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Damageanddimensionalprecisiononmillingcarbonfiber-reinforcedplasticsusingdesignexperimentsJ.PauloDavim∗,PedroReisDepartmentofMechanicalEngineering,UniversityofAveiro,CampusSantiago,3810-193Aveiro,PortugalReceived24June2002；receivedinrevisedform10May2004；accepted8June2004AbstractMillingcompositematerialsisarathercomplextaskowingtoitsheterogeneityandthenumberofproblems,suchassurfacedelamination,thatappearduringthemachiningprocess,associatedwiththecharacteristicsofthematerialandthecuttingparameters.Withthepurposeofunderstandingandreducingtheseproblems,thispaperpresentsastudythatevaluatesthecuttingparameters(cuttingvelocityandfeedrate)underthesurfaceroughness,anddamageinmillinglaminateplatesofcarbonfiber-reinforcedplastics(CFRPs).Aplanofexperiments,basedontheTaguchi’smethod,wasestablishedconsideringmillingwithprefixedcuttingparametersinanautoclaveCFRPcompositematerial.Ananalysisofvariance(ANOVA)wasperformedtoinvestigatethecuttingcharacteristicsofCFRPcompositematerialusingcementedcarbide(K10)endmills.TheobjectivewastoestablishamodelusingmultipleregressionanalysisbetweencuttingvelocityandfeedratewiththesurfaceroughnessanddamageinaCFRPcompositematerial.©2004ElsevierB.V.Allrightsreserved.Keywords:Milling；Carbonfiber-reinforcedplastics(CFRPs)；Dimensionalprecision；Taguchi’smethod；Orthogonalarrays；Analysisofvariance(ANOVA)1.Introduction1.1.Millingfiber-reinforcedplastics(FRPs)Millingisthemachiningoperationmostfrequentlyusedinmanufacturingpartsoffiber-reinforcedplastics,becausecomponentsmadeofcompositematerialsarecommonlyproducedbynet-shapethatoftenrequiretheremovalofexcessmaterialtocontroltolerances,andmillingisusedasacorrectiveoperationtoproduceawelldefinedandhighqualitysurfaces[1].Themachinabilityoffiber-reinforcedplasticsisstronglyinfluencedbythetypeoffiberembeddedinthecompositeandbyitsproperties.MechanicalandthermalpropertieshaveanextremelyimportanceonmachiningFRP.Thefiberusedinthecompositeshasagreaterinfluenceintheselectionofcuttingtools(cuttingedgematerialandgeometry)andmachiningparameters.Itisfundamentaltoensurethatthetoolselectedissuitableforthematerial.Theknowledgeofcuttingmechanismsisindispensableinviewofcuttingmechanicsandmachinabilityassessmentinmilling[1,2].Compositematerialssuchascarbonfiber-reinforcedplastics(CFRPs)madebyusingcarbonfibersforreinforcingplasticresinmatrices,suchasepoxy,arecharacterisedbyhavingexcellentpropertiesaslightweight,highstrengthandhighstiffness.Thesepropertiesmakethemespeciallyattractiveforaerospaceapplications[2].Surfaceroughnessisaparameterthathasagreaterinfluenceondimensionalprecision,performanceofmechanicalpiecesandonproductioncosts.Forthesereasons,researchdevelopmentshavebeencarriedoutwiththepurposeofoptimisingthecuttingconditionstoreachaspecificsurfaceroughness[3,4].Forachievingthedesiredqualityofthemachinedsurface,itisnecessarytounderstandthemechanismsofmaterialremoval,thekineticsofmachiningprocessesaffectingtheperformanceofthecuttingtools[5].Theworksofanumberofauthors[6–12],whenreportingonmillingofFRP,haveshownthatthetypeandorientationofthefiber,cuttingparametersandtoolgeometryhaveanessentialpaperonthemachinability.EverstineandRogers[6]presentedthefirsttheoreticalworkonthemachiningofFRPsin1971,sincethentheresearchmadeinthisareahasbeenbasedonexperimentalinvestigations.Koplevetal.[7],Kaneeda[8]andPuwandHocheng[9]concludedthattheprincipalcuttingmechanismscorrelatestronglytofiberarrangementandtoolgeometry.Santhanakrishmanetal.[10]andRamuluetal.[11]carriedoutastudyonmachiningofpolymericcompositesandconcludedthatanincreasingofthecuttingspeedleadstoabettersurfacefinish.Hochengetal.