Helical milling of CFRP–titanium layer compounds

CIRP Journal of Manufacturing Science and Technology 1 (2008) 64–69 Contents lists available at ScienceDirect CIRP Journal of Manufacturing Science and Technology journal homepage: www.elsevier.com/locate/cirpj Helical milling of CFRP–titanium layer compounds B. Denkena *, D. Boehnke, J.H. Dege ¨t ¨t Institute of Production Engineering and Machine Tools, Leibniz Universita Hannover, An der Universita 2, 30823 Garbsen, Germany A R T I C L E I N F O A B S T R A C T Article history: Available

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  Helical milling of CFRP–titanium layer compounds B. Denkena*, D. Boehnke, J.H. Dege Institute of Production Engineering and Machine Tools, Leibniz Universita¨ t Hannover, An der Universita¨ t 2, 30823 Garbsen, Germany 1. Introduction Due to their high strength-to-weight ratio carbon fiberreinforced plastics (CFRP) are highly attractive for use in theaircraft industry. They allow a weight reduction and thus decreasethe fuel consumption or increase the payload. Beneath theapplication of lightweight materials a lightweight design can alsoreducethemassofcomponents.Aspecialformofthisdesignistheconstruction of layer stack-ups with materials with very differentproperties. In a wide range of applications dissimilar materialstack-ups of CFRP, aluminum and titanium are used for highperformance structural components. In order to assemble theseparts, it is necessary to apply holes for various purposes such asbolt and rivet holes[1].An alternative to drilling these compounds with common twistdrillsishelicalmilling.Here,amillingtoolrotatesonahelicalpathand generates the borehole. The kinematics of this process allowsdrilling holes with different diameters independently of the tooldiameter and without changing the tool. More advantages of helicalmillingare:lowburrformation,littledelaminationinCFRP,low process forces as well as good chip transportation[2–6].The combination of titanium with CFRP has the followingadvantages over stack-ups made of aluminum and CFRP: similarthermal expansion, reduced galvanic corrosion issue and higherspecific strength[7]. In contrast machining of Ti–CFRP compositesintroducesauniquesetofproblems.Apartfromhightoolwearandfiber delamination, diameter tolerances caused by the differentmaterial propertiesof the layers oftenreduce the borehole qualityduring helical milling[8,9]. Hence the influence of the axial andtangential feed on the bore hole diameter during helical milling isinvestigated in this paper. 2. Kinematics of helical milling  Theconsiderabledifferencebetweenthedrillingandthehelicalmilling process results from the kinematical conditions. In drillingoperations the bore diameter is determined by the tool diameter.In contrast to this, in helical milling the bore diameter isdeterminedbythetooldiameterincombinationwiththediameterofthehelicalpath(Fig.1).Thisleadstoahighflexibilityconcerningthe bore diameter.To use this kinematic on a three axis machine tool the feedvelocity of the tool center point v f  and the depth per helicalrotation a à p or the helix angle a have to be calculated. The inputparameters for these calculations are the axial feed per tooth f  za ,thetangentialfeedpertooth  f  zt ,thediameterofthebore D B andthehelical path D h , the rotational speed n and the number of teeth of the end mill z  . To determine the feed velocity of the tool centerpoint v f  it is necessary to calculate the axial feed velocity of thehelix v fha ,thetangentialfeedvelocityofthecuttingedge v ft andthetangential feed velocity of the helix v fht at first. The equations aresummarized inFig. 2. During helical milling the tool center point(TCP) describes in general a movement on a small diameter ( D h ). If this movement is realized by the interpolation of two linear axesthe demands on the feed drive acceleration and the rigidity andstiffness of the machine tool are very high. A lack of theserequirementcanresult indeviations of the bore hole diameter androundness. CIRP Journal of Manufacturing Science and Technology 1 (2008) 64–69 A R T I C L E I N F O  Article history: Available online 12 November 2008 Keywords: Helical millingOrbital millingTitaniumCFRPCompounds A B S T R A C T Helical milling is used to generate boreholes by means of a milling tool being operated on a helical pathinto the workpiece. The bore diameter can be adjusted