- Email: [email protected]
Influence of cutting parameters on tool wear and hole quality in composite aerospace components drilling
Accepted Manuscript Influence of cutting parameters on tool wear and hole quality in composite aerospace components drilling J. Fernández Pérez, J.L. Cantero, J. Díaz Álvarez, M.H. Miguélez PII: DOI: Reference:
S0263-8223(17)31782-8 http://dx.doi.org/10.1016/j.compstruct.2017.06.043 COST 8630
To appear in:
Composite Structures
Received Date: Accepted Date:
5 June 2017 19 June 2017
Please cite this article as: Pérez, J.F., Cantero, J.L., Álvarez, J.D., Miguélez, M.H., Influence of cutting parameters on tool wear and hole quality in composite aerospace components drilling, Composite Structures (2017), doi: http:// dx.doi.org/10.1016/j.compstruct.2017.06.043
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
3rd International Conference on Mechanics of Composites – Bologna, Italy – July 2017
Influence of cutting parameters on tool wear and hole quality in composite aerospace components drilling J. Fernández Pérez (1) *, J. L. Cantero (1), J. Díaz Álvarez (2), M. H. Miguélez (1) (1)
Mechanical Engineering Department, Universidad Carlos III de Madrid, Av. de la Universidad 30, Leganés, Madrid (2) Bioengineering and Aerospace Engineering Department, Universidad Carlos III de Madrid, Av. de la Universidad 30, Leganés, Madrid *Corresponding author: [email protected]
ABSTRACT Composite Fiber Reinforced Plastics (CFRP) are characterized by their outstanding mechanical properties combined with reduced density and good resistance to corrosion and fatigue which make them suitable for aerospace components. During assembly procedures, one shoot drilling operations, usually including countersinking cycle, are required to minimize positional errors, enhance tight tolerances and reduce process time. Countersink drill bits were tested on CFRP test specimens, representative of aircraft components. Along testing, tool wear was monitored with an optical microscope to track its evolution and determine the dominant wear mechanism. On the other hand, hole quality was evaluated since tool life criterion is based on the assessment of machined surface quality. The influence of cutting speed and feed was analyzed with the objective of looking for extended tool life and more productive cutting parameters. The information gathered from monitoring tool wear and inspecting hole quality can be used for the enhancement of CFRP drilling and the improvement of the manufacturing process competitiveness, in terms of production cost and time. Keywords: Composites drilling, cutting parameters, quality inspection, tool wear
1. Introduction The employment of composite materials by aerospace industry has stood out in the last decade due to their outstanding mechanical properties, combined with reduced density and good resistance to corrosion and fatigue, until today when they shape over 50% of the structural weight of some aircraft. In particular, Composite Fiber Reinforced Plastics (CFRP) have high specific strength and superior fracture toughness [1]. Even though composite materials are characterized by its manufacturing process, which allows to produce near net shape, machining is unavoidable for assembly purposes, being drilling the most common operation. The different components are stacked together to perform the operation in single shot, obtaining the required tolerances of the hole and removing the reaming cycle. Usually, it is included countersinking to avoid interferences of the head of the rivet with the aerodynamic shape. This minimize positional errors, enhance tight tolerances and reduce process time. Although kinematics of composites machining process remains the same as in metal machining [2], there are several differences due to the fact that composites are inhomogeneous and anisotropic materials made from two constituents: the reinforcements which use to be brittle and the matrix which, for its part, tends to be more ductile. Hence, machining process is based on intermittent fractures and bouncing cutting forces. The drilling operation is produced by two different material removal processes, related with conventional drill bits geometry. The main cutting edges eliminates most of the material, analogously to an orthogonal cutting, which generates the bulk of the torque or cutting force. On the other hand, the chisel edge behaves as a blunt edge with a high rake angle which work as a punching process and producing most of the thrust force [3]. Regarding chip formation process, it is caused by multiple cutting edges which encounter the fibers at difference orientations despite of the material is made of
3rd International Conference on Mechanics of Composites – Bologna, Italy – July 2017
