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UnitTwoDieDesign模具与数控专业英语2.5BendingDieDesign模具与数控专业英语Bendingisofthemostcommonformingoperations．Wemerelyhavetolookatthecomponentsinanautomobileoranappliance—oratapapercliporafilecabinet—toappreciatehowmanypartsareshapedbybending．Bendingisusednotonlytoformflanges，seams，andcorrugationsbutalsotoimpartstiffnesstothepart（byincreasingitsmomentofinertia）．模具与数控专业英语TheterminologyusedinbendingisshowninFigure2-12．Notethat，Inbending，theouterfibersofthematerialareintension，whiletheinnerfibersareincompression．BecauseofthePoisson’sratio，thewidthofthepart（bendlength，L）intheouterregionissmaller,andintheinnerregionitislargerthantheoriginalwidth．Thisphenomenonmayeasilybeobservedbybendingarectangularrubbereraser.模具与数控专业英语Figure2-12Bendingterminology模具与数控专业英语Minimumbendradii---varyfordifferentmetals,generally,differentannealedmetalscanbebenttoaradiusequaltothethicknessofthemetalwithoutcrackingorweakening.AsR/Tdecreases(theratioofthebendradiustothethicknessbecomessmaller),thetensilestrainattheouterfiberincreases,andthematerialeventuallycracks(Figure2-13).模具与数控专业英语Figure2-13Poissoneffect模具与数控专业英语TheminimumbendradiusforvariousmaterialsisgiveninTable2-2anditisusuallyexpressedintermofthethickness,suchas2T,3T,4Tandsoon.模具与数控专业英语Table2-2MinimumBendRadiusforVariousMaterialsatRoomTemperatureT—thicknessofmaterial模具与数控专业英语BendallowanceasshowninFigure2-12isthelengthoftheneutralaxisinthebendandisusedtodeterminetheblanklengthforabentpart．Howeverthepositionoftheneutralaxisdependsontheradiusandangleofbend（asdescribedintextsonmechanicsofmaterials）．Anapproximateformulaforthebendallowance，LbisgivenbyLb=α(R+kT)(2-1)模具与数控专业英语WhereLb—bendallowance,in(mm);α—thebendangle(inradians)，deg;T—thesheetthickness，in(mm);R—theinsideradiusofbend，in(mm);k—0.33whenRislessthan2Tandis0.50whenRismorethan2T.模具与数控专业英语Bendmethodsarecommonlymadeuseofinpresstool.Metalsheetorstrip,supportedbyaVblock(Figure2-14a),isforcedbyawedge-shapedpunchintotheblock.Thismethod,termedVbending,producesabendhavinganincludedanglewhichmaybeacute,obtuse,orof90°.Frictionbetweenaspring-loadedknurledpinintheveeofadieandthepartwillpreventorreducesidecreepofthepartduringitsbending［3］.模具与数控专业英语(a)Vdie(b)WipingdieFigure2-14Bendingmethods模具与数控专业英语Edgebending(Figure2-14b)iscantileverloadingofabeam.Thebendingpunchforcesthemetalagainstthesupportingdie.Thebendaxisisparalleltotheedgeofthedie.Theworkpieceisclampedtothedieblockbyaspring-loadedpadbeforethepunchcontactstheworkpiecetopreventitsmovementduringdownwardtravelofthepunch.模具与数控专业英语BendingForcecanbeestimatedbyassumingthattheprocessisoneofsimplebendingofarectangularbeam．Thebendingforceinthatcaseisafunctionofthestrengthofthematerial.Thecalculationofbendingforceisasfollows:模具与数控专业英语P=(2-2)WhereP—bendingforce,tons(formetricusage,multiplynumberoftonsby8.896toobtainkilonewtons);K—dieopeningfactor:1.20foradieopeningof16timesmetalthickness,1.33foranopeningofeighttimesmetalthickness;L—lengthofpart,in;S—ultimatetensilestrength,tonspersquarein;W—widthofVorUdie,in;T—metalthickness,in.WKLST2模具与数控专业英语ForUbending(channelbending)pressureswillbeapproximatelytwicethoserequiredforVbending，edgebendingrequiresaboutone-halfthoseneededforVbending.Springbackinthatallmaterialshaveafinitemodulusofelasticity,plasticdeformationisfollowed,whenbendingpressureonmetalisremoved,bysomeelasticrecovery(seeFigure2-15).模具与数控专业英语Inbending,thisrecoveryiscalledspringback.Generallyspeaking,suchaspringbackvariesinsheetfrom0.5°to5°,dependinguponfinitemodulusofelasticity,modesofbending,clearanceofdieandsoon,phosphorbronzemayspringbackfrom10°to15°.模具与数控专业英语Methodsofreducingoreliminatingspringbackinbendingoperationscanbemadeaccordingtothefollowingoperations,showninFigure2-16,andpartsproducedinbendingdiearealsooverbentthroughanangleequaltothespringbackanglewithanundercutorrelievedpunch.