ve g u i d a n c e t o t h e

d e s i g n e r in r e d u c i n g th e p a r t c o u r t , th e D F M A

m e t h o d o l o g y 3 p r o v i d e s t h r e e c r i t e r i a a g a i n s t w h i c h e a c h

P r o d u c t d e s i g n f o r m a n u f a c t u r e a n d a s s e m b l y : G B o o t h r o y d

p a r t m u s t b e e x a m i n e d a s it i s a d d e d t o t he p r o d u c t

d u r i n g a s s e m b l y :

D u r i n g th e o p e r a t i o n o f t h e p r o d u c t , d o e s t h e p a r t

m o v e r e l a t i v e t o a l l t h e o t h e r p a r t s a l r e a d y

a s s e m b l e d ? O n l y g r o s s m o t i o n s h o u l d b e c o ns i d e r e d ;

s m a l l m o t i o n s t h a t c a n b e a c c o m m o d a t e d b y i nt e g ra l

e l a s t i c e l e m e n t s , f o r e x a m p l e , a r e n o t s u f f i c i e n t f o r

a p o s i t i v e a n s w e r .

M u s t t h e p a r t b e o f a m a t e r i a l t h a t is d i f f e r e n t

f r o m t h o s e o f a ll t he o t h e r p a r t s a l r e a d y a s s e m b l e d ,

o r m u s t i t b e i s o l a t e d f r o m t h e s e ? O n l y f u n d a m e n t a l

r e a s o n s r e l a t i n g t o m a t e r i a l s p r o p e r t i e s a r e a c c e p t a b l e .

M u s t th e p a r t b e s e p a r a t e f r o m a ll t h e o t h e r p a r t s

a l r e a d y a s s e m b l e d b e c a u s e n e c e s s a r y a s s e m b l y o r

d i s a s s e m b l y o f o t h e r s e p a r a t e p a r t s w o u l d o t h e r w i s e

b e i m p o s s i b le ?

T h e a p p l i c a t i o n o f t he s e c r i te r i a t o t h e p r o p o s e d d e s i g n

(see Figure 6) d u r i n g a s s e m b l y p r o c e e d s a s f o l lo w s :

T h e b a s e is a s s e m b l e d i n to a f i x tu r e , a n d , s in c e t h e r e

a r e n o o t h e r p a r t s w i t h w h i c h t o c o m b i n e i t, it i s a

t h e o r e t i c a l l y n e c e s s a r y p a r t .

T h e t w o b u s h e s d o n o t s a t is f y t h e c r i t e r ia , a n d c a n

t h e o r e t i c a l l y b e i n t e g r a l w i t h t h e b a s e .

T h e m o t o r is a s t a n d a r d s u b a s s e m b l y o f p a r t s w h i c h

i s a p u r c h a s e d i t e m . T h u s , t h e c r i t e r i a c a n n o t b e

a p p l i e d u n l e s s t h e a s s e m b l y o f t h e m o t o r i t s e l f i s

c o n s i d e r e d a s p a r t o f t h e a n a l y s i s . I n t h is e x a m p l e ,

w e a s s u m e t h a t t h e m o t o r a n d s e n s o r a r e n o t t o b e

a n a l y s e d .

I n v a r i a b l y , s e p a r a t e f a s t e n e rs s u c h a s t h e t w o m o t o r

s c r e w s d o n o t m e e t t h e c r i t e r i a , b e c a u s e a n i n t e g r a l

f a s t e n i n g a r r a n g e m e n t i s a l w a y s t h e o r e t i c a l l y p o s s i b le .

T h e s e n s o r i s a p u r c h a s e d it e m .

T h e s e t s c r e w i s t h e o r e t i c a l l y n o t n e c e s s a r y .

T h e t w o s t a n d o f f s d o n o t m e e t t h e c r i t e r ia ; t h e y c o u l d

b e i n c o r p o r a t e d i n t o t h e b a s e.

T h e e n d p l a t e m u s t b e s e p a r a t e f or r e a s o n s o f

a s s e m b l y .

T h e t w o e n d - p l a t e sc r e w s a r e t h e o r e t ic a l l y n o t

n e c e s s a r y .

T h e p l a s ti c b u s h c a n b e o f t h e s a m e m a t e r i a l a s , a n d

t h e r e f o r e c o m b i n e d w i t h , t h e e n d p l a t e .

T h e c o v e r c a n a ls o b e c o m b i n e d w i t h t h e e n d pl a te .

F i n a l l y , t h e f o u r c o v e r s c r e w s a r e t h e o r e t i c a l l y n o t

n e c e s a r y .

F r o m t h is a n a l y s is , it c a n b e s e e n t h a t , i f t h e m o t o r a n d

s e n s o r s u b a s s e m b l i e s c a n b e a r r a n g e d t o s n a p o r s c r e w

i n t o t h e b a s e , a n d a p l a s t i c c o v e r c a n b e d e s i g n e d t o s n a p

o n , o n l y f o u r s e p a r a t e i t e m s w i l l b e n e e d e d , i n s t e a d o f

1 9 . T h e s e f o u r i t e m s r e p r e s e n t t h e t h e o r e t i c a l m i n i m u m

n u m b e r n e e d e d t o s a ti s fy th e c o n s t r a i n t s o f t h e p r o d u c t

d e s i gn w i t h o u t c o n s i d e r a t i o n o f th e p r a c t i c a l l i m i t a ti o n s .

I t is n o w n e c e s s a r y f o r t he d e s i g n e r o r d e s i g n t e a m t o

j u s t i f y t h e e x i s t e n c e o f t h o s e p a r t s t h a t h a v e n o t s a t i s f i e d

t h e c r i t e r i a . J u s t i f i c a t i o n m a y a r i s e f r o m p r a c t i c a l ,

t e c h n ic a l o r e c o n o m i c c o n s i d e r a t i o n s . I n t h i s e x a m p l e , i t

c a n b e a r g u e d t h a t t w o m o t o r s c r e w s a r e n e e d e d t o s e c u r e

t h e m o t o r , a n d o n e s e t s c r e w i s n e e d e d t o h o l d t h e s e n s o r ,

b e c a u s e a n y a l t e r n a t i v e s w o u l d b e i m p r a c t i c a l f o r a

l o w - v o l u m e p r o d u c t s u c h a s t h is .

I t c a n b e a r g u e d t h a t t h e t w o p o w d e r m e t a l b u s h e s

a r e u n n e c e s s a r y , b e c a u s e t h e b a s e c o u l d b e m a c h i n e d

C o m p u t e r - A i d e d D e s i g n V o l u m e 2 6 N u m b e r 7 J u l y 1 9 9 4 5 0 7


4/16


f r o m a n a l t e r n a t i v e m a t e r i a l w i t h t h e n e c e s s a r y f r i c ti o n a l

c h a r a c t e r i s t i c s .

F i n a l l y , i t i s v e r y d i f f i c u l t t o j u s t i f y t h e s e p a r a t e

s t a nd o f f s, e n d p l a t e , c o v er , p l a s t i c b u s h a n d a s s o c i a t e d

s i x s c r ews .

N o w , b e f o r e a n a l t e r n a t i v e d e s i g n c a n b e c o n s i d e r e d ,

i t i s n e c e s s a r y t o h a v e e s t i m a t e s o f th e a s s e m b l y t i m e s

a n d c o s t s , s o t h a t a n y p o s s i b l e s a v i n g s c a n b e t a k e n i n t o

a c c o u n t w h e n c o n s i d e r i n g d e s i g n a l t e r n a t i v e s . U s i n g

D F M A t im e s t a n d a r d s a n d k n o w l e d g e b a s e s, i t i s p o s s i b l e

t o m a k e e s t i m a t e s o f a s s e m b l y c o s t s, a n d t h e n t o e s t i m a t e

t h e co s t o f t h e p a r t s a n d a s s o c i a t e d t o o l i n g , w i t h o u t

h a v i n g f i n a l d e t a i l d r a w i n g s o f th e p a r t s .

F i r s t , Ta b le 1 s h o w s t h e r e s u lt s o f t h e D F A a n a l y s i s;

t h e t o t a l a s s e m b l y t i m e i s e s t i m a t e d t o b e 1 6 0 s. I t is a l s o

p o s s i b l e t o o b t a i n a n a b s o l u t e m e a s u r e o f t h e q u a l i t y o f

t h e d e s ig n f o r e a s e o f a s s e m b l y . T h e t h e o r e t i c a l m i n i m u m

n u m b e r o f p a r t s i s f o u r, a s e x p l a i n e d a b o v e , a n d , i f t h e s e

p a r t s w e r e e a s y t o a s s e m b l e , t h e y w o u l d t a k e 3 s e a c h t o

a s s e m b l e o n a v e r a g e . T h u s , t h e t h e o r e t i c a l m i n i m u m ( o r

i d e a l ) a s s e m b l y t i m e i s 1 2 s , a f i g u r e w h i c h c a n b e

c o m p a r e d w i t h t h e e s t i m a t e d t i m e o f 1 6 0 s , g i v i n g a n

a s s e m b l y e f f ic i e n c y ( o r D F A i n d e x ) o f 1 2 /1 6 0 , o r 7 . 5 % .

