A
mechanistic
approach
to
investigate
drilling
of
UD-CFRP
laminates
with
PCD
drills
Y.
Karpat
a,*
,
O.
Bahtiyar
b,
B.
Deg˘er
b,
Bilgin
Kaftanog˘lu
(1)
c aBilkentUniversity,DepartmentofIndustrialEngineering,Bilkent,Ankara,Turkeyb
TurkishAerospaceIndustries(TAI),Kazan,Ankara,Turkey
c
AtılımUniversity,DepartmentofManufacturingEngineering,I˙ncek,Ankara,Turkey
1. Introduction
Inthenewgenerationofaircrafts,thepercentageofcarbonfiber reinforcedplastics(CFRP)hasreachedupto40%duetotheirability toresistcorrosionand withstandhigh loadswhilereducingthe weightofthestructuralparts.Drilling,themostcommonmachining operationinaircraftmanufacturingisthesubjectofthispaper.
CFRPsareknowntobedifficulttomachineduetotheabrasive natureandlow thermalconductivity.CFRPdrilling has beenthe subjectofmanystudiesintheliterature,mostofwhichconsiderthe influenceof drilling conditions on delamination [1,2]. A ‘‘critical thrustforce’’ concept (afterwhichdelamination starts)has been introducedbyKo¨nigetal.[3].Manydifferentmathematicalmodels havebeenproposedtopredictthecriticalthrustforceasafunctionof toolgeometryandFRPmaterialproperties[4,5].Ithasbeenshown that feed and tool geometryare the two most importantinput parametersaffectingthequalityofdrilledholes,andfeedanddrilling thrustforcearecloselyrelated.Ithasbeenrecommendedtosetfeed low.Theinfluenceofdrillingspeedontheholequalityisshowntobe minimal;therefore,highdrillingspeedsarerecommendedforbetter productivity[6].Rapidtool wearis commonwhile drillingCFRP laminates,whichleadstoincreaseddrillingforces.Increasedthrust forcesresultindelaminationattheexitofthedrilledholes.Lowfeed, whichisrecommendedtodecreasethrustforcesattheholeexit, increasesthetotalcontacttimebetweentoolandCFRPandtherefore results in faster tool wear [7]. In order to increase productivity, especially when drilling uni-directional CFRPs, diamond coated
tungstencarbideandpolycrystallinediamond(PCD)drillshavealso been used.PCD toolmaterial combineshighabrasionresistance, thermalconductivity,hardness,andimpacttoughness,anditcanalso beusedindrillingofmetalcompositestacks[8].Thecuttingedge radiusofPCDdrillsissharperthanitisondiamondcoatedcarbide drills thanks to their small grain size, which is an important considerationforholequality.Inaddition,theycanbereconditioned whentheyareworn.However,thereareonlyalimitednumberof studiesonCFRPdrillingwithPCDdrillsintheliterature.
Itmustbenoted thatthematerialpropertiesand hencechip formation mechanism ofmetals and fiber reinforcedplasticsare different.CFRPmaterialpropertiessuchasitsthickness,carbonfiber diameter,typeofresin,volumetric ratioofcarbonfiber,elasticity modulus,strength,curingconditions,etc.areknowntoaffectmaterial propertiesand,in turn,its drilling characteristics. Therefore,drill geometriesmaybecustomizeddependingontheCFRPmaterialand laminateproperties.ModelingofCFRPdrillingisthereforenecessary tounderstandthemechanicsoftheprocessandtoimprove/optimize drilldesignsforbetterholemakingperformance[9].
Thisstudyconductsanexperimentalinvestigationofdrilling CFRPswithPCDtools.Thethrustforceandtorquemeasurements are used to identify cutting force and edge coefficients while drillingCFRPs.Theseidentifiedcuttingandedgecoefficientsare thenvalidatedwithadditionaldrillingexperimentsperformedon singledirectionCFRPlaminates.