unidirectional layers [4]. Furthermore, the coefficients of thermal expansion of matrix and fibers can be quite dissimilar which may reduce the dimensional accuracy of the hole and the quality of machined surface. During composites machining, induced damage on the workpiece has to be taken into consideration since several failure modes can be produced, such as, fiber pullout, delamination, surface damage or burning being delamination most critical for the structural integrity and long term reliability of the component [5]. It is characterized by the separation of the layers caused by the low interlaminate strength of the composite structure and excessive cutting forces which are enhanced by tool wear, inadequate cutting speeds and feeds or improper tool geometry [6]. The level of tool wear is another important feature that have to be monitored on CFRP drilling operations. Most of the wear occurs at the cutting edge corner due to higher cutting speeds, although in the rest of the edges it has to be taken into account [7]. In some cases, the wear may not uniform due to difference mechanical properties of fibers and polymer matrix. The predominant wear mechanisms are abrasion, promoted by the abrasiveness of fibers, and chipping produced by the fluctuation of forces due to material inhomogeneity. These type of wear mechanisms are mitigated with different tool material properties: hardness is required for abrasive wear and toughness for chipping [8]. General recommendation for CFRP drill bits are sharp cutting edges and large positive rake angles to facilitate clean shaving of the fibers. Several authors have been researching about tool geometry for CFRP drilling and the associated wear produced. Mayuet et al. [9] showed that abrasion was identified to be the dominant wear mechanism on a conventional carbide geometry drill bit under different cutting conditions. The direct influence of tool wear on machining induced damage in woven materials was studied in [10] and correlations between delamination, cutting edge rounding and drilling loads were found. Karpat et al. [11] analyzed the performance of double point angle drill bits on fabric woven CFRP laminates and it was found that feed was the dominant parameter of tool wearing. The influence of the point angle was analyzed on woven CFRP in [12] showing that larger angles produce higher thrust force while the torque remains constant and that the increase of cutting speed does influence hole quality but increase thrust forces. On the other hand, the influence of cutting parameters on the axial and cutting force produced during dry drilling of woven CFRP composites was studied in [13]. Also, Khashaba et al. [14] have analyzed the effect of machining parameters on the cutting force and the quality of the workpiece on woven glass fiber-reinforced epoxy (GFRE) composites in terms of delamination size, surface roughness, and bearing strength. High speed drilling of woven graphite epoxy was investigated by Rawat and Attia in [15] and it was found chipping at the very beginning of the drilling process, followed by abrasion and adhesion of material. Finally, the behavior of coated versus uncoated carbide drills was studied by Iliescu et al. on [16]. It was shown that thrust force is more influenced by tool wear than torque and that uncoated tools have a power law dependence of tool wear with axial force while coated carbide tools have a linear dependence instead. The market for composite aerospace materials has duplicated the levels corresponding to 10-years ago, and it is expected to grow another 55% over the next following years [17]. According to Airbus data, the number of holes drilled on composite made aircraft components will be five times bigger in 2020. As a matter of fact, due to the huge number of holes required for the assembly of aeronautical components, machining operations must be designed ensuring competitive levels of productivity and cost per operation, while maintaining the required quality on the machined surface. So, the main target of this work is to study the wearing process of carbide countersink drill bits, on CFRP test specimens representative of aircraft components, with the objective of looking for extended tool life and more productive cutting parameters. For this purpose, an optical microscope was used to characterize the wear mechanism of the tools and a contact profilometer to assess the quality of the hole, as well as different measurement tools, such as calibers or inside micrometers.
3rd International Conference on Mechanics of Composites – Bologna, Italy – July 2017
2. Experimental procedure The study was focused on carbide drill bits employed on the assembly of aeronautical components made of CFRP. Wearing process was carried out in a controlled environment to replicate real production conditions. Along testing, tool wear was periodically monitored and hole quality was evaluated according to the engineering requirements that have to be controlled to ensure optimal performance of the manufacturing process, based on internal rules and standards defined by Airbus Group. In this way, it is possible to compare the behavior of several cutting parameters and analyze their influence on tool life and hole quality. 2.1 Tool geometry and workpiece specification The test specimens are made of CFRP laminates at different orientations. The upper surface of the coupon is covered by an epoxy preimpregnated expanded copper foil and in the lower part, it has a prepreg layer made from an E-glass fiber fabric preimpregnated with epoxy. The total thickness of the coupons is 9.5 mm. Regarding cutting tools, carbide countersink drill bits with diamond coating were tested. They are right hand cutting with two high positive rake angle cutting edges and 90⁰ point angle. The nominal diameter of tool studied is 4.825 mm. Figure 1 shows one of the countersink drill bits tested.
