外文翻译--切削加工工序和夹具设计.doc

英文原文CuttingprocessandfixturedesignMachinetoolshaveevolvedfromtheearlyfoot-poweredlathesoftheEgyptiansandJohnWilkinsonsboringmill.Theyaredesignedtoproviderigidsupportforboththeworkpieceandthecuttingtoolandcanpreciselycontroltheirrelativepositionsandthevelocityofthetoolwithrespecttotheworkpiece.Basically,inmetalcutting,asharpenedwedge-shapedtoolremovesarathernarrowstripofmetalfromthesurfaceofaductileworkpieceintheformofaseverelydeformedchip.Thechipisawasteproductthatisconsiderablyshorterthantheworkpiecefromwhichitcamebutwithacorrespondingincreaseinthicknessoftheuncutchip.Thegeometricalshapeofworkpiecedependsontheshapeofthetoolanditspathduringthemachiningoperation.Mostmachiningoperationsproducepartsofdifferinggeometry.Ifaroughcylindricalworkpiecerevolvesaboutacentralaxisandthetoolpenetratesbeneathitssurfaceandtravelsparalleltothecenterofrotation,asurfaceofrevolutionisproduced,andtheoperationiscalledturning.Ifahollowtubeismachinedontheinsideinasimilarmanner,theoperationiscalledboring.Producinganexternalconicalsurfaceuniformlyvaryingdiameteriscalledtaperturning,ifthetoolpointtravelsinapathofvaryingradius,acontouredsurfacelikethatofabowlingpincanbeproduced；or,ifthepieceisshortenoughandthesupportissufficientlyrigid,acontouredsurfacecouldbeproducedbyfeedingashapedtoolnormaltotheaxisofrotation.Shorttaperedorcylindricalsurfacescouldalsobecontourformed.Flatorplanesurfacesarefrequentlyrequired.Theycanbegeneratedbyradialturningorfacing,inwhichthetoolpointmovesnormaltotheaxisofrotation.Inothercases,itismoreconvenienttoholdtheworkpiecesteadyandreciprocatethetoolacrossitinaseriesofstraight-linecutswithacrosswisefeedincrementbeforeeachcuttingstroke.Thisoperationiscalledplanningandiscarriedoutonashaper.Forlargerpiecesitiseasiertokeepthetoolstationaryanddrawtheworkpieceunderitasinplanning.Thetoolisfedateachreciprocation.Contouredsurfacescanbeproducedbyusingshapedtools.Multiple-edgedtoolscanalsobeused.Drillingusesatwin-edgedflutedtoolforholeswithdepthsupto5to10timesthedrilldiameter.Whetherthedrillturnsortheworkpiecerotates,relativemotionbetweenthecuttingedgeandtheworkpieceistheimportantfactor.Inmillingoperationsarotarycutterwithanumberofcuttingedgesengagestheworkpiece.Whichmovesslowlywithrespecttothecutter.Planeorcontouredsurfacesmaybeproduced,dependingonthegeometryofthecutterandthetypeoffeed.Horizontalorverticalaxesofrotationmaybeused,andthefeedoftheworkpiecemaybeinanyofthethreecoordinatedirections.BasicMachineToolsMachinetoolsareusedtoproduceapartofaspecifiedgeometricalshapeandpreciseIsizebyremovingmetalfromaductilematerialintheformofchips.Thelatterareawasteproductandvaryfromlongcontinuousribbonsofaductilematerialsuchassteel,whichareundesirablefromadisposalpointofview,toeasilyhandledwell-brokenchipsresultingfromcastiron.Machinetoolsperformfivebasicmetal-removalprocesses:Iturning,planning,drilling,milling,andgrinding.Allothermetal-removalprocessesaremodificationsofthesefivebasicprocesses.Forexample,boringisinternalturning；reaming,tapping,andcounterboringmodifydrilledholesandarerelatedtodrilling；bobbingandgearcuttingarefundamentallymillingoperations；hacksawingandbroachingareaformofplanningandhoning；lapping,superfinishing.Polishingandbuffingarevariantsofgrindingorabrasiveremovaloperations.Therefore,thereareonlyfourtypesofbasicmachinetools,whichusecuttingtoolsofspecificcontrollablegeometry:1.lathes,2.planers,3.drillingmachines,lingmachines.Thegrindingprocessformschips,butthegeometryoftheabrasivegrainisuncontrollable.TheamountandrateofmaterialremovedbythevariousmachiningprocessesmaybeIlarge,asinheavyturningoperations,orextremelysmall,asinlappingorsuperfinishingoperationswhereonlythehighspotsofasurfaceareremoved.Amachinetoolperformsthreemajorfunctions:1.itrigidlysupportstheworkpieceoritsholderandthecuttingtool；2.itprovidesrelativemotionbetweentheworkpieceandthecuttingtool；3.itprovidesarangeoffeedsandspeedsusuallyrangingfrom4to32choicesineachcase.SpeedandFeedsinMachiningSpeeds,feeds,anddepthofcutarethethreemajorvariablesforeconomicalmachining.Othervariablesaretheworkandtoolmaterials,coolantandgeometryofthecuttingtool.Therateofmetalremovalandpowerrequiredformachiningdependuponthesevariables.Thedepthofcut,feed,andcuttingspeedaremachinesettingsthatmustbeestablishedinanymetal-cuttingoperation.Theyallaffecttheforces,thepower,andtherateofmetalremoval.Theycanbedefinedbycomparingthemtotheneedleandrecordofaphonograph.Thecuttingspeed(V)isrepresentedbythevelocityof-therecordsurfacerelativetotheneedleinthetonearmatanyinstant.Feedisrepresentedbytheadvanceoftheneedleradiallyinwardperrevolution,oristhedifferenceinpositionbetweentwoadjacentgrooves.Thedepthofcutisthepenetrationoftheneedleintotherecordorthedepthofthegrooves.TurningonLatheCentersThebasicoperationsperformedonanenginelatheareillustrated.Thoseoperationsperformedonexternalsurfaceswithasinglepointcuttingtoolarecalledturning.Exceptfordrilling,reaming,andlapping,theoperationsoninternalsurfacesarealsoperformedbyasinglepointcuttingtool.Allmachiningoperations,includingturningandboring,canbeclassifiedasroughing,finishing,orsemi-finishing.Theobjectiveofaroughingoperationistoremovethebulkofthematerialasrapidlyandasefficientlyaspossible,whileleavingasmallamountofmaterialonthework-pieceforthefinishingoperation.Finishingoperationsareperformedtoobtainthefinalsize,shape,andsurfacefinishontheworkpiece.Sometimesasemi-finishingoperationwillprecedethefinishingoperationtoleaveasmallpredeterminedanduniformamountofstockonthework-piecetoberemovedbythefinishingoperation.Generally,longerworkpiecesareturnedwhilesupportedononeortwolathecenters.Coneshapedholes,calledcenterholes,whichfitthelathecentersaredrilledintheendsoftheworkpiece-usuallyalongtheaxisofthecylindricalpart.Theendoftheworkpieceadjacenttothetailstockisalwayssupportedbyatailstockcenter,whiletheendneartheheadstockmaybesupportedbyaheadstockcenterorheldinachuck.Theheadstockendoftheworkpiecemaybeheldinafour-jawchuck,orinatypechuck.Thismethodholdstheworkpiecefirmlyandtransfersthepowertotheworkpiecesmoothly；theadditionalsupporttotheworkpieceprovidedbythechucklessensthetendencyforchattertooccurwhencutting.Preciseresultscanbeobtainedwiththismethodifcareistakentoholdtheworkpieceaccuratelyinthechuck.Verypreciseresultscanbeobtainedbysupportingtheworkpiecebetweentwocenters.Alathedogisclampedtotheworkpiece；togethertheyaredrivenbyadriverplatemountedonthespindlenose.OneendoftheWorkpieceismecained；thentheworkpiececanbeturnedaroundinthelathetomachinetheotherend.Thecenterholesintheworkpieceserveaspreciselocatingsurfacesaswellasbearingsurfacestocarrytheweightoftheworkpieceandtoresistthecuttingforces.Aftertheworkpiecehasbeenremovedfromthelatheforanyreason,thecenterholeswillaccuratelyaligntheworkpiecebackinthelatheorinanotherlathe,orinacylindricalgrindingmachine.Theworkpiecemustneverbeheldattheheadstockendbybothachuckandalathecenter.Whileatfirstthoughtthisseemslikeaquickmethodofaligningtheworkpieceinthechuck,thismustnotbedonebecauseitisnotpossibletopressevenlywiththejawsagainsttheworkpiecewhileitisalsosupportedbythecenter.Thealignmentprovidedbythecenterwillnotbemaintainedandthepressureofthejawsmaydamagethecenterhole,thelathecenter,andperhapseventhelathespindle.Compensatingorfloatingjawchucksusedalmostexclusivelyonhighproductionworkprovideanexceptiontothestatementsmadeabove.Thesechucksarereallyworkdriversandcannotbeusedforthesamepurposeasordinarythreeorfour-jawchucks.Whileverylargediameterworkpiecesaresometimesmountedontwocenters,theyarepreferablyheldattheheadstockendbyfaceplatejawstoobtainthesmoothpowertransmission；moreover,largelathedogsthatareadequatetotransmitthepowernotgenerallyavailable,althoughtheycanbemadeasaspecial.Faceplatejawsarelikechuckjawsexceptthattheyaremountedonafaceplate,whichhaslessoverhangfromthespindlebearingsthanalargechuckwouldhave.IntroductionofMachiningMachiningasashape-producingmethodisthemostuniversallyusedandthemostimportantofallmanufacturingprocesses.Machiningisashape-producingprocessinwhichapower-drivendevicecausesmaterialtoberemovedinchipform.Mostmachiningisdonewithequipmentthatsupportsboththeworkpieceandcuttingtoolalthoughinsomecasesportableequipmentisusedwithunsupportedworkpiece.LowsetupcostforsmallQuantities.Machininghastwoapplicationsinmanufacturing.Forcasting,forging,andpressworking,eachspecificshapetobeproduced,evenonepart,nearlyalwayshasahightoolingcost.Theshapesthatmayheproducedbyweldingdependtoalargedegreeontheshapesofrawmaterialthatareavailable.Bymakinguseofgenerallyhighcostequipmentbutwithoutspecialtooling,itispossible,bymachining；tostartwithnearlyanyformofrawmaterial,sotongastheexteriordimensionsaregreatenough,andproduceanydesiredshapefromanymaterial.Therefore.machiningisusuallythepreferredmethodforproducingoneorafewparts,evenwhenthedesignofthepartwouldlogicallyleadtocasting,forgingorpressworkingifahighquantityweretobeproduced.Closeaccuracies,goodfinishes.Thesecondapplicationformachiningisbasedonthehighaccuraciesandsurfacefinishespossible.Manyofthepartsmachinedinlowquantitieswouldbeproducedwithlowerbutacceptabletolerancesifproducedinhighquantitiesbysomeotherprocess.Ontheotherhand,manypartsaregiventheirgeneralshapesbysomehighquantitydeformationprocessandmachinedonlyonselectedsurfaceswherehighaccuraciesareneeded.Internalthreads,forexample,areseldomproducedbyanymeansotherthanmachiningandsmallholesinpressworkedpartsmaybemachinedfollowingthepressworkingoperations.PrimaryCuttingParametersThebasictool-workrelationshipincuttingisadequatelydescribedbymeansoffourfactors:toolgeometry,cuttingspeed,feed,anddepthofcut.Thecuttingtoolmustbemadeofanappropriatematerial；itmustbestrong,tough,hard,andwearresistant.Thetoolsgeometrycharacterizedbyplanesandangles,mustbecorrectforeachcuttingoperation.Cuttingspeedistherateatwhichtheworksurfacepassesbythecuttingedge.Itmaybeexpressedinfeetperminute.Forefficientmachiningthecuttingspeedmustbeofamagnitudeappropriatetotheparticularwork-toolcombination.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.Increaseincuttingspeed；however,reducetheamountofheatwhichpassesintotheworkpieceandthisincreasethetemperatureriseofthechipmprimarydeformation.Further,thesecondarydeformationzonetendstobesmallerandthishastheeffectofincreasingthetemperaturesinthiszone.Otherchangesincuttingparametershavevirtuallynoeffectonthepowerconsumedperunitvolumeofmetalremovedandconsequentlyhavevirtuallynoeffectonthecuttingtemperatures.Sinceithasbeenshownthatevensmallchangesincuttingtemperaturehaveasignificanteffectontoolwearrateitisappropriatetoindicatehowcuttingtemperaturescanbeassessedfromcuttingdata.Themostdirectandaccuratemethodformeasuringtemperaturesinhigh-speed-steelcuttingtoolsisthatofWright&.Trentwhichalsoyieldsdetailedinformationontemperaturedistributionsinhigh-speed-steelcuttingtools.Thetechniqueisbasedonthemetallographicexaminationofsectionedhigh-speed-steeltoolswhichrelatesmicrostructurechangestothermalhistory.Trenthasdescribedmeasurementsofcuttingtemperaturesandtemperaturedistributionsforhigh-speed-steeltoolswhenmachiningawiderangeofworkpiecematerials.Thistechniquehasbeenfurtherdevelopedbyusingscanningelectronmicroscopytostudyfine-scalemicrostructurechangesarisingfromovertemperingofthetemperedmartensticmatrixofvarioushigh-speed-steels.Thistechniquehasalsobeenusedtostudytemperaturedistributionsinbothhigh-speed-steelsinglepointturningtoolsandtwistdrills.WearsofCuttingToolDiscountingbrittlefractureandedgechipping,whichhavealreadybeendealtwith,toolwearisbasicallyofthreetypes.Flankwear,craterwear,andnotchwear.Flankwearoccursonboththemajorandtheminorcuttingedges.Onthemajorcuttingedge,whichisresponsibleforbulkmetalremoval,theseresultsinincreasedcuttingforcesandhighertemperatureswhichifleftuncheckedcanleadtovibrationofthetoolandworkpieceandaconditionwhereefficientcuttingcannolongertakeplace.Ontheminorcuttingedge,whichdeterminesworkpiecesizeandsurfacefinish,flankwearcanresultinanoversizedproductwhichhaspoorsurfacefinish.Undermostpracticalcuttingconditions,thetoolwillfailduetomajorflankwearbeforetheminorflankwearissufficientlylargetoresultinthemanufactureofanunacceptablecomponent.Becauseofthestressdistributiononthetoolface,thefrictionalstressintheregionofslidingcontactbetweenthechipandthefaceisatamaximumatthestartoftheslidingcontactregionandiszeroattheend.Thusabrasiveweartakesplaceinthisregionwithmoreweartakingplaceadjacenttotheseizureregionthanadjacenttothepointatwhichthechiplosescontactwiththeface.Thisresultinlocalizedpittingofthetoolfacesomedistanceupthefacewhichisusuallyreferredtoascateringandwhichnormallyhasasectio