Direction of vibration Eo Tool Figure 8: Mechanism of ultrasonic vibration cutting PCD ve= cutting speed f vibration frequency: IT cutting distance during one period of tool vibration [24] This cutting speed is called"critical cutting speed" in the vibration cutting and is calculated by ve 2raf, where a is the amplitude of the tool vibration and f is its frequency ( ir 100 this study Vc=110 m/min) Cutting speed'Vc(m/min) The mechanism of ultrasonic vibration cutting is shown in Figure 8. The performance of ultrasonic vibration cutting Figure 6: Tool life for carbide and pcd tools vs. cutting strongly depends on the cutting distance during one speed in turning of epoxy matrix CFRP(Vt= 40 %)[15] period of the tool vibration IT Vo/f. It was experimentally confirmed that lr must be smaller than the fibre diameter (7 um in this study) to take advantage of ultrasonic 100 vibration cutting. By making l smaller than the fibre diameter the matrix and the fibre, which have different O DCC mechanical properties, can be sheared separately Hereby, the fibres do not prevent the shearing of the plastic matrix and, consequently, the surface quality is 50 improved even if the angle between fibre orientation and cutting direction is 90 A comparison of surface roughness between conventiona and ultrasonic vibration cutting is shown in Figure 9. Wher Ir is larger than the fibre diameter, the surface roughness 8 in ultrasonic vibration cutting is similar to that of conventional cutting(Figure 9a). On the contrary, when IT is smaller than the fibre diameter, the roughness in ultrasonic vibration cutting becomes smaller than that of conventional cutting(Fi 103mmin3.103 Conventional cutting vibration cutting Cutting Speed Vc 8 Figure 7: Tool life of diamond-coated and uncoated 6 54 carbide tools vs. cutting speed in turning of polyamide matrix CFRP (V+=40%)[15] As regards cutting parameters, speed and feed primarily nfluence the life of the cutting edge(Figure 7 4590 0 45 The tool life of both uncoated and diamond-coated carbide Fibre orientation(deg) Fibre orientation(deg)D tools reveals that wear is reduced by the diamond layer (a)Ir =18 um>7 um An increase in thermal stress of the cutting edge is (b)hr=36μm<7pm connected with the increase in cutting speed. Due to the high thermal conductivity of the diamond layer, an Figure 9: Surface roughness for conventional and ultrasonic vibration cutting [24] increase in thermal load capacity is available and ccordingly, higher cutting speeds are allowed for because of the difficulty in machining CFRP composites 目 Conventional cutting■ Vibration cutting h high efficiency, in [24] it was proposed to apply ultrasonic vibrations in turning of CFRP pipes using a iamond-coated tool The performance of the ultrasonic vibration cutting was 810 evaluated in terms of cutting force, burr formation and 12 surface roughness Ultrasonic vibration cutting allows to obtain good surface quality when machining difficult-to-cut materials. This is due to the fact that the ultrasonic vibration avoids the 04590 continuous contact between the tool rake face and the Fibre orientation(deg) Fibre orientation(deg) hip. As reported in [25], when the cutting speed beco faster than the speed of the tool vibration, the tool rake Figure 10: Cutting forces for conventional and vibration face is not separated from the chip and consequently cutting for a cutting speed 4 m/m ultrasonic vibration cutting loses its effectiveness corresponding to IT =3.6 um [24
Figure 8: Mechanism of ultrasonic vibration cutting. vc = cutting speed; f = vibration frequency; IT = cutting distance during one period of tool vibration [24]. This cutting speed is called "critical cutting speed" in the vibration cutting and is calculated by vc = 2naf, where a is the amplitude of the tool vibration and f is its frequency (in this study vc = 110 m/min). The mechanism of ultrasonic vibration cutting is shown in Figure 8. The performance of ultrasonic vibration cutting strongly depends on the cutting distance during one period of the tool vibration IT = vJf. It was experimentally confirmed that IT must be smaller than the fibre diameter (7 pm in this study) to take advantage of ultrasonic vibration cutting. By making IT smaller than the fibre diameter, the matrix and the fibre, which have different mechanical properties, can be sheared separately. Hereby, the fibres do not prevent the shearing of the plastic matrix and, consequently, the surface quality is improved even if the angle between fibre orientation and cutting direction is 90". A comparison of surface roughness between conventional and ultrasonic vibration cutting is shown in