Figure 1. Countersink drill bit geometry 2.2 Experimental set up and machining conditions Machining experiments were carried out on a 3 axis CNC machine, KONDIA B500, with a numerical control Heidenhain. To position the test specimens and vacuum the chips and dust generated, a made on purpose tooling was designed and manufactured (Figure 2). The aspiration system was also intended to simulate the cutting conditions found on real production process, where the vacuum hose is positioned concentric to the cutting tool. The aspirator employed is a Nilfisk S2B with a cyclonic filter, an antistatic filter and a micro filter to retain the smallest particles.
3rd International Conference on Mechanics of Composites – Bologna, Italy – July 2017
Figure 2. Tooling designed to position the test specimens and vacuum dust generated In order to analyze the influence of cutting parameters on tool wear and hole quality, it was proposed a reference condition and then, the cutting speed and the feed were increased a 20% with respect to that reference (Table 1). Notice that machining was performed on dry conditions. Table 1. Cutting conditions employed: 1 Reference case, 2 Increase cutting speed, 3 Increase feed, 4 Increase cutting speed + feed Cutting conditions 1 2 3 feed [mm/rev] 0.10 0.10 0.12 Vc [m/min] 50.0 60.0 50.0 2.3 Inspection equipment and damage evaluation Parameter
4 0.12 60.0
The inspection equipment employed to monitor tool wear and hole quality was an optical microscope OLYMPUS SZ40 and a contact profilometer MARWIN XCR 20, respectively. Furthermore, different measurement tools were employed, such as calibers or inside micrometers. Figure 3 shows one of the profilometer probes measuring a countersink hole profile.
Figure 3. MARWIN XCR 20 profilometer equipment measuring a test specimen For a given application with specific cutting parameters, each tool has a certified life that ensures the quality of the produced hole will fulfill the following engineering requirements:
Hole geometry. Diameter must comply some acceptable limits which depends on the required tolerance of the hole. Also, the tool has to ensure capability indexes, to guarantee the stability of the process Countersink geometry. It is controlled by the countersink angle and the transition radius which is the part of the fastener between the head and the body. Hole integrity. It is evaluated as a function of surface finish roughness and the absence of machining induced damage.
Hence, the useful life of the tool is determined by one of the previously defined parameters that no longer comply with the required quality to guarantee optimal performance of the manufacturing process. In the case of CFRP drilling, delamination is usually the criteria defining end of tool life. It is caused by the low interlaminate strength of the material and large axial forces promoted by inadequate
3rd International Conference on Mechanics of Composites – Bologna, Italy – July 2017
cutting parameters and level of wear. Under proper conditions, tool life is reached gradually because of progressive wear (main driver of machining induced damage), so by monitoring machining process it may be possible to increment tool life or find optimum cutting parameters [2]. The quantification of the delamination damage extension was done with the concept of delamination factor which is defined as the quotient of maximum diameter damaged over the nominal diameter of the hole.
3. Results For each cutting condition, two test were carried out to guarantee the repeatability of the experiment. In this case, the tool life criterion is based on the assessment of the hole quality with respect to the requirements defined in Section 2.3. 3.1 Tool wear characterization Worn tools were inspected and discontinuous flakes on the main cutting edges were observed. So, main wear mechanism found was chipping. It is produced by the presence of highly abrasive fibers and the fluctuation of forces during the drilling process which promotes stress concentration regions weakening the cutting edge. Eventually, after some level of wear, the sharp edge is lost and the cutting geometry of the tool is modified towards negative rake angles, which significantly modifies material removal process producing higher machining induced damage. From this point, wearing process stabilizes, which smooths the breaks and rounds the edges [18]. Once the main wear mechanism was identified, tools were inspected at different levels of wear to analyze its evolution and compare the behavior of the four cutting parameters tested. Figure 4 shows the condition of the main cutting edge of the reference case tool after drilling a total distance of 4400 mm, in comparison with the new cutting edge.
Figure 4. Comparison of cutting edge condition between new (left) and worn tool after drilling a total distance of 4400 mm (right). Reference cutting conditions (case 1). Noticeable differences can be observed between the condition of the new and worn tool for the reference case in Figure 4. Chipping phenomena starts promoting abrasive wear and modifying the cutting geometry. Same behavior was produced for the rest of cutting conditions, although the level of wear found was slightly dissimilar. Table 2 shows the total cutting time and the equivalent linear distance covered by each cutting edge for the tests performed. For condition 2, the cutting speed was increased while maintaining constant the feed in terms of mm/rev, which implies having shorter cutting time since the overall feed increases and, despite of that, it was found almost the same wear as in the case 1. On the other hand, cutting condition 3, with higher feed and same cutting speed as in the reference case, has a smaller edge linear distance covered and a shorter drilling time and shows a smaller level of wear with respect to previous condition.