T h e e l i m i n a t i o n o f p a r t s n o t m e e t i n g t h e m i n i m u m

p a r t - c o u n t c r i t e r ia , a n d w h i c h c a n n o t b e j u s t i f ie d o n

p r a c t i c a l g r o u n d s , r e s u l ts in t h e d e s i g n c o n c e p t s h o w n i n

Figure 7 . H e r e , t h e b u s h e s a r e c o m b i n e d w i t h t h e b a s e ,

a n d t h e s t a n d o f f s , e n d p l a t e , c o v e r , p l a s t i c b u s h a n d s i x

a s s o c i a t e d s c r e w s a r e r e p l a c e d b y o n e s n a p - o n p l a s t i c

c o v e r . T h e e l i m i n a t e d i t e m s e n t a i l e d a n a s s e m b l y t i m e o f

9 7 .4 s . T h e n e w c o v e r t a k e s o n l y 4 s t o a s s e m b l e , a n d i t

a v o i d s t h e n e e d f o r a r e o r i e n t a t i o n . I n a d d i t i o n , s c r e w s

w i t h p i l o t p o i n t s a r e u s e d a n d t h e b a s e i s r e d e s i g n e d s o

t h a t t h e m o t o r i s s e l f - a l i g n i n g . Ta b le 2 p r e s e n t s t h e r e s u l t s

o f a D F A a n a l y s i s o f t h e r e d e s i g n e d a s s e m b l y ; th e n e w

a s s e m b l y t i m e i s o n l y 4 6 s , a n d t h e d e s i g n e f f i c i e n c y h a s

i n c r e a s e d t o 2 6 % .

F i n a l l y , Ta b le 3 c o m p a r e s t h e c o s t o f t h e p a r t s f o r t h e

t w o d e s i g n s . I t c a n b e s e e n t h a t t h e r e i s a s a v i n g o f $ 1 3. 7 1

i n p a r t s c o s t s . H o w e v e r . t h e t o o l i n g f o r t h e n e w c o v e r i s

, e s ti m a t e d t o b e $ 5 0 00 - - a n i n v e s t m e n t t h a t w o u l d h a v e

t o b e m a d e a t t h e o u ts e t . T h u s , t h e o u t c o m e o f t h i s s tu d y

i s a s e c o n d d e s i g n c o n c e p t t h a t r e p r e s e n t s a t o t a l s a v i n g

o f $1 4 .6 6 , o f w h i c h $ 0 .9 5 r e p r e s e n t s t h e s a v i n g s i n

a s s e m b l y t i m e .

F igure

7 Redesign of mo tor-dr ive assembly following design-for-

assembly analysis

Table 1 Results of D FA analysis for initial design of mo tor-driv e assembly

Item Num ber Theoretical Assembly Assembly

par t count time, s cost, US cents

Base 1 1 3.5 2.9

Bu sh 2 0 12.3 10.2

M oto r subassembly 1 1 9.5 7.9

M oto r screw 2 0 21.0 17.5

Sensor subassembly 1 1 8.5 7.1

Set screw 0 10.6 8.8

Stand off 2 0 16.0 13.3

End plate 1 8.4 7.0

End -plate screw 2 0 16.6 13.8

Plastic bush I 0 3.5 2.9

Threa d leads 5.0 4.2

Reo rient 4.5 3.8

Cov er 1 0 9.4 7.9

Cov er screw 4 0 31.2 26.0

To tals l 9 4 160.0 133.0

[Design efficiency=4 x 3 /160=7.5%. ]

Table 2 Results of DFA analysis for redesign of moto r-drive assembly

Item Num ber Theoretical Assembly Assembly

part coun t time, s cost, US cents

Base 1 l 3.5 2.9

Mo tor subassembly 1 l 4.5 3.8

Mo tor screw 2 0 12.0 10.0

Senso r subass emb ly 1 1 8.5 7.1

Set screw 1 0 8.5 7.1

Thr ead leads 5.0 4.2

Plastic cover 1 1 4.0 3.3

Tota ls 7 4 46.0 38.4

[Design efficiency= 4 x 3/46.0 = 26%.]

5 0 8 C o m p u t e r - A i d e d D e s i g n V o l u m e 2 6 N u m b e r 7 J u l y 1 9 9 4


5/16

Pr o d u c t d e s i g n f o r m a n u f a c t u r e a n d a s s e m b l y : G Bo o t h r o y d

Ta b le 3 Com par i s o n o f pa r t cos t s fo r m oto r -d r ive as s em bly des ign and r edes ign

Propos e d des ign Redes ign

I tem Cos t , US cen t s I t em Cos t , US cen t s

Base (alum inium ) 12.91

Bu sh (2) 2.40*

Mo tor screw (2) 0.20

Set screw 0.10"

Stan doff (2) 5 .19

End plate 5.89

End -pla te screw (2) 0.20*

Plas t i c bus h 0 .10"

Cover 8.05

Cov er screw (4) 0.40*

Total 35.44

Base (nylon) 13.43

Mo tor s c r ew (2 ) 0 .20*

Set screw 0.10"

P las t i c cover ( inc ludes too l ing ) 8 .00

Total 21.73

[*Pu rchas e d in quan t i ty . Pu rc has ed m ot o r an d s ens o r s ubas s em bl ies no t inc luded . Redes ign : too l ing cos t for p last i c cover = $5000. ]

gn concep t I~

design for assembly L

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e~arlySelection f materials

and processes and

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estimates

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, manufactur ing

,

I p r o d u c t i o o

Fi gu re

8 T yp ica l s t eps t aken in s im ul tan eous eng ineer ing s tudy us ing

D F M A

I t c a n b e n o t e d t h a t t h e r e d e s i g n s u g g e s t i o n s a r o s e

t h r o u g h t h e a p p l ic a t i o n o f t h e m i n i m u m p a r t - c o u n t

c r i te r i a d u r i n g t h e d e s i g n - f o r - a s se m b l y a n a l y s i s ; th e f i n a l

c o s t c o m p a r i s o n w a s m a d e a f t e r a s s e m b l y - c o s t a n d

p a r t s - c o s t e s t i m a t e s w e r e c o n s i d e r e d .

Figure 8 s u m m a r i z e s t h e s t e p s t a k e n w h e n u s i n g

D F M A d u r i n g d e s ig n . T h e d e s i g n - fo r - a ss e m b l y ( D F A )

ana lys i s i s conduc ted f i r s t , l eading to a s impl i f i ca t ion of

t h e p r o d u c t s t r u c t u r e . T h e n , e a r l y c o s t e s ti m a t e s f o r t h e

p a r t s a r e o b t a i n e d f o r b o t h t h e o r i g i n a l d e s i g n a n d t h e

n e w d e s i g n in o r d e r t o m a k e t r a d e o f f d e c i si o n s . D u r i n g

th i s proces s , t he bes t ma te r i a l s and proces ses to be used

f o r th e v a r i o u s p a r t s a r e c o n s i d e r e d . F o r e x a m p l e , w o u l d

i t b e b e t t e r t o m a n u f a c t u r e t h e c o v e r i n t h e n e w d e s i g n

f r o m s h e e t m e t a l ? O n c e t h e m a t e r i a l s a n d p r o c e s s es h a v e

b e e n f i n a l ly se l ec t ed , a m o r e t h o r o u g h a n a l y s i s f o r d e s i g n

f o r m a n u f a c t u r e ( D F M ) c a n b e c a r r i e d o u t f o r t h e d e t a i l

des ign of the pa r t s .

D E V E L O P M E N T O F D F A A N D D F M

M E T H O D S

A s e a r l y a s t h e 1 9 6 0 s , c o m p a n i e s w e r e d e v e l o p i n g

g u i d e l in e s fo r u s e d u r i n g p r o d u c t d e s i g n . P e r h a p s o n e o f

F i g u r e 9 Mis lead ing p roduc ib i l i ty r ecom m enda t ion4 : (a ) s ing le p iece ,

(b) mult iple pieces

[T wo- fo r -one par t des ign : the s ubs t i tu t ion o f a s m al l num b er o f s im ple

s hapes to p rov ide a func t ion r a ther than a s ing le com plex s hape . ]

t h e b e s t k n o w n e x a m p l e s i s t h e Manufacturin9

Producibility Handbook, w h i c h w a s p u b l i s h e d f o r i n t e r n a l

u s e b y G e n e r a l E l e c t r i c i n t h e

U S A 4 .