through the diameter of the helical path. Incomparison to conventional drilling operations this process often results in lower burr formation andfiber delamination. Therefore helical milling is used in the aircraft industry for cutting composites andcomposite-metal compounds. One of these compounds, which is regarded as difficult to machine, is alayer compound consisting of unidirectional carbon fiber reinforced plastic (CFRP) and TiAl6V4. Thispaper presents the impact of the axial and tangential feed during helical milling on process forces andborehole quality is shown. ß 2008 CIRP. * Corresponding author. E-mail address: dege@ifw.uni-hannover.de(B. Denkena). Contents lists available atScienceDirect CIRP Journal of Manufacturing Science and Technology journal homepage: www.elsevier.com/locate/cirpj 1755-5817/$ – see front matter ß 2008 CIRP.doi:10.1016/j.cirpj.2008.09.009  3. Undeformed chip geometry during helical milling processes The helical milling process consists of a peripheral face millingpart with a discontinuous cut on the radial cutting edge and adrilling part with a continuous cut on the axial cutting edge at thesame time. Regarding the radial cutting edge the axial depth of cut a p increases approximately linearly with the tool rotation angle w and reaches a maximum of  a à p . The undeformed chip thickness h tan shows a sinusoidal behaviour over the tool rotation angle w with amaximum of the tangential feed per tooth f  zt . The resulting Fig. 1. Kinematics of helical milling. Fig. 2. Calculation of relevant parameters for the helical milling process. Fig. 3. Dimensions of the undeformed chip. B. Denkena et al./CIRP Journal of Manufacturing Science and Technology 1 (2008) 64–69 65  complexformoftheundeformedchipleadstoadiscontinuouscut.On the axial cutting edge the cross section of the undeformed chipremains constant over the tool rotation angle w as in drillingoperations. In axial direction the undeformed chip thickness h ax isequivalenttotheaxialfeedpertooth  f  za whiletheundeformedchipwidth b ax equals half the tool diameter D Wz .Fig. 3shows theundeformed chip parameters in dependence of the tool rotationangle for an example process.Fig. 4shows an example of the geometry of the undeformedchip calculated by the parameters given inFig. 3after a toolrotation angle of  w = 180 8 . While the volume marked in red is cutcontinuously, the volume marked in blue and green is removed bya discontinuous cut. A further tool rotation by w = 180 8 causes aconstant removal of the continuously cut chip without anydiscontinuously cut part. The complex geometry of the unde-formed chip is, the diameter of bore and tool apart, mainlyinfluenced by the axial and tangential feed per tooth and theresulting depth per helical rotation.The superposition of continuous and discontinuous cut duringthe helical milling process results in two chip formationmechanisms. On the one hand long chips form continuously atthe axial cutting edge (Fig. 5). They show a fanfold surface andcould cause problems with chip removal. On the other hand thechipsgenerateddiscontinuouslyattheradialcuttingedgeareverysmall.Duetothedifferingflowdirectionduringchipformationthetwo chip types are separated. 4. Experimental setup The helical milling operations have been carried out on a fouraxis Heller MC16 machine tool in dry machining conditions, sincewet conditions can negatively affect the material properties of thecomposite. The CFRP chips are removed in process via a vacuumdust removal system. During the process the forces are measuredwith a Kistler 9257B dynamometer. Since the dynamometermeasures the process forces in the workpiece coordinate system( F   x , F   y ) it is necessary to transform the forces into the toolcoordinate system ( F  f  , F  fN ). Therefore, the spindle position isrecorded by a Polytec 303 laser vibrometer and the force data istransformed via a rotational matrix, depending on the spindle Fig. 5. Chip formation during the helical milling process. Fig. 4. Form of the undeformed chip. Fig. 6. Buildup of the CFRP-Ti layer compound. B. Denkena et al./CIRP Journal of Manufacturing Science and Technology 1 (2008) 64–69 66  position, into feed force F  f  and feed normal force F  fN . The bore holediameter in the layer compounds is measured with a Leitz PMM866 3D coordinate measuring machine. Every material layer ineach hole is measured in four planes at