5轴加工和3轴加工的比较

IntJAdvManufTechnol(2001)17:562–5692001Springer-VerlagLondonLimitedComparisonof5-Axisand3-AxisFinishMachiningofHydroformingDieInsertsP.Gray1,S.Bedi1,F.Ismail1,N.Rao1andG.Morphy21DepartmentofMechanicalEngineering,UniversityofWaterloo,Ontario,Canada;and2Vari-Form,Woodstock,Ontario,CanadaTherecentgrowthinhydroformingtechnologyhassparkedinterestinalternativemethodstothecurrentconventionaldiemanufacturingtechniques.Hydroformingdiestypicallyhaveshallowformingchannelsandopen,lowcurvaturesurfaces,makingthemideallysuitedfor5-axismachining.Tofullyappreciatethebenefitsandtoproperlydemonstratethecapa-bilitiesof5-axismachiningforhydroformingdies,acompari-sonof5-axisand3-axisfinishmachiningwasdone.Twohydroformingdieinsertsetsweremachinedona5-axismachinewithatilt/rotarytable.Thetoolpathsfor5-axismachiningweregeneratedusingcustomsoftwarebasedonamodifiedformofatoolpositioningstrategycalledtheprincipalaxismethod.Thequalityofgenerated3-axistoolpathswasverifiedagainstthemachiningtimesofathirdsetofdieinserts,similartothosemachinedin5-axis,byanindependentindustrialmouldanddiemanufacturerusinga3-axishigh-speedmachine.Acomparisonofthegenerated3-axispathsversusthe5-axispathsforoneofthedieinsertswasmadeusingtotalfinishmachiningtoolpathlengthstoeliminatedifferencesinmachines.Theresultsshowthatthegenerated3-axistoolpathsarelongerthanthe5-axispathsbyatleast247%.Thepaperdiscussesthedifferenttool-pathgenerationmethodsalongwiththegeometryofcuspformationandtheeffectoftoolselection.Methodstoimprovethe3-axisresultsarealsopresented.Keywords:5-axis;Finishmachining;High-speedmachining;Hydroforming;Sculpturedsurfaces;3-axis;Toolpath1.IntroductionTherearemanypublishedworksclaimingsignificantadvan-tagesof5-axismachiningofsculptedsurfacesover3-axis[1–6].Thefastermachiningtimesandbettersurfacefinishin5-axismachiningareachievedbyusingflatandradiused-cornerendmillsinsteadoftheball-noseendmillsfavouredinCorrespondenceandoffprintrequeststo:Dr.S.Bedi,DepartmentofMechanicalEngineering,UniversityofWaterloo,Waterloo,Ontario,Canada,N2L3G1.E-mail:sbediKsurya.uwaterloo.ca3-axisfinishmachining.Theextratwoaxesofa5-axismachineareusedtoadjustthetiltandrotation(i.e.orientation)ofthetoolwithrespecttothesurfacenormalatthemachiningpointsontheworkpiece.Whenaflatorcorner-radiusendmillistilted,thecurvaturecutbytheedgegeometryofthetoolontheworkpieceisaltered.Thismeansthatthecurvatureofthedesignedsurfaceatthepointbeingmachined,canbematchedmoreclosely,whichreducesthenumberofpassesrequiredtomachinethesurfaceandlowersthecuspheightanddensity.TwomethodsoftoolpositioninghavebeendevelopedattheUniversityofWaterloo:multi-pointmachiningandtheprincipalaxismethod.Inmulti-pointmachining(MPM)aflatorradiused-cornerendmillispositionedonasurfacesuchthatitgeneratesthedesignedsurfaceattwoseparatepointssimul-taneously[1].Theprincipalaxismethod(PAM)extractscurva-turedatafromthedesignsurfaceandcalculatestherequiredtiltingofaflatorradiused-cornerendmilltomatchthecurva-tureatthepointbeingmachined[2,3].AnothermethodistheSturzmethod,sometimesreferredtoastheinclinedtoolmethod[4,7].Inthismethod,thetoolaxisisinclinedataconstantanglewithrespecttothesurfacenormalfortheentiresurface.Workersinvolvedintheabovemethodshavefavouredtheapproachofcomparingcomputersimulationsof3-and5-axismethodsbecauseofthecostinvolvedinactualmachining,and,inmostcases,thelimitedaccesstofullsimultaneous5-axismillingmachines.Thesamplesthathavebeenusedforcomparisonareusuallysimplesurfacesorafractionofanactualindustrialpart.Theissuesinvolvedwithdieandmouldmanufacturingsuchasinterference,gouging,depthofthesurfaces,depthofcut,andthemachiningofseveralconnectedsurfacesfoundinrealpartsarenotallsimultaneouslyaddressedbythesesimulationsandlaboratorysurfaces.Thispaperdocu-mentsthemachiningofhydroformingdieinsertsin5-axis,foracomparisonwith3-axisfinishmachining.Theinsertshavesincebeenusedtoproducetestpartsbythecompanythatoriginallyissuedthestudy.Figures1and2showsamplesofthefemaleandmalesidesofthehydroformingdieinserts.Thebasisforcomparingthetoolpathsin3-axisand5-axiswillbethetotalfinishmachiningtool-pathlength.Toolpathsfor3-axisweregeneratedwithoneoftheleadingcommercialcomputer-aidedmanufacturing(CAM)packages.Aninde-pendentmouldanddiemanufacturerwascommissionedtoFinishMachiningofHydroformingDieInserts563Fig.1.Femalehydroformingdieinsert.Fig.2.Malehydroformingdieinsert.machineasimilarsetofdieinsertsusinga3-axishigh-speedmachine.Themachiningtimesrecordedbythemanufacturerwereusedtoverifythemachiningtimeofthe3-axispathsgeneratedforthecomparison.2.5-AxisMachiningofHydroformingDiesThepurposeofhydroformingistochangethecross-sectionofatubefromroundtorectangular.Thoughothercross-sectionsarepossible,theyarerare.Waterpressureisusedtoexpandthetubewallstoconformtothatoftheformingchannelofthehydroformingdie.Thus,thedeepestdepthinthedieisapproximatelythediameteroftheformedtube,thelargestbeingtypicallynomorethan150mmindiameter.Theseshallowdepthscanbemachinedeasilywith5-axismachineswithouttheproblemsofinterferenceofthetoolshank(crashing)whenorientingthetool.Sincethecross-