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Machining of CFRP via Short Amplitude Torsion Pendulum Drilling
Available online at www.sciencedirect.com
ScienceDirect Procedia CIRP 66 (2017) 169 – 174
1st Cirp Conference on Composite Materials Parts Manufacturing, cirp-ccmpm2017
Machining of CFRP via short amplitude torsion pendulum drilling Lukas Hebergera,*, Benjamin Kirscha, Bert Hennb, Jan C. Auricha a
University of Kaiserslautern, Institute for Manufacturing Technology and Production Systems, Gottlieb-Daimler-Str., 67663 Kaiserslautern, Germany b Philipp Persch Nachf. KG - Diamantwerkzeuge, Zur Rothheck 16, 55743 Idar-Oberstein, Germany
* Corresponding author. Tel.: +49-631-205-5482; fax: +49-631-205-3238. E-mail address: [email protected]
Abstract The machining of carbon fiber reinforced polymer (CFRP) can cause damages such as delamination or fiber protrusion. Those damages weaken the structure and thus can result into premature failure of CFRP parts. In this paper, a novel machining process for machining CFRP is proposed. The tool oscillates in a small angle at a high frequency. This cutting motion is superimposed by a feed motion in the tool’s axial direction. The tools are hollow drills equipped with an abrasive body. In the experiments, different tool designs as well as different feed rates of this short amplitude torsion pendulum drilling are examined. © Authors. Published by Elsevier B.V. This ©2017 2017The The Authors. Published by Elsevier B.V.is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 1st Cirp Conference on Composite Materials Parts Manufacturing. Peer-review under responsibility of the scientific committee of the 1st Cirp Conference on Composite Materials Parts Manufacturing Keywords: drilling; grinding; fiber reinforced polymers
1. Introduction Composite materials like Carbon-fiber-reinforced-polymers (CFRP) have a good strength-to-mass ratio, making them suitable for applications in aerospace [2] and the automotive [3]. E.g. the Airbus A380 is made of 25 % weight percent CFRP [4], the Airbus A350 even 53 % [5]. Greater holes in CFRP, needed e.g. for repairing damaged CFRP parts or the panels in aircrafts, are commonly manufactured via cutting processes. Such processes for machining CFRP being drilling, circular milling or grinding are still not fully controlled. They can, due to the material’s anisotropic properties with respect to the fiber orientation [1], cause damages to the CFRP like delamination or fiber protrusion [6]. Those damages weaken the material and can cause premature failure [7]. In recent papers, some possibilities have proved to be useful in order to prevent those failures. For example, high cutting speed with low feed rate can reduce delamination while drilling [12]. In addition to optimizing the parameters, laser pre-scoring of the workpiece’s top layer before machining [11] can decrease the premature failure. The rotary motion of the tools often results into bending of the fibers in the direction of motion. A large field of application are hand-guided machines to manufacture holes e.g. for repair
issues in aerospace. Damaged segments of an aircraft are commonly patched with CFRP plates that are attached to the aircraft via rivet connections. The holes for these connections are drilled by workers on site. This application raises some issues. The rotation of the tools can cause injuries. Small chips are formed [8] that can be in the range of a few microns [9]. Those chips or particles are dispersed to the environment by the rotary motion of the tool and can impair electrical devices in vicinity or harm the worker [10]. In addition, they necessitate to clean the parts after machining [8]. Finally, centering with hand-guided machines is an issue, especially for free-formed surfaces and oblique contacts as given by the aforementioned case of repairing of aircrafts. In this paper, a novel machining process for machining CFRP is proposed to cope with the mentioned issues. Instead of a common tool rotation, the tool is rotated in a small angle at a high frequency, similar to a torsion pendulum. This cutting motion is superimposed by a slow feed motion in the tool’s axial direction. The tools are hollow drills equipped with an abrasive body (galvanic bonded diamond). In the experiments, different tool designs as well as different feed rates of the short amplitude torsion pendulum drilling process are examined. The quality of the machined holes is examined with respect to fiber protrusion and delamination.
