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A n a u t o m a t i c c o m p a r a t o r f o r s l i pg a u g e c a l ib r a tio nP . J . E . A l d r e d *
The m anufacture o f s l ip gauges is a h igh ly sk i l led process,the end resu l ts o f wh ich are batches o f gauges which mustbe measured and, depending on the i r de v ia t ions f romnom ina l s ize , ass igned to the cor rec t Br i t i sh or D IN s tan-dard grade. There are a l together ten o f these grades formet r ic gauges and e ight fo r imper ia l gauges. I t i s par t icu-l a r l y im po r tan t t ha t t he ca l i b r a t i on p r ocess shou ld becapable o f iden t i fy ing those gauges which meet therequire me nts of th e highest grades since high grade gaugescommand a premium pr ice . Present ca l ib ra t ion methodsusing mechanical c om para tors are, however, ba rely capableof meet ing the accuracy requ i red s ince, fo r the h ighestgrade, the to lerances on length , f la tness and para l le l ismare 50 nm. T I Research Laborator ies have there foredeve loped , on beha l f o f C oven t r y G auge L im i t ed , t heinst rum ent wh ich is described in th is paper, w i th thepr ime ob ject ive o f improv ing the accuracy o f postmanufacture ca l ib ra t ion o f s l ip gauges.M e a s u r e m e n t r e q u i r e m e n t sIn order to c lass i fy a gauge in to the cor rec t grade, i tslength, the p aral le l ism of i ts measur ing faces and thef latness of each measur ing face must be measured. Thelength is def ined (BS 4311) as the d is tance between thecentre of oqe measur ing face and a surface onto which thegauge has been wrung for measurement . Para l le lism er roris def ined as the v ar ia t ion in length o ver the measuringface and the f la tness er ror o f e ach face is the separat ionbetween two para l le l p lanes, w i t h in which the face canjust be enclosed. In pract ice, measurement at the centreand four corners o f each measur ing face (pos i t ions 1A -5A, 1 B - 5B in F ig 1) permi ts ca lcu la t ion o f these er rors.Met r ic gauges are ass igned to one o f ten Br i t i sh or D INstandard grades and im per ia l gauges to o ne of e ight Br i t is hor Am erica n standard grades (see Tab le 1). The tolerancesfor smal l gauges are t igh ter than those for la rge gauges, therange o f m et r ic gauges be ing d iv ided in to th i r tee n groupsand im per ia l gauges in to f ive groups. Each group hasd i f fe r ent to lerances and, o f course, fo r any gauge theapprop r ia te to lerances must be used for c lass i f ica t ion.Any inst rument des igned to make the measurementson which the grad ing o f s l ip gauges is based must sat is fyth ree im po r tan t r equ i r emen ts : High measur ing accuracy on gauge length, paral lel ismand f latness. The length tolerance on the highest grade(00) in BS 4311 is -+50 nm. Ide al ly, in orde r to achievea good grad ing e f f ic iency, the inst rument shou ldmeasure to w i th in -+5 nm. Lack o f damage to the h igh qua l i ty f in ish o f the measur-ing faces of the gauges. Th is is essent ial since even ana r r ow sc ra tch ( 1 - 2 / Jm w ide ) , w h i ch may no t a f f ec tthe accuracy o f the s l ip gauge, w i l l show c lea r ly aga instthe background f in ish and is com me rc ia l ly unacceptab le .*Physics Departme nt, TI Research Laboratories, Hi nxt on H all,Hinx ton, Saf f ron Walden, Essex CB 10 1RH , UK
