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Technical NoteA n E f fic ie n t C N C P r o g r a m m i n g A p p r o a c hB a s e d o n G r o u p T e c h n o l o g yM a n o c h e r D j a s s em i , D e p t . o f I n d u s t r ia l a n d E n g i n e e r i n g T e c h n o l o g y , M u r r a y S t a t e U n i v e r si t y,M u r r a y , K e n t u c k y , U S A

Ab st ractMan y s tud ies and repor ts suppo r t the s ign i f ican t impac t o fthe app l ica t ion o f g roup techno logy on var ious aspec ts o fp roduc t des ign an d m anufactu r ing . T he fac t tha t g roup tech-no logy is the common so lu t ion to s im i la r p rob lems is the mainsource o f such impac t . In th is paper , one o f the less f requen t-ly u t i l ized fea tu res o f CNC techno logy known as parametr icpar t p rogramming is used to imp lem ent the comm on so lu t ionfea tu re o f g roup techno logy in p rocess ing a se t o f s im i la rpar ts . Us ing th ree par t fami l ies , the advan tages o f b lend ingthe concep t o f g roup techno logy and parametr ic p rogramm ingfo r CNC mach in ing opera t ions a re i l lus t ra ted . The e f f ic iencyimprovement due to the use o f pa rametr ic p rogramming inte rms o f n umb er o f se tups and s ize o f p rogram is d iscussed.K e yw o r d s :Grou p Technology, Com puter Num erical Control,Parametric Part Programming

IntroductionG r o u p T e c h n o l o g y ( G T ) i s a b r o a d c o n c e p t t ha tu s e s c o m m o n a t tr ib u t es t o r e d u c e r e d u n d a n t w o r k i n

des i gn and m anufac t u r i ng and r e s u l t s i n s ho r t e rp r o d u c t d e v e l o p m e n t a n d p r o d u c t i o n t i m e . T h is G Tf e a t u r e - - s h o r t e r p r o d u c t d e v e l o p m e n t a n d p r o d u c -t i o n t i m e - - p r o v i d e s a s i g n i f ic a n t a d v a n ta g e f o r m a n -u fac t u re r s by enab l i ng t hem t o r e s pon d f a s t e r t o m ar -k e t d e m a n d / c h a n g e s a n d r u n p r o d u c t i o n m o r e e c o -n o m i c a l l y i n t o d a y 's h i g h l y c o m p e t i ti v e m a n u f a c t u r -i n g e n v i r o n m e n t .A s i gn i f i can t am oun t o f r e s ea rch and cas e s t ud i esi n g r o u p t e c h n o l o g y s h o w s t h a t G T i m p r o v e s p r o d u c -t i v it y i n one o r m o re a s p ec t s J a The app l i ca t i on o f GTcover s a r eas a s d i ve r s e a s des i gn , p roces s p l ann i ng ,t oo l i ng , s chedu l i ng , pa r t p rog ram m i ng , and m a t e r i a lhand l i ng . In a l l cas es , GT i s u s ed t o f i nd a com m onsolu t ion to a se t of s imi lar problems, that i s , des ign-i ng s i m i l a r p roduc t s o r p roces s i ng s i m i l a r pa r t s .L i kew i s e , GT i s u s ed t o c l a s s i fy pa rt des i gns , a l l ow-i ng t he e f f i c i en t g roup i ng o f s i m i l a r des i gns tha t canb e m a n u f a c t u r e d o n t h e s a m e m a c h i n e o r m a c h i n ecel l . F i rs t, the app roach avoids dup l icat ion of data ina database; and second, i t resu l t s in s igni f icant reduc-

