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外文翻译--基本加工工序和切削技术.doc

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外文翻译--基本加工工序和切削技术.doc

英文原文BasicMachiningOperationsandCuttingTechnologyMachinetoolshaveevolvedfromtheearlyfootpoweredlathesoftheEgyptiansandJohnWilkinsonsboringmill.Theyaredesignedtoproviderigidsupportforboththeworkpieceandthecuttingtoolandcanpreciselycontroltheirrelativepositionsandthevelocityofthetoolwithrespecttotheworkpiece.Basically,inmetalcutting,asharpenedwedgeshapedtoolremovesarathernarrowstripofmetalfromthesurfaceofaductileworkpieceintheformofaseverelydeformedchip.Thechipisawasteproductthatisconsiderablyshorterthantheworkpiecefromwhichitcamebutwithacorrespondingincreaseinthicknessoftheuncutchip.Thegeometricalshapeofworkpiecedependsontheshapeofthetoolanditspathduringthemachiningoperation.Mostmachiningoperationsproducepartsofdifferinggeometry.Ifaroughcylindricalworkpiecerevolvesaboutacentralaxisandthetoolpenetratesbeneathitssurfaceandtravelsparalleltothecenterofrotation,asurfaceofrevolutionisproduced,andtheoperationiscalledturning.Ifahollowtubeismachinedontheinsideinasimilarmanner,theoperationiscalledboring.Producinganexternalconicalsurfaceuniformlyvaryingdiameteriscalledtaperturning.Ifthetoolpointtravelsinapathofvaryingradius,acontouredsurfacelikethatofabowlingpincanbeproducedor,ifthepieceisshortenoughandthesupportissufficientlyrigid,acontouredsurfacecouldbeproducedbyfeedingashapedtoolnormaltotheaxisofrotation.Shorttaperedorcylindricalsurfacescouldalsobecontourformed.Multipleedgedtoolscanalsobeused.Drillingusesatwinedgedflutedtoolforholeswithdepthsupto5to10timesthedrilldiameter.Whetherthedrillturnsortheworkpiecerotates,relativemotionbetweenthecuttingedgeandtheworkpieceistheimportantfactor.Inmillingoperationsarotarycutterwithanumberofcuttingedgesengagestheworkpiece,whichmovesslowlywithrespecttothecutter.Planeorcontouredsurfacesmaybeproduced,dependingonthegeometryofthecutterandthetypeoffeed.Horizontalorverticalaxesofrotationmaybeused,andthefeedoftheworkpiecemaybeinanyofthethreecoordinatedirections.IntroductionofMachiningMachiningasashapeproducingmethodisthemostuniversallyusedandthemostimportantofallmanufacturingprocesses.Machiningisashapeproducingprocessinwhichapowerdrivendevicecausesmaterialtoberemovedinchipform.Mostmachiningisdonewithequipmentthatsupportsboththeworkpieceandcuttingtoolalthoughinsomecasesportableequipmentisusedwithunsupportedworkpiece.LowsetupcostforsmallQuantities.Machininghastwoapplicationsinmanufacturing.Forcasting,forging,andpressworking,eachspecificshapetobeproduced,evenonepart,nearlyalwayshasahightoolingcost.Theshapesthatmaybeproducedbyweldingdependtoalargedegreeontheshapesofrawmaterialthatareavailable.Bymakinguseofgenerallyhighcostequipmentbutwithoutspecialtooling,itispossible,bymachiningtostartwithnearlyanyformofrawmaterial,solongastheexteriordimensionsaregreatenough,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.ThemostdirectandaccuratemethodformeasuringtemperaturesinhighspeedsteelcuttingtoolsisthatofWrightTrent,whichalsoyieldsdetailedinformationontemperaturedistributionsinhighspeedsteelcuttingtools.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,wheretheweartendstobenonuniformithasbeenfoundthatthemostmeaningfulandreproducibleresultscanbeobtainedwhenthewearisallowedtocontinuetotheonsetofcatastrophicfailureeventhough,ofcourse,inpracticeacuttingtimefarlessthanthattofailurewouldbeused.Theonsetofcatastrophicfailureischaracterizedbyoneofseveralphenomena,themostcommonbeingasuddenincreaseincuttingforce,thepresenceofburnishedringsontheworkpiece,andasignificantincreaseinthenoiselevel.MechanismofSurfaceFinishProductionTherearebasicallyfivemechanismswhichcontributetotheproductionofasurfacewhichhavebeenmachined.ThesearelThebasicgeometryofthecuttingprocess.In,forexample,singlepointturningthetoolwilladvanceaconstantdistanceaxiallyperrevolutionoftheworkpieccandtheresultantsurfacewillhaveonit,whenviewedperpendicularlytothedirectionoftoolfeedmotion,aseriesofcuspswhichwillhaveabasicformwhichreplicatestheshapeofthetoolincut.2Theefficiencyofthecuttingoperation.Ithasalreadybeenmentionedthatcuttingwithunstablebuiltupedgeswillproduceasurfacewhichcontainshardbuiltupedgefragmentswhichwillresultinadegradationofthesurfacefinish.Itcanalsobedemonstratedthatcuttingunderadverseconditionssuchasapplywhenusinglargefeedssmallrakeanglesandlowcuttingspeeds,besidesproducingconditionswhichleadtounstablebuiltupedgeproduction,thecuttingprocessitselfcanbecomeunstableandinsteadofcontinuousshearoccurringintheshearzone,tearingtakesplace,discontinuouschipsofuneventhicknessareproduced,andtheresultantsurfaceispoor.Thissituationisparticularlynoticeablewhenmachiningveryductilematerialssuchascopperandaluminum.3Thestabilityofthemachinetool.Undersomecombinationsofcuttingconditionsworkpiecesize,methodofclamping,andcuttingtoolrigidityrelativetothemachinetoolstructure,instabilitycanbesetupinthetoolwhichcausesittovibrate.Undersomeconditionsthisvibration

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