An operat ing range which covers the range o f s ize o fme t r i c s l i p gauges f r om 1 - 1 00 mm and impe r i a l gaugesf rom 0.01 - 4 in and takes in to account the d i ffe rencein cross-sectional area between small gauges and largegauges.As we l l as meet ing these bas ic cr i te r ia , an autom atedinst rum ent des igned to ca l ib ra te gauges a f ter p rodu ct ionmus t measur e gauges su f f i c i en t l y qu i ck l y t ha t t heca l ib ra t io n throu ghp ut matches or exceeds the ra te o fproduct ion o f gauges.I n s t r u m e n t d e s ig nG e n e r a l c o n s i d e r a t i o n sThe in i t ia l des ign cons iderat ion is whether the measurementshou ld be abso lu te or comparat ive . For th is ins t rument thedecis ion is s t ra ight forw ard s ince the advantage l ies soc lear ly w i th com parat ive measurement and i ts less s t r ingentrequ i rements for therma l s tab i l i t y and long term mechanica ls tab i l i t y .The r ma l d r i f t i s an obv ious p r ob lem w h ich mus t betack led in the ea r ly s tages o f des ign. The approach we haveadopted is to bu i ld la rge therma l iner t ia in to the machineand to ensure a min ima l d is turbance o f the a i r sur round ingthe m easur ing region. Th e m achine (see Fig 2) has dime nsion sof ap prox ima te ly 1 x 1 x 2 m, weighs 1 tonnes and isinsta l led in a room w i th c lose con t ro l o f both a mp l i tude andrate o f change in temperature . In add i t ion , the gauges mustbe le f t fo r some t ime a f te r hand l ing to reach thermalequ i l ib r ium w i th the i r sur round ings. The genera l organ i -sat ion o f the c om parator is there fore th at each set o f gaugesis loaded in to a casset te together w i th a prev ious ly ca l ib ra tedmaster gauge of the sa me nomina l size. The cassettes are
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N umber o f d i f f e r en t N umber o f N umber o fnom ina l s izes groups g rad esMe tr ic 184 13 10(BS+ DIN)gaugesImper ia l 134 5 8(BS+A mer ican)gauges
P R E C I S I O N E N G I N E E R I N G 0 1 4 1 - 6 3 5 9 / 8 1 / 0 2 0 1 0 9 - 0 5 $ 0 2 . 00 1 9 8 1 I PC B u sin es s P re ss 1 0 9
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Fig 4 Schema t ic v iew o f the beam and ram, showing agauge a t the measur ing pos i t ion
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Measuring eadFig 5 "No dding" mechanism for raising the uppe r measuringheadi s in a f ixed p os i t ion and the gauges are brought up to themeasur ing posit ion by advancing the cassette along a beamwh ich passes hrough slots in the side of the ram (see alsoF ig 4) . When the upper measuring hea d is a t the c or rec the igh t , as ind ica ted by sensors wh ich de tec t the uppersur face o f the s l ip gauge, the ram is c lamped to the cy l in derby app ly ing hydrau l ic p ressure to the annu lar c lampga l le r ies in the cy l in der l iner .
A second mechanism, shown in more detai l in Fig 5,is used to ra ise and lo wer th e up per measur ing head fo rgauge inser t ion . The measuring head is suppor ted on twosteel tapes, one o f wh ich ac ts as a f lexure h inge. The o the rtape, w h ich has a longer f ree length , can be def lec ted bya p is ton to ra ise one end o f the measur ing head. When thep i s t on i s w i t hd r awn , t he head r e tur ns t o a pos i t i on de f i nedby the lengths o f the tw o tapes. Measurements o f th erepe atab i l i t y o f th is 'nod d ing ' a c t ion have shown tha t i ti s 1 .4 nm (s tandard dev ia t ion) , wh ich i l lus t ra tes the exce l -lent results that can be achieved by f lexure devices.The sequence fo r inser t ing a gauge between themeasuring heads is tha t the head is f i r s t ra ised and thenthe beam is ra ised to l i f t the p rev ious gauge c lear o f thelowe r measuring head. The m ovem ent o f the beam is inf ac t a r o t a t i on abo u t a f l e xu r e p i vo t s i tua ted a t t he l e f thand end of the beam as dep icted in Fig 3. The cassetteis then advanced one step and the beam is lowe red top lace the n ext gauge on the measuring head. F ina l ly , the