t i o n i n b o th n u m b e r a n d v a r i e ty o f C A D f i le s , p ro c e s sp l an s , a n d N C p a r t p r o gr a m s . M a n y c o m p a n i e s in t h eUS and Eu rope have adop t ed GT fo r s t andard i za t i ono f d e s i g n a n d p r o d u c t io n t e c h n i q u e s ?T h i s s t u d y i s f o c u s e d o n t h e i m p l e m e n t a t i o n o f t h eG T c o n c e p t to a s p e c i f i c a s p e c t o f c o m p u t e r - a i d e dm anufac t u r i ng , t ha t i s , num er i ca l con t ro l (NC ) p ro -g r a m m i n g o f m a c h i n i n g a n d t u r n i n g c e n t e r s . T h ec o m m o n s o l u t io n f o r s i m i la r p r o b l e m s ( C S S P ) f e a -t u re o f G T a n d t h e p a r a m e t r ic p r o g r a m m i n g c a p ab i l-i ty o f c o m p u t e r n u m e r i c a l c o n tr o l ( C N C ) m a c h i n e sw i l l b e u s e d t o d e v e l o p a s in g l e N C p r o g r a m f o r e a c hpar t f am i l y . F o r C NC m ach i ne u s e r s , t he com bi na -t i o n o f C S S P a n d p a r a m e t r i c p r o g r a m m i n gapp roache s i s expec t ed t o c rea t e the opp o r t un i t y tog e n e r a t e N C c o d e s a n d o p e r a te C N C m a c h i n e s w i t hg rea t e r e f f ic i ency . C NC m ach i n i ng o r t u rn i ng cen t e r sl e n d t h e m s e l v e s to t h e C S S P c o n c e p t i n t w o w a y s :c o m m o n to o l in g a n d c o m m o n p r o g r a m m i n g f o r af am i l y o f pa r ts . W he n a f am i l y o f pa r t s i s p roces s edo n t h e s e m a c h i n e s , t o o l i n d e x i n g a n dl o a d i n g / u n l o a d i n g a r e p e r f o r m e d l e s s f r e q u e n t lyb e c a u s e m o s t p a r t s c a n b e m a c h i n e d b y a s e t o fcom m on t oo l s . S i m i l a r l y , t he pa ram et r i c p rog ram -m i n g f e a tu r e o f m o d e r n C N C m a c h i n e s a l lo w s ac o m m o n p r o g r a m t o b e u s e d f o r m a c h i n in g a n u m b e ro f s i m i l a r pa r t s.

C o n s i d e r i n g t h e p o t e n t i a l b e n e f i t s o f s u c h a na p p l ic a t io n f o r g r o u p t e c h n o l o g y a n d C N C m a c h i n eus e r s , a s tudy t o l ink the pa ram et r i c p rog ram m i n gt echn i que t o GT s ys t em i s app rop r i a t e . Us i ng t h reee x a m p l e s , t h e e f f i c i e n c y i m p r o v e m e n t d u e t o t h ej o i n t ap p l i ca t i on o f the t wo t ech no l og i es i s d i s cus s edi n t he fo l l owi ng s ec t i ons .

Parametric Part ProgrammingTraditionally, separate part prog ram s are wri t ten forindiv idual par t s wi th in a par t fami ly ; then the pro-g ram s a re l oaded t o t he m ach i ne con t ro ll e r one by one .

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Mo s t C N C ma c h i n e s h a v e a s p e c ia l f e a tu r e k n o w na s p a r a m e t r i c p r o g r a m m i n g , a l s o r e f e r r e d t o a sma c r o , 4 i n w h i c h a p a r t p r o g r a m c a n b e w r i t t e n u s i n gvar i ab les and paramet r i c express ions to rep resen t them ach in e ax i s pos i t ion (x , y , z, a , e t c . ), feed , and s peedf u n c ti o n s , s S i mi l a r t o c o m p u t e r p r o g r a m mi n g l a n -guages such as Pasca l o r C, computer - re l a t ed fea tu ressuch as var i ab les , a r i thmet i c , l og ic s t a t ement s , andl o o p i n g c a n b e i mp l e me n t e d in a p a r a me t r i c p r o g ra m.T h i s p r o g r a mmi n g f e a t u r e a l l o w s t h e u s e r t o l o a d as i n g le p a r t p r o g r a m f o r a f a mi l y o f p a r t s t o t h e CN Ccont ro l l e r . The par t p rogram i s then ca l l ed up fo rmach in ing a s imi la r par t o r s imi la r fea tu re on d i f fe r -en t part s . The p roc ess invo lves a s imple en t ry o f para-me ter va lues in to the ma ch ine con t ro l l e r . For examp le ,s e v e ra l c y l i n d ri c a l p a r t s ma y h a v e t w o c o m m o n p a r a -meters , such as d iameter and overa l l l eng th . A s ing lep a r a me t r i c p a r t p r o g r a m c a n b e c a l l e d u p f r o m a ma i np r o g r a m f o r ma c h i n i n g s u c h a g r o u p o f s i m i la r p ar ts .U p o n l o a d i n g th e ma i n p r o g r a m, t h e v a l u e s o f t h e t w oparameters a re en te red ; then these va lues a re t rans -f e r r e d t o p a r a me t r i c s u b p r o g r a ms . T h i s a p p r o a c hc o u l d m i n i m i z e t h e n u m b e r o f p r o g r a m c h a n ge o v er s ,r e d u c e t h e r e d u n d a n t c o d e s i n th e p a r t p r o g r a m, a n ds h o r t e n t h e l e n g t h o f t h e p r o g r a m.

