Wear 302 (2013) 1560–1567 Contents lists available at SciVerse ScienceDirect Wear journal homepage: www.elsevier.com/locate/wear Tribo-functional design of double cone drill implications in tool wear during drilling of copper mesh/CFRP/woven ply Redouane Zitoune a,n, Mohamed El Mansori b, Vijayan Krishnaraj c a Institut Clément Ader (ICA), ‘‘INSA, UPS, Mines Albi, ISAE’’, Université de Toulouse, Toulouse, 133c, avenue de Rangueil, 31077 Toulouse, cedex 04, France Laboratoire de Mécanique et Procédés de Fabrication (LMPF), Arts et Métiers ParisTech, Châlons-En-Champagne, France c PSG College of Technology, Coimbatore, India b a r t i c l e i n f o a b s t r a c t Article history: Received 22 August 2012 Received in revised form 15 January 2013 Accepted 17 January 2013 Available online 24 January 2013 This article presents an experimental analysis of drilling using carbide drills on carbon fiber reinforced plastic (CFRP) laminates specially made with copper mesh on one side and woven carbon fabric on the other side using carbide drills. The objective of this study was to improve the machining performance during drilling of sandwiched composite using various dimensions of double cone drill. The drill design in terms of its tribological characteristics (i.e. friction, wear) are analyzed including cutting force, life, chip form and hole quality. Results have shown that double cone drills generated less thrust force compared to standard twist drills. No delamination was found in the holes at high feed rates (above 0.1 mm/rev). This could be attributed to the presence of thermoplastic nodules between the layers of the CFRP laminate and the presence of woven fabric ply at the bottom of the laminate. Furthermore a relationship between the feed per cutting edge and the thickness of the thermoplastic layer has been developed to promote the formation of continuous chips during drilling. Finally, SEM observations reveal several damage when standard twist drills were used and less damage was observed with double cone drills. These damaged areas were observed in the plies of fibers oriented at  451 and 901 compared with peripheral cutting speed of the drill. & 2013 Elsevier B.V. All rights reserved. Keywords: Wear Defects Damage mechanics Fiber/matrix bond Delamination 1. Introduction Drilling is one of the most fundamental and widely used machining operations for composite materials used in structural assembly in the aerospace industry. In general, a single aircraft requires approximately ten million holes drilled for the whole fuselage structural joining. Composite materials especially Carbon Fiber Reinforced Plastics (CFRP) are often used to a larger extent due to their higher specific strength and stiffness. However, due to their laminated architecture several damages, like matrix cratering, thermal alteration, fiber pull-out and fuzzing, are induced when drilling [1]. In addition, geometrical defects similar to those found in metallic drilling are found depending on cutting conditions [2]. These damages are more pronounced when drilling composite and metal structures (sandwich) during assembly. This is the case of the composite and metal parts used to prevent damage from lightning or other high-voltage currents. During assembly, about 60% of the rejections are due to the defects in the holes. These defects would significantly reduce structural stiffness, resulting in varying the dynamic performance of the entire n Corresponding author. E-mail address: Redouane.Zitoune@iut-tlse3.fr (R. Zitoune). 0043-1648/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.wear.2013.01.046 structure. Many of these problems are due to the use of nonoptimal cutting tool designs, rapid tool wear, and improper machining conditions [3–6]. Hocheng and Puw [7,8] performed drilling on CFRP thermo set and thermoplastic materials using high speed steel (HSS) drills for varying feed rates. They reported that continuous and curly chips were observed during drilling in thermoplastics. However, a fracture with discontinuous chips and delamination was reported at the exit of the hole in a thermoset composite. Similar observations have been reported during experimental and numerical studies of orthogonal cutting on unidirectional specimens made of carbon fiber and epoxy resin [9–11]. Persson et al. [12] compared the performance of the KTH (Kungl Tekniska Hogskolan) method (also called as orbital machining) with axial drilling. With the KTH method, diamond coated tools were used for hole generation. For axial drilling, PCD (Poly Crystalline Diamond) drills and dagger drills were used. The results of