head is lowe red and th e gauge can be measured. The pr i-ma ry powe r source fo r a l l the mechan isms is pneum at ic ,bu t in o rder to min imise the genera t ion o f heat in theregion of th e measur ing heads, the actua l mechanismsa re a l l ope r a ted hyd r au l i ca l l y v i a a i r / o i l conve r t e rs mou n tedremote f rom the measur ing heads.A genera l v iew o f the measuring zone o f the com -para tor is shown in Fig 6. The cassette is appro achingthe measur ing p os i t ion and the lower m easuring headcan be seen inset in the beam. In the background are the:on t ro l conso le and the desk top co mp uter used fo r da taprocessing.C o n t r o lThere are a number o f mechan isms in th is ins t rum entwh ich must ac t in the cor rec t sequence to avo id damageto s l ip gauges or the measuring heads. A l thou gh a desktop com puter has been incorpora ted f o r da ta processing ,i t i s no t used fo r m ach ine cont ro l wh ich is hand led insteadby a purpose -bu i l t hard-wi red cont ro l sys tem. Th is approache l im ina tes t he r i s k o f i nadve r t en t co r r up t i on o f con t r o l byp r og r amming mod i f ica t i ons .The con t ro l ph i loso phy is tha t each step o f thecon t ro l sequence is on ly in i t ia ted wh en a s igna l f rom asensor has ind ica ted the com ple t ion o f the prev ious ac t ion .Most o f these sensors are op t ica l source /de tec tor pa i rswi th the led sources run a t low power to min imise heatgenerat ion. One exce pt ion is the sensor wh ich dete ctswh ether the uppe r measur ing he ad has raised. Here, amic ros witc h is needed since the sensor is in closep r o x im i t y t o t he co ll aps ing s t r u t mechan ism and no hea tsource cou ld be to le ra ted . For th e sam e reason, the secondstage he igh t se t t ing de tec to r (F ig 3 ) moun ted ad jacentto the m easur ing probes is capa cit ive.
Where necessary, fa i l -safe technique s have been usedin order to ensure tha t in the event o f fa i lu re o f a sensor,no damage wi l l resu l t . An exam ple o f th is is in he igh tse t t ing where the ram is in i t i a l l y ad justed us ing an opt ica lsensor mounted on the ou ts ide o f the ram. Th is a l lowsfu l l t rave l o f the ram wi thout r isk o f mechan ica l contac twi th the gauge. The ram is c lamped w h i le m ov ing upw ardsso tha t any de lay in c lamping w i l l leave ex t ra , ra ther thanless, c learance. The second stage, precise heig ht s et t ingusing the cap acit ive sensor also involves raising the ram
Fig 6 V iew o f the measuring zone w i th the cover openedand ram ra ised to sh ow deta i l s
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D e t e c t o r sFig 7 The measuring and co ntrol systemsby a smal l am oun t so tha t , aga in, any de lay in c lamp ingwi l l leave an excess c learance and wi l l not resu l t in damagecaused by t r y in g to inse r t a gauge in to too sma ll a gap .
The con t ro l sequence inc ludes au tom at ic load ing o fcasset tes f rom the storage rack onto the beam, ad justmento f the he igh t o f the ram to g ive the co rrec t gap be tweenthe gauge and the upper measuring head, p lac ing of eachgauge in tu rn a t the m easur ing pos i t ion and un load ing thecassette f rom the beam af ter a l l the gauges have beenmeasured. When a gauge has been posi t io ned ready fo rmeasuremen t , the con t r o l sys tem hands ove r to thec o mp u t e r w h i c h mu l t i p l e x e s to e a c h p ro b e i n t u rn a n dstores the readings. When all the probes have been read, thecom pu te r s ignals to the c on t ro l sys tem tha t the measure -men ts a re comp le ted and the con t ro l sys tem then sequencesthe ope ra t ions to pos i t ion the ne x t gauge fo r measurement .
