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Unit10Unit10TEXTLEARNTOSPEAKNEWWORDSANDEXPRESSIONSNOTESEXERCISESREADINGMATERIALPROFESSIONALWORDSANDPHRASES TolerancesAlthoughstandardizedmeasurementisessentialtomodernindustry,perhapsevenmoreimportantarethedesignspecificationsthatindicatethedimensionsofapart.Thesedimensionscontrolthesizeofapart,itsfeatures,and/ortheirlocationonthepartorrelativetootherparts.Throughthis,partsareabletobeinterchangedandmatedtoeachothertoformcompleteassemblies.Thepurposeofthisunitistointroducethebasicterminologyoftolerancesandfits.Sinceitisimpossibletomachineaparttoanexactsize,adesignermustspecifyanacceptablerangeofsizesthatwillstillpermittheparttofitandfunctionasintended.Themaximumandminimumsizesinpartdimensionsthatareacceptablearelimits,betweenwhichtheactualpartdimensionmustfall.Thedifferencebetweenthemaximumandminimumlimitsistolerance,orthetotalamountbywhichapartdimensionmayvary.Tolerancesondrawingsareoftenindicatedbyspecifyingalimit,orbyplusandminusnotations(asshowninFig.10-1).Withplusandminustolerance,whenthetoleranceisbothaboveandbelowthenominal(truetheoretical)size,itissaidtobebilateral(twosides).Whenthetoleranceisindicatedallononesideofnominal,itissaidtobeunilateral(oneside).Fig.10-1Tolerancenotations LEARNTOSPEAKMr.Wang: Goodmorning,Mr.Yang.Mr.Yang: Goodmorning,Mr.Wang.Mr.Wang: Itseemsyouareverybusynow.Mr.Yang: Yes,alittle.Mr.Wang: Whatareyoudoing?Mr.Yang: Iammeasuringapart.Mr.Wang: Areallsizesofthepartcorrect?Mr.Yang: No,Ifindtheholeisbiggerthanthemaximumlimitofthesize.Mr.Wang: Canitbedoneoveragain?Mr.Yang: No.Ithasbecomeawasteproduct?Mr.Wang: Whydoesthedesignergiveatoleranceofapart?Mr.Yang: Becauseitisvirtuallyi
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