机械制造技术基础课件-chapter3-section9-2012 (2)

3.9MetalRemoval,DimensionalChainsandEconomicTolerance3.9.1ConceptofmetalremovalThewholethicknessofthisremovedmetallayeriscalledthetotalmetalremoval,andthemetalremovedineachoperationiscalledoperationremoval.TherelationshipbetweentotalmetalremovalZtandeachoperationremovalZiiseasilyestablished,whichis,here,nisthenumberofjobsorsub-jobsforthesurface.Therateofmetalremovaldirectlyinfluencesthemachiningquality,productionefficiencyandtheeconomicresult.ifthemetalremovalistoolarge,itwillwastebothmaterialandtimeanditwillgeneratelargetemperaturesandforces,whichwillcausetheworktodeform,andthusaffectthemachiningquality.niZZZZZ21总2021/6/231SCUTRotatingaxistheworkpieceAxisAftermachiningBeforemachining2021/6/232SCUTFig.3.453.9.2Thefactorsaffectingtherateofmetalremoval(1)Affectedbythesurfacequalitiesproducedbytheprecedingoperation,whicharetheroughness（Ra）anddefectivethickness(ta）Thelayers,includingthemicroroughlayer,thecoldplasticlayer,oxidizedlayerandcrackinglayerleftbytheprecedingoperation,mustberemovedbythecurrentoperation.ThesizeofRaandtaisrelatedtothemachiningmethod,forinstancetable3.11showsthereferencevaluesofta.2021/6/233SCUTTable3.11Valueoftainvariousmachiningmethod(μm)MachiningmethodtaMachiningmethodtaMachiningmethodtaClose-dieforging500Roughenlarginghole40～60Finishplaning25～40Colddrawing80～100Finishenlarginghole30～40Roughslotting50～60Hotrolling150Roughreaming25～30Finishslotting35～50Highaccuracyrolling300Finishreaming10～20Roughmilling40～60Metaldieforging100Roughboring30～50Finishmilling25～40***Finishboring25～40broaching10～20Externalorinternalroughturning40～60externalcylindricalGrinding15～25Cuttingoff60Externalorinternalfinishturning30～40Internalholegrinding20～30lapping3～5Endsurfaceroughturning40～60Endfacegrinding15～35Super-lapping0.2～0.3Endsurfacefinishturning30～40Surfacegrinding20～30polishing2～5drilling40～60Roughplaning40～50Thetoleranceofincludesthegeometricalshapeerror,suchasthedegreeofconing,ellipsingandsurfaceflatness.Thesizeofcanbedecidedaccordingtothetoleranceoftheselectedmachiningmethod,whichcanbefoundinhandbooksandininformationbookletsprovidedbymetalcuttingmanufacturers.Ta2021/6/235SCUT(2)ItisaffectedbythetoleranceTaoftheaccurateoperationTa3.9.2Thefactorsaffectingtherateofmetalremovalmax(a)minLbb(LLbL(bminaLmaxaLL(a)δ))ZbδbZbminmaxamaxmin)LaLamin(b)((LaLbmaxbL(LbbLδbZmaxδaminZbb))图5-12加工余量与工序公差的关系ZbFig3.44therelationshipbetweenmetalremovalandoperationaltolerancebabLLZFig.(a)showstheoperationonanexternalcylindricalsurface(herecalledsurroundedSurface被包容面).thecurrentmetalremovalZbis：2021/6/236SCUT3.9.2ThefactorsaffectingtherateofmetalremovalThemaximummetalremoval:Zbmax=Lamax-LbminTheminimummetalremoval:Zbmin=Lamin-LbmaxForthesurroundingsurfaces,themaximummetalremovalandminimummetalremovalare:Zbmax=Lbmax-LaminZbmin=Lbmin-LamaxItisobviousthattheprecedingoperationaltolerancewillinfluencethesizeofthecurrentoperation,sincethetoleranceofthemetalremovalisZ=Zmax-Zmin=Ta+Tb2021/6/237SCUT(3)Itisaffectedbytherelativepositionerror（）betweentheprecedingmachinedsurfacesTheerrorofincludesthedegreeofthelinearity,theparallelismbetweenaxes,theperpendicularitybetweenaxisandsurface,coaxialitybetweeninnerandexternalsurfaces,andflatnessofsurface.Thesekindsoferrorshouldbecorrectedinthecurrentoperationsoastoensurethemachiningquality.2021/6/238SCUT3.9.2Thefactorsaffectingtherateofmetalremoval(4)Itisaffectedbythecurrentclampingerror（）Theerrorincludesboththepositioningandclampingerror,aswellasthemanufacturingerrorofthefixture.Itsvalueisthevectorsumoftheabovethreevariables.Itwilldirectlyaffecttherelativepositionbetweenthesurfaceandcuttingtool.Avectorsumshouldbeusedtoevaluatetheirresultantinfluenceonmetalremoval.TheinfluenceofvariousfactorsonmetalremovalisschematicallyshowninFig3.25bb2021/6/239SCUT3.9.2ThefactorsaffectingtherateofmetalremovalFig.3.45FactorsinfluencingthemetalremovalRotatingaxistheworkpieceAxisAftermachiningBeforemachining2021/6/2310SCUT3.9.3Methodofplanningthemetalremoval(1)calculationmethodThefollowingformulacanbeobtainedbasedontheanalysisoftheabovefactorsaccordingtoFig.3.45.Thebasicbilateralmetalremoval(双边余量）forasymmetricalsurface,suchasaholeoraxis,is:orThebasicunilateralmetalremoval（单边余量）foranunsymmetricalsurface(suchasaplane)is:)()(2baaaabtRTZε△ρ)(2)(22baaabtRTZεκ△ρ()()baaaabZTRtερ△2021/6/2311SCUT(2)MethodoflookinguptablesThismethodiswidelyusedinfactories.Thetableispreparedaccordingtotheproductionpracticeandexperimentalresearchworkofthefactory,aswellasmodificationafterconsideringthespecificmachiningcondition.Theappropriaterateofmetalremovalcanbefoundinhandbooksandininformationbookletsprovidedbymetalcuttingmanufacturers.2021/6/2312SCUT3.9.3MethodofplanningthemetalremovalThismethodismainlyusedinsingleorsmallbatchtypeproduction,andthemetalremovalrateisestimatedbyexperience,thusitisnotveryaccurate.Itcauseshighrejectrates（废品率）.2021/6/2313SCUT(3)Experientialmethod3.9.3Methodofplanningthemetalremoval3.9.4DimensionalChains1.TheconceptofadimensionalchainInmechanicaldesignandtechnicalwork,itisof