D at a hand l i ngA H e w l e tt -P a c ka rd 9 8 2 5 A d es k t o p c o m p u t e r i s u s e d f o rthe da ta hand l ing since i t com b ines the necessa ry com pu t in gand s to rage capab i l i ti es w i th ease o f in te r fac ing to the o the re lements of the system (Fig 7).
When a new casset te of gauges is loaded o nto thebeam, an ide n t i f i ca t ion code e tched on the mas te r gaugeis read by f ib re o p t i c de tec to rs and the p rocesso r m us t thense lec t the app rop r ia te mas te r ca l ib ra t ion and to le rance da taf rom i t s memory f i l es . In add i t ion , depend ing on the c ross -sect ional area of the gauge, the correct probes for measure-ment must be se lected. The readings on the master gaugemus t be s to red and then fo r each rem a in ing gauge in thecasset te , the grade and ca l ibrat ion must be ca lcu la ted andprin ted. The master gauges have a l l been ca l ibr ated by as t an d a rd i n t e r f e ro m e t r i c m e t h o d w h i l e w ru n g t o a p la t t e n .The refore, a l thou gh gauges are in the f ree state wh enmeasured in the compara to r , a con ta c t e r ro r (o r wr in g ingf i lm th ickness ) w i l l , as requ i red , be inc luded in the leng thmeasurement.
C o m p a r a t o r p e r fo r m a n c eThe f i rs t s tages in the deve lopm en t o f the co mpara to rc o n c e n t ra te d o n a n s w e r i n g t w o c r i t ic a l q u e s t i o n s : -( i ) Can we ach ieve su f f i c ie n t t inea r i t y and s tab i l i t y f ro mthe t ransduce rs and m easur ing e lec t ron ics? ( i i ) Can thenodd in g head ' mechan ism ach ieve good rep ea tab i l i t y?The answ er to b o th these ques t ions is yes . Measuremen to t t r a n s d u c er l i n e a r i t y s h ow e d a ma x i mu m e r ro r o f o n l y1 nm ove r a 5 / lm range and the sho r t te rm s tab i l i t y o fthe m easur ing sys tem as measured by the re pea tab i l i t y o freadings on a single gauge is 1.4 n m (s.d.).
Th e re p e a t a b i l i ty o f me a s u reme n t w h e n t h e n o d d i n ghead is operated betwe en readings is 2 .0 nm (s.d. ), f ro mwh ich the rep ea tab i l i t y o f the nod d ing head i t se l f canbe ca lcu la ted as 1.4 nm (s.d. ).
Some repea tab i l i t ies o f leng th measurem en t on arange of gauges are l is ted in Table 2. F or the sma l ler gaugesthe resul ts for the last gauge to be measured (posi t ion 24in casset te) are as good as those fo r the f i rs t gauge to bemeasured af ter the master gauge (posi t ion 1 in casset te).The la rge r s tandard dev ia t ion on the 100 mm gaugein pos i t ion 24 does , however , ind ica te the l imi ta t ionsT a b l e 2 R e s u l t s o f r e p e a t a b i l i t y t es t s o n a r an g e o f d i f f e r e n tsize gaugesNom ina l Pos i t ion in S tandard dev ia t ion Per fo rmancesize, mm cassette of length requ ired,
measuremen t , am nm1 3 . 0 52 24 2 .4 51 2 . 9 55 24 2 .2 51 2.4 510.5 24 2 .7 51 2.9 151 0 0 24 5 .5 15
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imposed by the rm a l d r i f t resu l t ing f rom tempe ra tu re changesin the labo ra to ry in wh ich the mach ine was tes ted p r io rt o i ts t r a n sf e r t o a te mp e ra t u re c o n t ro l l e d ro o m.