2212-8271 © 2017 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 1st Cirp Conference on Composite Materials Parts Manufacturing doi:10.1016/j.procir.2017.03.222
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2. Experimental design The hollow drill tools did not rotate in the conventional sense, but oscillated like a torsion pendulum with a frequency of 333.33 Hz. The angle of deflection was 3.4°. The workpiece material used was a carbon fiber reinforced polymer (CFRP). The workpieces were plates with eight layers [0°, 90°, 45°, -45°, -45°, 45°, 90°, 0°]. It was manufactured from the unidirectional prepreg CE 1007-150-38 supplied by SGL. The plates had a thickness of 1.2 - 1.4 mm with a fiber content of 60 Vol.-%.
of an individual grain, was hence smaller than the nominal grain diameter. 2.2. Manufacturing Process The hollow drills were attached on a commercial handheld oscillator by Fein1, which again was mounted on the main spindle of a 5-axis machining center, see Fig. 2. In doing so, reproducible conditions were ensured. The CFRP plates were fastened via a clamping device to a dynamometer by Kistler1.
2.1. Tools The hollow drills were manufactured by Philipp Persch Nachf. KG. The tool body material consisted of a casehardened steel (16MnCr5). The wall thickness of all hollow drills was 1 mm. The tools were electroplated with diamond abrasives. The diamond grains had a diameter of D356 (corresponding to a mesh size of 45/50). Tools with differing diameter were examined: 65, 50, 25, and 10 mm. For each diameter, tools with a cylindrical shape (corresponding to a “zero teeth” drill) and a “V-shape” (corresponding to two teeth) were applied. For an overview of the tools see Fig. 1. Due to the smaller contact area of the two teethed hollow drills, higher feed forces of the individual abrasive grain were generated - while the global feed force did not change compared to the zero toothed hollow drill. Fig. 2. Experimental setup.
The manufacturing process was carried out in two steps, see Fig. 3 (b): In step 1, the hollow drill moved downwards in a straight line in the direction of the tool axis. The further the progress, the more the lateral areas of the teeth came into contact with the CFRP plate. Those lateral areas do not cut but push and rub against the plate and result into friction. For this reason, step 1 was stopped as soon as the tooth tips came out of the CFRP plate. In step 2 the tool moved along a helix, i.e. the linear feed motion was superimposed with a rotation around the tool axis. x step 1: Hollow drill moves down in tool axis direction 1.5 mm after contacting the specimen with a feed rate of 2 mm/min x step 2: Hollow drill moves in a helix with 135°/min with superimposed feed rate in tool axis direction at 1.35 mm/min Fig. 1. Overview of the tools with a detailed view of the abrasive diamond grains.
The diameter has a decisive influence: A variation of the diameter changes the cutting speed and, due to a constant angle of deflection, the cutting path. The smaller the diameter, the smaller the cutting path. For the tool with a diameter of 10 mm this resulted into a cutting path of 297 µm. The cutting path, equivalent to the cutting path
The feed rate of both steps was doubled in a second series of tests. During all manufacturing steps, the tool oscillated with an amplitude of 1.7°, see Fig. 3 (a).
Lukas Heberger et al. / Procedia CIRP 66 (2017) 169 – 174
3. Results
Fig. 3. (a) Short amplitude torsion pendulum; (b) 2 steps of manufacturing process (view from below).
Experiments with described critical tools, like the small diameter 10 mm (cutting path smaller than nominal grain diameter) and tools without teeth (a high force was necessary because of the large contact area), directly began with step 2 to reduce the load on the clamping device. The oscillation frequency was 333.33 Hz for all experiments. The process conditions are listed in table 1.
With the kinematics chosen, no hole could be manufactured with the 10 mm diameter hollow drills. This is due to the small cutting path (297 mm) in conjunction with the size of the abrasives (nominal diameter of ~350 µm). Even with perfect juxtaposition of the abrasive grains, the distance to each other would be greater than the cutting path. Due to the shape of the abrasives, the material between the grains cannot be cut. This could be compensated by the additional helix angle, as implemented in step 2 of the kinematics. Very low feed rates in conjunction with high helix speeds would be needed, not chosen in the experiments. With the tools with zero teeth, high feed forces resulted due to the large contact areas. Drilling was interrupted at 120 N peak force. With this limitation, no holes could be manufactured for any diameter, see Fig. 4. In addition, the abraded CFRP could not be conveyed away from the contact zone with the zero teeth tools, increasing the load of the tools.
