机械制造技术基础课件-chapter3-section9-2012

3.9MetalRemoval,DimensionalChainsandEconomicTolerance3.9.1ConceptofmetalremovalThewholethicknessofthisremovedmetallayeriscalledthetotalmetalremoval,andthemetalremovedineachoperationiscalledoperationremoval.TherelationshipbetweentotalmetalremovalZtandeachoperationremovalZiiseasilyestablished,whichis,here,nisthenumberofjobsorsub-jobsforthesurface.Therateofmetalremovaldirectlyinfluencesthemachiningquality,productionefficiencyandtheeconomicresult.ifthemetalremovalistoolarge,itwillwastebothmaterialandtimeanditwillgeneratelargetemperaturesandforces,whichwillcausetheworktodeform,andthusaffectthemachiningquality.niZZZZZ21总2021/6/281SCUTRotatingaxistheworkpieceAxisAftermachiningBeforemachining2021/6/282SCUTFig.3.453.9.2Thefactorsaffectingtherateofmetalremoval3.9.2.1Affectedbythesurfacequalitiesproducedbytheprecedingoperation,whicharetheroughness（Ra）anddefectivethickness(ta）Thelayers,includingthemicroroughlayer,thecoldplasticlayer,oxidizedlayerandcrackinglayerleftbytheprecedingoperation,mustberemovedbythecurrentoperation.ThesizeofRaandtaisrelatedtothemachiningmethod,forinstance,tables4—7，4—8,and4—9showsomereferencevaluesofRaachievedbydifferentmachiningmethod(这里书上没有),andtable3.11showsthereferencevaluesofta.2021/6/283SCUTTable3.11Valueoftainvariousmachiningmethod(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.TherelationshipbetweenmetalremovalandoperationdimensionaltoleranceisshowninFig.3-9-2.Ta2021/6/285SCUT3.9.2.2ItisaffectedbythetoleranceTaoftheaccurateoperationTaFig3.44therelationshipbetweenmetalremovalandoperationaltolerancebabLLZFig.(a)showstheoperationonanexternalcylindricalsurface(herecalledsurroundedSurface被包容面).thecurrentmetalremovalZbis：（a)movie(b)moviemax(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Zb2021/6/286SCUTThemaximummetalremoval:Zbmax=Lamax-LbminTheminimummetalremoval:Zbmin=Lamin-LbmaxForthesurroundingsurfaces,themaximummetalremovalandminimummetalremovalare:Zbmax=Lbmax-LaminZbmin=Lbmin-LamaxItisobviousthattheprecedingoperationaltolerancewillinfluencethesizeofthecurrentoperation,sincethetoleranceofthemetalremovalisZ=Zmax-Zmin=Ta+Tb2021/6/287SCUT3.9.2.3Itisaffectedbytherelativepositionerror（）betweentheprecedingmachinedsurfacesTheerrorofincludesthedegreeofthelinearity,theparallelismbetweenaxes,theperpendicularitybetweenaxisandsurface,coaxialitybetweeninnerandexternalsurfaces,andflatnessofsurface.Thesekindsoferrorshouldbecorrectedinthecurrentoperationsoastoensurethemachiningquality.2021/6/288SCUT3.9.2.4Itisaffectedbythecurrentclampingerror（）Theerrorincludesboththepositioningandclampingerror,aswellasthemanufacturingerrorofthefixture.Itsvalueisthevectorsumoftheabovethreevariables.Itwilldirectlyaffecttherelativepositionbetweenthesurfaceandcuttingtool.Avectorsumshouldbeusedtoevaluatetheirresultantinfluenceonmetalremoval.TheinfluenceofvariousfactorsonmetalremovalisschematicallyshowninFig.3.45bb2021/6/289SCUTFig.3.45FactorsinfluencingthemetalremovalmovieRotatingaxistheworkpieceAxisAftermachiningBeforemachining2021/6/2810SCUT3.9.3Methodofplanningthemetalremoval3.9.3.1calculationmethodThefollowingformulacanbeobtainedbasedontheanalysisoftheabovefactorsaccordingtoFig.5-13,Thebasicbilateralmetalremoval(双边余量）forasymmetricalsurface,suchasaholeoraxis,is:orThebasicunilateralmetalremoval（单边余量）foranunsymmetricalsurface(suchasaplane)is:)()(2baaaabtRTZε△ρ)(2)(22baaabtRTZεκ△ρ()()baaaabZTRtερ△2021/6/2811SCUT3.9.3.2MethodoflookinguptablesThismethodiswidelyusedinfactories.Thetableispreparedaccordingtotheproductionpracticeandexperimentalresearchworkofthefactory,aswellasmodificationafterconsideringthespecificmachiningcondition.Theappropriaterateofmetalremovalcanbefoundinhandbooksandininformationbookletsprovidedbymetalcuttingmanufacturers.2021/6/2812SCUTThismethodismainlyusedinsingleorsmallbatchtypeproduction,andthemetalremovalrateisestimatedbyexperience,thusItisnotveryaccurate.Itcauseshighrejectrates（废品率）.2021/6/2813SCUT3.9.3.3Experientialmethod3.9.4DimensionalChains3.9.4.1TheconceptofadimensionalchainInmechanicaldesignandtechnicalwork,itisoftennecessarytoanal