fatigue bearing tests carried out on various specimens showed that failure stress of the specimens machined by the KTH method are 19% to 27% higher when compared to the failure stress of the specimens machined with PCD drill. Authors [13,14] have reported that it is possible to significantly reduce the thrust force by reducing the chisel edge width when using a pre hole. Hocheng and Dharan [15] noted that as the end layers of the laminate display considerably lesser resistance R. Zitoune et al. / Wear 302 (2013) 1560–1567 to deformation, maximum delamination occurs at the entry and exit side of the hole. This is mainly because of peel-up and push-out action of the tool. In order to predict the critical thrust force responsible for the delamination at the hole exit, Zitoune and Collombet [16] developed a numerical model. The authors demonstrated that when 60% of the thrust force is generated by the chisel edge of the drill. The experimental results and the numerical model developed show a close correlation. Krishnaraj et al. [17] drilled composites at high spindle speeds and studied the influence of tool geometry. They reported that a double cone drill offers better surface finish when compared to a standard twist drill or a Brad & Spur drill. Chen [18] and Davim and Reis [19] introduced the concept of delamination factor in their experimental investigation on drilling of CFRP material. They reported that delamination free drilling is achievable by proper selection of tool geometry and drilling parameters. The quality of the machined surface is based on cutting speed, depth of cut, and direction of fiber orientation with respect to cutting speed [9–11]. Sakuma et al. [20] drilled holes using four drill materials and investigated drill wear pattern, flank wear width and cutting forces. They concluded that K10 drill material has the highest wear resistance among the drill materials investigated. Tsao et al. [21,22] studied the delamination factor by using twist drill, candle stick drill and saw drill under various cutting conditions. Analytically, the effect of various drill geometries on thrust force was predicted and compared to a twist drill. Tsao and Hocheng [23] also concluded that saw drills and core drills produce a lesser delamination than the twist drills by distributing the drilling thrust toward the hole periphery. Saw and core drills showed better quality during drilling. Chen [18] investigated the effect of double cone, Brad & Spur at high spindle speed during drilling of glass fiber reinforced plastics. Many researchers investigated the effect of tool geometry on drilling of polymer composite materials. Many of the modified geometries (Zhirov point, Brad and Spur, step drill, candle stick, saw drill, etc.) are difficult to regrind. Among the various tool geometries investigated, double cone drills have been found to offer many advantages when compared to the modified geometries. Only a few investigations on drilling of CFRP laminates using double cone drill have been reported. Literature reveal studies where double cone drills are optimized for drilling of metallic materials (steel or aluminum). However, there is little information about the influence of the geometry of the double cone drill on the quality of holes machined. The main objective of this study was to improve the drilling performance of CFRP sandwiched composites using various geometries of double cone drills. The drill designs (lip length of the double cone) in terms of its tribological properties (i.e. friction, wear, and cutting induced-heat) are analyzed 1561 including cutting force reduction, drill life, chip form and hole quality. For this, an experimental study on drilling of CFRP laminate sandwiched with copper mesh has been carried out. 2. Experimentation 2.1. Workpiece details The CFRP composite specimen used in the investigation was 6.86 mm thick. The laminate was made out of 34 unidirectional plies of 0.196 mm thickness each and one woven ply with 0.1 mm of thickness. The 34 unidirectional plies are made of carbon/ epoxy prepreg and manufactured by Hexcel Composite Company with the reference T700-M21. The following was the staking sequence [45/-45/0/90/45/0/0/-45/0/0/-45/0/0/45/90/-45/45]s. At the top of the laminate a copper mesh has been placed in order to improve the electrical conductivity (Ref. Fig. 1). In aeronautics, copper mesh is used on the top of the laminate in order to avoid the damages of the composite parts if struck by lightning. A thin layer of carbon/epoxy woven fabric (0.1 mm) has been used at the bottom of the laminate, in order to avoid delamination at the exit of the hole during drilling. These materials were compacted using a vacuum pump in a controlled atmosphere. A mold for the laminate was prepared and placed inside the autoclave. The cure cycle consisted of raising temperature at the rate of 2 1C per minute up to 180 1C, and maintaining at this temperature for duration of 120 min and brought back to room temperature by reducing temperature at the rate of 2 1C per minute. The whole cycle was carried out at a pressure of 7 bars and  0.7 bars of vacuum. Fig. 1 shows the laminate used for conducting the experiments. 