Table 1. Tool geometry and process conditions. Tool geometry parameters Tool diameter [mm]: 10, 25, 50, 65
Teeth: 0, 2
Fig. 4. Process results for the zero tooth tools.
Grain size: D356
With the two teeth hollow drills holes could be manufactured, except for the 10 mm diameter drill, see Fig. 5.
Process parameters Step 1: feed rate [mm/min]: 2, 4 Step 2 (helix motion): 135°/min ר1.35 mm/min; 270°/min ר2.7 mm/min
Cutting path [µm]: 297 (for tool diameter 10 mm); 741 (for tool diameter 25 mm); 1483 (for tool diameter 50 mm); 1928 (for tool diameter 65 mm)
Actuator spindle rotational frequency [Hz]:333.33
For every cutting condition, three holes were manufactured. The process forces were measured by means of a dynamometer (sampling rate 2048 Hz) by Kistler1. The force signal was filtered with a moving average. The moving average had an interval width of 100 measuring points. The hole quality was investigated by analyzing the delamination and the fiber protrusion via microscope. The delamination was described using the delamination factor Fd. This factor Fd is defined by the quotient of the maximum diameter of delamination and the hole diameter. The fiber protrusion was described by the two-dimensional percentage overlap of the uncut fibers over the hole. The overlap was measured using a MATLAB-routine as described in [6]. The surface morphology of the machined hole walls was analyzed via a confocal 3D microscope by Nanofocus1. After drilling, tool wear was determined qualitatively. For this purpose, the tools were cleaned in an ultrasonic bath and examined in a scanning electron microscope (SEM).
Fig. 5. Process results for different tool diameters with two teeth tools.
3.1. Feed force The feed forces are depicted in Fig. 6. with regard to the tool diameter. The feed forces over the process time show a sharp increase for both, high and low feed rate for the 25 mm hollow drill towards the end of the cycle. This was due to the separated cylindrical core of the CFRP plate. This core was stuck in the hollow drill and pushed onto the clamping device, when the hollow drill continued moving downwards after the hole was manufactured. This effect did not occur for the diameter of 50 mm and 65 mm. It seemed as if the stronger curvature of the hollow drills with smaller diameter rather clamped the core. This presumption must, however, be investigated in further studies.
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The course of the feed forces for the diameter 50 mm and 65 mm differed only slightly. The maximum feed forces detected during one respective cycle are shown in Fig. 6. The maximum values correspond to the forces during drilling, neglecting the peaks due to stuck of the core mentioned above. The maximum feed force changed with the tool diameter. The smaller the tool diameter, the higher the feed force. A big change was determined for the diameter variation from 25 mm to 50 mm and only a slight change from 50 mm to 65 mm. The greater tool diameter led to higher cutting speeds at constant oscillation frequency and bigger cutting paths at constant angles of deflection. A higher cutting speed resulted in a better transportation of the CFRP-dust-like chips out of the contact area. The larger cutting path resulted in a greater relative movement between carbon fibers and cutting edges of the abrasive grains. These effects led to a favorable condition, the amount of rubbing and pushing was reduced, the cutting action improved, resulting in lower feed forces.
Fig. 6. (a) Feed forces against process time at low and high feed rate; (b) Actual maximum feed force against tool diameter at low and high feed rate.
When the feed rate was doubled, the maximum feed force increased for all diameters. Since only the feed rate was doubled but the spindle speed was not, twice as much material per oscillation had to be cut. As a result, the necessary energy for material separation increased and hence also the force. 3.2. Machining results - hole quality The fiber protrusion of the machined holes for low and high feed rates can be seen in Fig. 7. As mentioned above, only the hollow drills with two teeth and a sufficiently large diameter
(from 25 mm) were able to machine holes with the selected cutting conditions.
Fig. 7. Fiber protrusion of the machined holes.
The fiber protrusion was highest for the hollow drill diameter of 25 mm. For the 65 mm tools, the fiber protrusion was slightly higher than for 50 mm. For all three diameters, the fiber protrusion was higher when the feed rate was increased. Due to the higher feed rate, the amount of fibers not being cut but rather being pushed aside was increased. This, in conjunction with higher feed forces, resulted in a bigger fiber protrusion. The delamination decreased with an increase of the tool diameter (see Fig. 8). The delamination factor was measured at the hole entrance and the hole exit.