Figure 9. When IT is larger than the fibre diameter, the surface roughness in ultrasonic vibration cutting is similar to that of conventional cutting (Figure 9a). On the contrary, when IT is smaller than the fibre diameter, the roughness in ultrasonic vibration cutting becomes smaller than that of conventional cutting (Figure 9b). Figure 6: Tool life for carbide and PCD tools vs. cutting speed in turning of epoxy matrix CFRP (Vf= 40 %) [I51 Figure 7: Tool life of diamond-coated and uncoated carbide tools vs. cutting speed in turning of polyamide matrix CFRP (Vf = 40%) [I 51. As regards cutting parameters, speed and feed primarily influence the life of the cutting edge (Figure 7). The tool life of both uncoated and diamond-coated carbide tools reveals that wear is reduced by the diamond layer. An increase in thermal stress of the cutting edge is connected with the increase in cutting speed. Due to the high thermal conductivity of the diamond layer, an increase in thermal load capacity is available and, accordingly, higher cutting speeds are allowed for. Because of the difficulty in machining CFRP composites with high efficiency, in [24] it was proposed to apply ultrasonic vibrations in turning of CFRP pipes using a diamond-coated tool. The performance of the ultrasonic vibration cutting was evaluated in terms of cutting force, burr formation and surface roughness. Ultrasonic vibration cutting allows to obtain good surface quality when machining difficult-to-cut materials. This is due to the fact that the ultrasonic vibration avoids the continuous contact between the tool rake face and the chip. As reported in [25], when the cutting speed becomes faster than the speed of the tool vibration, the tool rake face is not separated from the chip and consequently ultrasonic vibration cutting loses its effectiveness. Figure 9: Surface roughness for conventional and ultrasonic vibration cutting [24]. Fibre orientation (deg) Fibre orientation (deg) Figure 10: Cutting forces for conventional and vibration cutting for a cutting speed = 4 m/min corresponding to IT = 3.6 pm [24]
Feed direction Feed direction Cutting direction 88 c2 0.1mm 0.1mm (a) conventional cutting (b)Vibration cutting Figure 11: Microscopic observation of the cut surface when fibre orientation is 0"[24] 7.5 Typical thrust force vs. time plot for a single drilling operation on CFRP [26] (a) Conventional cutting b)Vibration cutting Figure 12: Microscopic observation of the edge when : fibre orientation is 90[24] Cutting forces were also investigated to confirm that the critical limit of hr in CFRP cutting was correct. The cutting forces for different fibre orientations in conventional and ultrasonic vibration cutting are shown in Figure 10. In cutting of CFRP the thrust force is higher than the principal force. The average thrust force in ultrasonic vibration cutting becomes less than half of the thrust force in conventional cutting if IT <7 um The improvement of the surface quality was confirmed by microscopic photographs(Figures 11 and 12). Arrows in 7.5 the figure show the cutting and the feed direction For fibre Time t(s) orientation 0, in conventional cutting lots of fibres are Figure 14: Typical torque vs. time plot for a single drilling ulled out Figure 11a). In ultrasonic vibration cutting operation on CFRP [26] those fibres are absent (Figure 11b). In addition, for fibre orientation 90, in conventional cutting fibres are not cut a This is followed by a sharp reduction of the force and a he edge of the surface( Figure 12a). In ultrasonic vibration slight drop of the torque due to the fact that the tip of the cutting, however, those fibres are not visible(Figure 12b) tool has broken through the back face of the workpiece Drilling of FRP A number of research workers have investigated the the laminate, the reduction in force becomes more gradual drilling of different FRP composite materials using various and the torque is seen to slightly increase cutting tool materials Finally, the force and the torque drop to zero as reaming In [26], the use of high performance carbide drills in drilling takes place CFRP epoxy matrix composites was studied. To reduce As the number of holes drilled increases so does the high wear rate of the carbide drills, speciality coatings magnitude of both the maximum torque and the the including titanium nitride(TiN)and diamond-like-carbor force values (DLC)can be used. The performance of the coatings was Similar profiles were noted for both the uncoated and the analysed in terms of damage to the composite and thrust coated tools force and torque produced during drilling Figure 15 shows a combination of