2.2. Experimental details The workpiece was mounted on the dynamometer to measure the thrust force and torque, and the later was fixed on the table of a precision milling machine. Two spindle speeds and four feed rates were selected to study the effect of the machining parameters. Values of spindle speeds used in this study represent on the one side the maximum values available with the machine tool used and on the other hand are representative of those can provide an (UPA) automatic drilling unit (electric) commonly used in Airbus. Moreover from the previous studies carried out by the authors it is found that spindle speed has less effect on the quality of the hole [24]. High speed reduces the life of the drill and not suitable for CFRP. Hence spindle speeds are selected in such a Copper mesh at the top of the laminate Sensors to monitor temperature Rigid mould Vacuum CFRP laminate Fig. 1. Manufacturing of the composite part by the autoclave process, (a) Mold in the autoclave after curing, (b) composite part after demolding ‘‘laminate with the copper mesh’’. 1562 R. Zitoune et al. / Wear 302 (2013) 1560–1567 way that they suit the requirements of drilling of CFRP. Table 1 summarizes the experimental conditions. Spindle speeds were selected to suit the requirements of drilling of CFRP. All the trials were conducted without the use of coolant. The thrust force and torque during machining were measured using a piezo-electric dynamometer (type Kistler 9272). The charge amplifier (model 5019) converts the resulting charge signals, which are proportional to the force, to voltage and managed through the data acquisition system. Each experimental condition was repeated 5 times to get average representative values. To remove the influence of tool wear, each experiment was performed with a new drill. The quality of the machined surface (wall of the hole) was quantified using the SEM observation and the surface roughness. The surface roughness (Ra) of the hole was measured by surface roughness tester (Mitutoya SJ 500) with a sampling length (cut-off) of 0.8 mm. The total length of measurement through the hole was 4 mm (0.8  5¼4 mm). Hole diameters and circularity were measured using co-ordinate measuring machine (MC 1200 C) with F 2 mm ruby probe. 2.3. Drill geometry Table 2 represents the geometric characteristics of the standard twist drill (ref-tool) used for drilling composite materials by Air-Bus Company and the modified double cone drills. The modification of Table 1 Experimental condition used for drilling of the composite part. Machine tool Milling machine, Spindle power 3 kW Workpiece Drill Parameter’s CFRP (58% Vf, 6.86 mm thick) C 6.35 Carbide drills (Grade K20) Spindle speed (rpm) 2020 and 2750, Feed rate (mm/rev) 0.05, 0.1, 0.15 and 0.3 the tool geometry is achieved using a 5-axis grinding machine. The double cone tools measure 901 and 1321 point angles with cutting edge lengths, namely L1 and L2 (Ref. Fig. 2)—while L1 represents the size of the principal cutting edge number 1 which is ground at a point angle of 1321, L2 represents the size of principal cutting edge number 2 which is ground at a point angle of 901. The double cone drills are ground with different L2/L1 ratios, in which M1, M2 and M3 represent L2/L1 ratios of 0.33, 1 and 3.1 respectively. Drilling trials were carried out using a 6.35 mm diameter drills made of tungsten carbide (K20). 