Fig. 8. Delamination factor of the machined holes.
For all depicted diameters, the delamination factor on the hole exit was higher than on the hole entrance. This can be led back to the feed motion of the hollow drills. The abrasive grains pushed the CFRP’s lower layers when exiting the CFRP-plates and separated those layers from the rest of the CFRP-plates. Although not acceptable for the 25 mm diameter, both the fiber protrusion and the delamination factor for the two big diameters (50 mm and 65 mm) were in a good range. An example measurement of the hole wall via the confocal 3D microscope is shown in Fig. 9. The surface investigation showed a symmetrical relief-like wall. This symmetrical form fitted to the CFRP-layup with eight layers. The relief means that the CFRP was separated differently at different layers. This
Lukas Heberger et al. / Procedia CIRP 66 (2017) 169 – 174
fact led to a layer-oriented removal behavior of this machining process but this requires further investigations.
it stays in the cutting zone. It can adhere to the tool, as shown Fig. 10. 4. Conclusion and Outlook
Fig. 9. Surface of the hole wall measured with a confocal 3D microscope (65 mm diameter).
After usage, the tools were cleaned in an ultrasonic bath and the wear was qualitatively examined in the SEM. Fig. 10 shows a representative SEM picture for the wear of the tools after drilling six holes.
A novel process design for machining holes in CFRP was introduced. In this process, the tool rotates in a small angle at a high frequency, similar to a torsion pendulum. This short amplitude torsion pendulum drilling copes some issues of drilling with hand-guided machines like risk of injuries or dispersion of the chips. An inherent issue of this process is the roundness of the holes. For conventional machining, the envelope curve of the tool defines the roundness of the hole. This can cause issues of dimensional deviations; while a round shape is maintained. For the short amplitude torsion pendulum drilling, the shape of the tool is reproduced in the hole due to the missing tool rotation. Hence the accuracy of the roundness of the tools has to be very high. The process was suitable for machining holes of diameters bigger than 50 mm. For smaller diameters, the cutting path (oscillation amplitude) or cutting speed (oscillation frequency) in conjunction with the chosen feed rate was too low. In future works, tools with different abrasive body (grain size, grain concentration, grain protrusion) as well as different tool bodies (number of teeth, shape of teeth) will be examined. Also, further oscillation amplitudes and frequencies and their interplay with the tool design will be researched. The effect of the stucked cores depending on the hollow drill’s diameter, will be further investigated. Strategies will be devised to prevent sticking, for example by omitting the abrasive layer on the inner wall of the hollow drill. Since the relief of the hole wall seems to depend on the orientation of the fiber layers, the influence of this orientation on the material removal behavior will be investigated in future investigations. Acknowledgements This research was funded by the State Research focus “Advanced Materials Engineering (AME)” at the University of Kaiserslautern. References
Fig. 10. Wear of the hollow drills after usage.
The wear was examined at the tip of the teeth, the area of the tool which first immersed in the material. There, the highest wear can be expected because of the greatest loads. As can be seen in Fig. 10, hardly any wear can be detected. No attritious wear of the abrasives or grain fractures occurred. Some isolated grain breakouts on the tool were found, common for electroplated tools after first usage. Another aspect to mention is the formation of “dust-like” chips. This CFRP dust remained near the cutting zone and was not dispersed in the air or the environment. This is very positive concerning the operator’s health and protection of guides of machine. The downside of the dust not being dispersed is that
[1] Voß R, Henerichs M, Kuster F, Wegener K. Chip root analysis after machining carbon fiber reinforced plastics (CFRP) at different fiber orientations. Procedia CIRP 2014;14:217-222. [2] Hufenbach W, Biermann D, Seliger G. Serientaugliche Bearbeitung und Handhabung moderner faserverstärkter Hochleistungswerkstoffe (sefawe)Untersuchungen zum Forschungs- und Handlungsbedarf. Institut für Leichtbau und Kunststofftechnik. Technische Universität Dresden 2008. [3] Jaeger H. Carbonfaserverstaerkte Werkstoffe im Automobilbau. VCI Forum “Chemie macht Elektromobil”. Berlin; 2011. [4] Sheikh-Ahmad JY. Machining of polymer composites. Springer, Abu Dhabi. 2009. [5] Airbus S.A.S. A350 MSN2 rolls out of paint hangar with special ‚Carbon‘ livery.