different heights.Theboreholeswithadiameterof  D B = 10 mmarehelicalmilledwith TiAlN coated solid carbide end mills featuring three teeth,an overall length of  l OA = 65 mm, an cutting edge length of  l CE = 22 mm, a diameter of  D Wz = 8 mm, a helix angle of  d = 45 8 , aclearance angle of  a = 20 8 and a rake angle of  g  = 9 8 . The tools arechanged after the slightest appearance of tool wear to exclude theinfluence of the wear. The axial feed per tooth is varied in a rangefrom  f  za = 2–12 m mwhereasthe tangentialfeedisrangedbetween  f  zt = 40–120 m m. All cutting tests have been carried out at aconstant cutting speed of  v c ¼ 40m = min and are repeated twotimes. For tool retraction after the helical milling process, the pathof the tool center point describes half circle to the bore hole centerand then retracts on a linear path outof the bore hole.Fig. 6showsthe machined compound consisting of a bimodal titanium alloy(TiAl6V4)layerandanunidirectional,quasiisotropicCFRPlayer.Allholes are milled from the CFRP layer into the titanium layer. 5. Impact of the feed on process forces and borehole quality  Fig.7showstheinfluenceoftheaxialfeed  f  za andthetangentialfeed f  zt per tooth on the feed force F  f  , the feed normal force F  fN andthe axial force F  a in the titanium and the CFRP layer. Thecorresponding geometries of the undeformed chips are illustratedschematicallybelowthediagrams.Anincreaseintheaxialfeedataconstant tangential feed of  f  zt = 60 m m leads to an increased pitchof the helical tool path. This results in a rising height of theundeformed chip and, regarding the titanium layer, thus in higherfeed and feed normal forces. An increase in the tangential feed pertooth f  zt at a constant axial feed of  f  za = 6 m m causes, unlike theaxial feed, a reductionof the heightof the undeformed chip,whichleads to decreasing feed and feed normal forces. The axial forceremains nearly constant over the axial and tangential feed pertooth.TheprocessforcesintheCFRPlayeraresignificantlylowerincomparison to the titanium layer due to the material properties.That is why the scale of the Y  -axis differs from the previousdiagram. Similar to the titanium layer, the feed and feed normalforces in the CFRP layer increase with increasing axial feed pertooth  f  za anddecreasewithariseofthetangentialfeedpertooth  f  zt .The axial force is not considerably influenced by the axial andtangential feed. In contrast to the titanium layer the level of theaxial force in the CFRP layer is far above the feed and feed normalforces. This behaviour can be explained by an unfavourable chipformation and increased friction on the axial flank face due to anaxial feed per tooth f  za which is in the same range of the fiberdiameter of  d f  = 7 m m.A main aspect concerning workpiece quality is the precision of the bore diameter. In order to produce bore holes within apredicted diameter tolerance, the diameter difference at thetransition between the two material layers has to be studiedcarefully.Fig. 8shows the mean diameter of each layer over theaxial feed f  za and the tangential feed f  zt per tooth. Independentfrom the feed rates the bore hole diameter in the CFRP layer isalways higher than in the titanium layer. An increase in the axialfeed per tooth accompanied by higher process forces leads to areduction of the bore diameter in the CFRP layer as well as in thetitanium layer. On the other hand a gain in the tangential feed pertooth with decreasing process forces results in higher borediameters in both materials. This behaviour is mainly caused bya tool deflection due to the occurring feed normal forces.Thefeednormalforcesactonthetoolcenterpointanddeflectitin the direction of the bore hole center point. Hence the actualdiameter is machined instead of the programmed target diameter.The diagram on the right side of Fig. 9shows the bore diameterover the feed normal force F  fN for the CFRP and the titanium layer.Inbothmaterialsanearlylinearinterrelationbetweenfeednormalforces and bore hole diameter is visible. This leads to theconclusion that the tool deflection is the main factor of thediameter deviations. Fig. 7. Process forces in the titanium and the CFRP layer in dependence on the axial and tangential feed per tooth. B. Denkena et al./CIRP Journal of Manufacturing Science and Technology 1 (2008) 64–69 67
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