CIRP Journal of Manufacturing Science and Technology 1 (2008) 64–69

Helical milling of CFRP–titanium layer compounds

B. Denkena *, D. Boehnke, J.H. Dege

Institute of Production Engineering and Machine Tools, Leibniz Universitat Hannover, An der Universitat 2, 30823 Garbsen, Germany

A R T I C L E I N F O

Article history:

Available online 12 November 2008

Keywords:

Helical milling

Orbital milling

Titanium

CFRP

Compounds

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 path

into the workpiece. The bore diameter can be adjusted through the diameter of the helical path. In

comparison to conventional drilling operations this process often results in lower burr formation and

fiber delamination. Therefore helical milling is used in the aircraft industry for cutting composites and

composite-metal compounds. One of these compounds, which is regarded as difficult to machine, is a

layer compound consisting of unidirectional carbon fiber reinforced plastic (CFRP) and TiAl6V4. This

paper presents the impact of the axial and tangential feed during helical milling on process forces and

borehole quality is shown.

� 2008 CIRP.

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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 alsoreduce the mass of components. A special form of this design is theconstruction 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 twistdrills is helical milling. Here, a milling tool rotates on a helical pathand generates the borehole. The kinematics of this process allowsdrilling holes with different diameters independently of the tooldiameter and without changing the tool. More advantages ofhelical milling are: low burr formation, little delamination in CFRP,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 compositesintroduces a unique set of problems. Apart from high tool wear andfiber delamination, diameter tolerances caused by the different

* Corresponding author.

E-mail address: dege@ifw.uni-hannover.de (B. Denkena).

1755-5817/$ – see front matter � 2008 CIRP.

doi:10.1016/j.cirpj.2008.09.009

material properties of the layers often reduce the bore hole 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

The considerable difference between the drilling and the helicalmilling 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 isdetermined by the tool diameter in combination with the diameterof the helical path (Fig. 1). This leads to a high flexibility concerningthe bore diameter.

To use this kinematic on a three axis machine tool the feedvelocity of the tool center point vf and the depth per helicalrotation a�p or the helix angle a have to be calculated. The inputparameters for these calculations are the axial feed per tooth fza,the tangential feed per tooth fzt, the diameter of the bore DB and thehelical path Dh, the rotational speed n and the number of teeth ofthe end mill z. To determine the feed velocity of the tool centerpoint vf it is necessary to calculate the axial feed velocity of thehelix vfha, the tangential feed velocity of the cutting edge vft and thetangential feed velocity of the helix vfht at first. The equations aresummarized in Fig. 2. During helical milling the tool center point(TCP) describes in general a movement on a small diameter (Dh). Ifthis 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 theserequirement can result in deviations of the bore hole diameter androundness.

Fig. 1. Kinematics of helical milling.

Fig. 2. Calculation of relevant parameters for the helical milling process.

B. Denkena et al. / CIRP Journal of Manufacturing Science and Technology 1 (2008) 64–69 65

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 the

Fig. 3. Dimensions of th

same time. Regarding the radial cutting edge the axial depth of cutap increases approximately linearly with the tool rotation angle wand reaches a maximum of a�p. The undeformed chip thickness htan

shows a sinusoidal behaviour over the tool rotation angle w with amaximum of the tangential feed per tooth fzt. The resulting

e undeformed chip.

Fig. 5. Chip formation during the helical milling process.

Fig. 4. Form of the undeformed chip.

B. Denkena et al. / CIRP Journal of Manufacturing Science and Technology 1 (2008) 64–6966

complex form of the undeformed chip leads to a discontinuous cut.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 hax isequivalent to the axial feed per tooth fza while the undeformed chipwidth bax equals half the tool diameter DWz. Fig. 3 shows theundeformed chip parameters in dependence of the tool rotationangle for an example process.

Fig. 4 shows an example of the geometry of the undeformedchip calculated by the parameters given in Fig. 3 after a toolrotation angle of w = 1808. 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 = 1808 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 formation

Fig. 6. Buildup of the CFR

mechanisms. 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 thechips generated discontinuously at the radial cutting edge are verysmall. Due to the differing flow direction during chip formation thetwo 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(Fx, Fy) it is necessary to transform the forces into the toolcoordinate system (Ff, FfN). Therefore, the spindle position isrecorded by a Polytec 303 laser vibrometer and the force data istransformed via a rotational matrix, depending on the spindle

P-Ti layer compound.