In this ,

m a n u f a c t u r i n g d a t a w a s a c c u m u l a t e d i n t o o n e l a r g e

r e f er e n c e v o l u m e , w i t h t h e i d e a t h a t d e s i g n e r s w o u l d h a v e ,

a t t h e i r f i n g e r t i p s , t h e m a n u f a c t u r i n g k n o w l e d g e

necessa ry for e f f ic i en t des ign . How ever , t he em pha s i s was

o n t h e d e s i g n o f i n d i v i d u a l p a r t s f o r ' p r o d u c i b i li t y ' , a n d

l i t t l e a t t en t ion was g iven to the as sembly proces s . Thi s

a p p r o a c h l ed , f o r ex a m p l e , to t h e c u r i o u s r e c o m m e n d a t i o n

s h o w n i n

Figure 9:

' s u b s t i t u t e a s m a l l n u m b e r o f s i m p l e

s h a p e s t o p r o v i d e a f u n c t i o n r a t h e r t h a n a s i ng l e c o m p l e x

s h a p e ' . I n f a c t , w h e n o n e c o n s i d e r s t h e m e a n s w h e r e b y

the separa te s imple pa r t s in Figure 9 m i g h t b e s e c u r e d ,

o n e c a n s ee t h a t t h e t o t a l c o s t o f th i s r e c o m m e n d e d d e s i g n

w o u l d b e f a r g r e a t e r t h a n t h a t o f th e s i n g l e p a r t .

I t h a s n o w b e c o m e c l e a r t h a t t h e o b j e c t i v e s h o u l d b e

t o s i m p l i fy th e p r o d u c t s t r u c t u r e t o r e d u c e a s s e m b l y c o s t

and reduce the to t a l pa r t s cos t . In fac t , des ign for a s sembly

( D F A ) s h o u l d a l w a y s b e t h e f i r s t c o n s i d e r a t i o n .

D E S I G N F O R A S S E M B L Y

S i g n i f i c a n t b e n e f i t s d e r i v e d f r o m t h e u s e o f D F A w e r e

n o t r e a li z e d u n t i l s y s t e m a t i c - a n a l y s i s t o o l s b e c a m e

a v a i l a b l e a r o u n d 1 9 8 0 . T h e r e a s o n w a s t h a t d e s i g n

g u i d e li n e s , e v e n i f t h e y p r o v i d e s o u n d r e c o m m e n d a t i o n s ,

d o n o t h e l p t h e d e s ig n e r a n y m o r e t h a n b y s a y i n g ' t r y t o

C o m p u t e r -A i d e d De s i g n Vo l u m e 2 6 Nu m b e r 7 J u l y 1 9 94 5 0 9


6/16


design so that the product is easy to assemble'. Examples

of changes made to simplify assembly in other products

never seem to apply to the product under consideration,

and, in order to cover a reasonable proportion of possible

design changes, the design-guideline handbook would be

huge, leaving the designer to thumb through numerous

examples with little chance of success in the end.

Interestingly, most of the first efforts to develop

systematic procedures for assembly analysis concentrated

on product design for ease of

automatic assembly.

The

Hitachi Assembly Evaluation Method (AEM) 5 described

further below was directed at simplifying the automatic

insertion of parts. The Boothroyd Dewhurst design-for-

assembly (DFA) method 3 grew out of collaborative

research on design for automatic feeding and automatic

insertion carried out at the University of Massachusetts,

USA, and the University of Salford Industrial Centre,

UK. This emphasis arose from the fact that, when a

company desired to automate the assembly of a product,

it was forced to reconsider its design. There are many

examples of products for which automatic assembly is

simply not feasible without redesign, but none where

manual assembly is not feasible. Also, when redesign for

automatic assembly was undertaken, it was frequently

found that the resulting product was so easy to assemble

manually that automatic assembly could no longer be

justified. The IBM Pro-Printer is an example of this

experience 6.

It is now the application of design for

manual

assembly

that is resulting in staggering cost savings in many

products, because of the resulting simplification of the

product and the reductions in

to tal

manufacturing and

assembly costs.

As with the method just described, the idea behind

most systematic DFA methods is to consider each part

in turn as it is inserted into the product, gauge the

difficulty of the assembly process, and then sum the results

to obtain a numerical rating of assembly difficulty.

Ideally, different individuals analysing the same product

will obtain similar ratings, thus providing the means for

independent evaluation of a design.

HITACHI AEM METHOD

In 1980, the Okochi Memorial Prize was awarded for the

development of an automatic-assembly system for

tape-recorder mechanisms 7. In the process of developing

this system, the product design was considered carefully

using the Assembly Evaluation Method developed at

Hitachi. This method is based on the principle of 'one

motion for one part'. For more complicated motions, a

point-loss standard is used, and the assemblability of

the whole product is evaluated by subtract ing points lost.

The AEM method, described in 1986 by Miyakawa

and Ohashi 5, uses two indices at the earliest possible

stage of design, namely the assembly-evaluation score E,

which is used to assess the design quality or the difficulty

of assembly, and the assembly-cost ratio K, which is used

to project assembly costs relative to current assembly

costs. The method does not distinguish between manual,

robot or automatic assembly, because, Myakawa and

Ohashi believe, there is a strong correlation between the

degrees of assembly difficulty using these three methods.

In the AEM, approximately 20 symbols represent the

various assembly operations. Each symbol has an index

Elementaloperation AEM sym bo l Pen al t y core

Fr---h

4) /q

Downward

movement

Soldering 20

Figure 10 Examplesof AEM symbolsand penalty scores~

Product s t ructure and assembly operat ions

S e t c has s i s

2 Br ing down block

and hold i t to

maintain its

orientation

t Set chass is

2 Br ing Clown block

(orientation is

maintained by

spot-facing )

3 Fasten screw

1 S e t c has s i s

2 B r ing d ow n and

pressf it block

Part

assemblabi l it y Assemb[abi~i ty Assem bly Part to be

evaluat ion score evaEuat ion score cost rat io improved

1 0 0

50

7 3 1 b l o c k

6 5

1 0 0

1 0 0 8 8 A p p r o x s c r e w

0 8

65

100

80

A p p r o x

8 9 0 5 b l o c k

Figure 11 Assemblability valuationand improvementexamples8

to assess the assemblability of the part under

consideration. Examples of the symbols and penalty

scores 8 are given in

Figure 10,

and examples of their

application are given in

Figure 11.

By 1986, more than 1500 engineers at Hitachi had been

trained to use this method, and it was claimed that the

method was saving tens of millions of dollars annually.

B O O T H R O Y D D E W H U R S T D F A M E T H O D

The development of the Boothroyd Dewhurst DFA 3

method, which was described above, started in 1977 with

funding from the US National Science Foundation, and

it was first introduced in handbook form in 1980. Soon

afterwards, the Salford University Industrial Centre

produced a UK version of the handbook 9 which was

authored by K G Swift. These handbooks included

analysis methods and databases for both manual and

high-speed automatic assembly. For each process, the

handling of the parts and their insertion were considered

separately. The original procedure for design for

automatic assembly was the result of collaboration

between the author of this paper and A H Redford and

K G Swift in Salford.

Later, in the USA, the author and his colleague

P Dewhurst developed a personal computer program for

DFA which was introduced in 1982. In 1983, a new

handbook, based on the lessons learned in implement ing

DFA in industry, was introduced, and, since then, design

for robot assembly and PCB assembly have been added 3.

5 10 C o m p u t e r - A i d e d D e s i g n V o l u m e 2 6 N u m b e r 7 J u l y 1 99 4


7/16

M O R E R E C E N T D F A A N A L Y S IS M E T H O D S

S o m e t e n y e a r s a f t er t he i n t r o d u c t i o n o f th e H i t a c h i A E M

a n d t h e B o o t h r o y d D e w h u r s t D F A m e t h o d s , v a r ia t i on s

o n t h e s e s t a r t e d t o a p p e a r . O n e o f t h e f ir s t w a s t h a t o f

W a r n e c k e a n d B a s s l e r a t t h e U n i v e r s i t y o f S t u t t g a r t ,

G e r m a n y . I n t h e i r m e t h o d l , w h i c h t h e y n a m e d

A s s e m b l y - O r i e n t e d P r o d u c t D e s i g n , t h e y a s s e s s e a c h

p a r r s u s e f u ln e s s o r f u n c t i o n a l v a l u e . T h u s , b o t h a s s e m b l y

d i f fi c u l ty a n d f u n c t i o n a l v a l u e a r e e v a l u a t e d , a n d a

c o m b i n e d r a t i n g i s g i v e n . T h i s m e a n s t h a t p a r t s w h i c h

h a v e l i t t l e f u n c t i o n a l v a l u e ( s u c h a s s e p a r a t e f a s t e n e r s )

a n d t h a t a r e d i f fi c u l t t o a s s e m b l e a r e g i v e n t h e l o w e s t

r a t in g s . T h e s e r a t i n g s a r e t h e n u s e d a s g u i d e s t o r e d e s i g n

(see

Figure 12).

I n t h e i r p a p e r , B L M i l e s a n d K G S w i f t d e s c r i b e t h e

a p p l i c a t i o n o f t h e L u c a s m e t h o d d e v e l o p e d a t t h e

U n iv er s i ty o f H u l l , U K , d u r in g th e la te 1 9 80 s 1 1.

I n t h e L u c a s m e t h o d , t h e t h r e e s t e p s a r e a s f o l l o w s :

A f u n c t i o n a l a n a ly s i s i s c a r r i e d o u t i n w h i c h p a r t s

a r e c a t e g o r i z e d i n t o A p a r t s ( d e m a n d e d b y t h e d e s i g n

s p e c i fi c a t i on ) , a n d B p a r t s ( r e q u i r e d b y t h a t p a r t i c u l a r

d es ig n so lu t io n ) . A ta r g e t i s s e t f o r d es ig n e f f ic ien cy ,

w h ich i s

A / ( A + B )

an d i s ex p r essed as a p e r cen tag e .

