外文翻译---尺寸控制.doc

附录附录1英文翻译DimensionalControlIntheearlydaysofengineer,thematingofpartswasachievedbymachiningonepartasnearlyaspossibletotherequiredsize,machiningthematingpartnearlytosize,andthencompletingitsmachining,continuallyofferingtheotherparttoit,untilthedesiredrelationshipwasobtained.Ifitwasinconvenienttoofferoneparttotheotherpartduringmachining,thefinalworkwasdoneatthebenchbyafitter,whoscrapedthematingpartsuntilthedesiredfitwasobtained,thefitterthereforebeingafitterintheliteralsense.Itisobviousthatthetwopartswouldhavetobedonealloveragain.Inthesedays,weexpecttobeabletopurchaseareplacementforabrokenpart,andforittofunctioncorrectlywithouttheneedforscrapingandotherfittingoperations.Whenonepartcanbeusedofftheshelftoreplaceanotherofthesamedimensionandmaterialspecification,thepartsaresaidtobetobeinterchangeable.Asystemofinterchangeabilityusuallylowerstheproductioncosts,asthereisnoneedforanexpensivefiddlingoperation,anditalsobenefitsthecustomerintheeventoftheneedtoreplacewornparts.Italso,however,demandsthatthedimensionofmatingpartsbespecified,andthatdimensionalvariations,duetomachineandoperatorshortcomings,betakenintoaccount.Someformofinspectionmustbeintroducedtoensurethatthemanufactureiscontrolled；thisisparticularlyimportant,becausedimensionalerrorsmaynotberevealeduntilsometimehaselapsed,andoftenmanymilesfromtheplacewherethemachiningwasdone.1.ToleranceandLimitsofsizeSinceitisacceptedthatitisvirtuallyimpossibletomanufactureapartwithouterror,orintherareeventofapartbeingwithouterror,tobeabletoproclaimittobeperfect(becausethemeasuringinstrumentsaresubjecttoerrors),itisnecessarytoindicatethemaximumerrorspermitted.Thedraughtsmanmustindicatethelargestandsmallestsizesthatthelimitsofsize,andthedifferencebetweenthemiscalledthetolerance,theactualtolerancemustbeincreasedwithsize.Thetoleranceshouldbeaslargeaspossible,tokeepthecosttoaminimum.Themethodofindicating,onadrawing,thepermittedtolerancedependsmainlyuponthetypeofoperationinvolved,butlocalpreferencemustalsobetakenintoaccount.Thefollowingexampleswillillustratesomeofthemethodsused,(1)Unilaterallimits.Theseareusuallyusedwhenthedistancebetweentwofaces,orthediameterofaholeorshaftidspecified.Forexample,whenadiameterisbeingground,themachinistwouldprefertoaimatthelargestsizepermitted,sothat,intheeventofhisreachingadiameterthatisjustalittlelargerthanthemaximumsizepermitted,hecantakeanothercut,knowingthathecanuseupthewholeofthetolerancebeforethejobisrejected.Adraughtsmanmightdimensionanominal75-0.012mmdiametershaftasD75.Similarly,anominal75mmholemightdimensionedasD75-0.012,thesamereasoningappliesasforshafts.(2)Bilaterallimits.Theseareusuallyappliedwhen,forexample,thepositionofaholeisspecified.Themachineoperatormaypositionheholenearerthedatumorfurtherfromthedatumthanintended,hemustaimbetweenthelimitsofposition,sothatthemaximumerrorcanbemadewithoutcausingtheparttoberejected.Thecenterdistancebetweentwoholeswouldthereforebespecifiedas,forexample,100+0.02mm.2.Fitsareconcernedwiththerelationshipbetweentwoparts.Considerashaftandaholecombination:iftheshaftislargerthanthehole,theconditionissaidtobeofinterference；andifsmallerthanthehole,theconditionissaidtobeofclearance.Theinterferencemaybesuchthatthetwopartscanbeassembledonlybyshrinking,oritmaybeveryslight,sothatthepartscanbeassembledbyhand-operatedpress.Similarly,theclearancecanbeslight,sothattheshaftcanrotateeasilyinthehole,orbelarge,sothatthereisampleclearanceforboltstopassthrough.Inorderthatthepreciseconditionisensured,thelimitsofsizeofboththeshaftandtheholemustbestipulated.