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1、UnitFOURPartⅠPassageDimensionalControl1.Intheearlydaysofengineering,thematingofpartswasachievedbymachiningonepartasnearlyaspossibletotherequiredsize,machiningthematingpartnearlytosize,andthencompletingitsmachining,continuallyofferingtheotherparttoit,untilthedesire
2、drelationshipwasobtained.12.Ifitwasinconvenienttoofferoneparttotheotherpartduringmachining,thefinalworkwasdoneatthebenchbyafitter,whoscrapedthematingpartsuntilthedesiredfitwasobtained,thefitterthereforebeinga“fitter”intheliteralsense.Itwasobviousthatthetwopartswou
3、ldhavetoremaintogether,andintheeventofonehavingtobereplaced,thefittingwouldhavetobedonealloveragain.3.Inthesedays,weexpecttobeabletopurchaseareplacementforabrokenpart,andforittofunctioncorrectlywithouttheneedforscrapingandotherfittingoperations.Whenonepartcanbeuse
4、d“offtheshelf”toreplaceanotherofthesamedimensionandmaterialspecification,thepartissaidtobeinterchangeable.24.Asystemofinterchangeabilityusuallylowerstheproductioncosts,asthereisnoneedforanexpensive“fiddling”operation,anditalsobenefitsthecustomersintheeventofthenee
5、dtoreplacewornparts.Italso,however,demandsthatthedimensionsofmatingpartsbespecified,andthatdimensionalvariations,duetomachineandoperatorshortcomings,betakenintoaccount.5.Someformofinspectionmustbeintroducedtoensurethatthemanufactureiscontrolled;thisisparticularlyi
6、mportant,becausedimensionalerrorsmaynotberevealeduntilsometimehaselapsed,andoftenmanymilesfromtheplacewherethemachiningwasdone.6.ToleranceandlimitsofsizeSinceitisacceptedthatitisvirtuallyimpossibletomanufactureapartwithouterror,orintherareeventofapartbeingwithoute
7、rror,3tobeabletoproclaimittobeperfect(becausethemeasuringinstrumentsaresubjecttoerrors),itisnecessarytoindicatethemaximumerrorspermitted7.Thedraughtsmanmustindicatethelargestandsmallestsizesthatcanbepermittedwithoutthepartfunctioningincorrectly.Theextremedimension
8、sarecalledthelimitsofsize,andthedifferencebetweenthemiscalledthetolerance.8.Themagnitudeofthetolerancedependsuponthetypeofoperationinvolved,theskillofth