MethodologyIn th i s s tudy , th ree par t fami l i es a re cons idered toi n v e s t i g a t e t h e e f f e c t s o f b l e n d i n g CSSP a n d p a r a -me t r i c p r o g r a mmi n g i n i mp r o v i n g t h e e f f i c i e n c y o fC N C o p e r a t i o n s . T h e e f f i c i e n c y i m p r o v e m e n t i sd e t e r m i n e d b a s e d o n t w o f a c t o rs : ( a ) n u mb e r o f p r o -g r a m c h a n g e o v e rs a n d ( b) n u m b e r o f N C f i le s a n ds i z e o f p a r t p r o g r a m. A s t h e n u m b e r o f p a r t t y p e sw i t h i n a p a r t f a mi l y i n c r e a s e s, th e n u m b e r o f p r o g r a mc h a n g e o v e r s o r s e t u p s i s e x p e c t e d t o i n c r e a s e . T h en u m b e r o f N C f i l e s a n d th e s i z e o f t h e p a r t p r o g r a mc a n h a v e a s i g n i f i c a n t e f f e c t o n f i l e ma n a g e m e n t a n dm e m o r y s p a c e w h e n t h e n u m b e r o f p a r t ty p e s w i t h ina par t fami ly i s re l a t ive ly h igh . Based on the a fo re-me n t i o n e d f a c t o r s , a n e f f i c i e n c y i mp r o v e me n t ( E I )me a s u r e i s d e f i n e d a s f o l lo w s :

E I - n - 1 * W a ( y ~ ; N i / n ) - P * ( 1 - W )n ( Z : U , / n )w h e r e

n i s the par t fami ly s izeN; i s the n um ber o f NC ins t ruc t ions ( l ines ) in the

c o n v e n t i o n a l N C p r o g r a m f o r a p a r t t y p e i i n t h ef a mi l y

P i s the num ber o f NC ins t ruc t ions ( l ines ) in thep a r a me t r i c p r o g r a m f o r t h e s a me p a r t f a mi l yW is the weig h t fac to rT h e me t h o d o l o g y c o n s i s ts o f t h e f o l l o w i n g s te p s :

S t e p 1 .S t e p 2 .S t e p 3 .S t e p 4 .

D e v e l o p C A D f i l e s f o r t h e t h r e e p a r tfami l i es .G e n e r a t e N C c o d e s u si n g C A D / C A M s of t-w a r e 6 fo r the par t s ' C AD f i l es .W r i t e a s in g l e p a ra me t r i c p r o g r a m f o r e a c hpart family . 7Co m p a r e t h e t w o t y p e s o f p a r t p r o g r a msdeveloped in s t eps 2 and 3 us ing the EIm e a s u r e .

The weigh t fac to r , W, i s empi r i ca l ly de te rminedu s i n g a Br i d g e p o r t CN C mi l l a n d a PC . I t w a so b s e r v e d t h a t t h e r a t i o b e t w e e n t h e t i me s t o c h a n g ep r o g r a ms , i n c l u d i n g c l e a r i n g t h e c o n t r o l me mo r y ,a n d l o c a t i n g a p r o g r a m t o p r o g r a m d o w n l o a d i n g ti mefrom a PC to CN C i s roug h ly 4:1 (W = 80% ). Th i sind ica tes tha t fo r the cases s tud ied here , the e f fec t o fp rogram leng th on EI i s no t as s ign i f i can t as thee f f e c t o f ma c h i n e s t o p p a g e f o r p r o g r a m c h a n g i n g(see Table 1) .

Analysis of Re sultsPar t fam i ly I cons i s t ed o f f ive par t s (n : 5 ) , as

i l lus t ra ted in F i g u r e 1 . A s ing le paramet r i c par t p ro -g r a m f o r ma c h i n i n g t h i s p a r t f a mi l y i s s h o w n i n t h eA p p e n d i x .

T h e n u mb e r s o f i n s t r u c t i o n s i n c o n v e n t i o n a l N Cprograms fo r ind iv idua l par t t ypes were 66 , 37 , 132 ,104, and 132. The nu m ber o f execu tab le NC ins t ruc-t i o n s i n t h e c o r r e s p o n d i n g p a r a me t r i c p r o g r a m w a s45 l ines . The overa l l EI due to app l i ca t ion o f para-

Table 1P r o g r a m C h a n g e o v e r a n d D o w n l o a d i n g T i m e s ( s e c o n d s )P ar t F am i ly

I I1 IIIC h a n g e o v e r t i m e 4 0 3 5 2 7D o w n l o a d i n g t i m e 11 8 8

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F ig u re 1Part Family 1 F ig u re 3Part Family in

F ig u re 2Part Family II

m e t r i c p r o g r a m m i n g w a s 7 4 % . H o w e v e r , i f th e n u m -ber o f pa r t t ypes i n t he pa r t f am i l y g rows and t he n ewpar t m e m b ers a re d i f f e ren t f rom t he o l d ones on l y i nt e rm s o f va l ues o f s pec i f i ed pa ram et e r s i n t he p ro -g ram , s uch as d i am et e r and ove ra l l l eng t h , t hen EIy i e l d s g r e a t e r e f f i c i e n c y i n p r o g r a m m i n g t h e C N Cmachine. For example , for 15 par t types (n = 15) theEI i nc reas es f rom 75% t o 85%.