2. Experimentalprocedure
Afive-axismachiningcenterwasusedindrillingexperiments whichwereperformedunderwetconditionswithemulsiontype coolant. Analuminumbackingplatewith8mmdiameterholes
CIRPAnnals-ManufacturingTechnology63(2014)81–84
ARTICLE INFO Keywords:
Machining Drilling
Fiberreinforcedplastic
ABSTRACT
Carbonfiberreinforcedplastics(CFRPs)possessdesirablematerialpropertiesthatsatisfytheaerospace industry’shighstrengthtoweightratioobjective.Therefore,CFRPsarecommonlyusedinstructural parts,eitheraloneortogetherwithaluminumandtitaniumalloys.DrillingofCFRPshasbeenstudied extensively in the literature in recent years, with special emphasis on process parameters and delamination.Thisstudyidentifiesmechanicalpropertiesofuni-directionalCFRPsthroughdrillingtests. Drillingofuni-directionalCFRPplateswithandwithoutpilotholeshasbeenperformed,andcuttingand edgeforcecoefficientsareidentified.Apolycrystallinediamond(PCD)drillwasusedintestssincethis typeofdrilliscommonlyusedinpractice.Finally,validationtestsonmultidirectionalCFRPlaminates havebeenperformedandgoodresultshavebeenobtained.
ß2014CIRP.
*Correspondingauthor.Tel.:+903122902263. E-mailaddress:ykarpat@bilkent.edu.tr(Y.Karpat).
ContentslistsavailableatScienceDirect
CIRP
Annals
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Manufacturing
Technology
j o u r n a lh o m e p a g e :h t t p : / / e e s . e l s e v i e r . c o m / c i r p / d e f a u l t . a s p
http://dx.doi.org/10.1016/j.cirp.2014.03.077 0007-8506/ß2014CIRP.
wasplacedundertheCFRPlaminate.Thedrillingforcesandtorque weremeasuredbyaKistler9123rotatingdynamometerandits chargeamplifier.Drillingspeediscarefullyselectedtoobtainan acceptablelevelofcuttingspeedwithoutexcitingthedynamicsof the rotational force dynamometer. Intermediate modulus UD-CFRPlaminatesof10mmthicknessareproducedfordrillingtests. UD-CFRPlaminateshave0.14mmplythicknessandtheyconsistof 72layerswithequalfiberdirectionsrepeatinginasequenceof08/ 458/908/1358withtwolayersof458and1358laminatesonthetop and bottom surfaces. The intermediate modulus carbon fiber reinforceepoxyresinunidirectionaltapeusedinthisstudyhas59% fibervolumewith2690MPatensilestrength.Thegeometryofthe doublepointanglePCDdrillusedinthisstudyisshowninFig. 1a andb.ManydifferentPCDdrilldesignsareavailablewithvarying cuttingedgelengthsandangles.
Ageneralizeddoublepointangletoolgeometryisshownin
Fig. 2.Inthisfigure;O0-Orepresentsthechiseledgeregion,OA
shows the primary drilling region, AB shows the secondary drilling regionand BCshows the tertiary drilling region. The lengthandangleofthechiseledgearerepresentedwithLand
g
, respectively.Thetooltipanglescorrespondingtoprimaryandsecondary drillingregionsareshownwith
a
andb
.Thedrillhaszerohelix angleand1208and308tooltipangles.Table1summarizestool geometry measurements. The tool tip measurements were performedusingKeyenceVHX1000digitalmicroscope.Uncut chipthicknesses tAB andtOAare different duetodouble pointangledesign,andtheycanbecalculatedasafunctionoffeed( f) peredge.
3. Thrustforceandtorquecharacteristicsofdrillingwith doublepointanglePCDdrills
Importantinformationabouttheinfluenceoftoolgeometryand drilling conditions can be deduced from force and torque measurementsduringdrilling.Fig. 3illustrates a drillingcycle withthedoublepointanglePCDdrillusedinthisstudy,andphases I–VIIareidentifiedonthefigure.Thepoint(I)correspondstothe chiseledgeofthedrillenteringthematerial.Thepoint(II)indicates thesituationwhentheprimarycuttingedge(OA)entersthehole. Inthisregion,whilethrustforceincreasesrapidly,theincreasein torqueissmall.Whenthesecondarydrillingzoneentersthehole (AB), the rate of increase in thrust force is lower due to chip thinningasaresultoflowerdrillangle(
b
).Peakthrustforce(III)is reached at theend of thesecondary drillingzone, andit stays almoststeadyuntilthechiseledgeofthedrillreachesthebottom ofthelaminate(IV).TheregionBCdoesnotcarryanythrustforce, butthetorquereachesitspeakvaluehere.Thrustforcedecreases rapidlywithasimilarrateobservedattheholeentryassoonasthe chiseledge(IV–V)andprimarydrillingedge(V–VI)leavethecut. Thetorque staysalmostthesamebetween IVand VI sincethe secondaryandtertiarycuttingedgesarestillincontactwiththe material. The thrust force and torque decrease continuously betweenVIandVIIuntilthedrillcompletelyleavesthecut.4. Modelingofdrillingforcesandtorques
Thetotalthrustforce(Fz)andtorqueactingonthetoolcanbe
represented with Eq. (1). The first term (Fzch) represents the
influenceofthechiseledge,thesecondterm(FzOA)representsthe
influenceontheprimarydrillingedge,andthelast term(FzAB)
representstheinfluenceofthesecondarydrillingedge.Theeffect oftheBCregiononthethrustforcesisneglected.Asimilarequation canalsobewrittenfortotaldrillingtorque(T)alsoshowninEq.
(1). X Fz¼FzchþFzOAþFzAB X T¼TOAþTABþTBC (1)
Amechanisticforcemodelingapproachhasbeenusedtomodel drilling thrust forces and torques where machining forces are related tothe uncut chiparea [10,11].Average forceand edge coefficients represent theinfluences of drills’ cuttingedge and materialpropertiesontheforces.Eq.(2)representsthe mechanis-ticforcemodeling(thrustandcuttingforceperedge)adaptedto drillingwithadoublepointangletool.Thesubscriptprepresents
Fig.1.DoubletippointanglePCDdrillgeometry.
Fig.2.DoubletippointanglePCDdrillgeometry.
Table1
Toolgeometrymeasurements. Diameter
(2.rB)(mm)
2a(8) b(8) L(mm) g(8) rA(mm) OA(mm) AB(mm)
6.4 120 30 1250 37 0.82 0.46 4.4
Fig.3.Atypicalthrustforceandtorquemeasurementduringdrillingwithdouble pointanglePCDtoolsat5000rpmand0.04mm/revfeed(datalowpassfilteredat 100Hz).
Y.Karpatetal./CIRPAnnals-ManufacturingTechnology63(2014)81–84 82
primary(OA) andsubscripts representssecondary (AB)drilling regions.Thevariables inEq.(2)areshownaboveinFig. 2.The thrustandcuttingforcescanbecalculatedforknown valuesof averageforceandedgecoefficients.
Fzp¼OA K¯zp f 2sin
a
þ ¯Kzep ; TOA¼Fcp:¯rOA¼OA K¯cp f 2sina
þ ¯Kcep Fzs¼AB K¯zs f 2sinb
þ ¯Kzes ; TAB¼Fcs:¯rAB¼AB K¯cs f 2sinb
þ ¯Kces TBC¼FBC:rBC¼BCð ¯KpÞ (2) Drillingtests canbeused tocalculatetheaverage values of thrustandcuttingforce(Kzp,Kcp,Kzs,Kcs)andedge(Kzep,Kcep,Kzes,KcesKp)coefficientsinreversefashion.Characteristicthrustforces
and torques at drillingpoints (asshown I toIV in Fig. 3)are identifiedfromthemeasurementsandshowninFig. 4.