T h e o b j e c t i v e i s t o e x c e e d a n a r b i t r a r y 6 0 % t a r g e t

v a l u e b y t h e e l i m i n a ti o n o f c a t e g o r y B p a r t s t h r o u g h

r e d e s i g n . T h e a u t h o r s e m p h a s i z e a s s e m b l y - c o s t

r e d u c t i o n a n d p a r t s - c o u n t r e d u c t i o n , a n d i n c l u d e t he

u s e o f t h e B o o t h r o y d D e w h u r s t m i n i m u m - p a r t s

c r i t e r i a i n a ' t r u t h ' t a b l e t o a s s i s t i n p a r t - c o u n t

r e d u c t i o n .

A h a n d l i n g a n d f e e d i n g a n a l y s i s i s c a r r i e d o u t i n

w h i c h t h e p a r t s a r e s c o r e d o n t h e b a s is o f t h r e e a r e a s:

th e s ize an d w e ig h t o f th e p a r t , h an d l in g d i f f icu l t i e s,

a n d t h e o r i e n t a t i o n o f t h e p a rt . T h e s c o r e is s u m m e d

t o g i v e t h e t o t a l s c o r e f o r t h e p a r t , a n d a

h a n d l i n g / f e e d i n g r a t i o i s c a l c u l a t e d w h i c h i s g i v e n

t h e t o t a l s c o r e d i v i d e d b y t h e n u m b e r o f A p a r ts . A

t a r g e t o f 2 .5 i s r e c o m m e n d e d .

A f i t t in g an a ly s i s is ca r r ie d o u t w h ich is b ased o n th e

p r o p o s e d a s s e m b l y s e q u e n c e . E a c h p a r t i s s c o r e d o n

t h e b a s i s o f w h e t h e r i t r e q u i r e s h o l d i n g i n a f i x tu r e ,

t h e a s s e m b l y d i r e c t i o n , a l i g n m e n t p r o b l e m s , r e s t r i c te d

v i s i o n , a n d t h e r e q u i r e d i n s e r t i o n f o r c e . T h e t o t a l

s c o r e is d i v i d e d b y t h e n u m b e r o f A p a r t s t o g i v e th e

f i t ti n g r a ti o . A g a i n , it is r e c o m m e n d e d t h a t t h i s r a t i o

s h o u l d a p p r o a c h 2 . 5 f o r a n a c c e p t a b l e d e s i g n .

T h i s L u c a s m e t h o d i s b a s e d , in p a r t , o n t h e o r i g i n a l 1 98 0

c o l l a b o r a t i v e w o r k o f t he a u t h o r a n d P r o f e s s o r S w if t

d e s c r i b e d a b o v e .

I n a n o t h e r m e t h o d , S o n y C o r p o r a t i o n c l a i m s t o h a v e

d e v e l o p e d a u n i q u e s e t o f r u l e s f o r i n c r e a s e d p r o d u c t i v i t y

i n v o l v i n g d e s i g n - f o r - a s s e m b l y c o s t e f f e c t i v e n e s s ( D A C ) .

I n h i s p ap er , Y am ig iw a 12 r e i t e r a te s th e v iew th a t i t is

i m p o s s i b l e t o d e s i g n f o r a s s e m b l y e a s e u n l e s s o n e s t a r t s

a t t h e t i m e o f c o n c e p t i o n b e f o r e t h e b l u e p r i n t f o r t h e

p r o d u c t i s d r a w n u p . T h e i m p r o v e m e n t o f a d e s i g n a t i ts

i n c e p t i o n i s r e f e r r e d t o a s t h e c o n c e p t o f f e e d - f o r w a r d

d e s i g n , a s o p p o s e d t o m a k i n g i m p r o v e m e n t s l a t e r w i t h

f e e d b a c k f r o m t h e m a n u f a c t u r i n g p r o c e s s .

I n t h e D A C m e t h o d , f a c t o r s f o r e v a l u a t i o n a r e

c l a s si f ie d i n t o 3 0 k e y w o r d s . T h e e v a l u a t i o n s a r e d i s p l a y e d

o n a d i a g r a m u s i n g a 1 0 0 - p o i n t s y st e m f o r e a c h o p e r a t i o n ,

t h u s m a k i n g j u d g m e n t a t a g l a n c e e a s y . A l i s t o f


A S S E M B L Y P R O C E D U R E

[ n s e r t r e c e i v e r b a s e

i n r e a d i n e s s

i n s e r t h o l d e r f o r r e c e i v e r

c a p i n r e c e i v e r b a s e

I n s e r t h o l d e r

for

m i c r o p h o n e i n r e c e i v e r b a s e

I n s e r t m i c r o p h o n e i n

r e c e i v e r b a s e

I n s e r t r e c e i v e r c a p i n

r e c e i v e r b a s e

C l a m p r e c e i v e r c a p a n d

m i c r o p h o n e w i t h h o l d e r

I n s e r t a n t i - k i n k d e v i c e i n

r e c e i v e r b a se , m o u n t c a b l e

L o c k r e c e i v e r t o p i n

~osi t ion

Screw

r e c e i v e r b a s e t o

r e c e i v e r t o p

0 05 1 15 2 2.5 3 3.5

F M : f u n c t i o n a l c o n t e n t C h a r a c t e r i s t i c v a l u e o f s u b a s s e m b l y :

AM: a s s e m b l y e x p e n d i t u r e K 5 = 0 . 2 50

K M : c h a r a c t e r is t i c v a l u e o f t h e W e a k t e c h n i c a l a s p e c t o f a s s e m b l y ~ 1

a s s e m b l y

procedure

Figure 12 Evaluation of subassembly from technical viewpoint of

assembly (University of S tuttgart. Germany)

Sh a h ~

- - O -

Insert ion from above

B e f o r e i m p r o v e m e n t

DAC dlagrarl- o,mat

Core

operation DAC 50

Shaft press f i t P 80 /

Bearing press f i t T 90

C r i n g 2 0 [ ]

Screw t ig h len~rlg 90

80

Figure

13 Design-for-assem bly cost-effectiveness (DA C) exam ple

(Sony Corporation) 2

o p e r a t i o n s i s p r e s e n t e d o n t h e D A C d i a g r a m , a n d a b a r

i s d r a w n t h a t r e p r e s e n t s t h e s c o r e f o r t h a t p a r t i c u l a r

o p e r a t i o n ( s e e

Figure 13).

O p e r a t i o n s w i t h l o w s c o r e s a r e

e a s i l y i d e n t i f i e d . S i n c e 1 9 8 7 , D A C h a s b e e n i n t r o d u c e d

i n v a r i o u s c o m p a n i e s i n J a p a n a n d o v e r s e a s . E m p h a s i s

i s g i v e n t o th e e a s e w i t h w h i c h a n o p e r a t i o n c a n b e c a r r i e d

o u t a u t o m a t i c a l l y , a n d t h e m e t h o d i s u s e d t o i l l u s t r a t e

p r o b l e m s w i t h t h e e f f i ci e n c y o f t h e a s s e m b l y s y s t e m .

P C B D E S I G N F O R A S S E M B L Y

I n 1 9 8 5, A d ach i

et al.

o f t h e N E C C o r p o r a t i o n r e p o r t e d 13

t h a t t h e y w e r e d e v e l o p i n g t e c h n i q u e s f o r d e s i g n f o r e a s e

o f a s s e m b l y o f p r i n te d - c i r c u i t b o a r d s. T h e i r p r i m a r y

i n t e r e s t s i n d e s i g n f o r a s s e m b l y w e r e i n r e d u c i n g

p r o d u c t - s t r u c t u r e c o m p l e x i t y t o a v o i d c o m p l i c a t e d

a s s e m b l y m o t i o n s , a n d r e d u c i n g t h e v a r i e t y o f p a r t s s o

t h a t t h e y c o u l d b e a c c o m m o d a t e d i n a u t o m a t i c f ac il it ie s .

T h u s , a p r o d u c t d e s i g n w h i c h h a s t h e f o l l o w i n g t w o

a t t r i b u t e s i s d e f i n e d a s ' a p r o d u c t d e s i g n e d f o r e a s e o f

a s s e m b l y ' :

T h e p r o d u c t c a n b e a s s e m b l e d b y a f e w s i m p l e

m o t i o n s .

T h e v a r i e t y o f p a r t s a n d s u b a s s e m b l i e s h a s b e e n

m i n i m i z e d .

C o m p u t e r - A i d e d D e s i g n V o l u m e 2 6 N u m b e r 7 J u l y 1 9 9 4 5 11


8/16

Produc t des ign fo r manu fac ture and assembly : G Boothroyd

I

] P r 0 d u c t a s p e c t ~

I

P r o d u c t

Q u a l i t y

A p r o d u c t d e s i g n f o r e a s e o f a s s e m b l y I

I

i

I

h l . .. .. . .. .. .. .. .. .