P ar t f am i l y I I cons i s t ed o f fou r pa r t s ( F i g u r e 2 ) .The n um ber o f NC i n s t ruc t i ons fo r ind i v i dua l pa r ttypes were 27 , 53 , 8 , and 34 l ines for th i s group. Thenum ber o f NC i n s t ruc t i ons i n t he co r res pon d i ng para -m et r i c p rog ram was 25 l i nes ( s ee t he Appe nd i x ) . Theus e o f pa ram et r i c p rog ram m i n g res u lt ed i n a 64%i m p r o v e m e n t i n p r o g r a m m i n g e f f i c i e n c y . W h e n al a rge r num ber o f s i m i l a r pa r ts (n = 15) were i nc l udedby chan g i ng t he va l ue o f the pa ram et e r s , i nc l ud i ngex t e rna l r ad iu s , t h i cknes s , d i am e t e r o f cen t e r pocke t ,a n d n u m b e r o f h o l e s , t h e n t h e E I i m p r o v e d f r om 6 4 %t o 78%.Par t fami ly I I I cons i s ted of four par t s ( F i g u r e 3 ) .The num ber o f NC i n s truc t ions i n t he conven t iona lpar t program remained a t 53 for a l l par t types . Thenum ber o f NC i n s truc t ions i n t he co r res pond i ng pa ra -met r ic progra m w as 25 l ines . The use of paramet r icp rog ram m i ng res u l ted i n a 64% i m provem en t i n p ro -gra mm ing eff ic ienc y for th i s par t fami ly . By ch angin gthe value o f four parameters , including length , width ,th ickness , and num ber o f s lo ts , addi t ional par t s may b ei nc l uded i n t h is g roup . Inc reas ing t he num ber o f pa r tt ypes t o 15 i m proved the E I f rom 64% t o 85%.

T a b l e 2S u m m a r y o f N u m e r i c a l R e s u lt s

No. of Avg. No. of No. ofPart Instruction Instruction inNo. of Types No. of Lines per ParametricPart Program (n) Part Program Part Family Program

EfficiencyImprovement(El)/ /=5

EfficiencyImprovement(EI)n=15*I 5 5 94 45 54% 850/0lI 4 4 30 25 25% 78%IIl 4 4 53 24 53% 85%

*The number of part types in each family increased by 15 by changing the value of parameters.

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Table 2 summarizes the comparison results of thetwo methods of NC programming applied to the threepart families under study.

ConclusionThe results of this study support the findings ofmany other reports and studies regarding the effectsof adopting group technology in improving the effi-ciency of manufacturing operations. The parametricprogramming approach was used as a means toimplement the common solution for similar problems(CSSP) feature of group technology in CNC machin-ing operations where there are some similaritiesamong the parts. The numerical results showed animprovement in efficiency of NC programming afterapplying the parametric programming approach tothree part families. Such improvement is more signif-icant when the number of part types within a partfamily is relatively high. It is recommended that GTusers adopt parametric programming for large partfamilies or whenever there is a growing trend in thesize of the part family to minimize programchangeovers and the number of similar NC files. Athreshold value for part family size can be deter-mined based on a cost-benefit analysis consideringthe time for program changeover and the time to writethe NC program in conventional and parametric fash-ions.In today's competitive economy, manufacturingcompanies have no alternative other than takingadvantage of the new technologies in improving theefficiency of their operation. Parametric program-ming as the best-kept secret of modern CNCmachines is already at the machine tool users' dis-posal. Surveys on successful applications of paramet-ric part programming in group technology facilitieswould be an appropriate extension to this study.

