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外文翻译--切削加工工序和夹具设计.doc

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外文翻译--切削加工工序和夹具设计.doc

英文原文CuttingprocessandfixturedesignMachinetoolshaveevolvedfromtheearlyfootpoweredlathesoftheEgyptiansandJohnWilkinsonsboringmill.Theyaredesignedtoproviderigidsupportforboththeworkpieceandthecuttingtoolandcanpreciselycontroltheirrelativepositionsandthevelocityofthetoolwithrespecttotheworkpiece.Basically,inmetalcutting,asharpenedwedgeshapedtoolremovesarathernarrowstripofmetalfromthesurfaceofaductileworkpieceintheformofaseverelydeformedchip.Thechipisawasteproductthatisconsiderablyshorterthantheworkpiecefromwhichitcamebutwithacorrespondingincreaseinthicknessoftheuncutchip.Thegeometricalshapeofworkpiecedependsontheshapeofthetoolanditspathduringthemachiningoperation.Mostmachiningoperationsproducepartsofdifferinggeometry.Ifaroughcylindricalworkpiecerevolvesaboutacentralaxisandthetoolpenetratesbeneathitssurfaceandtravelsparalleltothecenterofrotation,asurfaceofrevolutionisproduced,andtheoperationiscalledturning.Ifahollowtubeismachinedontheinsideinasimilarmanner,theoperationiscalledboring.Producinganexternalconicalsurfaceuniformlyvaryingdiameteriscalledtaperturning,ifthetoolpointtravelsinapathofvaryingradius,acontouredsurfacelikethatofabowlingpincanbeproducedor,ifthepieceisshortenoughandthesupportissufficientlyrigid,acontouredsurfacecouldbeproducedbyfeedingashapedtoolnormaltotheaxisofrotation.Shorttaperedorcylindricalsurfacescouldalsobecontourformed.Flatorplanesurfacesarefrequentlyrequired.Theycanbegeneratedbyradialturningorfacing,inwhichthetoolpointmovesnormaltotheaxisofrotation.Inothercases,itismoreconvenienttoholdtheworkpiecesteadyandreciprocatethetoolacrossitinaseriesofstraightlinecutswithacrosswisefeedincrementbeforeeachcuttingstroke.Thisoperationiscalledplanningandiscarriedoutonashaper.Forlargerpiecesitiseasiertokeepthetoolstationaryanddrawtheworkpieceunderitasinplanning.Thetoolisfedateachreciprocation.Contouredsurfacescanbeproducedbyusingshapedtools.Multipleedgedtoolscanalsobeused.Drillingusesatwinedgedflutedtoolforholeswithdepthsupto5to10timesthedrilldiameter.Whetherthedrillturnsortheworkpiecerotates,relativemotionbetweenthecuttingedgeandtheworkpieceistheimportantfactor.Inmillingoperationsarotarycutterwithanumberofcuttingedgesengagestheworkpiece.Whichmovesslowlywithrespecttothecutter.Planeorcontouredsurfacesmaybeproduced,dependingonthegeometryofthecutterandthetypeoffeed.Horizontalorverticalaxesofrotationmaybeused,andthefeedoftheworkpiecemaybeinanyofthethreecoordinatedirections.BasicMachineToolsMachinetoolsareusedtoproduceapartofaspecifiedgeometricalshapeandpreciseIsizebyremovingmetalfromaductilematerialintheformofchips.Thelatterareawasteproductandvaryfromlongcontinuousribbonsofaductilematerialsuchassteel,whichareundesirablefromadisposalpointofview,toeasilyhandledwellbrokenchipsresultingfromcastiron.MachinetoolsperformfivebasicmetalremovalprocessesIturning,planning,drilling,milling,andgrinding.Allothermetalremovalprocessesaremodificationsofthesefivebasicprocesses.Forexample,boringisinternalturningreaming,tapping,andcounterboringmodifydrilledholesandarerelatedtodrillingbobbingandgearcuttingarefundamentallymillingoperationshacksawingandbroachingareaformofplanningandhoninglapping,superfinishing.Polishingandbuffingarevariantsofgrindingorabrasiveremovaloperations.Therefore,thereareonlyfourtypesofbasicmachinetools,whichusecuttingtoolsofspecificcontrollablegeometry1.lathes,2.planers,3.drillingmachines,and4.millingmachines.Thegrindingprocessformschips,butthegeometryoftheabrasivegrainisuncontrollable.TheamountandrateofmaterialremovedbythevariousmachiningprocessesmaybeIlarge,asinheavyturningoperations,orextremelysmall,asinlappingorsuperfinishingoperationswhereonlythehighspotsofasurfaceareremoved.Amachinetoolperformsthreemajorfunctions1.itrigidlysupportstheworkpieceoritsholderandthecuttingtool2.itprovidesrelativemotionbetweentheworkpieceandthecuttingtool3.itprovidesarangeoffeedsandspeedsusuallyrangingfrom4to32choicesineachcase.SpeedandFeedsinMachiningSpeeds,feeds,anddepthofcutarethethreemajorvariablesforeconomicalmachining.Othervariablesaretheworkandtoolmaterials,coolantandgeometryofthecuttingtool.Therateofmetalremovalandpowerrequiredformachiningdependuponthesevariables.Thedepthofcut,feed,andcuttingspeedaremachinesettingsthatmustbeestablishedinanymetalcuttingoperation.Theyallaffecttheforces,thepower,andtherateofmetalremoval.Theycanbedefinedbycomparingthemtotheneedleandrecordofaphonograph.ThecuttingspeedVisrepresentedbythevelocityoftherecordsurfacerelativetotheneedleinthetonearmatanyinstant.Feedisrepresentedbytheadvanceoftheneedleradiallyinwardperrevolution,oristhedifferenceinpositionbetweentwoadjacentgrooves.Thedepthofcutisthepenetrationoftheneedleintotherecordorthedepthofthegrooves.TurningonLatheCentersThebasicoperationsperformedonanenginelatheareillustrated.Thoseoperationsperformedonexternalsurfaceswithasinglepointcuttingtoolarecalledturning.Exceptfordrilling,reaming,andlapping,theoperationsoninternalsurfacesarealsoperformedbyasinglepointcuttingtool.Allmachiningoperations,includingturningandboring,canbeclassifiedasroughing,finishing,orsemifinishing.Theobjectiveofaroughingoperationistoremovethebulkofthematerialasrapidlyandasefficientlyaspossible,whileleavingasmallamountofmaterialontheworkpieceforthefinishingoperation.Finishingoperationsareperformedtoobtainthefinalsize,shape,andsurfacefinishontheworkpiece.Sometimesasemifinishingoperationwillprecedethefinishingoperationtoleaveasmallpredeterminedanduniformamountofstockontheworkpiecetoberemovedbythefinishingoperation.Generally,longerworkpiecesareturnedwhilesupportedononeortwolathecenters.Coneshapedholes,calledcenterholes,whichfitthelathecentersaredrilledintheendsoftheworkpieceusuallyalongtheaxisofthecylindricalpart.Theendoftheworkpieceadjacenttothetailstockisalwayssupportedbyatailstockcenter,whiletheendneartheheadstockmaybesupportedbyaheadstockcenterorheldinachuck.Theheadstockendoftheworkpiecemaybeheldinafourjawchuck,orinatypechuck.Thismethodholdstheworkpiecefirmlyandtransfersthepowertotheworkpiecesmoothlytheadditionalsupporttotheworkpieceprovidedbythechucklessensthetendencyforchattertooccurwhencutting.Preciseresultscanbeobtainedwiththismethodifcareistakentoholdtheworkpieceaccuratelyinthechuck.Verypreciseresultscanbeobtainedbysupportingtheworkpiecebetweentwocenters.Alathedogisclampedtotheworkpiecetogethertheyaredrivenbyadriverplatemountedonthespindlenose.OneendoftheWorkpieceismecainedthentheworkpiececanbeturnedaroundinthelathetomachinetheotherend.Thecenterholesintheworkpieceserveaspreciselocatingsurfacesaswellasbearingsurfacestocarrytheweightoftheworkpieceandtoresistthecuttingforces.Aftertheworkpiecehasbeenremovedfromthelatheforanyreason,thecenterholeswillaccuratelyaligntheworkpiecebackinthelatheorinanotherlathe,orinacylindricalgrindingmachine.Theworkpiecemustneverbeheldattheheadstockendbybothachuckandalathecenter.Whileatfirstthoughtthisseemslikeaquickmethodofaligningtheworkpieceinthechuck,thismustnotbedonebecauseitisnotpossibletopressevenlywiththejawsagainsttheworkpiecewhileitisalsosupportedbythecenter.Thealignmentprovidedbythecenterwillnotbemaintainedandthepressureofthejawsmaydamagethecenterhole,thelathecenter,andperhapseventhelathespindle.Compensatingorfloatingjawchucksusedalmostexclusivelyonhighproductionworkprovideanexceptiontothestatementsmadeabove.Thesechucksarereallyworkdriversandcannotbeusedforthesamepurposeasordinarythreeorfourjawchucks.Whileverylargediameterworkpiecesaresometimesmountedontwocenters,theyarepreferablyheldattheheadstockendbyfaceplatejawstoobtainthesmoothpowertransmissionmoreover,largelathedogsthatareadequatetotransmitthepowernotgenerallyavailable,althoughtheycanbemadeasaspecial.Faceplatejawsarelikechuckjawsexceptthattheyaremountedonafaceplate,whichhaslessoverhangfromthespindlebearingsthanalargechuckwouldhave.IntroductionofMachiningMachiningasashapeproducingmethodisthemostuniversallyusedandthemostimportantofallmanufacturingprocesses.Machiningisashapeproducingprocessinwhichapowerdrivendevicecausesmaterialtoberemovedinchipform.Mostmachiningisdonewithequipmentthatsupportsboththeworkpieceandcuttingtoolalthoughinsomecasesportableequipmentisusedwithunsupportedworkpiece.LowsetupcostforsmallQuantities.Machininghastwoapplicationsinmanufacturing.Forcasting,forging,andpressworking,eachspecificshapetobeproduced,evenonepart,nearlyalwayshasahightoolingcost.Theshapesthatmayheproducedbyweldingdependtoalargedegreeontheshapesofrawmaterialthatareavailable.Bymakinguseofgenerallyhighcostequipmentbutwithoutspecialtooling,itispossible,bymachiningtostartwithnearlyanyformofrawmaterial,sotongastheexteriordimensionsaregreatenough,andproduceanydesiredshapefromanymaterial.Therefore.machiningisusuallythepreferredmethodforproducingoneorafewparts,evenwhenthedesignofthepartwouldlogicallyleadtocasting,forgingorpressworkingifahighquantityweretobeproduced.Closeaccuracies,goodfinishes.Thesecondapplicationformachiningisbasedonthehighaccuraciesandsurfacefinishespossible.Manyofthepartsmachinedinlowquantitieswouldbeproducedwithlowerbutacceptabletolerancesifproducedinhighquantitiesbysomeotherprocess.Ontheotherhand,manypartsaregiventheirgeneralshapesbysomehighquantitydeformationprocessandmachinedonlyonselectedsurfaceswherehighaccuraciesareneeded.Internalthreads,forexample,areseldomproducedbyanymeansotherthanmachiningandsmallholesinpressworkedpartsmaybemachinedfollowingthepressworkingoperations.PrimaryCuttingParametersThebasictoolworkrelationshipincuttingisadequatelydescribedbymeansoffourfactorstoolgeometry,cuttingspeed,feed,anddepthofcut.Thecuttingtoolmustbemadeofanappropriatematerialitmustbestrong,tough,hard,andwearresistant.Thetoolsgeometrycharacterizedbyplanesandangles,mustbecorrectforeachcuttingoperation.Cuttingspeedistherateatwhichtheworksurfacepassesbythecuttingedge.Itmaybeexpressedinfeetperminute.Forefficientmachiningthecuttingspeedmustbeofamagnitudeappropriatetotheparticularworktoolcombination.Ingeneral,thehardertheworkmaterial,theslowerthespeed.Feedistherateatwhichthecuttingtooladvancesintotheworkpiece.Wheretheworkpieceorthetoolrotates,feedismeasuredininchesperrevolution.Whenthetoolortheworkreciprocates,feedismeasuredininchesperstroke,Generally,feedvariesinverselywithcuttingspeedforotherwisesimilarconditions.Thedepthofcut,measuredinchesisthedistancethetoolissetintothework.Itisthewidthofthechipinturningorthethicknessofthechipinarectilinearcut.Inroughingoperations,thedepthofcutcanbelargerthanforfinishingoperations.TheEffectofChangesinCuttingParametersonCuttingTemperaturesInmetalcuttingoperationsheatisgeneratedintheprimaryandsecondarydeformationzonesandtheseresultsinacomplextemperaturedistributionthroughoutthetool,workpieceandchip.Atypicalsetofisothermsisshowninfigurewhereitcanbeseenthat,ascouldbeexpected,thereisaverylargetemperaturegradientthroughoutthewidthofthechipastheworkpiecematerialisshearedinprimarydeformationandthereisafurtherlargetemperatureinthechipadjacenttothefaceasthechipisshearedinsecondarydeformation.Thisleadstoamaximumcuttingtemperatureashortdistanceupthefacefromthecuttingedgeandasmalldistanceintothechip.Sincevirtuallyalltheworkdoneinmetalcuttingisconvertedintoheat,itcouldbeexpectedthatfactorswhichincreasethepowerconsumedperunitvolumeofmetalremovedwillincreasethecuttingtemperature.Thusanincreaseintherakeangle,allotherparametersremainingconstant,willreducethepowerperunitvolumeofmetalremovedandthecuttingtemperatureswillreduce.Whenconsideringincreaseinunreformedchipthicknessandcuttingspeedthesituationismorecomplex.Anincreaseinundeformedchipthicknesstendstobeascaleeffectwheretheamountsofheatwhichpasstotheworkpiece,thetoolandchipremaininfixedproportionsandthechangesincuttingtemperaturetendtobesmall.Increaseincuttingspeedhowever,reducetheamountofheatwhichpassesintotheworkpieceandthisincreasethetemperatureriseofthechipmprimarydeformation.Further,thesecondarydeformationzonetendstobesmallerandthishastheeffectofincreasingthetemperaturesinthiszone.Otherchangesincuttingparametershavevirtuallynoeffectonthepowerconsumedperunitvolumeofmetalremovedandconsequentlyhavevirtuallynoeffectonthecuttingtemperatures.Sinceithasbeenshownthatevensmallchangesincuttingtemperaturehaveasignificanteffectontoolwearrateitisappropriatetoindicatehowcuttingtemperaturescanbeassessedfromcuttingdata.ThemostdirectandaccuratemethodformeasuringtemperaturesinhighspeedsteelcuttingtoolsisthatofWright.Trentwhichalsoyieldsdetailedinformationontemperaturedistributionsinhighspeedsteelcuttingtools.Thetechniqueisbasedonthemetallographicexaminationofsectionedhighspeedsteeltoolswhichrelatesmicrostructurechangestothermalhistory.Trenthasdescribedmeasurementsofcuttingtemperaturesandtemperaturedistributionsforhighspeedsteeltoolswhenmachiningawiderangeofworkpiecematerials.Thistechniquehasbeenfurtherdevelopedbyusingscanningelectronmicroscopytostudyfinescalemicrostructurechangesarisingfromovertemperingofthetemperedmartensticmatrixofvarioushighspeedsteels.Thistechniquehasalsobeenusedtostudytemperaturedistributionsinbothhighspeedsteelsinglepointturningtoolsandtwistdrills.WearsofCuttingToolDiscountingbrittlefractureandedgechipping,whichhavealreadybeendealtwith,toolwearisbasicallyofthreetypes.Flankwear,craterwear,andnotchwear.Flankwearoccursonboththemajorandtheminorcuttingedges.Onthemajorcuttingedge,whichisresponsibleforbulkmetalremoval,theseresultsinincreasedcuttingforcesandhighertemperatureswhichifleftuncheckedcanleadtovibrationofthetoolandworkpieceandaconditionwhereefficientcuttingcannolongertakeplace.Ontheminorcuttingedge,whichdeterminesworkpiecesizeandsurfacefinish,flankwearcanresultinanoversizedproductwhichhaspoorsurfacefinish.Undermostpracticalcuttingconditions,thetoolwillfailduetomajorflankwearbeforetheminorflankwearissufficientlylargetoresultinthemanufactureofanunacceptablecomponent.Becauseofthestressdistributiononthetoolface,thefrictionalstressintheregionofslidingcontactbetweenthechipandthefaceisatamaximumatthestartoftheslidingcontactregionandiszeroattheend.Thusabrasiveweartakesplaceinthisregionwithmoreweartakingplaceadjacenttotheseizureregionthanadjacenttothepointatwhichthechiplosescontactwiththeface.Thisresultinlocalizedpittingofthetoolfacesomedistanceupthefacewhichisusuallyreferredtoascateringandwhichnormallyhasasectionintheformofacirculararc.Inmanyrespectsandforpracticalcuttingconditions,craterwearisalesssevereformofwearthanflankwearandconsequentlyflankwearisamorecommontoolfailurecriterion.However,sincevariousauthorshaveshownthatthetemperatureonthefaceincreasesmorerapidlywithincreasingcuttingspeedthanthetemperatureontheflank,andsincetherateofwearofanytypeissignificantlyaffectedbychangesintemperature,craterwearusuallyoccursathighcuttingspeeds.Attheendofthemajorflankwearlandwherethetoolisincontactwiththeuncutworkpiecesurfaceitiscommonfortheflankweartobemorepronouncedthanalongtherestofthewearland.Thisisbecauseoflocalisedeffectssuchasahardenedlayerontheuncutsurfacecausedbyworkhardeningintroducedbyapreviouscut,anoxidescale,andlocalisedhightemperaturesresultingfromtheedgeeffect.Thislocalisedwearisusuallyreferredtoasnotchwearandoccasionallyisverysevere.Althoughthepresenceofthenotchwillnotsignificantlyaffectthecuttingpropertiesofthetool,thenotchisoftenrelativelydeepandifcuttingweretocontinuetherewouldbeagoodchancethatthetoolwouldfracture.Ifanyformofprogressivewearallowedtocontinue,dramaticallyandthetoolwouldfailcatastrophically,i.e.thetoolwouldbenolongercapableofcuttingand,atbest,theworkpiecewouldbescrappedwhilst,atworst,damagecouldbecausedtothemachinetool.Forcarbidecuttingtoolsandforalltypesofwear,thetoolissaidtohavereachedtheendofitsusefullifelongbeforetheonsetofcatastrophicfailure.Forhighspeedsteelcuttingtools,however,wheretheweartendstobenonuniformithasbeenfoundthatthemostmeaningfulandreproducibleresultscanbeobtainedwhenthewearisallowedtocontinuetotheonsetof

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