C o n t r o l ( W o r k ) a s p e c t I

: E l - : : : : . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

o To f fi w i t h s pec i f i c a t i on o f

p r oduc t i on f ac i l i t i e s

o B a s e d o n a p r o d u c t t r e n d

Q u a n t i t y o T o m i n i m i z e t h e v a r i e t y o f

p a r ts a n d s u b - a s s e m b l i e s

D e l i v e r y o T o s y n c h r o n i z e w i t h d e v e l o p -

m e n t o f p r o d u c t i o n f a c il it ie s

C o s t o

T o s i m p l i f y th e s t r u c t u r e

o T o u s e s t a n d a r d p a r t s

D e s i g n

T o m i n im i z e d e s i g n c h a n g e s

T o a d j u s t d e s i g n e n g i n e e r i n g

t o p r o d u c t i o n e n g i n e e r i n g

T o p r e - e v a l u a t e i n p r o d u c t

d e s i g n p h a s e

T o b e s t a n d a r d i z e d

T o s h o r t e n t h e d e s i g n t e r m

T o r e f l e c t f e e d b a c k i n f o r m a -

t i o n f r o m p r o d u c t i o n

p r o c e s s e s q u i c k l y

T o o p t i m i z e t h e t r a d e o f f

b e t w e e n f u n c t i o n a n d c o s t

T o m i n i m i z e i m p a c t s o n

p r o d u c t io n p r o c e s s e s

- 1

i

D

Q u a l i t y o

0

0

Quantity o

D e l i v e r y o

0

C o s t o

0

Produc t ion Cont l~ : v ,

Q u a l i t y

Q u a n t i t y

0

T o b e a b l e t o i m p r o v e c o n t r o l

q u a l i t y

f i b be eas y t o c on t r o l and

p u r c h a s e p a d s

O

D e l i v e r y o f b s ho r t en t he c on t r o l t er m ,

Cost o T o m i n i m i z e t h e c o n t r o i

m a n - h o u r

Q u a l i t y

Q u a n t i t y

D e l i v e r y

C o s t

0

Work

T0 m a k e p r o d u c t

q u a l i t y s t a b l e

o To m ak e* , m an - ho u r

r e q u i r e m e n t s s t a b le

o T o m a k e o u t p u t

s t ab l e i

. . . . . . . . . . . . . . . . . . . . . . . . . . . i

o 7 0 p r o d u c e i u s t

i n t i m e

o T o s p e e d u p t h e

w ork t im { :

o T o b e e a s y t o w o r k

( b e a u t o m a t e d )

o T o e l i m i n a t e r e w o r k s

Figure 14 Product-design equirements for ease of assembly IN E(,') ~

The authors present a chart (see Figure 14) which lists all

the product -desi gn features that lead to ease of assembly.

The PCB evaluation tool was developed first because

the proportionate cost of PCBs had been increasing. The

tool is based on a 100-point evaluation method, with

demerit marks being given for five factors that would

hamper automation. In this method, PCB designers

evaluate the level of ease of automation. The five factors

that would hamper automation are as follows:

There are man y parts that canno t be inserted

automatically.

Man y different parts are used.

There is much soldering and retouching.

There are man y parts which must be inserted after

soldering.

There are numer ous wire harnesses.

These factors are quanUfied on ,i worksheet, where .~

formula is used to calculate demerit marks t,; bc

subtracted from the initial 100 point~.

This evaluation tool has been applied m sevcraJ

NEC Corporation divisions, and it has resulted i.q

improvements in automation insertion ratios, and

achieved improved cooperation between design and

production,

However, it appears that the NEt_ tool was not the

first systematic analysis tool for PCBs. In fact, some six

years earlier, the Xerox Corporation had published a

meth od for assessing the manufac turabili ty of PCBs ~

In this method, ten leading cost drivers (attributes) in the

design of PCBs are identified. The designer gives a rating

of 1 5 for each attribute which is then multiplied by at

coefficient developed from historical data.

Tabh, 4

lists

the attributes and coefficients. The sums of the products

512 Com puter -A ided Des ign Vo lume 26 Numbe r 7 Ju ly 1994


9/16

Table 4 Xerox manufacturability index for PCB assemblies:

polynomial coefficients and attributes 15

i K i Attributes

0 -2.823 8 Constant value

1 0.4034 Piggybacks

2 0.6177 Solderside component s

3 0.1105 Heat tolerance

4 0.8445 Large assemblies

5 0.0731 Screws and mechanica ls

6 0.1477 Harnesses

7 0.8485 Componen t spacing

8 0.6004 % autoinsert

9 0.0676 Orientation

10 0.1105 Component size

P r o d u c t d e s ig n f o r m a n u f a c t u r e a n d a s s e m b l y : G B o o t h r o y d

E x i s t i ng

Design Redesign

C os t o f

one cavity and core

$ 8,032 $11,625

Cycle Time (s) 42.8 13.3

N um b er o f c av i t ies r equ i r ed 6 2

Co s t o f p rod uc t i on mo ld $36,383 $22,925

C os t pe r pa r t

( inc . 5 cents fo r mate r i a l ) 25 .1 cents 16.8 cents

F i g u r e 15 DFM analysis of injection-moulded heater cover22

of a t t ribu tes and coef f ic ien ts g ive the m anufac tu rab i l i ty

index fo r the PCB.

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

Des ign fo r assembly has genera ted a r evo lu t ion in des ign

prac t ices , no t p r inc ipa l ly because i t u sual ly r educes

assembly cos t s , bu t because i t has a f a r g rea ter impact

o n t h e t o t a l m an u f ac t u r in g co s t s o f a p r o d u c t . T h e r ea s o n

is tha t DFA s impl i f ies the p roduct s t ruc tu re , r educes the

n u m b er o f p a r ts , an d t h e r eb y r ed u ces th e t o t a l co s t o f

the par t s . How ever , to judg e the ef fec ts o f DF A a t the

ea r l y d e s i g n s t ag e , co m p an i o n m e t h o d s f o r t h e ea r l y

es t imat ion o f par t c os t s mu s t be made avai lab le , and ,

acco r d i n g l y , m an y o f t h o s e w h o h av e d ev e l o p ed D F A

m et h o d s a r e n o w t u r n i n g t h e i r a t t en t i o n t o m e t h o d s o f

assessme nt o f par t -man ufac tu r ing d i ff icu lt ies .

Fo r exam ple , the H i tach i r esearcher s 15 have in t rodu ced

a M ach i n i n g P r o d u c i b i l i t y E v a l u a t i o n M e t h o d w h i ch,

com bined w i th the i r AE M , g ives an overa l l p roducib i l i ty -

ev a l u a t i o n m e t h o d ( P E M ) .

S i m i la r ly , T o s h i b a C o r p o r a t i o n 16 h a s d ev e l o p ed a

P r o ces s ab i l it y E v a l u a t io n M e t h o d w h i ch , co m b i n ed w i t h

o t h e r m e t h o d s , i n c l u d i n g an a s s em b l ab i l i t y - ev a l u a t i o n

m e t h o d , p r o v i d es an o v e r a l l p r o d u c i b i l i t y - ev a l u a t i o n

method . P rocessab i l i ty i s def ined as be ing p ropor t ional

to the par t ' s cos t . The par t ' s cos t i s de termined by the

se lec t ion o f the par t -p rocess ing m ethod , and then by the

des ign o f the par t shape . V ar ious p rocess ing meth ods ar e

cons idered fo r a par t i cu lar par t . The par t ' s cos t i s then

determ ined fo r a ll comb inat ion s o f the se lec ted p rocess ing

methods and su i tab le mater ia l s . Then the par t ' s des ign

is eva lua ted to see whe ther i t f it s a par t i cu lar p rocess ing

me thod , and , f ina lly, a p rocessab i l i ty eva lua t ion i s

car r ied ou t .

S i nce 1 9 8 5 , B o o t h r o y d , D ew h u r s t an d K n i g h t h av e

d ev e l o p ed m e t h o d s f o r d e s i g n e rs t o o b t a i n co s t e s t im a t e s

fo r par t s and too l ing dur ing the ear ly phases o f design .

Stud ies ha ve been com ple ted fo r ma ch ined par t s 17,

in jec t ion-mo ulded par t s ~s, d ie- cas t par t s 19, shee t -meta l

s tampings 2 and pow der -m eta l par t s 2~ . The ob jec t ive o f

t h e s e s t u d i e s w as t o p r o v i d e m e t h o d s w i t h w h i ch t h e

des igner o r des ign team can qu ick ly ob ta in in fo rmat ion

on cos t s befo re de ta i led des ign has taken p lace . For

example , an analys i s 22 o f an in jec t ion-m oulded hea ter

cover gave the r esu l t s shown in

Figure 15.

I t was ev iden t

tha t cer ta in wal l th icknesses were too la rge , and tha t ,

th rough som e fa i r ly minor des ign changes , the p rocess ing

co s t co u l d b e r ed u ced b y 3 3 %. I f t h e s e s tu d i e s h ad t ak en

p lace a t the ear ly des ign s tage , the des igner cou ld a l so

have cons idered the cos t fo r an equ iva len t shee t -meta l

par t fo r example . In f ac t , the use o f these analys i s

techn iques i s now a l lowing des igner s and purchas ing

manager s to chal lenge supp l ie r s ' es t imates . I n one

ex am p l e , i t h a s b een r ep o r t ed t h a t P o l a r o i d C o r p o r a t i o n

has save d $16 000 -20 000 on the cos t o f too l ing fo r an

in jec t ion-m oulded par t 23.