3. Results and discussions 3.1. Effect of speed and feed on thrust and torque forces Fig. 3 shows the effect of feed rate and tool geometry (standard twist drill and double cone drills) on thrust force for the two spindle speeds used. It can be observed that, drilling with twist drill leads to higher thrust force compared to double cone drills. From Fig. 3a, it can be noticed that when the feed rate is increased from 0.05 mm/rev to 0.3 mm/rev, drilling using twist drill is also subjected to an increase of 120% of the initial thrust force while drilling at a spindle speed of 2020 rpm. A similar increase of thrust force was observed when drilling with double cone drills as well. When drilling of double cone drills was compared with twist drills, double cone drills produced 20% to 30% lesser thrust force. This can be explained by the fact that adding a secondary point angle (901) reduces the average chip thickness by 15% for the same machining parameters, Normally, the end of lip is subject to higher cutting speed (Peripheral cutting speed is the function of radius of drill and spindle speed (rpm)). During drilling at a spindle speed of 2750 rpm, when the feed rate was increased from 0.05 mm/rev to 0.3 mm/rev, the thrust force increased by 150%. For the small feed rate tested ( o0.1 mm/rev), increasing the spindle speed from 2020 rpm to 2750 rpm was found to cause a small reduction in the thrust force. Increasing the spindle speed Table 2 Characteristics of twist drills and double cone drills. Geometric characteristic of tools Twist drill Double cone tool type M1 Double cone drill type M2 Double cone drill-M3 Diameter (mm) Web thickness (mm) Point angle no. 1 (1) Point angle no. 2 (1) Clearance angle (1) Helix angle (1) Ratio L2/L1 6.35 0.16 136 – 8.58 32.5 0 6.35 0.16 136 90 8.65 32.5 0.33 6.35 0.16 136 90 8.65 32.5 1 6.35 0.16 136 90 8.65 32.5 3.1 L1 1.6 mm 1.6 mm L1 L2 Principal cutting edge Principal cutting edge Secondary cutting edge Fig. 2. Schematic showing the active portion (point) of the drilling tools with (a) Standard twist drill (b) Double cone tool type M2. R. Zitoune et al. / Wear 302 (2013) 1560–1567 250 70 60 150 100 Standard twist drill Double cone drill type M1 Double cone drill type M2 Double cone drill type M3 50 Torque (N.mm) 200 Thurst force (N) 1563 0.05 0.1 0.15 0.2 0.25 0.3 40 30 Standard twist drill Double cone drill type M1 Double cone drill type M2 Double cone drill type M3 20 10 0 0 50 0 0.35 0 0.05 0.1 Feed rate (mm/rev) 0.15 0.2 0.25 0.3 0.35 Feed rate (mm/rev) 250 80 70 200 150 100 Standard twist drill Double cone drill type M1 Double cone drill type M2 Double cone drill type M3 50 Torque (N.mm) Thrust froce (N) 60 50 40 30 Standard twist drill Double cone drill type M1 Double cone drill type M2 Double cone drill type M3 20 10 0 0 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 Feed rate (mm/rev) 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 Feed rate (mm/rev) Fig. 3. Influence of feed rate versus thrust force for twist drill and double cone drills: (a) spindle speed of 2020 rpm, (b) spindle speed of 2750 rpm. Fig. 4. Influence of torque versus feed rates: (a) spindle speed N ¼2020 rpm, (b) spindle speed N ¼2750 rpm. helps to raise the temperature of machining due to the friction between the tool and CFRP, thus resulting in softening of the material and a subsequent reduction in thrust force. During drilling CFRP at low feed rates (0.05–0.1 mm/rev), all the drills were subject to more or less same torque (Ref. Fig. 4a and b). When the feed rate was increased, double cone drill (M3) was subjected to higher torque. Since this double cone geometry has higher cutting edge length, the torque required to cut the hole is also higher when compared to standard twist drill. From Fig. 4b it can be observed that the double cone drill type M3, (with L2/L1 equal to 3.1) which has a higher secondary cutting edge length is subjected to higher torque during drilling at higher feed rate. When drills are subjected to higher torque, they have a negative impact on the tool life as well as the quality of machined holes. From the SEM observation, it can be noticed that the holes machined with twist drill show several damaged areas (Fig. 6a and Fig. 7a). These damaged areas were mainly observed at  451 and 901 fiber orientations (See Fig. 6a and Fig. 7a). However, when machined with double cone drills, the damage area and damage depths were observed lesser when compared to twist drill. Further, it was found that the damages are uniformly distributed (Fig. 6b). The increasing of the feed rate from 0.05 mm/rev to 0.3 mm/rev show that, the extent of the damaged areas located on the wall of the hole increased when drilling was done with standard twist drills (Ref. Fig. 7a). However, with double cone tools, the damaged area was found to be lesser (Ref. Fig. 7b). From the experimental analysis of drilling of the composite material, it can be inferred that the increase the feed rate does not cause any delamination or any fiber pull out at the exit of the hole. This can be due to the use of woven ply at the bottom of the CFRP during fabrication of laminate, and also because of the thermoplastics nodules present between each ply. The presence of thermoplastic nodules lead to an increase in the