the maximum thrust Figures 13 and 14 show typical thrust force and torque force and torque for uncoated and coated drills profiles, in the case of an uncoated tool. The general form Also included are the flank wear results which show wear of the thrust force and torque profile, comprises six main in the order of 0.07 mm after 32 drilling operations stages. Initially there is a sharp increase in thrust force The maximum thrust force, maximum torque and flank nd torque due to the initial entry of the drill into the ear curves for the three drill types exhibit similar trends composite. This is followed by a further increase in the Both the thrust force and torque curves rise sharply in the force and torque as the second cutting edge enters the nitial stages after which the subsequent rate of increase is workpiece. The maximum force and torque occur as the seen to reduce tip of the tool breaks through the bottom ply of the A change in form of both these curves is apparent in the aminate region 5<n<10, drilled holes
Figure 13: Typical thrust force vs. time plot for a single drilling operation on CFRP [26]. Figure 12: Microscopic observation of the edge when fibre orientation is 90" [24]. Cutting forces were also investigated to confirm that the critical limit of IT in CFRP cutting was correct. The cutting forces for different fibre orientations in conventional and ultrasonic vibration cutting are shown in Figure 10. In cutting of CFRP the thrust force is higher than the principal force. The average thrust force in ultrasonic vibration cutting becomes less than half of the thrust force in conventional cutting if IT < 7 pn. The improvement of the surface quality was confirmed by microscopic photographs (Figures 11 and 12). Arrows in the figure show the cutting and the feed direction. For fibre orientation O", in conventional cutting lots of fibres are pulled out (Figure Ila). In ultrasonic vibration cutting, those fibres are absent (Figure 11 b). In addition, for fibre orientation go", in conventional cutting fibres are not cut at the edge of the surface (Figure 12a). In ultrasonic vibration cutting, however, those fibres are not visible (Figure 12b). Drilling of FRP A number of research workers have investigated the drilling of different FRP composite materials using various cutting tool materials. In [26], the use of high performance carbide drills in drilling CFRP epoxy matrix composites was studied. To reduce the high wear rate of the carbide drills, speciality coatings including titanium nitride (TIN) and diamond-like-carbon (DLC) can be used. The performance of the coatings was analysed in terms of damage to the composite and thrust force and torque produced during drilling. Figures 13 and 14 show typical thrust force and torque profiles, in the case of an uncoated tool. The general form of the thrust force and torque profile, comprises six main stages. Initially, there is a sharp increase in thrust force and torque due to the initial entry of the drill into the composite. This is followed by a further increase in the force and torque as the second cutting edge enters the workpiece. The maximum force and torque occur as the tip of the tool breaks through the bottom ply of the laminate. Figure 14: Typical torque vs. time plot for a single drilling operation on CFRP [26]. This is followed by a sharp reduction of the force and a slight drop of the torque due to the fact that the tip of the tool has broken through the back face of the workpiece. When the first chisel edge breaks through the back face of the laminate, the reduction in force becomes more gradual and the torque is seen to slightly increase. Finally, the force and the torque drop to zero as reaming takes place. As the number of holes drilled increases, so does the magnitude of both the maximum torque and the thrust force values. Similar profiles were noted for both the uncoated and the coated tools. Figure 15 shows a combination of the maximum thrust force and torque for uncoated and coated drills. Also included are the flank wear results, which show wear in the order of 0.07 mm after 32 drilling operations. The maximum thrust force, maximum torque and flank wear curves for the three drill types exhibit similar trends. Both the thrust force and torque curves rise sharply in the initial stages after which the subsequent rate of increase is seen to reduce. A change in form of both these curves is apparent in the region 5 < n < 10, drilled holes