R E S U L T S O F D F M A A P P L I C A T I O N S

D F M A p r o v i d es a s y s t em a t i c p r o ced u r e f o r an a l y s i n g

p r o p o s ed d e s ig n s f r o m t h e p o i n t o f v iew o f a s sem b l y an d

manufac tu re . Th is p rocedure r esu l t s in s impler and more

re l iab le p roducts which ar e l ess expens ive to assemble

and manufac tu re . In add i t ion , any r educt ion in the

n u m b er o f p a r t s i n an a s s em b l y p ro d u ces a s n o w b a l l

eff ec t on co s t r educt ion , beca use o f the d rawings and

speci f ica tions tha t a r e no longer needed , the vendor s tha t

ar e no longer needed , and the inven to ry tha t i s e liminated .

Al l o f these fac to r s have an im por ta n t e ff ec t on overhead s ,

which , in man y cases , fo rm the la rges t p rop or t ion o f the

to ta l p roduct cos t .

D F M A t o o l s en co u r ag e d i a l o g u e b e t w een d e s i g n e r s

an d t h e m an u f ac t u r i n g en g i n ee r s an d an y o t h e r

ind iv iduals who p lay a par t in de termin ing f ina l p roduct

cos t s dur ing the ea r ly s tages o f design. Th is mea ns th a t

team work ing i s encouraged , and the benef i t s o f

s imul tane ous o r concu r ren t eng ineer ing can be ach ieved .

Defence con t r ac to r s have an especia l ly d i f f icu l t

p r o b l em i n ap p l y i n g d e s i g n f o r m an u f ac t u r e an d

assembly . Of ten , the des igner s do no t know who wi l l

b e m an u f ac t u r i n g t h e p r o d u c t t h ey a r e d e s ig n i n g b ecau s e

the des ign wi l l even tua l ly go ou t fo r b id af te r i t has

been fu l ly de ta i led . Under these c i r cumstances , com-

m u n i ca t i o n s b e t w een de s i gn an d m an u f ac t u r i n g a r e n o t

poss ib le . I n add i t ion , defence con t r ac to r s do no t have

the norm al incen t ives w i th r egard to min imiz ing the f ina l

p r o d u c t co s t . W e h av e a ll h ea r d h o r r o r s t o r i e s ab o u t t h e

r id icu lous ly h igh c os t o f seeming ly s imple i t ems such as

to i le t sea t s and door l a tches used by the mi l i t a ry . Th is

mean s tha t the defence indus t ry in genera l i s an ex t r em ely

fer ti le a r ea fo r the success fu l app l ica t ion o f DF M A.

T e x a s I n s t r u m e n t s

The f i r s t case s tudy descr ibed here has been p rov ided by

t h e D e f en s e S y s tem s an d E l ec t ro n i c s G r o u p o f T ex as

C o m p u t e r - A i d e d D e s i g n V o l u m e 2 6 N u m b e r 7 J u l y 1 99 4 5 13


10/16

Pro d u c t d e s ig n fo r ma n u fa c tu re a n d a s s e mb ly : G B o o th ro y d

~ ~ . RETAININGING

4 PLS

SPRINGSE*fS

j - ~ 5 PLS

. - . .ET2,22.G

tN PLS

SCREW COUPLING P

2 PLS {DFIILL

(LOCKIITE AND PIN

OPERATION OPERATION

REQUIRED) REQUIRED)

Figure 16 Or igina l design of reticle assem bly for thermal gunsighP4

RETICLE i ~ -.~_ ,

Figure 17 New design of reticle assem bly after DFM A 2a

I n s t r u m e n t s , D a l l a s , T X , U S A 2 ~ . T h e o r i g i n a l d e s i g n

s h o w n i n Figur e 16 i s f o r a r e t i c l e a s s e m b l y f o r a t h e r m a l

g u n s i g h t u s e d i n a g r o u n d - b a s e d a r m o u r e d v e h ic l e. I t i s

u s e d t o t r a c k a n d s i g h t t a r g e t s a t n i g h t , u n d e r a d v e r s e

b a t t l e f i e l d c o n d i t i o n s , a n d i t i s u s e d t o a l i g n t h e v i d e o

p o r t i o n o f t h e s y s t e m w i t h t h e t r a j e c t o r y p a t h o f t h e

v e h i c le ' s w e a p o n t o e n s u r e a c c u r a t e r e m o t e - c o n t r o l l e d

a i m i n g . I t m a k e s s t e a d y , p r e c is e a d j u s t m e n t s o f a c r i ti c a l

o p t i c a l e l e m e n t , w h i l e h a n d l i n g b a l l i s t i c s h o c k f r o m t h e

v e h i c le ' s w e a p o n s y s t e m s a n d m e c h a n i c a l v i b r a t i o n s

g e n e r a t e d b y t h e v e h i cl e ' s e n g i n e a n d r o u g h t e r ra i n . I t

m u s t a l s o b e l i g h t w e i g h t , a s t h i s i s a m a j o r c o n s i d e r a t i o n

f o r a l l s u c h s y s t e m s .

T h e a s s e m b l y c o n s is t s o f a c a r r i a g e s u b a s s e m b l y .

h o u s i n g , d r iv e s h a f t a n d c o u p l i n g , c o n n e c t o r b r a c k e t , t w o

s h a ft s , s p r in g s , a n d a s s o c i a t e d h a r d w a r e . T h e c o u p l i n g i s

d r i v e n b y a s i m i l a r c o u p l i n g o n t h e s y s t e m w h i c h d r i v e s

t h e c a r r i a g e s u b a s s e m b l y i n a l a t e r a l d i r e c t i o n o n t h e t w ~

s h a f t s . S p r i n g s a r e u s e d t o n e g a t e a n y b a c k l a s h i n t h e

g e a r s . T h e c u r r e n t d e s i g n r e q u i r e s o v e r 12 h ~ ,i

m e t a l - f a b r i c a t i o n t i m e . a n d m o r e t h a n 2 h o f a s s e m b l y

t im e ,

T h e g l g r o u p p e r f o r m e d a d e s i g n -l k )r - a ss e m b l y a na l y, ~ h

l o d e t e r m i n e w h a t c o u l d b e d o n e t o s i m p l if y t h e d e s ig n

a n d m a k e i t l e ss e x p e n s i v e . T h e r e s u l t s o f t h e a n a l y s i s

s h o w e d t h a t f a s t e n e rs a n d r e o r i e n t a t i o n s o f t h e a s s e m b b

w e r e t h e t w o m a i n c o n t r i b u t o r: ~ ~ o t h e a s s e m b l y t i m e

S p e c i a l o p e r a t i o n s f o r d ri l li n g a n d p i n n i n g c o u p l e r s a n d

a p p l y i n g a d h e s i v e t o s c r e w s w e r e a l s o m a j o r c o n t r i b u t o r s .

T h e m a i n o b j e c t iv e d u r i n g t h e r e d e s i gn w a s t o r e d u c e

h a r d w a r e , e l i m i n a t e u n n e c e s s a r y p a r t s , s t a n d a r d i z e t h e

r e m a i n d e r , a n d r e d u c e o r e l i m i n a t e r e o r i e n t a t i o n s . O n c e

t h e a n a l y s i s h a d b e g u n , s e v e r a l d e s i g n a l t e r n a t i v e s w e r e

p r o p o s e d w i t h i n a m a t t e r o f h o u rs . E v e n t u a l l y , t he b e s t

f e a t u r e s o f t h e a l t e r n a t iv e p r o p o s a l s w e r e c o m b i n e d ~ o

p r o d u c e a n e w d e s i g n ( s e e Fiqur c 71 .

T h e n e w d e s i g n i n c o r p o r a t e d t h e u s e o f a c a m ~,7~

p r o v i d e t h e c o n v e r s i o n f r o m r o t a t i o n a l t o l i n e a r m o t i o n .

