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外文翻译--机械设计及加工工艺.doc

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外文翻译--机械设计及加工工艺.doc

附录MechanicalDesignandManufacturingProcessesMechanicaldesignistheapplicationofscienceandtechnologytodeviseneworimprovedproductsforthepurposeofsatisfyinghumanneeds.Itisavastfieldofengineeringtechnologywhichnotonlyconcernsitselfwiththeoriginalconceptionoftheproductintermsofitssize,shapeandconstructiondetails,butalsoconsidersthevariousfactorsinvolvedinthemanufacture,marketinganduseoftheproduct.Peoplewhoperformthevariousfunctionsofmechanicaldesignaretypicallycalleddesigners,ordesignengineers.Mechanicaldesignisbasicallyacreativeactivity.However,inadditiontobeinginnovative,adesignengineermustalsohaveasolidbackgroundintheareasofmechanicaldrawing,kinematics,dynamics,materialsengineering,strengthofmaterialsandmanufacturingprocesses.Asstatedpreviously,thepurposeofmechanicaldesignistoproduceaproductwhichwillserveaneedforman.Inventions,discoveriesandscientificknowledgebythemselvesdonotnecessarilybenefitpeopleonlyiftheyareincorporatedintoadesignedproductwillabenefitbederived.Itshouldberecognized,therefore,thatahumanneedmustbeidentifiedbeforeaparticularproductisdesigned.Mechanicaldesignshouldbeconsideredtobeanopportunitytouseinnovativetalentstoenvisionadesignofaproduct,toanalyzethesystemandthenmakesoundjudgmentsonhowtheproductistobemanufactured.Itisimportanttounderstandthefundamentalsofengineeringratherthanmemorizemerefactsandequations.Therearenofactsorequationswhichalonecanbeusedtoprovideallthecorrectdecisionsrequiredtoproduceagooddesign.Ontheotherhand,anycalculationsmademustbedonewiththeutmostcareandprecision.Forexample,ifadecimalpointismisplaced,anotherwiseacceptabledesignmaynotfunction.Gooddesignsrequiretryingnewideasandbeingwillingtotakeacertainamountofrisk,knowingthatifthenewideadoesnotworktheexistingmethodcanbereinstated.Thusadesignermusthavepatience,sincethereisnoassuranceofsuccessforthetimeandeffortexpended.Creatingacompletelynewdesigngenerallyrequiresthatmanyoldandwellestablishedmethodsbethrustaside.Thisisnoteasysincemanypeopleclingtofamiliarideas,techniquesandattitudes.Adesignengineershouldconstantlysearchforwaystoimproveanexistingproductandmustdecidewhatold,provenconceptsshouldbeusedandwhatnew,untriedideasshouldbeincorporated.Newdesignsgenerallyhavebugsorunforeseenproblemswhichmustbeworkedoutbeforethesuperiorcharacteristicsofthenewdesignscanbeenjoyed.Thusthereisachanceforasuperiorproduct,butonlyathigherrisk.Itshouldbeemphasizedthat,ifadesigndoesnotwarrantradicalnewmethods,suchmethodsshouldnotbeappliedmerelyforthesakeofchange.Duringthebeginningstagesofdesign,creativityshouldbeallowedtoflourishwithoutagreatnumberofconstraints.Eventhoughmanyimpracticalideasmayarise,itisusuallyeasytoeliminatethemintheearlystagesofdesignbeforefirmdetailsarerequiredbymanufacturing.Inthisway,innovativeideasarenotinhibited.Quiteoften,morethanonedesignisdeveloped,uptothepointwheretheycanbecomparedagainsteachother.Itisentirelypossiblethatthedesignwhichisultimatelyacceptedwilluseideasexistinginoneoftherejecteddesignsthatdidnotshowasmuchoverallpromise.Psychologistsfrequentlytalkabouttryingtofitpeopletothemachinestheyoperate.Itisessentiallytheresponsibilityofthedesignengineertostrivetofitmachinestopeople.Thisisnotaneasytask,sincethereisreallynoaveragepersonforwhichcertainoperatingdimensionsandproceduresareoptimum.Anotherimportantpointwhichshouldberecognizedisthatadesignengineermustbeabletocommunicateideastootherpeopleiftheyaretobeincorporated.Communicatingthedesigntoothersisthefinal,vitalstepinthedesignprocess.Undoubtedlymanygreatdesigns,inventions,andcreativeworkshavebeenlosttomankindsimplybecausetheoriginatorswereunableorunwillingtoexplaintheiraccomplishmentstoothers.Presentationisasellingjob.Theengineer,whenpresentinganewsolutiontoadministrative,management,orsupervisorypersons,isattemptingtosellortoprovetothemthatthissolutionisabetterone.Unlessthiscanbedonesuccessfully,thetimeandeffortspentonobtainingthesolutionhavebeenlargelywasted.Basically,thereareonlythreemeansofcommunicationavailabletous.Thesearethewritten,theoral,andthegraphicalforms.Thereforethesuccessfulengineerwillbetechnicallycompetentandversatileinallthreeformsofcommunication.Atechnicallycompetentpersonwholacksabilityinanyoneoftheseformsisseverelyhandicapped.Ifabilityinallthreeformsislacking,noonewilleverknowhowcompetentthatpersonisThecompetentengineershouldnotbeafraidofthepossibilityofnotsucceedinginapresentation.Infact,occasionalfailureshouldbeexpectedbecausefailureorcriticismseemstoaccompanyeveryreallycreativeidea.Thereisagreatdealtobelearnedfromafailure,andthegreatestgainsareobtainedbythosewillingtoriskdefeat.Inthefinalanalysis,therealfailurewouldlieindecidingnottomakethepresentationatall.Tocommunicateeffectively,thefollowingquestionsmustbeanswered1Doesthedesignreallyserveahumanneed2Willitbecompetitivewithexistingproductsofrivalcompanies3Isiteconomicaltoproduce4Canitbereadilymaintained5WillitsellandmakeaprofitOnlytimewillprovidethetrueanswerstotheprecedingquestions,buttheproductshouldbedesigned,manufacturedandmarketedonlywithinitialaffirmativeanswers.Thedesignengineeralsomustcommunicatethefinalizeddesigntomanufacturingthroughtheuseofdetailandassemblydrawings.Quiteoften,aproblemwilloccurduringthemanufacturingcycle3.Itmaybethatachangeisrequiredinthedimensioningortolerancingofapartsothatitcanbemorereadilyproduced.Thisfailsinthecategoryofengineeringchangeswhichmustbeapprovedbythedesignengineersothattheproductfunctionwillnotbeadverselyaffected.Inothercases,adeficiencyinthedesignmayappearduringassemblyortestingjustpriortoshipping.Theserealitiessimplybearoutthefactthatdesignisalivingprocess.Thereisalwaysabetterwaytodoitandthedesignershouldconstantlystrivetowardsfindingthatbetterway.Designingstartswithaneed,realorimagined.Existingapparatusmayneedimprovementsindurability,efficiently,weight,speed,orcost.Newapparatusmaybeneededtoperformafunctionpreviouslydonebymen,suchascomputation,assembly,orservicing.Withtheobjectivewhollyorpartlydefined,thenextstepindesignistheconceptionofmechanismsandtheirarrangementsthatwillperformtheneededfunctions.Forthis,freehandsketchingisofgreatvalue,notonlyasarecordofonesthoughtsandasanaidindiscussionwithothers,butparticularlyforcommunicationwithonesownmind,asastimulantforcreativeideas.tofluctuatingstress,particularattentionisgiventoareductioninstressconcentration,andtoanincreaseofstrengthatfillets,threads,holes,andfits.Stressreductionaremadebymodificationinshape,andstrengtheningmaybedonebyprestressingtreatmentssuchassurfacerollingandshallowhardening.Hollowshaftsandtubing,andboxsectionsgiveafavorablestressdistribution,togetherwithstiffnessandminimumweight.Sufficientstiffnesstomaintainalignmentanduniformpressurebetweencontactingsurfacesshouldbeprovidedforcrank,cam,andgearshafts,andforenclosuresandframescontainingbearingsupports.Thestiffnessofshaftsandothercomponentsmustbesuitabletoavoidresonantvibrations.3.Usezsicequationstocalculateandoptimizedimensions.Thefundamentalequationsofmechanicsandtheothersciencesaretheacceptedbasesforcalculations.Theyaresometimesrearrangedinspecialformstofacilitatethedeterminationoroptimizationofdimensions,suchasthebeamandsurfacestressequationsfordetermininggeartoothsize.Factorsmaybeaddedtoafundamentalequationforconditionsnotanalyticallydeterminable,e.g.,onthinsteeltubes,anallowanceforcorrosionaddedtothethicknessbasedonpressure.Whenitisnecessarytoapplyafundamentalequationtoshapes,materials,orconditionswhichonlyapproximatetheassumptionsforitsderivation,itisdoneinamannerwhichgivesresultsonthesafeside.Insituationswheredataareincomplete,equationsofthesciencesmaybeusedasproportioningguidestoextendasatisfactorydesigntonewcapacities.4.Choosematerialsforacombinationofproperties.Materialsshouldbechosenforacombinationofpertinentproperties,notonlyforstrengths,hardness,andweight,butsometimesforresistancetoimpact,corrosion,andloworhightemperatures.Cost

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