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[11] Hintze W, Cordes M, Geis T, Blühm M, Emmelmann C, Canisius M. Laser Scored Machining of Fiber Reinforced Plastics to Prevent Delamination. Procedia Manufacturing 2016;6:1-8. [12] Melentiev R, Priarone P, Robiglio M, Settineri L. Effects of tool geometry and process parameters on delamination in CFRP drilling: An overview. Procedia CIRP 2016;45:31-43. 1
„Naming of specific manufacturers is done solely for the sake of completeness and does not necessarily imply an endorsement of the named companies nor that the products are necessarily the best for the purpose.“
ScienceDirect Procedia CIRP 66 (2017) 169 – 174
1st Cirp Conference on Composite Materials Parts Manufacturing, cirp-ccmpm2017
Machining of CFRP via short amplitude torsion pendulum drilling Lukas Hebergera,*, Benjamin Kirscha, Bert Hennb, Jan C. Auricha a
University of Kaiserslautern, Institute for Manufacturing Technology and Production Systems, Gottlieb-Daimler-Str., 67663 Kaiserslautern, Germany b Philipp Persch Nachf. KG - Diamantwerkzeuge, Zur Rothheck 16, 55743 Idar-Oberstein, Germany
* Corresponding author. Tel.: +49-631-205-5482; fax: +49-631-205-3238. E-mail address: [email protected]
Abstract The machining of carbon fiber reinforced polymer (CFRP) can cause damages such as delamination or fiber protrusion. Those damages weaken the structure and thus can result into premature failure of CFRP parts. In this paper, a novel machining process for machining CFRP is proposed. The tool oscillates in a small angle at a high frequency. This cutting motion is superimposed by a feed motion in the tool’s axial direction. The tools are hollow drills equipped with an abrasive body. In the experiments, different tool designs as well as different feed rates of this short amplitude torsion pendulum drilling are examined. © Authors. Published by Elsevier B.V. This ©2017 2017The The Authors. Published by Elsevier B.V.is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 1st Cirp Conference on Composite Materials Parts Manufacturing. Peer-review under responsibility of the scientific committee of the 1st Cirp Conference on Composite Materials Parts Manufacturing Keywords: drilling; grinding; fiber reinforced polymers
1. Introduction Composite materials like Carbon-fiber-reinforced-polymers (CFRP) have a good strength-to-mass ratio, making them suitable for applications in aerospace [2] and the automotive [3]. E.g. the Airbus A380 is made of 25 % weight percent CFRP [4], the Airbus A350 even 53 % [5]. Greater holes in CFRP, needed e.g. for repairing damaged CFRP parts or the panels in aircrafts, are commonly manufactured via cutting processes. Such processes for machining CFRP being drilling, circular milling or grinding are still not fully controlled. They can, due to the material’s anisotropic properties with respect to the fiber orientation [1], cause damages to the CFRP like delamination or fiber protrusion [6]. Those damages weaken the material and can cause premature failure [7]. In recent papers, some possibilities have proved to be useful in order to prevent those failures. For example, high cutting speed with low feed rate can reduce delamination while drilling [12]. In addition to optimizing the parameters, laser pre-scoring of the workpiece’s top layer before machining [11] can decrease the premature failure. The rotary motion of the tools often results into bending of the fibers in the direction of motion. A large field of application are hand-guided machines to manufacture holes e.g. for repair
issues in aerospace. Damaged segments of an aircraft are commonly patched with CFRP plates that are attached to the aircraft via rivet connections. The holes for these connections are drilled by workers on site. This application raises some issues. The rotation of the tools can cause injuries. Small chips are formed [8] that can be in the range of a few microns [9]. Those chips or particles are dispersed to the environment by the rotary motion of the tool and can impair electrical devices in vicinity or harm the worker [10]. In addition, they necessitate to clean the parts after machining [8]. Finally, centering with hand-guided machines is an issue, especially for free-formed surfaces and oblique contacts as given by the aforementioned case of repairing of aircrafts. In this paper, a novel machining process for machining CFRP is proposed to cope with the mentioned issues. Instead of a common tool rotation, the tool is rotated in a small angle at a high frequency, similar to a torsion pendulum. This cutting motion is superimposed by a slow feed motion in the tool’s axial direction. The tools are hollow drills equipped with an abrasive body (galvanic bonded diamond). In the experiments, different tool designs as well as different feed rates of the short amplitude torsion pendulum drilling process are examined. The quality of the machined holes is examined with respect to fiber protrusion and delamination.