T h e c a m t a k e s u p l e s s r o o m t h a n t h e g e a r b o x

a r r a n g e m e n t ; t h i s a l l o w s t h e d r i v i n g p o i n t t o b e m o v e d

f r o m t h e e n d o f t h e c a r r i a g e a s s e m b l y t o w a r d s t h e m i d d l e ,

r e d u c i n g t o r q u e o n t h e c a r r i a g e a n d r e s u l t i n g i n a

s m o o t h e r m o t i o n . T h e c a m a l s o e l i m i n a t e s t h e n e e d f o r

a c o u p l i n g a n d a d r i l l - a n d - p i n o p e r a t i o n . V i r t u a l l y a l l

t h e f a s te n e r s w e r e e l i m i n a t e d b y r e d u c i n g t h e n u m b e r o f

p a r t s t h a t n e e d e d t o b e s e c u r e d , a n d i n c o r p o r a t i n g t h e

u s e o f s e l f - s e c u r i n g p a r t s , s u c h a s p r e s s - f i t s h a f t s a n d

b u s h i n g s . T h e t w o m a j o r m e t a l - f a b r i c a t i o n i t e m s w e r e

c h a n g e d t o c a s t a l u m i n i u m l i n j e c t i o n - m o u l d e d p l a s t i c

w a s r u le d o u t b e c a u s e o f l o w p r o d u c t i o n v o l u m e a n d

b a l l i s t i c - s h o c k r e q u i r e m e n t s ) , a n d t h e c o n n e c t o r b r a c k e t

w a s i n c o r p o r a t e d i n t o t h e h o u s i n g , t h u s e l i m i n a t i n g t w o

p a r t s , a s s o c i a t e d h a r d w a r e , a n d a s p e c i a l o p e r a t i o n t o

a p p l y a d h e s i v e t o t h e s c r e w s . T h e f a b r i c a t i o n t i m e w a s

a l s o g r e a t l y r e d u c e d , o w i n g t o t h e u s e o f a c a s t i n g r a t h e r

t h a n a m a c h i n e d c o m p o n e n t , a n d t h e e l i m i n a t i o n o f

u n n e c e s s a r y p a r t s a s i n d i c a t e d d u r i n g t h e d e s i g n - f o r -

a s s e m b l y a n a l y s i s . T h i s n e w d e s i g n w a s a l s o a n a l y s e d

u s i n g th e d e s i g n - f o r - a s s e m b l y p ro c e d u r e , a n d T ab le 5 l i s t s

t h e r e s u l t s f o r t h e o r i g i n a l d e s i g n a n d f o r t h e r e d e s ig n .

I t c a n b e s e e n f r o m T abh , 5 t h a t v e r y i m p r e s s i v e r e s u l t s

w e r e o b t a i n e d i n a ll a s p e c t s o f th e m a n u f a c t u r e o f th i s

a s s e m b l y . I n a d d i t i o n , t h e s a v i n g s in o v e r h e a d s , w h i c h

a r e p a r t i c u l a r l y h i g h i n t h e d e f e n c e i n d u s t r y , w i l l b e

e n o r m o u s . I n t h e o r i g i n a l d e s i g n , t h e r e w e r e 2 4 d i f f e r e n t

p a r t s , a n d i n t h e n e w d e s i g n t h e r e a r e o n l y e i g h t . T h i s

m e a n s t h a t t h e d o c u m e n t a t i o n , a c q u i s i t io n a n d i n v e n t o r y

o f 1 6 p a r t t y p e s h a s b e e n e l i m i n a t e d . O n e c a n o n l y

i m a g i n e w h a t t h e p o t e n t i a l s a v i n g s w o u l d b e if D F M A

w e r e a p p l i e d t h r o u g h o u t t h e d e f e n c e i n d u s t r y

B r o w n S h a r p e

T h e n e e d f o r a l o w - c o s t , h i g h - a c c u r a c y c o o r d i n a t e -

m e a s u r i n g m a c h i n e ( C M M ) w a s th e i m p e t u s b e h in d t h e

d e v e l o p m e n t o f t h e M i c r o V a l p e r s o n a l C M M b y B r o w n

5 1 4 Co m p u te r - A id e d De s ig n Vo lu m e 2 6 Nu m b e r 7 J u ly 1 9 94


11/16

Product design for manufacture and assembly: G Boothroyd

Table

5 Results of DFM A redesign of reticle assembly25

Original Redesign Improve ment, %

design

Assem bly time , h 2.15 0.33 84.7

Num ber of different parts 24 8 66.7

Tota l num ber O parts 47 12 74.5

Tota l num ber of operations 58 13 77.6

M etal fab rica tion time , h 12.63 3.65 71.1

W eight, lb 0.48 0.26 45.8

& S h a r p e 25. T h e p r i m a r y d e s i g n c o n s i d e r a t i o n w a s t o

p r o d u c e a C M M w h i c h w o u l d s e ll f o r o n e - h a l f o f t h e p ri c e

o f t h e e xi s ti n g p r o d u c t . T h e C M M w a s t o c o m p e t e w i t h

l o w - p r i c e d i m p o r t s w h i c h h a d p e n e t r a t e d t h e C M M

m a r k e t t o a n e v e n g r e a t e r e x t e n t t h a n i m p o r t s h a d i n

t h e a u t o m a t i v e i n d u s t r y . S in c e t h e C M M c u s t o m e r i s n o t

d r i v e n b y p ri c e al o n e , t h e n ew C M M w o u l d h a v e t o b e

m o r e a c c u r a t e t h a n t h e c u r r e n t d e s i g n , w h i l e a l s o b e i n g

eas ie r to in s ta l l , u se , ma in ta in an d r ep a i r .

B r o w n & S h a r p e s t a r t e d w i t h a c l e a n s h e e t o f p a p e r .

I n s t e a d o f d e s i g n i n g t h e b a s i c e l e m e n t s o f th e m a c h i n e

a n d t h e n a d d i n g o n p a r t s w h i c h w o u l d p e r f o r m s p e c i f i c

f u n c t i o n s r e q u i r e d f o r t h e o p e r a t i o n o f t h e m a c h i n e , i t

w a s d e c i d e d t o b u i l d a s m a n y f u n c t i o n s i n t o t h e r e q u i r e d

e l e m e n t s a s w a s f e a s i b l e . T h i s c o n c e p t w a s c a l l e d

i n t e g r a t e d c o n s t r u c t i o n . H o w e v e r , u n t i l t h e D F A

m e t h o d o l o g y w a s a p p l i e d , t h e c o s t o b j e c t i v e s c o u l d n o t

b e m e t w i t h t h e o r i g i n a l d e s i g n p r o p o s a l . A f t e r D F A , f o r

e x a m p l e , t h e s h a p e o f t h e Z r ai l w a s c h a n g e d t o a n

e l o n g a t e d h e x a g o n , t h u s p r o v i d i n g t h e n e c e s s a r y

a n t i r o t a t i o n f u n c t i o n . A s a r es u l t , t h e n u m b e r o f p a r t s

r e q u i r e d t o p r o v i d e t h e a n t i r o t a t i o n f u n c t i o n w a s

r e d u c e d f r o m 5 7 t o f o u r . I n a d d i t i o n , t h e t i m e r e q u i r e d

t o a s s e m b l e a n d a l i g n t h e a n t i r o t a t i o n r a i l w a s

e l i m i n a t e d . S i m i l a r s a v i n g s w e r e m a d e i n o t h e r a r e a s ,

s u c h a s t h e l i n e a r - d i s p l a c e m e n t m e a s u r i n g s y s t e m a n d

t h e Z - r a i l c o u n t e r b a l a n c e s y s t e m . O n i t s i n t r o d u c t i o n a t

t h e

Qual i ty Show

in Ch icag o , I L , U SA , in 1 9 8 8 , th e

m a c h i n e b e c a m e a n i n s t a n t s u c c e s s , s e t t i n g n e w i n d u s t r y

s t a n d a r d s f o r pr i c e a n d e a s e o f o p e r a t i o n . T h e p r o d u c t

h a s p r o v e d p o p u l a r n o t o n l y i n th e U S A a n d E u r o p e ,

b u t a l s o i n J a p a n .

N C R

F o l l o w i n g a y e a r - l o n g c o m p e t i t i o n f o r t h e U S A ' s

' o u t s t a n d i n g e x a m p l e o f a p p l ie d a s s e m b l y t e c h n o l o g y a n d

t h i n k i n g ' , Assembly Engineering m a g a z i n e s e l e c t e d B i l l

S p r a g u e o f N C R C o r p o r a t i o n , C a m b r i d g e , O H , U S A , a s

t h e PAT ( P r o d u c t i v i t y T h r o u g h T e c h n o l o g y ) r e ci p ie n t.

S p r a g u e , a s e n i o r a d v a n c e d - m a n u f a c t u r i n g e n g i ne e r , w a s

r e c o g n i z e d f o r h i s c o n t r i b u t i o n i n d e s i g n i n g a n e w

p o i n t - o f - s a l e t e r m i n a l c a l l e d t h e N C R 2 7 6 0 . T h e D F A

m e t h o d o l o g y , u s e d i n c o n j u n c t i o n w i t h s o l i d m o d e l l i n g ,

a s s i s t e d N C R e n g i n e e r s i n m a k i n g s i g n i f i c a n t c h a n g e s

f r o m t h e p r e v i o u s d e s i g n . T h o s e c h a n g e s t r a n s l a t e d i n t o

d r a m a t i c r e d u c t i o n s a n d s a v i n g s , a s f o l l o w s 2 6 :

6 5 % f ew er su p p l ie r s ,

7 5 % l es s a s s e m b l y t i m e ,

1 0 0 % r e d u c t i o n i n n u m b e r o f a s s e m b l y t o o l s ,

8 5 % f e w e r p a r t s ,

a t o t a l l i f e ti m e m a n u f a c t u r i n g c o s t r e d u c t i o n o f 4 4 %

( t r an s la t in g in to sav in g s o f mi l l io n s o f U S d o l la r s ) .

