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W hy T u n g s te nCarbide?J o n a t h a n S . K i em a n , T.Eng. (CEI), MIQA, qual ity a s suran ce m anage rof S e l e c t G aug e s Ltd , e xam ine s the grow ing popularity of t u n g s t e nc a r b id e g a u g e b l o c k s i n p r e fe r e n c e t o s t e e l .

THE int roduct ion of tungs tencarbide as a suitable material forprecision gauge blocks began almostby accident.Several years ago leadingmanufacturers, concerned at thesusceptibi l i ty of ordinary steelgauges to damage and excessivewear, started providing tungstencarb ide protec tor b locks as apractical means of overcoming thisproblem.So successful was the move inhelping to maintain the accuracy ofsteel gauges and extending theirworking life, that the next logicalstep was to prod uce com plete sets oftungsten carbide gauges ateconomical prices.Today there is a rapidly growingtrend in this direction. A number ofhigh-tech manufacturing industries,including branches of aircraftengineering, now prefer tungstencarbide gauge blocks to steel, a factwhich appears to apply equallywh ethe r the gauges are to measuringtolerances during manufacture orverifying them at the quality controlstage.Why should this be and what arethe part icular advantages oftungsten carbide over steel?Several major benefitsimm ediately come to mind, of whichthe most important are economy,thermal and dimensional stabiity,performance and durability.E c o n o m yAlthough many steel gauges costless than their tungsten carbideequ ivale nts, this is certainly not trueamong higher grade blocks wheretungsten carbide often shows verysignificant price savings.

Longevity is another importantfactor. It has been estimated thattungsten carbide gauge blocks canoutlast steel blocks by a ratio ofbetween 10:1 and 100:1 dependingupon the working environment andapplication.Because tungsten carbide blocks

enjoy this exceptionally long life,they need recalibrating much lessoften than steel blocks. This iseviden ced in guidelines issued by theBrit ish Calibrat ion Service toApproved Laboratories and whichrecommend calibrating steel blocksafter the first 12 months to verifytheir stability and thereafter every 2years. The period recommended fortungsten carbide gauges is fouryears.

This extended calibration periodnot only shows a direct saving of 50per cent in real terms but, equallyimportant to small companies withonly a limited numb er of gauge sets,it also cuts production downtimewhen gauges are away at thecalibration laboratory.Similarly, the superior wearcharacteristics of tungsten carbideensure that blocks made of thismaterial retain their accuracy muchlong er than steel equivalents and, asa result , require less frequentrenewal.

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Dimen sional stabili tyTungsten carbide is a s interedmaterial and extremely stable; theauthor is aware of no documentaryevidence to show that it is prone tosecular change. On the other hand,steel requires careful stabilisingtreatment before being used inprecision gauge work and it is veryimportant that this operation iscorrectly carried out.Th ermal stabil ityTungsten carbide has a lowcoefficient of linear expansion, only(4 .230 .10)x 10 - 6 per degree Cagainst steel's 11.7x10 -6 per degreeC.Despite the obvious advantageshown by these figures, it issometimes argued that tungstencarbide is in fact at a disadvantagebecause its coefficient of expansiondiffers too much from that ofmater ia ls general ly used incomponent manufacture.The accompanying graph showsthe linear expansion rate of somecommonly used p roduc t ionma terials and it is interesting to n otethat there is a greater differencebetween aluminium and steel thanthere is between steel and tungstencarbide.The graph shows the wide rangeof expansion coefficients betweenmaterials but as steel is the mostcommon reference point whencomparisons are being made, let usnow examine the errors introduceddue to temperature when tungstencarbide gauge blocks are used tomeas ure steel components:

Coefficient of steelCoefficient of tungsten carbideDifference

0.00001170perC0.00000423 0.00000747

Thus the temperature inducederr or in the m easurement of a 10mm(0.4in) s teel component a t atem pe rat ur e of 30C would be(0 .000 007 47x(30-20))x10mm=0.000 747mm (0.00003in) , a var iat ion which isinsignificant in many manufacturingoperations.M aster gauge accuracyTh e prop erties and characteristics oftungsten carbide make it the idealmaterial for master gauge work,wh ere the em phasis is very much onaccuracy. The combinat ion of

dimensional stability and a lowcoefficient of expansion not onlyreduces the margin of error duringcal ibrat ion, i t a lso means lessrecalibration in subsequent years, afactor which helps to encourage userconfidence in the m aster gauge.W r in g in gIt is most important that a gaugeblock w rings easily to an other block.On the shop floor, good wringingproperty helps to avoid error andovercomes frustrat ion whenhandling packs of blocks; in thelaboratory i t is an essent ia lrequ i rement when measur ingreference master gauges on aninterfero me ter using the wavelengthof light.Tung sten carbide is a particularlyhard and homogenous material andwhen it is machined, usingspecialised lapping techniques, avery high quality surface finish isachieved. In addi t ion, themeasu ring face geom etry of a gaugeblock (flatness and parallelism) canbe held to very close tolerances.These properties combine to givea wring of such good quality that firstt ime users are of ten surprised,particularly as the blocks virtuallyfuse toge ther un aided b y thetraditional wipe across the handassociated with steel gauges. Afurther bonu s is that this good wringis held for a long time.DurabilityThe grade of tungsten carbide used

in gauge block manufacturecom bines good wear resistance withlow porosity, yet is not undulybrittle.It is an extremely durablematerial. For instance, if a tungstencarbid e gauge block is dropped, it isunlikely to suffer dam age, whereas asteel block would almost certainly'bruise', distorting measuring facesclose to the p oint of contact. Such ablow can physically alter thecalibrated length of the gauge and itis essential therefore to check allmeasu ring faces for flatness and alsorecalibrate the length.Because tungsten carbide is lessductile than steel, it does not sufferthe same degree of damage. Themost common problem is when twoblocks receive a blow from eachother, raising a minute localisedburr . When the burr has beenremoved, the gauge can be reusedimmediately, without the need forrecalibration.Squ are gauge b locksTungsten carbide square gaugeblocks are becoming increasinglypopular in Bri t ish industry,complementing rectangular blocks.Although relatively new to thiscountry, they are widely used inAmerica, where they are known as'Hok e blocks'.The primary advantage of thesquare block lies in its 24.1mm(0.950in) cross-sect ion which,compared to a standard rectangularblock, offers more than twice theme asurab le surface area . This largerarea not only gives improvedstability when used for surface tablework, it also reduces the possibilityof the gauge falling over when aheight dimension is beingtransferred using a transfer standand transducer.

A central hole accepts a threadedrod which, when lightly tightened,enables large packs to be heldtogether easily and with completesecurity. It also assists the use ofaccessories such as end jaws for gapgauge work.Whether the choice is for squareor re ctangu lar gauge blocks, there isno doubt that tungsten carbide hasalready proved i tself a viable ,economic alternative to steel intod ay's aircraft industry.Select Gauges LtdSe lect W orks, TorpointCornwal lPL112PT

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