Theprimaryedgethrustforceiscalculatedbysubtractingthe chiseledgeforcefromthethrustforcemeasurements(Fzp=FII–FI
asshowninFigs. 3and4).Similarly,thesecondaryedgedrilling thrustforceiscalculatedby subtractingtheprimaryedgeforce fromthepeakforce(Fzs=FIII–FII).Alinearfithasbeenobtainedfor
boththrustloadandtorqueatprimary,secondaryandtertirary drilling regions as shown in Fig. 4. The chisel edge torque is neglectedsinceitisverylow.Table2summarizesthecuttingand edge force coefficients obtained from thrust force and torque measurements. A linear curve fitting approach is also used to representtheinfluenceofchiseledge(FI)onthethrustforceasa
functionoffeed[10]whichisalsoshowninTable2.
Theedgeforcecoefficientsontheprimarydrillingregion(Kzep
andKcep)arequitelarge. Thisisexpectedsinceeffectivedrilling
cannotbeperformedinthisregionbecauseofitsclosenesstodrills’ center.Forcecoefficientfortorque(Kcs)islargerthanthatofthrust
force(Kzs)forthesecondarydrillingedge.Thisisalsoexpected,
sincemostofthetorqueiscarriedonthesecondarydrillingedge. TheedgecoefficientonthetertiaryzoneKpislargerthanthatof
secondary drilling edge coefficient (Kces) indicating a possible
plowingaction,whichmayhelpimproveboththequalityofthe holesurfaceandthestabilityoftheprocess.Averagecuttingand edgecoefficientscanbeusedtocalculatedrillingthrustforcesand torques.Asimplecomputercodeisdevelopedtocalculatedrilling timebasedondrillgeometryandfeed.Fig. 5showsthepredicted drillingthrustforceandtorquesbasedoncalculatedcuttingand edgeforcecoefficients.Theinfluenceofthelaminate’sdecreasing stiffnessasthenumberoflaminatesdecreasestowardtheendof the process is neglected. The peak force and peak torque are predicted as 170N and 0.38Nm, respectively. These are in agreementwiththevaluespresentedinFig. 4(a)and(b).
5. Cuttingforcecoefficientsasafunctionoffiberdirections
Inordertoobservetheinfluenceoffibercuttingangleduring thedrillingprocess,additionaldrillingtestsareperformedona singledirectionUD-CFRPlaminate.Thedefinitionoffibercutting angle(
u
)andthevariationoffibercuttingangleduringdrilling cycleasafunctiondrill’srotationareshowninFig. 6.Thecorrespondingdrillingthrustforceandtorque measure-mentsareshowninFig.7.Thevariationofthrustforceandtorque measurements have been enlarged in the same figure. The averagethrustforceandtorqueareingoodagreementwiththe predictions for multi direction laminate shown in Fig. 5. Experimenting on single direction CFRP laminates allow for obtainingcuttingforcecoefficientsasafunctionoffibercutting angle[12,13].Thesameapproachisusedheretorepresentthe relationship between thefiber cutting angle andforce coeffi-cients. Additionaldrillingexperimentsare performedonpilot holes of2.48mmdiameterwheretheinfluencesof chiseland primaryedgesareremovedfromthrustforceandmeasurements. Highfeeddrillingtestswerealsoperformedwherethelargest feed of 0.15mm/rev was used which is larger than the ply thickness(0.13mm)ofthelaminate.Average thrustforceand
Fig.4.(a)Thrustforceand(b)torquemeasurementswithlinearfitequations.
Fig.5.Drillingthrustforceandtorquepredictionsat5000rpmand50mm/revfeed.