2212-8271 © 2017 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 1st Cirp Conference on Composite Materials Parts Manufacturing doi:10.1016/j.procir.2017.03.222
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2. Experimental design The hollow drill tools did not rotate in the conventional sense, but oscillated like a torsion pendulum with a frequency of 333.33 Hz. The angle of deflection was 3.4°. The workpiece material used was a carbon fiber reinforced polymer (CFRP). The workpieces were plates with eight layers [0°, 90°, 45°, -45°, -45°, 45°, 90°, 0°]. It was manufactured from the unidirectional prepreg CE 1007-150-38 supplied by SGL. The plates had a thickness of 1.2 - 1.4 mm with a fiber content of 60 Vol.-%.
of an individual grain, was hence smaller than the nominal grain diameter. 2.2. Manufacturing Process The hollow drills were attached on a commercial handheld oscillator by Fein1, which again was mounted on the main spindle of a 5-axis machining center, see Fig. 2. In doing so, reproducible conditions were ensured. The CFRP plates were fastened via a clamping device to a dynamometer by Kistler1.
2.1. Tools The hollow drills were manufactured by Philipp Persch Nachf. KG. The tool body material consisted of a casehardened steel (16MnCr5). The wall thickness of all hollow drills was 1 mm. The tools were electroplated with diamond abrasives. The diamond grains had a diameter of D356 (corresponding to a mesh size of 45/50). Tools with differing diameter were examined: 65, 50, 25, and 10 mm. For each diameter, tools with a cylindrical shape (corresponding to a “zero teeth” drill) and a “V-shape” (corresponding to two teeth) were applied. For an overview of the tools see Fig. 1. Due to the smaller contact area of the two teethed hollow drills, higher feed forces of the individual abrasive grain were generated - while the global feed force did not change compared to the zero toothed hollow drill. Fig. 2. Experimental setup.
The manufacturing process was carried out in two steps, see Fig. 3 (b): In step 1, the hollow drill moved downwards in a straight line in the direction of the tool axis. The further the progress, the more the lateral areas of the teeth came into contact with the CFRP plate. Those lateral areas do not cut but push and rub against the plate and result into friction. For this reason, step 1 was stopped as soon as the tooth tips came out of the CFRP plate. In step 2 the tool moved along a helix, i.e. the linear feed motion was superimposed with a rotation around the tool axis. x step 1: Hollow drill moves down in tool axis direction 1.5 mm after contacting the specimen with a feed rate of 2 mm/min x step 2: Hollow drill moves in a helix with 135°/min with superimposed feed rate in tool axis direction at 1.35 mm/min Fig. 1. Overview of the tools with a detailed view of the abrasive diamond grains.
The diameter has a decisive influence: A variation of the diameter changes the cutting speed and, due to a constant angle of deflection, the cutting path. The smaller the diameter, the smaller the cutting path. For the tool with a diameter of 10 mm this resulted into a cutting path of 297 µm. The cutting path, equivalent to the cutting path
The feed rate of both steps was doubled in a second series of tests. During all manufacturing steps, the tool oscillated with an amplitude of 1.7°, see Fig. 3 (a).
Lukas Heberger et al. / Procedia CIRP 66 (2017) 169 – 174
3. Results
Fig. 3. (a) Short amplitude torsion pendulum; (b) 2 steps of manufacturing process (view from below).
Experiments with described critical tools, like the small diameter 10 mm (cutting path smaller than nominal grain diameter) and tools without teeth (a high force was necessary because of the large contact area), directly began with step 2 to reduce the load on the clamping device. The oscillation frequency was 333.33 Hz for all experiments. The process conditions are listed in table 1.