AppendixP a r a m e t r i c P a r t P r o g r a m sO001G20GO T0101G97 $1500 M3G96 $1200(CHECK RIGHT SHOULDER)

(PART FAMILY I )

(P 10 EXTERNAL DIAMETER OF BAR)(P11 LENGTH OF RIGHT SHOULDER)(P 13 SHOULDER DIAMETER)IF P 11 =0 GOTO N3GO X[P10]+.I Z.I (MACHININGRIGHTSHOULDER)G71 P1 Q2 I0 K0 R0 D.05N1GO X [P13]G1 Z0Z [-P11]X [P10]N20N3(P12(P13(P14(P15

(CHECK THE THREAD)# OF PASSES)MAJOR DIAMETER)# OF THREADS/INCH)LENGTH OF THREAD)IF P 12=0 GOTO N4GO T0202 (MACHINING THE UNC THREAD,H=,6495P)GO Z.5P6=0WHILE P6 LT [PI2]P6=P6+I

GO X[P 13-[P6*[[.6495/PI 4]/P12]]]G32 Z[-P15] F.005GO X[P13]G0 Z.1WENDN4 (CHECK LEFT SHOULDER)(P16 DISTANCE FROM RIGHT FACE TO)(SHOULDER)(P17 OVERALLLENGTH OF PART)(P18 DIAMETER OF LEFT SHOULDER)N3 IF P16=PI7 GOTO N10GO T0303 (MACHININGLEFT SHOULDER)GO X[P10]Z[-P16]G71 P5 Q6 I0 K0 R0 D.05N5GO X[P18]GI Z[-P16]Z[-P17]X[PlS]N6N10GO T0404 (CUTOFF THE PART)GO X[P 10]+. 1

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Z[-P17]G1 X0G1 X[PI0]+IM5M300002(P10(Pll(P12(P13

(PART FAMILY II)# OF HOLES)RADIUS OF HOLE CIRCLE)EXTERNAL DIAMETER)THICHNESS OF PART)

(P14 Z INCREMENT)(P150 CENTER POCKET RADIUS)(P155 CUT WIDTH)G92 X[PI1] Y0S1000 M03 D1 (EXTERNAL CONTOUR)M06 T01 G43 H1GO Z0GO X[-P12] Y0G1 Z[-P13] F15G02 X[P12] Y0 R[P12] F80X[-P12]GO Z.1GO X0 Y0 (CENTER POCKET)F20G24 Z[-PI3] Q[P14] F10M06 T02 (BOLT HOLE PATTERN)GO X[Pll] YOGS1 Z[-Pl3] R.1 F20GO AA0 AB0 R[Pll]AA0 AB[360/P10]P6=0WHILE P6 LT [[P10]-I]P6=P6+lGO AA[[360/[P 10]]*P6]AB[[360/[PlO]]*[P6+l]] R[Pl l] F20WENDM05M300003(P10(Pll(P12(P13(P14

(PART FAMILY III)# OF SLOTS)THICKNESS OF PART)LENGTH OF PART)WIDTH OF PART)# OF PASSES IN Z)

(P 15 DEPTH OF SLOTS)G54T01 M6SI000 M3GO X0 Y0 Z0P6=0WHILE P6 LT [P14]P6=P6+lG1 Z[-[Pll/P14]*P6] F5v[P13] F50P7=0WHILE P7 LT [[P10]]P7=P7+lGI X[[P12/[[P10]+l]]*P7]G91Y[-P15]Y[P15]G90WENDX[P12]Y0X0GO Z.1WENDM30References:

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6 . C N C S o f t w a r e, I n c . , " M a s t e r C A M M i ll A p p l i c a ti o n M a n u a l "(Holland , CT: 1993).

7 . Mi l t ron i cs Mfg . Co . , "Cen tu r ion V Opera t ion Ma nual , Vers ion 1 .3" '(Chan hassen , MN: Dec . 1990) .

Author's BiographyM a n o c h e r D j a s s e m i r e c e i v e d a B S I E f r o m t h e U n i v e r si t y o f S c i e n c e a n dTechno logy in Teh ran , I ran , and an MS and P hD f rom the U n ivers i t y o fWiscons in -M i lwaukee . H e i s an as s i s tan t p ro fes so r i n t he Dep t . o f Indus tr i a land E ng ineer ing Techno logy a t Murray St a t e Un ivers i ty . He has a l so t augh tin t he i ndus t r ia l s t ud i es depar tm en t a t t he Un ivers i t y o f Wisc ons in -Pla t t ev i ll e . Dr . D jas semi has f i ve years o f i ndus t r ia l expe r i ence and i s a ce r -t i f i e d m a n u f a c t u r i n g e n g i n ee r . H i s p r i m a r y a r e a s o f t e a c h i n g a n d r e s e a r c ha r e c o n v e n t io n a l a n d C N C m a c h i n e t o o l s, r o b o t ic s , C A D / C A M , g r o u p t e c h -no logy , and ce l l u l a r manufac tu r ing .

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