(1)Classesoffit.Theseareclassifiedasfollows.Clearancefit.Whenthelimitsofsizeofboththeholeandtheshaftaresuchthattheshaftisalwayssmallerthanthehole,thefitidsaidtobeaclearancefit.Interferencefit.Whenthelimitsofsizeofboththeholeandtheshaftaresuchthattheshaftisalwayslargerthanthehole,thefitidsaidtobeainterferencefit.Transitionfit.Whenthelimitsofsizeofboththeholeandtheshaftaresuchthattheconditionmaybeclearanceorinterference,thefitidsaidtobeatransitionfit.(2)Hole-basedsystemandShaft-basedsystem.Inordertoobtainarangeofdegreesofclearance,anddegreesofinterference,itisnecessarytouseawidevariationofholesizesandshaftsizes.Forexample,amanufacturingcompanycouldbemakinganumberofparts,allofanominal25-mmdiameter,butwhichareallslightlydifferentinactuallimitsofsize,tosuittheactualfitrequiredofeachpairofparts.Thissituationcouldmeanthatalargenumberofdrills,reamers,gauges,etc.wererequired.Itislogicalthat,toreducethisnumber,astandardholecouldbeusedforeachnominalsize,andthevariationoffiteobtainedbymakingthematingshaftsmallerorlargerthanthehole.Thisisknownasahole-basedsystem.Alternatively,astandardshaftcouldbeusedforeachnominalsize,andthevariationoffitisobtainedbymakingthematingholelargerorsmaller,asrequired.Thisisknownasashaft-basedsystem.ahole-basedsystemisusuallypreferred,becauseitstandardizes“fixedsize”equipmentsuchasreamerandpluggauges；butashaft-basedsystemisusuallyalsoprovided,becausesometimesitismoreconvenienttoemployacommonshafttowhichanumberofcomponentsisassembled,eachwithadifferentfit,andsometimesitisconvenienttousebarstockwithoutfurthermachining.3.SystemsoflimitsandfitsItisconvenienttoestablishastandardizedsystemoflimitsandfits,notonlytoeliminatetheneedforthedraughtsmantodeterminethelimitseachtimeanassemblyisdetailed,butalsotostandardizethetoolsandgaugesrequired.Asystemoflimitsandfitsshouldcaterforawiderangeofnominalsizes.Tosatisfythevariousneedsofindustry,andshouldcaterforawiderangeofqualityofwork.Thesystemshould,ifpossible,betabulated,tosavetheuserthetroubleofhavingtocalculatethelimitsofsizetosuitoftheclassoffit,thequalityofthework,andthesizeofthepart.4.BritishStandard4500:1969,ISOlimitsandfitsThisstandardreplaceBS4500isessentiallyarevisionofBS1961tobringtheBritishStandardintolinewiththelatestrecommendationsoftheInternationalOrganizationforStandardization(ISO).Thesystemreferstoholesandshafts,butthesetermsdonotonlyapplytocylindricalpartsbutcanequallywellbeappliedtothespacecontainedby.Orcontaining,twoparallelfacesortangentplanes.Thesystemistabulated,andcoverssizesupto3150mm.TheNumericalControlsdevelopmentThefirstelectroniccomputerintheworldemergedin1946,thisindicatesthemankindhascreatedthetoolthatcanstrengthenandreplacethementallabourpartly.It,andmankindthosethatcreatetostrengthentoolofmanuallaborcompareamongagriculture,industrialsociety,thequalitativeleaphasarisen,hasestablishedthefoundationthatthemankindenterstheinformation-intensivesociety.6yearslater,namelyin1952,thetechnologyofthecomputerwasappliedtothelathe,thefirstnumericalcontrollathehasemergedinU.S.A.Fromthenon,thetraditionallathehadproducedthechangeofthequality.Innearlyhalfacentury,thenumericalcontrolsystemwentthroughtwostagesandsixgenerationsdevelopmentThenumericalcontrollatheisregardedastheintegratedtypicalproductsofelectromechanics,playanenormousroleamongmechanicalmanufacturingindustry,solvestructurecomplicated,accurate,batchlittle,changeableprocessingproblemofpartinthemodernmachine-buildingwell,andcanstabilizetheprocessingqualityoftheproducts,improveproductionefficiencybyalargemargin.Butseenfromsituationwhichenterprisesfaceatpresent,becausethenumericalcontrollathepriceisrelativelyexpensive,ambassadorsenterprisesareunabletodowhatonewantsverymuchtodotorelativelymaketheinvestmentonceonly.Ourcountrycanyetberegardedasakindofbettergoo