Repea tab i l i t y o f measuremen t i s no t , o f cou rse ,synonymous w i th accu racy s ince sys temat ic e r ro rs maybe p resen t. I n o rde r to tes t the accu racy o f the com para -to r , a num ber o f gauges p rev iou s ly ca l ib ra ted in te r fe ro -me t r i ca l l y by a BCS approved labo ra to ry have beenmeasured. Ana lys is o f the resu l t s showed tha t the re wereno s ta t i s t i ca l l y s ign i f i can t d i f fe rences be tween the twose ts o f measurements. Fo l low ing ex tens ive labo ra to ryt r ia ls cove r ing ma ny thousands o f measuremen ts theins t rum en t has been t rans fe r red to Cov en t ry Gauge andis cu r ren t l y be ing in teg ra ted in to the p rodu c t ion inspec t ionsystem.
A c k n o w l e d g e m e n t sTh e a u t h o r w i s h e s t o a c k n o w l e d g e t h e ma j o r c o n t r i b u t i o n sto th is wor k made by co l leagues a t T I Research La bora to r ies ,pa r t i cu la r D .M. Marsh , T .S.R. Po t te r and Mrs P.T. Humm,and to thank the Cha i rm an o f Tube Inves tmen ts fo rpermiss ion to publ ish th is paper. I t is based on a paperp re se nt ed a t N E L E X '8 0 h e l d o n 7 -9 O c t o b e r 1 9 8 0 a t t h eNa t iona l Eng inee r ing Labora to ry , Eas t K i lb r ide , G lasgowG 7 5 0 Q U , U K .R e f e r e n c e s1. Birch K.G . Uncertainties in the Measurementof Gauge Blocksby Interferometry. NPL R eport MOM 29, 19782. Siddal lG.J. and Willey P.C.T . Flat-Surface Wringing and ContactError Variability. J. Phys. D (Appl. Phys.), 1970, 3, 8 - 2 8
Open-br idge cmmOpen b r idge con f igu ra t ion has beena d a p te d t o p ro v i d e b o t h c o n v e n i e n tload ing o f large eng inee r ing com-p o n e n t s a nd s t ru c t u ra l r i g i d i t y f o r t h ere c e n t l y i n t ro d u c e d c n c Ma u se r K M Z-S co -o rd ina te measur ing mach ines .
Travel o f the largest vers ion oft h e Ma u s e r K MZ-S i s 3 0 0 0 m m i n th ex ax is , 2000mm in the y ax is and1600ram in thez ax is . When g rea termeasur ing capac i t y i s requ i red , thedesign is a l tered, and the machineis th e n d e s ig n a te d K MZ-B . Th e t w otypes o f mach ine , however , a re s imi la rin the i r equ ipmen t , accu racy and pe r-fo rman ce . Reso lu t ion in each ax is i s1/1m and repeatabi l i ty is sa id to bebe t te r tha n 3 /~m.I n t h e o p e n -b r id g e c o n f i g u ra t i o n ,the ho r izon ta l beam o f the t rave l l ingb r idge is suppor ted a t one end by ara ised gu idew ay , wh ich i s i t se l f a b r idge .The axes are c la imed to bep e rp e n d i c u l a r t o o n e a n o t h e r t o w i t h i n2 a rc-seconds . Tran s is to r -c on t ro l led dcmoto rs ac t ing th rough p re - loaded ba l lsc rews d r ive the mach ine . Measuremen tin each axis is performed by a ZeissMin i -P hocos in op t i ca l scale. The re fe r -ence scale is an in terference grat ingon g lass, and m achine t rave l is reg is-te red by reader heads w i th l igh t emi t -t ing d iodes. The vert ica l s l ide carr iesa un ive rsa l p robe head wh ic h sensesi n d u c t i v e l y t h e p ro b e d e f l e c t io n i n t hre eaxes . Wi th the p robe in con ta c t w i t h thework -p iece , the mach ine moves to b r ingp robe de f lec t ion to ze ro . Da ta i s co l lec -ted on ly when the mach ine i s a t as tands t i l l , wh ich i s c la imed to improveaccu racy by e l im ina t in g e r ro rs due toma c h i n e mo t i o n .