critical energy release rate in mode I and mode II (GIc and GIIc) which increase the critical thrust force responsible for the delamination at the exit side of hole. These aspects in the CFRP material used for the current study could have retarded the propagation of the delamination (Ref. Fig. 8) which is a normal phenomenon in drilling CFRP. Fig. 9 presents a comparison of the measured diameters (measurements are carried out at the mid of the part thickness) obtained after drilling with different machining parameters and different drill geometries. It can be noted that, for all the machining parameters and types of drills used, the measured values of diameters are higher when compared to the nominal diameter (d¼6.35 mm) of the drills. However, it can be noticed that, when standard twist drill is used, the diameter variations are 3.2. Quality of hole machined Fig. 5 shows the effect of drill geometry on surface finish at various spindle speeds and feed rates. Experimental results reveal that at low feed rate (o 0.1 mm/rev), the quality of the machined surface is better for both the spindle speeds used (2020 rpm as well as 2750 rpm). In this case, the measured roughness values are smaller (o0.4 mm). Further, it can also be observed that, for all machining parameters used the values of the surface roughness obtained with the twist drills are higher when compared to those obtained with double cone drills. This difference can be linked to the interaction between the chip thickness and the drill point angle. In addition, double cone drills offer stable surface roughness (around 0.4 mm) between the feed rates of 0.1-0.3 mm/rev at 2020 rpm. Drilling at a feed rate of 0.15 mm/rev using the double cone drill (M2) offers a lower roughness at both the spindle speed used. The stable surface roughness value beyond 0.1 mm/rev could be due to the wiping effect of the primary cutting edge L2. 1564 R. Zitoune et al. / Wear 302 (2013) 1560–1567 1 Copper mesh area 0.9 Standard twist drill Double cone drill type M1 Double cone drill type M2 Double cone drill type M3 0.8 Ra (µm) 0.7 Woven ply 0.6 Damage areas (matrix degradation) 0.5 0.4 Tool feed 0.3 400 µm 0.2 0.1 Damage areas (matrix degradation) 0 0 0.05 0.1 0.15 0.2 0.25 0.3 Woven ply 0.35 Feed rate (mm/rev) 1 Tool feed Standard twist drill Double cone drill type M1 Double cone drill type M2 Double cone drill type M3 0.9 0.8 Ra (µm) 0.7 0.6 0.5 Fig. 7. SEM observation of the wall of the holes after drilling with twist drill and double cone drill type M2. (a) Twist drill, (b) double cone drill type M2. With N¼ 2750 rpm and f¼ 0.3 mm/rev. 0.4 0.3 0.2 Holes drilled with feed speed of 0.05 mm/rev 0.1 0 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 Feed rate (mm/rev) Holes drilled with feed speed of 0.1 mm/rev Holes drilled with feed speed of 0.15 mm/rev Fig. 5. Evolution of surface roughness versus feed rate for twist drill and double cone drills: (a) at spindle speed of 2020 rpm, (b) at spindle speed of 2750 rpm. Holes drilled with feed speed of 0.3 mm/rev Copper mesh area Damage areas (matrix Fig. 8. Photos showing the state of the exit holes after machining with standard twist drill and for various feed rates at a spindle speed of 2020 rpm. Woven ply this case is 6 mm/side when a nominal diameter of bolt/rivet is used, which is accepted in the aeronautic field. 3.3. Effect of tool wear Tool feed 400 µm (a) Copper mesh area Damage areas (matrix degradation) Tool feed Woven ply 400 µm (b) Fig. 6. SEM observation of the wall of the holes after drilling with twist drill and double cone drill type M2. Twist drill, (b) double cone drill type M2. With N ¼2020 rpm and f¼ 0.05 mm/rev. between 5.5 mm to 10.4 mm. When the double cone drills are used they varied from 4.5 mm to 19.24 mm. The maximum values can be observed for the double cone drill type M3. The maximum clearance available during assembly in In order to study the effect of tool wear on the thrust forces and the quality of machining, drilling tests were carried out at a spindle speed of 2020 rpm and at a feed rate of 0.1 mm/rev. Fig. 10 shows the influence of the number of holes on the thrust force with twist drill and with double cone drill of type M2. It was observed that, in the case of both tools after 105 holes were drilled, the thrust force increased to 65% (127–212 N for the twist drill and 107–176 N for the double cone drill M2). This increase can be attributed to the wear generated at the cutting edge of the drills tested. Fig. 11 presents the evolution of the wear land (VB) versus the number of holes drilled. It can be observed that, the rate of wear on the