[12]studiedtheeffectofthefiberorientationonthecutquality,cuttingforcesandtoolwearonthemachinability.Insummary,itcanbenoticedthattheworkscarriedoutonthemachinabilityofFRP,arebasicallyrelatedonthewearofcuttingtoolsandthequalityonthesurfaces,asafunctionofthecuttingconditions,thedistributionofstaplefibersinthepolymericmatrixandtheangleofinclinationofstaplefibers.Thecurrentpaperinvestigatestheinfluenceofcuttingparameters(cuttingvelocityandfeedrate)onthesurfaceroughness(Ra),delaminationfactor(Fd),andinternationaldimensionalprecision(IT),onCFRPcompositematerialusingcementedcarbideendmills,withthepurposetoestablishaempiricalrelationshipbetweencuttingparameters(Vandf)andsurfaceroughness(Ra)anddelaminationfactor(Fd).1.2.AutoclaveprocessTheautoclaveprocessiswidelyusedtoproducehigh-performancelaminatesusuallywithfibersreinforcedepoxysystems.Compositematerialsmanufacturedbyautoclaveareparticularlyimportantforaerospaceapplications.Thisprocessusesapressurisedvesseltoapplypressureandheattobothpartsthathavebeensealedinavacuumbag.Nextitcanbeseentheseveralstagesofthisprocess.Onthefirststage,theprepregcarbon-fiber–epoxymaterialiscarefullylaidoutonatabletoensurethatfiberorientationmeetsthedesignrequirement,wheretheprepregmaterialconsistsofunidirectionallongcarbonfibersinapartiallycuredepoxymatrix.Onthesecondstage,piecesoftheprepregmaterialarecutoutandplacedontopofeachotheronashapedtooltoformalaminate.Thelayerscouldbeplacedindifferentdirectionstoproducethedesiredstrengthpatternsincethehigheststrengthofeachlayerisindirectionparalleltothefibers.Aftertherequirednumberoflayershasbeenproperlyplaced,thetoolingandtheattachedlaminatearevacuum-bagged,forremovingtheentrappedairfromthelaminatedpart.Finally,thevacuumbagandthetoolingisputintoanautoclaveforthefinalcuringoftheepoxyresin.Afterremovedfromtheautoclave,thecompositematerialisreadyforfurtherfinishingoperations[2,13].2.Experimentalprocedure2.1.MethodandmaterialsInordertoreachtheobjectiveofthisexperimentalwork,mainlytheestablishmentofthecorrelationsbetweencuttingparameters(Vandf)andsurfaceroughness(Ra)anddelaminationfactor(Fd),machiningissueswereperformedunderdifferentcuttingconditionsontheCFRPcompositematerial.Thecompositematerialusedinthetests(epoxymatrixreinforcedwith55%ofcarbonfiber),suppliedbyINEGI,wasproducedbyautoclavewithafiberorientationof0/90◦,ascanbeobservedinFig.1.Theexperimentshavebeencarriedoutinalaminateplate,madeupwith16alternatinglayersoffiberswith4mmofthickness,usingtwocementedcarbide(K10)endmills,presentedinFig.2,with6mmofdiameter.Bothcementedcarbideendmills,two-flute(R216.32-06030-AC10P-1020)andsix-flute(CCT-GSR-D0635),weremanufacturedaccordingtoISO.Thetwo-fluteendmillpresentsthefollowinggeometry:ahelixangleof30◦,arakeangleof10◦30_,aclearanceangleof9◦andaflutelengthof10mm.Thesix-fluteendmillpresentsaneutralhelixand20mmofflutelength.ThedepthofthecutonCFRPcompositematerialwas2mm.Amillingmachine“LC-11/2VSFirst”with2.2kWspindlepowerandamaximumspindlespeedof2500rpmwasusedtoperformtheexperiments.Thefixationofthecompositematerial(plate)wasmadeasobservedinFig.3,tomakesurethatvibrationsanddisplacementdidnotexist.Thesurfaceroughnesswasevaluated(accordingtoISO4287/1)withaHommeltesterT1000profilometer,ascanbeobservedinFig.4.Foreachtestfivemeasurementsweremadeovermillingsurfaces,accordingtoFig.5.Consideringthenumberofmeasurementstobecarriedout,aprogrammabletechniquewasused,bypreviouslyselectingaroughnessprofile,thecut-off(0.8mm)andtheroughnessevaluatorparameter(Ra)accordingtoISO.Dataacquisitionsweremadethroughpro-filometer,byinterfaceRS-232toPCusingthesoftwareHommeltesterTurbo-Datawin.Thedamagecausedonthecompositematerialwasmeasuredwithashopmicroscope,MitutoyoTM500®,with30×magnificationand1_mresolution.2.2.PlanofexperimentsTaguchi’smethodhasbeenwidelyusedinengineeringanalysisandconsistsofaplanofexperimentswiththeobjectiveofacquiringdatainacontrolledway,inordertoobtaininformationaboutthebehaviourofagivenprocess.TheTaguchi’smethodfortwofactorsatthreelevelswasusedfortheelaborationoftheplanofexperiments.Table1indicatesthefactorsstudiedandtheassignmentofthecorrespondinglevels.Bylevelsismeantthevaluestakenbythefactors.TheorthogonalarrayL9(24),wasselectedasshowninTable2,whichhasninerowscorrespondingtothenumberoftests(8degreesoffreedom)withtwocolumnsatthreelevels.Thefactorsandtheinteractionsareassignedtothecolumns.Theplanofexperimentswasmadeofninetests(arrayrows),wherethefirstcolumnwasassignedtothecuttingvelocity(V)andthesecondcolumntothefeedrate(f)andtheremainingwereassignedtotheinteractions.Theoutputsstudiedweresurfaceroughness(Ra)anddelaminationfactor(Fd),intheCFRPcompositematerial.

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