0

i G B E FO R E

F igure 1 8 O l d a n d n e w d e s i g n s of D i g i t a l m o u s e 2 7

A F T E R

I n d e e d , S p r a g u e e s t i m a t e d t h a t t h e r e m o v a l o f o n e s i n g l e

s c r e w f r o m t h e o r i g i n a l d e s i g n w o u l d r e d u c e l i f e t i m e

p r o d u c t c o s t s b y a s m u c h a s $ 1 2 5 00 .

D i g i t a l E q u i p m e n t

A m u l t i f u n c t i o n a l d e s i g n t e a m a t D i g i t a l E q u i p m e n t

C o r p o r a t i o n r e d e si g n e d t he c o m p a n y ' s c o m p u t e r m o u s e z 7.

T h e y b e g a n w i t h t h e c o m p e t i t i v e b e n c h m a r k i n g o f

D i g i t a l ' s p r o d u c t s a n d m i c e m a d e b y o t h e r c o m p a n i e s .

T h e y us e d D F M A s o f t w a r e t o c o m p a r e s u c h f a c to r s as

a s s e m b l y t i m e s , p a r t c o u n t s , a s s e m b l y o p e r a t i o n s , l a b o u r

c o s t s, . a nd t o t a l c o s t s o f t h e p r o d u c t s . T h e y a l s o c o n s u l t e d

w i t h h o u r l y - p a i d p e o p l e w h o a c t u a l l y a s s e m b l e d t h e m i ce .

G o r d o n L e w is , t h e D F M A c o o r d i n a t o r a n d t e a m l e a de r ,

s t a t e d t h a t D F M A g i v es t h e d e si g n t e a m a ' f o c a l p o i n t

s o t h a t [ t h e y ] c a n g o i n a n d p i n p o i n t t h e p r o b l e m s f r o m

a m a n u f a c t u r i n g p e r s p e c t i v e a n d a d e s i g n p e r s p e c t i v e ' .

' I t ' s t h e 8 0 / 2 0 r u l e ' , s a i d M r L e w i s . ' Y o u s p e n d 8 0 % o f

y o u r t i m e o n 2 0 % o f y o u r p r o b le m s ' . ' D F M A i s o n e o f

t h e t o o l s t h a t h e l p s d e s i g n t e a m s i d e n t i f y t h e r i g h t 2 0 %

o f t h e p r o b l e m s t o w o r k o n ' , h e s a id .

Figure 18

s h o w s t h e o l d a n d n e w m i c e . I n t h e n e w

D F M A d e s i gn , 1 3 0 s o f a s s e m b l y f o r a b a l l - c a g e d e v i c e

h a s b e e n r e d u c e d t o 1 5 s f o r t h e d e v i c e t h a t h a s r e p l a c e d

Computer-Aided Design Volume 26 Number 7 July 1994 515


12/16

P r o d u c t d e s i g n f o r m a n u f a c t u r e a n d a s s e m b l y : G Boothroyd

i t . O t h e r c h a n g e s t o t h e p r o d u c t s t r u c t u r e h a v e a l s o

b r o u g h t c o s t s a v in g s . F o r i n s t a n c e , t h e a v e r a g e o f s e v e n

s c r e w s i n t h e o r i g i n a l m o u s e h a s b e e n r e d u c e d t o z e r o

w i t h s n a p f i t s . T h e n e w m o u s e a l s o r e q u i r e s n o a s s e m b l y

a d j u s t m e n t s , w h e r e a s t h e a v e r a g e n u m b e r f o r p r e v i o u s

d e s i g n s w a s ei g h t . T h e t o t a l n u m b e r o f a s s e m b l y

o p e r a t i o n s h a s d e c r e a s e d f r o m 8 3 i n t h e o l d p r o d u c t t o

5 4 i n t h e n e w m o u s e . A l l t h e s e i m p r o v e m e n t s a d d u p t o

a m o u s e t h a t i s a s s e m b l e d i n 2 7 7 s , r a t h e r t h a n 5 9 2 s f or

t h e c o n v e n t i o n a l o n e . C y c l e t i m e , t o o , h a s b e e n r e d u c e d

b y D F M A . A s e c o n d d e v e l o p m e n t p r o j e c t t h a t a d h e r e d

t o t h e n e w m e t h o d o l o g y w a s f i n i s h e d i n 1 8 w e e k s ,

i n c l u d i n g t h e h a r d - t o o l i n g c y c l e . ' T h a t ' s u n b e l i e v a b l e ' .

a d m i t t e d M r L e w i s . ' N o r m a l l y i t t a k e s 18 w e e k s t o d o

h a r d t o o l i n g a l o n e ' .

M o t o r o l a

D F M A m e t h o d s h a v e b e e n u s e d a t M o t o r o l a t o s im plit~ ,'

p r o d u c t s a n d r e d u c e a s s e m b l y c o s t s . A s p a r t o f t h c

c o m m i t m e n t t o t o t a l c u s t o m e r s a ti s fa c t io n , M o t o r o l a h a s

e m b r a c e d t h e s i x - s i g m a p h i l o s o p h y f o r p r o d u c t d e s i g n

a n d m a n u f a c t u r i n g . I t s e e m e d o b v i o u s t h a t s i m p l e r

a s s e m b l y s h o u l d r e s u l t i n i m p r o v e d a s s e m b l y q u a l i t y .

W i t h t h e s e p r e c e p t s i n m i n d , t h e y s e t a b o u t d e s i g n i n g

t h e n e w g e n e r a t i o n o f v e h i c u l a r a d a p t o r s z S.

T h e p o r t a b l e - p r o d u c t s d i v i s i o n o f M o t o r o l a d e s i g n s

a n d m a n u f a c t u r e s p o r t a b l e 2 - w a y H a n d i - T a l k i e TM r a d i o s

f o r t h e l a n d m o b i l e - r a d i o m a r k e t . T h i s i n c l u d e s s u c h u s e rs

a s p o l i c e , f i r e m e n a n d o t h e r p u b l i c - s a f e t y s e r v i c e s , m

a d d i t i o n t o t h e c o n s t r u c t i o n a n d u t i l i t y f i e l d s . T h e s e

r a d i o s a r e b a t t e r y - o p e r a t e d , a n d a r e c a r r i e d a b o u t b y t he

user .

T h e d e s i g n t e a m e m b r a c e d t h e i d e a t h a t d e s i g n i n g a

p r o d u c t w i t h a h i g h a s s e m b l y e f fi c i e n cy w o u l d r e s u l t in

l o w e r m a n u f a c t u r i n g c o s t s , a n d t h e p r o v i s i o n o f t h e h i g h

D ef ec t s , Pa r t s / M i l l i on

I ]

10000

1000

100

1

0 10 20 30 40 50 60

M a n u a l A s s e m b l y E f f i c i e n c y %

Figure

19 Im pro ved assembly-design efficiency results m term s (H

increased reliability at Mo torola2s

Table 6 Redesign

of vehicular adaptor (Motorola) ~

Old New Improvem ent,

p r o d u c t p r o d u c t %

DFA assem bly efficiency,% 4 36 800

Assem bly time , s 2742 354 87

Assem bly cou nt 217 47 78

Faste ners 72 0 100

a s s e m b l y q u a l i t y d e s i r e d . T h e y a l s o c o n s i d e r e d t h a t a n

i m p o r t a n t p a r t o f a n y d e s ig n w a s t o b e n c h m a r k

c o m p e t i t o r s " p r o d u c t s a s w e l l a s t h e i r o w n , A t t h e t i m e .

M o t o r o l a p r o d u c e d t w o t y p e s o f v e h i c u la r a d a p t o r c a l le d

C o n v e r t - a - C o m TM ( C V C ) f o r d i f f e r e n t r a d i o p r o d u c t s .

Sev er a l o f th e i r co m p e t i to r s a l so o f f e r ed s imi la r u n i t s f o ,:

t h e i r r a d i o p r o d u c t s . T h e r e s u lt s o f th e r e d e s i g n e ff o r ts

w e r e s o e n c o u r a g i n g t h a t M o t o r o l a s u r v e y e d s e v e r a l

p r o d u c t s w h i c h h a d b e e n d e s i g n e d u s i n g t h e D F A

m e t h o d o l o g y t o s e e i f t h e r e m i g h t b e a g e n e r a l c o r r e l a t i o n

o f a s s e m b l y e f f ic i e n c y w i t h m a n u f a c t u r i n g q u a l i t y .

Fiqur,

I 9 s h o w s w h a t t h e y f o u n d . T h e d e f e c t l e ve l s a r e r e p o r t e d

a s d e f e c ts p e r m i l l i o n p a r t s a s s e m b l e d , w h i c h a l l o w s J

q u a l i t y e v a l u a t i o n t o b e m a d e t h a t i s i n d e p e n d e n t o f t h e

n u m b e r o f p a r t s i n t h e a ss e m b ly ' . M o t o r o l a ' s s i x - s i g m a

q u a l i ty g o a l i s 3.4 de /ec ts per mi l l ion part ,s assembled.

Each r esu l t in

Figure 19

r e p r e s e n t s a p r o d u c t w i t h a n

a n a l y

computer-aided design volume 26 issue 7 1994

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