B. Denkena et al. / CIRP Journal of Manufacturing Science and Technology 1 (2008) 64–69 67

position, into feed force Ff and feed normal force FfN. 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.

The bore holes with a diameter of DB = 10 mm are helical milledwith TiAlN coated solid carbide end mills featuring three teeth,an overall length of lOA = 65 mm, an cutting edge length oflCE = 22 mm, a diameter of DWz = 8 mm, a helix angle of d = 458, aclearance angle of a = 208 and a rake angle of g = 98. 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 fza = 2–12 mm whereas the tangential feed is ranged betweenfzt = 40–120 mm. All cutting tests have been carried out at aconstant cutting speed of vc ¼ 40 m=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 out of the bore hole. Fig. 6 showsthe machined compound consisting of a bimodal titanium alloy(TiAl6V4) layer and an unidirectional, quasiisotropic CFRP layer. Allholes are milled from the CFRP layer into the titanium layer.

5. Impact of the feed on process forces and borehole quality

Fig. 7 shows the influence of the axial feed fza and the tangentialfeed fzt per tooth on the feed force Ff, the feed normal force FfN andthe axial force Fa in the titanium and the CFRP layer. Thecorresponding geometries of the undeformed chips are illustratedschematically below the diagrams. An increase in the axial feed at aconstant tangential feed of fzt = 60 mm 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 fzt at a constant axial feed of fza = 6 mm causes, unlike theaxial feed, a reduction of the height of the undeformed chip, whichleads to decreasing feed and feed normal forces. The axial forceremains nearly constant over the axial and tangential feed per

Fig. 7. Process forces in the titanium and the CFRP layer in

tooth. The process forces in the CFRP layer are significantly lower incomparison 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 fza and decrease with a rise of the tangential feed per tooth fzt.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 fza which is in the same range of the fiberdiameter of df = 7 mm.

A main aspect concerning workpiece quality is the precision ofthe 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. 8 shows the mean diameter of each layer over theaxial feed fza and the tangential feed fzt 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.

The feed normal forces act on the tool center point and deflect itin 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. 9 shows the bore diameterover the feed normal force FfN for the CFRP and the titanium layer.In both materials a nearly linear interrelation between feed normalforces and bore hole diameter is visible. This leads to theconclusion that the tool deflection is the main factor of thediameter deviations.

dependence on the axial and tangential feed per tooth.

Fig. 8. Bore diameter in the CFRP-Ti compound in dependence of the feed.

Fig. 9. Impact of the process forces on the bore diameter.

B. Denkena et al. / CIRP Journal of Manufacturing Science and Technology 1 (2008) 64–6968

However, it is not the only one, since otherwise both best fitlines would be identical. Possible factors influencing the diameterdeviations may be the different Young’s moduli of titanium andCFRP as well as deviations in the surface roughness caused by thematerial specific chip formation mechanisms. These points will beanalyzed in further investigations.

6. Conclusion and outlook

This paper presents the modelled undeformed chip geometriesfor the process helical milling. This knowledge is used to explainthe impact of the axial and tangential feed per tooth on the processforces. The interrelation between process forces and bore diametercould be verified.

During helical milling the combination of axial feed per tooth fza

and tangential feed per tooth fzt has a strong influence on theappearance of the undeformed chip. High tangential feeds at aconstant axial feed and low axial feeds at a constant tangential feedresult in flat undeformed chip geometries. These generate low feedand feed normal forces leading to small tool deflections and lowdiameter deviations (Fig. 9).

In future the influence of the Young’s moduli of the workpieceand the surface roughness on the diameter deviations have to befurther investigated. At the same time the influence of the toolmacrogeometry on process forces and bore hole quality is to be

researched with the aim to reduce the diameter difference at thetransition between the CFRP and the titanium layer.

Acknowledgement

We would like to thank the German Research Foundation (DFG)for the financial support within the project ‘‘Herstellung,Bearbeitung und Qualifizierung hybrider Werkstoffsysteme (GRK1378/1)’’.

References

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