Thrust Force %0一 88见 Torque (a)n=1 (b)n=1000 5101520253035 Figure 17: Hole exit in drilled GFRP(Ve Work thickness: 10 mm: drill; fish tail carbide drill dial Number of holes drilled n 10 mm; feed: 0. 1 mm; cutting speed: 163 m/min (301 Figure 15: Variation of maximum thrust force torque and In [30], the problem of burr generation in drilling of GFRP flank wear with number of drilled holes [26] composites with different cutting tools is studied. The fish x Uncoated tool, o dlc tool tin coated tool tail drill is found to be very effective in suppressing the generation of burr. Several grades of carbide materials In [27 the tool life of uncoated and diamond-coated were tested as fish tail drills. Among the tested carbides carbide tools in drilling of GFRP composites was studied K01 and K10 showed the highest cutting performance and (Figure 16). The comparison of the tool life of the different drill wear depended only on the fibre type and volume types of tools illustrates the protective effect of the lese drills, tool wear causes diamond layer. In addition to the protection against generation of burr after a certain length of drilling. Figure abrasive wear, the diamond layer also protects against 17 shows an example of burr after drilling 1000 holes on a thermal wear. a shift of the tool life line towards highe values is obtained for higher cutting speeds. Nevertheless mainly caused by the outer corner wear of the drill the tool life curve of the diamond-coated carbide bends at To get a longer tool life, it is necessary to use higher wear high cutting speeds. This indicates the thermal failure of resistant tool materials and diamond is the most suitable the substrate material. An increase in cutting speed is a trial diamond endmill with sintered diamond blades connected with an increase in cutting temperature On the Compared with carbide drills, the wear of diamond development and, on the other, by the decrease in tool life endmills was very small and after drilling 1000 holes the for uncoated carbide tools due to the insufficient heat burr was scarcely generated. In addition, the torque and esistance of the substrate [28] thrust for diamond endmills was less than a half of that for [29] an overview of the potential uses of PCD in FRP carbide drills. The roughness of the hole wall drilled with composite drilling was shown and PCD tools were carbide drills and diamond endmills was compared. Holes compared to carbide tools in terms of both economics and drilled with fish tail carbide drills have a high roughness uality. It was found that drilling processes performed on with Rmax 30 um after drilling 1000 holes. Holes drilled with diamond endmills have a low roughness with rmax <5 implemented. PCD is an economical alternative to carbide um even ater drilling 1000 holes despite the higher cost because tool life is longer and As regards the geometry of fish tail drills, clearance angle higher processing speeds can be used point angle and helix angle were examined. As the elastic deformation of composites is rel machining, the contact area between the flank of the drill and the finished surface may become quite large when the 100 cle To find out the suitable clearance angle, drilling tests were conducted with fish tai carbide drills and variable clearance, point and helix angles. The most suitable angles for drilling gFrP were clearance angle 15, point angle 75, helix angle 35 DCC Figure 18 shows the photos of the exit side of holes aft drilling 1000 holes on 10 mm thick GFRP with each drill The effect of machining parameters on the cut quality and the mechanical behaviour of GFRP composites was verified in [31] during drilling tests. A correlation between width of the damage zone and drilling speed and feed ratio was found: the higher the ratio, the better the cut quality Carbide Dynamic modelling and adaptive predictive control of thrust force in the drilling of CFRP composite laminates and control of CFRP composite laminates were presented n[33] the thrust force defined by the discrete Hocheng Dharan equations was compared with the experimental Cutting speed vc(m/min) thrust force at the pre-exit drilling phase. A theoretical study and a series of experiments were conducted to Figure 16: Tool life of carbide and diamond-coated carbide develop a dynamic model of the process which was used tools vs cutting speed in drilling of GFRP(V+=55%) to design a supervisory adaptive predictive controller that Drill diameter: 10 mm: work thickness: 18 mm. estimates model parameters and applies predictive control feed: 0.08 mm[271 for force regulation