D i re c t i o n a n d f o rc e o f p ro b i n ga re c o n t ro l l e d b y a l o g i c l i n k b e t w e e nprobe head and compu te r . Th is ac t i vepro bin g system g ives se l f -c entr in gunder compu te r con t ro l , eas ing measure -men t o f to o th spac ing o f gea rs and
M a u s e r K M Z - S c o o r d i n a t e m e a s u r i ngm a c h i n elocat ion of centres of smal l bores, fore x a mp l e .C o n t r o l l in g t h e m a c h i n eS t o re d i n th e me mo ry o f t h e c o mp u t e ris the Ze iss /Mauser so f tware kno wnas UMESS-S. Developed to a id theo p e ra t o r i n c o mmu n i c a t i n g w i t h t h emachine, th is sof tware a lso reducesthe da ta co l lec ted by the mach inein to eas i ly in te rp re ted resu lt s.
UMESS-S is designed to measurecom b ina t ion s o f fea tu res such as p lanesurfaces, cy l inder s, e l l ipses, cones andspheres . Add i t ion a l s o f tware packagesar e ava i lab le fo r sp ec i f i c app l i ca t ionsinc lud ing the measurem en t o f sp ira lbevel gears, cams, tur b in e b lades,impel lers and genera l ised curved sur-faces.
I l l u s t ra t in g t h e p o w e r o f t h esof tware is i ts ab i l i ty to recognise theat t i tudes of spat ia l axes and p lanes aswe l l as to d i f fe re n t ia te be twee n in te rna land ex te rna l c i rc les and cy l inde rs . Se t -t i n g u p a w o rk -p i e c e i s q u i c k l y d o n ebecause i ts a l ignment can be adjusteda u t o ma t i c a l l y w i t h i n t h e c o mp u t e r .
The p rocesso r is a Hewle t t -Packard HP 9825 T. Resul ts are recordedby h igh-speed prin ter and x - y p l o t t e r .A l t h o u g h d a t a a re n o rma l l y re c o rd e dc o n c u r re n t l y w i t h me a s u reme n t , th emachine cycle can be speeded byus ing an o f f - l i ne compu te r sys tem fo rp r in tou t . Mauser spec i f y the measur-ing unce r ta in ty in mm fo r the la rges tKMZ-S pe r co -o rd ina te in one p laneat 20 C as:
LUgs = A + - -2 5 0 0 0 0Where A, the s tandard dev ia t ion , i s0 .006m m in the x ax is , and 0 .004 mmin the y and z ax is .
Hahn & K o lb (GB) L td , Le i -cester Road, R ugby , Warw icksh i reC V 2 1 1 N Y , U K or Mauser-WerkeO b e r n d o r f GmbH, Pos t fach 1349u n d 1 3 6 0 , D -7 2 3 8 O b e rn d o r f , FR GTransdu cer m iniatur isedSubs tan t ia l reduc t ion in the d imens ionsof the t ransducer head of the Fastrac200 0 l ine ar meas uring system havebeen ach ieved w i th ou t loss o f reso lu-t ion or t raverse speed.
Th e c y l i n d r i c a l 5 5 2 -2 0 t ra n s-duce r head , wh ich i s 20mm in d iamete rb y 2 0 mm l o n g (c o mp a re d w i t h 2 7 x37mm approx ) de tec ts in f ra red rad i -a t i o n re f le c t e d fro m a c h ro m i u m o nglass grat ing . A t t raverse speeds of1m/s, re so lu t ion and c la imed re-pea tab i l i ty are 1/~m. Special systems toresolve to 0 .5/ lm at t raverse speedsup to 20m/s can a lso be supp l ied w i ththe 552 -20 .
The m anu fac tu re rs , wh o desc ribed r i f t and s tab i l i t y cha rac te r is t i cs o f 552 -20 as exce l len t , sugges t tha t the equ ip -men t i s pa r t i cu la r l y su i ted fo r app l i -ca t ion in coo rd ina te tab les , inspecz ione q u i p m e n t a n d p h o t o g ra mm e t ry t a b l es .
Me t ro n i c Te c h n o l o g y L t d , P O B o x404 , K ings Lang ley , Her t fo rdsh i reW D 4 8 N D , U K
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