twist drill is higher compared to the double cone drill of type M2. This difference can be certainly linked to the difference between the cutting edge length of the drills, point angles of double cone drill and reduction in chip thickness during drilling using double cone drill. In addition, the evolution of the average roughness obtained with twist drill versus the number of the hole drilled was almost identical to those obtained with the double cone drill (M2), when the number of the drilled hole is lesser or equal to 85 (Ref. Fig. 12). The wear land values (VB) measured on the twist drill and on double cone drill (M2) are lesser than 0.2 mm when the number of holes drilled was less than 85. However, when the wear land VB increased beyond 0.2 mm, a significant discrepancy was observed between the roughness values obtained between twist drill and double cone drill M2. R. Zitoune et al. / Wear 302 (2013) 1560–1567 1565 Fig. 9. The measured diameter of the CFRP specimen. 4.5 250 4 3.5 Rougnhess Ra (µm) Thrust force (N) 200 150 Standard twist drill Double cone drill type M2 100 Standard twist drill Double cone drill type M2 3 2.5 2 1.5 1 50 0.5 0 0 0 20 40 60 80 100 120 0 20 40 60 80 100 120 Number of holes Number of holes Fig. 12. Evolution of the roughness (Ra) versus number of holes drilled using twist drill and double cone drill M2. Fig. 10. Influence of the number of holes drilled on the thrust force. 0.25 Standard twist drill Double cone drill type M2 3.4. Chips analysis VB (mm) 0.2 0.15 0.1 0.05 0 0 20 40 60 80 100 120 Number of holes Fig. 11. Evolution of the flank wear (VB) versus number of holes drilled. This difference can be explained by the fact that the point angle (901) of the double cone drill finishes the hole much better by thinning the chip when compared to twist drill. This leads to a change in the energy dissipated by friction at the interface of cutting edges/microchip or rake face/chip. This phenomenon increases the wear on the standard twist drill when compared to double cone drill. Various forms of chips were observed during the visual analysis of the chips produced (Ref. Fig. 13). When the feed rate is small ( o0.1 mm/rev), tangled chips were seen (Ref. Fig. 13a and b). In this case, chips were found to be tangling on the body of the tool (Ref. Fig. 13a). While stable and continuous chips were produced, surface finish and quality of the surface is better. However the friction of chip on the rake face of the tool may be higher (Ref. Fig. 5). In addition to this, friction during continuous cutting increases the axial force. When the feed rate is greater or equal to 0.1 mm/rev, chips are broken into small segments (Ref. Fig. 13c). Increasing the feed rate favours formation of discontinuous chips (the stiffness of the chip increases with the feed rate), which further increase the roughness (Ref. Fig. 5). Similar results were obtained for all the tested tools (twist or double cone) as well. From the literature it can be noticed that during machining CFRP materials referenced under T700-M21 GC the observed chips were in the fine powder form [9–11]. This is because the mechanism of the material removal is affected by the orientation of the fibers at each ply [9–10]. The increase in the cross sectional area of the chip also favours discontinuous chips while drilling ductile materials. The increase in cross sectional area during drilling is a function of feed rate and point angle of the drill which is given below (cf. Fig. 14). 1566 R. Zitoune et al. / Wear 302 (2013) 1560–1567 CFRP plate Dril Tangled chips 50 mm 5 mm 5 mm Fig. 13. Photos showing the shape and the size of the chips obtained after drilling. (a) Tangled chips and hanging on the body of the tool, (b) tangled chips after drilling with reference tool and 2020 rpm of spindle speed and 0.05 mm/rev of feed rate, and (c) broken chips with millimeter size. Fig. 14. Dimension of the chip thickness vs. tool geometry and the feed rate [26]. Thermoplasticlayer CFRP ply Fig. 15. Optical microscope observation in the thickness of the CFRP part showing the different plies and the different thermoplastics layers. It is important to mention that, in conventional drilling (with twist drill) the theoretical thickness of the chip depends mainly on two parameters (feed rate by cutting edge and point angle of the tool), these information are well detailed below. The theoretical thickness of the chip during drilling (Fig. 1) is given by the following equation: h1 ¼ fz: sinðj=2Þ with h1: theoretical thickness of the chip (mm), h2: real thickness of the chip (mm), fz: feed rate by tooth (mm/rev/tooth), j: point angle of the tool, ð1Þ In order to better understand the relation between the feed rate and the form of chips obtained, SEM observations were taken on specimens removed from the mother composite plate used for the drilling tests. The presence of the thermoplastics layers were observed between the plies of the specimens (Ref. Fig. 15). These thermoplastics layers have a thickness of 20 mm to 50 mm. Hence it can be seen that, when feed rate of the drill per cutting edge are lesser or equal to the thickness of the thermoplastic layer the chips produced have a continuous form and tend to tangle around the drill body. However for feed rates greater than the thickness of thermoplastic layer per revolution per cutting edge, the chips produced have a discontinuous (broken) form with a considerable size. This implies that the presence of these nodules gives some flexibility to chips, which prevent their breaking in powder form. R. Zitoune et al. / Wear 302 (2013) 1560–1567 1567 4. Conclusions References From the experimental study carried out during drilling of a multi layered composite material made of Copper mesh/CFRP/ woven ply, using standard and double cone drills, the following conclusions can be drawn: [1] T. Richardson, Composites—A Design Guide, Industrial Press Inc., New York, 1987. [2] S. Abrate, Machining of composites, in: P.K. Mallick (Ed.), Composites Engineering Hand Book, Marcel Deckker Inc., New York, 1997, pp. 777–807. [3] W. Konig, Ch. Wulf, P. Graß, H. Willercheid, Machining of fibre reinforced plastics, Annals of the CIRP 38 (1985) 119–124. [4] R. 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[26] BM 7 088, technique de l’Ingénieur (by SANDVIK-COROMANT Company), /http://www.techniques-ingenieur.fr/S.  This paper deals with tribo-functional aspect of drilling multi-       material aeronautic components (Copper mesh/CFRP laminate/ Carbon-Epoxy fabric layer). To assess the functional design, from a tribological point of view, of new generation of cutting tool for drilling CFRP was analyzed at the macroscopic level to prove their industrial feasibility. For this reason, the tribo-drilling performances were studied primarily by analyzing the machining induced friction and wear. This demonstrated at the macroscopic scale the tribo-implications of the new designed tools which led significantly to i) reduce wear at the cutting edges ii) decrease friction during both cutting action (reduced thrust forces) and residual drilling (reduced cutting torques). This tribological performance manifests itself clearly in terms of the tool-chip interface of the new designed tools namely i) better hole quality (as consequence of reduced frictional component of torque) ii) chip formation process (as a consequence of loading contact modification). Drilling using double cone drill presents a lesser thrust force when compared to drilling using the standard twist drill. Moreover, drilling using double cone drill (Type M2 with L2/ L1¼1) resulted in reduced average roughness values when compared to other double cone drills and twist drill at 0.15 mm/rev feed rate. Also when drilling an isotropic material such as steel or titanium with double point angle, it was recommended [25] that the ratio of the second cutting edge to the principal cutting edge (L2/L1) has to be equal to 0.15. One can conclude that it is suggested to use a double cone drill with L2/L1 equal to 1 when drilling CFRP materials. The standard twist drill commonly used presents higher wear rate compared to the double cone drill M2 and this twist drill is not capable of assuring good quality of the machined hole after 60 holes are drilled. However, with the double cone drill M2 the wear rate is less when compared to the twist drill. Moreover the quality of the machined surface can be assured until 106 holes. No push-out delamination was observed even at feed rate above 0.1 mm/rev. This can be attributed to the woven ply at the bottom of the laminate and also to the presence of thermoplastics layers between the composite plies. The copper mesh at the top of the composite material totally eliminates any peel up delamination at the top of the hole. Thermoplastics nodules present in the CFRP favor the formation of continuous chips at small feed rates. When the feed rate increases, the chips were broken into small segments. This phenomenon of chip formation both continuous and broken is comparable to the one observed during drilling of metallic material. From the health of the operator point of view machining with continues chips or broken chips (no dust chips) is highly beneficial to the machine operators.