(a) n = 1 (b) n=1000 Figure 17: Hole exit in drilled GFRP (Vf = 60%). Work thickness: 10 mm; drill: fish tail carbide; drill diam.: 10 mm; feed: 0.1 mm; cutting speed: 163 mhin [30]. Figure 15: Variation of maximum thrust force, torque and flank wear with number of drilled holes [26]. x Uncoated tool, DLC tool, + TIN coated tool. In [27], the tool life of uncoated and diamond-coated carbide tools in drilling of GFRP composites was studied (Figure 16). The comparison of the tool life of the different types of tools illustrates the protective effect of the diamond layer. In addition to the protection against abrasive wear, the diamond layer also protects against thermal wear. A shift of the tool life line towards higher values is obtained for higher cutting speeds. Nevertheless, the tool life curve of the diamond-coated carbide bends at high cutting speeds. This indicates the thermal failure of the substrate material. An increase in cutting speed is connected with an increase in cutting temperature. On the one hand, this can be explained by a crater wear development and, on the other, by the decrease in tool life for uncoated carbide tools due to the insufficient heat resistance of the substrate [28]. In [29] an overview of the potential uses of PCD in FRP composite drilling was shown and PCD tools were compared to carbide tools in terms of both economics and quality. It was found that drilling processes performed on FRP composites are strongly dependent on the tools implemented. PCD is an economical alternative to carbide despite the higher cost because tool life is longer and higher processing speeds can be used. Cutting speed vc (rnhin) Figure 16: Tool life of carbide and diamond-coated carbide tools vs. cutting speed in drilling of GFRP (Vf = 55%). Drill diameter: 10 mm; workthickness: 18 mm; feed: 0.08 mm [27]. In [30], the problem of burr generation in drilling of GFRP composites with different cutting tools is studied. The fish tail drill is found to be very effective in suppressing the generation of burr. Several grades of carbide materials were tested as fish tail drills. Among the tested carbides, KO1 and K10 showed the highest cutting performance and drill wear depended only on the fibre type and volume fraction. Even with these drills, tool wear causes the generation of burr after a certain length of drilling. Figure 17 shows an example of burr after drilling 1000 holes on a 10 mm thick GFRP laminate with Vf = 60 %. The burr is mainly caused by the outer corner wear of the drill. To get a longer tool life, it is necessary to use higher wear resistant tool materials and diamond is the most suitable. A trial diamond endmill with sintered diamond blades brazed on a carbide substrate was used for drilling GFRP. Compared with carbide drills, the wear of diamond endmills was very small and after drilling 1000 holes the burr was scarcely generated. In addition, the torque and thrust for diamond endmills was less than a half of that for carbide drills. The roughness of the hole wall drilled with carbide drills and diamond endmills was compared. Holes drilled with fish tail carbide drills have a high roughness with R, = 30 pm after drilling 1000 holes. Holes drilled with diamond endmills have a low roughness with R, < 5 pm even after drilling 1000 holes. As regards the geometry of fish tail drills, clearance angle, point angle and helix angle were examined. As the elastic deformation of composites is relatively large during machining, the contact area between the flank of the drill and the finished surface may become quite large when the clearance angle is small. To find out the suitable clearance angle, drilling tests were conducted with fish tail carbide drills and variable clearance, point and helix angles. The most suitable angles for drilling GFRP were: clearance angle 15", point angle 75", helix angle 35". Figure 18 shows the photos of the exit side of holes after drilling 1000 holes on 10 mm thick GFRP with each drill. The effect of machining parameters on the cut quality and the mechanical behaviour of GFRP composites was verified in [31] during drilling tests. A correlation between width of the damage zone and drilling speed and feed ratio was found: the higher the ratio, the better the cut quality. Dynamic modelling and adaptive predictive control of thrust force in the drilling of CFRP composite laminates were developed in [32]. The characterisation, modelling and control of CFRP composite laminates were presented in [33]; the thrust force defined by the discrete HochengDharan equations was compared with the experimental thrust force at the pre-exit drilling phase. A theoretical study and a series of experiments were conducted to develop a dynamic model of the process which was used to design a supervisory adaptive predictive controller that estimates model parameters and applies predictive control for force regulation