With the kinematics chosen, no hole could be manufactured with the 10 mm diameter hollow drills. This is due to the small cutting path (297 mm) in conjunction with the size of the abrasives (nominal diameter of ~350 µm). Even with perfect juxtaposition of the abrasive grains, the distance to each other would be greater than the cutting path. Due to the shape of the abrasives, the material between the grains cannot be cut. This could be compensated by the additional helix angle, as implemented in step 2 of the kinematics. Very low feed rates in conjunction with high helix speeds would be needed, not chosen in the experiments. With the tools with zero teeth, high feed forces resulted due to the large contact areas. Drilling was interrupted at 120 N peak force. With this limitation, no holes could be manufactured for any diameter, see Fig. 4. In addition, the abraded CFRP could not be conveyed away from the contact zone with the zero teeth tools, increasing the load of the tools.
Table 1. Tool geometry and process conditions. Tool geometry parameters Tool diameter [mm]: 10, 25, 50, 65
Teeth: 0, 2
Fig. 4. Process results for the zero tooth tools.
Grain size: D356
With the two teeth hollow drills holes could be manufactured, except for the 10 mm diameter drill, see Fig. 5.
Process parameters Step 1: feed rate [mm/min]: 2, 4 Step 2 (helix motion): 135°/min ר1.35 mm/min; 270°/min ר2.7 mm/min
Cutting path [µm]: 297 (for tool diameter 10 mm); 741 (for tool diameter 25 mm); 1483 (for tool diameter 50 mm); 1928 (for tool diameter 65 mm)
Actuator spindle rotational frequency [Hz]:333.33
For every cutting condition, three holes were manufactured. The process forces were measured by means of a dynamometer (sampling rate 2048 Hz) by Kistler1. The force signal was filtered with a moving average. The moving average had an interval width of 100 measuring points. The hole quality was investigated by analyzing the delamination and the fiber protrusion via microscope. The delamination was described using the delamination factor Fd. This factor Fd is defined by the quotient of the maximum diameter of delamination and the hole diameter. The fiber protrusion was described by the two-dimensional percentage overlap of the uncut fibers over the hole. The overlap was measured using a MATLAB-routine as described in [6]. The surface morphology of the machined hole walls was analyzed via a confocal 3D microscope by Nanofocus1. After drilling, tool wear was determined qualitatively. For this purpose, the tools were cleaned in an ultrasonic bath and examined in a scanning electron microscope (SEM).
Fig. 5. Process results for different tool diameters with two teeth tools.
3.1. Feed force The feed forces are depicted in Fig. 6. with regard to the tool diameter. The feed forces over the process time show a sharp increase for both, high and low feed rate for the 25 mm hollow drill towards the end of the cycle. This was due to the separated cylindrical core of the CFRP plate. This core was stuck in the hollow drill and pushed onto the clamping device, when the hollow drill continued moving downwards after the hole was manufactured. This effect did not occur for the diameter of 50 mm and 65 mm. It seemed as if the stronger curvature of the hollow drills with smaller diameter rather clamped the core. This presumption must, however, be investigated in further studies.
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The course of the feed forces for the diameter 50 mm and 65 mm differed only slightly. The maximum feed forces detected during one respective cycle are shown in Fig. 6. The maximum values correspond to the forces during drilling, neglecting the peaks due to stuck of the core mentioned above. The maximum feed force changed with the tool diameter. The smaller the tool diameter, the higher the feed force. A big change was determined for the diameter variation from 25 mm to 50 mm and only a slight change from 50 mm to 65 mm. The greater tool diameter led to higher cutting speeds at constant oscillation frequency and bigger cutting paths at constant angles of deflection. A higher cutting speed resulted in a better transportation of the CFRP-dust-like chips out of the contact area. The larger cutting path resulted in a greater relative movement between carbon fibers and cutting edges of the abrasive grains. These effects led to a favorable condition, the amount of rubbing and pushing was reduced, the cutting action improved, resulting in lower feed forces.
Fig. 6. (a) Feed forces against process time at low and high feed rate; (b) Actual maximum feed force against tool diameter at low and high feed rate.
When the feed rate was doubled, the maximum feed force increased for all diameters. Since only the feed rate was doubled but the spindle speed was not, twice as much material per oscillation had to be cut. As a result, the necessary energy for material separation increased and hence also the force. 3.2. Machining results - hole quality The fiber protrusion of the machined holes for low and high feed rates can be seen in Fig. 7. As mentioned above, only the hollow drills with two teeth and a sufficiently large diameter
(from 25 mm) were able to machine holes with the selected cutting conditions.
Fig. 7. Fiber protrusion of the machined holes.
The fiber protrusion was highest for the hollow drill diameter of 25 mm. For the 65 mm tools, the fiber protrusion was slightly higher than for 50 mm. For all three diameters, the fiber protrusion was higher when the feed rate was increased. Due to the higher feed rate, the amount of fibers not being cut but rather being pushed aside was increased. This, in conjunction with higher feed forces, resulted in a bigger fiber protrusion. The delamination decreased with an increase of the tool diameter (see Fig. 8). The delamination factor was measured at the hole entrance and the hole exit.
Fig. 8. Delamination factor of the machined holes.
For all depicted diameters, the delamination factor on the hole exit was higher than on the hole entrance. This can be led back to the feed motion of the hollow drills. The abrasive grains pushed the CFRP’s lower layers when exiting the CFRP-plates and separated those layers from the rest of the CFRP-plates. Although not acceptable for the 25 mm diameter, both the fiber protrusion and the delamination factor for the two big diameters (50 mm and 65 mm) were in a good range. An example measurement of the hole wall via the confocal 3D microscope is shown in Fig. 9. The surface investigation showed a symmetrical relief-like wall. This symmetrical form fitted to the CFRP-layup with eight layers. The relief means that the CFRP was separated differently at different layers. This
Lukas Heberger et al. / Procedia CIRP 66 (2017) 169 – 174
fact led to a layer-oriented removal behavior of this machining process but this requires further investigations.
it stays in the cutting zone. It can adhere to the tool, as shown Fig. 10. 4. Conclusion and Outlook
Fig. 9. Surface of the hole wall measured with a confocal 3D microscope (65 mm diameter).
After usage, the tools were cleaned in an ultrasonic bath and the wear was qualitatively examined in the SEM. Fig. 10 shows a representative SEM picture for the wear of the tools after drilling six holes.
A novel process design for machining holes in CFRP was introduced. In this process, the tool rotates in a small angle at a high frequency, similar to a torsion pendulum. This short amplitude torsion pendulum drilling copes some issues of drilling with hand-guided machines like risk of injuries or dispersion of the chips. An inherent issue of this process is the roundness of the holes. For conventional machining, the envelope curve of the tool defines the roundness of the hole. This can cause issues of dimensional deviations; while a round shape is maintained. For the short amplitude torsion pendulum drilling, the shape of the tool is reproduced in the hole due to the missing tool rotation. Hence the accuracy of the roundness of the tools has to be very high. The process was suitable for machining holes of diameters bigger than 50 mm. For smaller diameters, the cutting path (oscillation amplitude) or cutting speed (oscillation frequency) in conjunction with the chosen feed rate was too low. In future works, tools with different abrasive body (grain size, grain concentration, grain protrusion) as well as different tool bodies (number of teeth, shape of teeth) will be examined. Also, further oscillation amplitudes and frequencies and their interplay with the tool design will be researched. The effect of the stucked cores depending on the hollow drill’s diameter, will be further investigated. Strategies will be devised to prevent sticking, for example by omitting the abrasive layer on the inner wall of the hollow drill. Since the relief of the hole wall seems to depend on the orientation of the fiber layers, the influence of this orientation on the material removal behavior will be investigated in future investigations. Acknowledgements This research was funded by the State Research focus “Advanced Materials Engineering (AME)” at the University of Kaiserslautern. References
Fig. 10. Wear of the hollow drills after usage.
The wear was examined at the tip of the teeth, the area of the tool which first immersed in the material. There, the highest wear can be expected because of the greatest loads. As can be seen in Fig. 10, hardly any wear can be detected. No attritious wear of the abrasives or grain fractures occurred. Some isolated grain breakouts on the tool were found, common for electroplated tools after first usage. Another aspect to mention is the formation of “dust-like” chips. This CFRP dust remained near the cutting zone and was not dispersed in the air or the environment. This is very positive concerning the operator’s health and protection of guides of machine. The downside of the dust not being dispersed is that
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