Table2
Identifiedaveragecuttingandedgecoefficients. ChiseledgeforceFz_ch=1254f+1.71(N)
Kzp(N/mm2) Kzep(N/mm) Kzs(N/mm2) Kzes(N/mm)
67 18 427 4.2
Kcp(N/mm2) Kcep(N/mm) Kcs(N/mm2) Kces(N/mm) Kp(N/mm)
1317 22 810 2.38 7
Fig.6.Definitionoffibercuttingangle,andvariationoffibercuttingangleduring onerotationofthedrill.
torquemeasurementsobtainedonapilotholearegiveninFig.8
withlinearfitequations.
Kcs¼689þ345:sinð2
u
þ180Þ N=mm2;Kces¼2:88 N=mmKzs¼143þ72:sinð2
u
þ180Þ N=mm2;Kzes¼3:58 N=mm(3)
Theaveragevaluesofthesinewaveiscalculatedfromslopeand interceptvaluesgiveninFig.8usingEq.(2).Theamplitudeofthe sine wave is calculated from single direction laminate force fluctuationsasshowninFig. 7.Fig. 9(a)showsthevariationofKcs
asafunctionoffibercuttingangle.Thereisalmostathreetimes
differenceinforcecoefficientsbetween458and1358fibercutting angles.Theadverseeffectofhighlevelsofforcecoefficients(from 908to1358fibercuttingangle)ontheholeexitqualityisshownin
Fig. 9(b).Whilethesecondarydrillingedgecutting(Kcs)andedge
forcecoefficients (Kces,Kzes)are incloseagreements withthose
listedinTable2,cuttingforcecoefficientcorrespondingtothrust forceKzsiscalculatedtobelower.Thrustforcemeasurementsin
Fig. 8revealthatrateofincreaseinthrustforcedecreasewith increasingfeed.Thismaybeexplainedwiththebrittlefracturingof thefibersasaresultoflargethrustforces.
6. Conclusions
A mechanistic model for drilling of multidirectional CFRP laminatesusingdoublepointanglePCDdrillsisproposed.Cutting andedgeforcecoefficientsarecalculatedbasedonthrustforceand torquemeasurements.Thevariationofcuttingforcecoefficientsas afunctionoffibercuttingangleisalsocalculatedforthesecondary drillingregion. Itallowscalculationofthrustforcesandtorques accordingtolaminateconfiguration.Theproposedmodelenables investigatingtheinfluenceofdrillgeometryanddrilling param-etersonprocessoutputssuchasthrustforcesandtorquesatthe holeexit.Thismodel,combinedwithexperimentaldelamination analysis, can be used to optimize drilling conditions and drill geometry.Chiseledgelengthandangle,primaryandsecondary edge lengths and angles can be considered as drill geometry parametersforthePCDdrillusedinthisstudy.
Acknowledgement
Theauthorswouldliketoacknowledgefinancialsupportfrom the Scientific and Technological Research Council of Turkey (Tubitak-Teydeb3090513).
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Fig.7.DrillingofsingledirectionCFRPlaminateat7500rpmand50mm/revfeed andvariationofthrustforcesandtorquesasafunctionofdrillrotation.
y = 3x + 0,05 y = 280x + 28 -5 5 15 25 35 45 55 65 75 0 0,1 0,2 0,3 0,4 0,5 0,6 0 0,05 0,1 0,15 0,2 Th ru st F o rce (N ) To rq u e (N m ) Feed (mm/rev) T Fz
Fig.8.Averagepeakthrustforceandtorquemeasurementsasafunctionoffeed whiledrillingwithpilotholeof2.48mmdiameter.Forceandedgecoefficientsfor thesecondarydrillingedgecanbecalculatedasafunctionoffibercuttingangleas inEq.(3).
Fig.9. (a)Cuttingforcecoefficientasafunction offibercuttingangleinthe secondarydrillingregion;(b)holeexitqualityasafunctionoffibercuttingangleat 5000rpmand20mm/revfeed.
Y.Karpatetal./CIRPAnnals-ManufacturingTechnology63(2014)81–84 84