3rd International Conference on Mechanics of Composites – Bologna, Italy – July 2017
Finally, in condition 4, the cutting speed and the feed were increased in the same proportion, hence the drilling time is further reduced, and the tool wear obtained was even smaller. Regarding the countersink cutting edge, it was observed a more uniform abrasive wear, so the differences between cutting conditions are more subtle which complicates the quantification of the wear extension. In general, flank is the main mechanism, although chipping is also appearing. Table 2. Summary of test characteristics. 1 Reference case, 2 Increase cutting speed, 3 Increase feed, 4 Increase cutting speed + feed Cutting condition 1 2 3 4
Distance drilled (mm)
Cutting time (min)
Edge linear distance (m)
2500
7.7
382.8
4400
13.4
670.8
2500
6.4
382.8
4400
11.2
670.8
2500
6.4
319.0
4400
11.2
559.0
2500
5.3
319.2
4400
9.3
559.2
Vc (m/min)
Vf (mm/rev)
Feed (mm/min)
50.0
0.10
330
60.0
0.10
396
50.0
0.12
396
60.0
0.12
475
3.2 Hole quality assessment After characterizing the type of wear on the tools, the quality of the holes on the test specimens was analyzed. Hole diameter was measured with an inside micrometer and transition radius, countersink angle and surface finish with the profilometer equipment. Machining induced damage was evaluated by visual inspection and with the optical microscope. Figure 5 shows the differences between the conditions of the holes made with a new tool and with a tool after drilling a total distance of 4400 mm.
Figure 5. Comparison of the exit surface between holes made by a new and a worn tool with reference cutting parameters Diameter and countersink geometry were monitored along the tool life and it was verified that these parameters remain around the nominal values in the four cutting conditions. Roughness was also measured on the hole and on the countersink surface and it was found out that it is still under typical quality demands which used to be below 4 μm. Main problem arises from machining induced damage at the exit of the hole, experiencing severe delamination and flaking. It can be seen in Figure 5 that quality delivered by new tool is very good without any evidence of machining induced damage, but holes made with tools after drilling a distance of 4400 mm show severe delamination at the exit surface. Notice, that delamination is not axisymmetric, instead, it is produced in preference directions which are linked to fiber directions of the
3rd International Conference on Mechanics of Composites – Bologna, Italy – July 2017
exit layer. [7] Since the quality of the hole at the beginning of the tool life is quite good, the machining induced damage produced is directly related with the wear mechanism of the tool. Hence, the delamination at the exit of the hole starts appearing when wear deforms the cutting geometry of the tool edges towards negative rake angles, which is the main difference between the four conditions. In the reference case, delamination appears after drilling a distance of 1400 mm. Regarding the tests at higher cutting speed, delamination is produced with an 85% of the thickness, and at the condition of higher feed, with a 130%. On the other hand, the cutting condition which increases the feed and the cutting speed produced the first delamination with a 160% of the reference distance. From this point, wear stabilizes and delamination increases progressively. In the reference cutting condition it was produced a critical delamination factor, which defines the end of tool life, after drilling a total distance of 4400 mm. For the rest of cutting conditions, the distance drilled at which critical delamination factor was produced follows the same proportion as the appearance of first delamination explained in previous paragraph.
4. Conclusions The main objective of this work was to analyze the influence of cutting parameters on CFRP drilling process and assess its influence on tool wear and hole quality. For this purpose, it was defined a reference test and then, the cutting speed and the feed were increased individually. Finally, both were incremented in the same proportion towards more productive cutting parameters. Each test was performed two times to ensure the repeatability and enhance the reliability of the results. The tool wear mechanism observed in the four cutting conditions was very similar being chipping main phenomena followed by a progressive abrasive wear on the main cutting edges. Regarding the countersink edge, it was observed a more uniform and smooth abrasive wear. The quality of the holes was similar in all cases with the only difference of the machining induced damage. It was observed that the initiation of the delamination at the exit of the hole is linked to the level of wear at which the cutting geometry is modified from a sharp edge towards negative rake angles. Furthermore, it was found that the condition of higher cutting speed and feed showed a reduced level of wear, delaying the appearance of delamination and, in consequence, increasing the amount of distance drilled before reaching the critical delamination factor which defines the tool life. The results fulfill a twofold objective: reducing the cycle time by employing more productive cutting parameters and increasing the tool life due to a reduced level of wear on the tool, always ensuring the demanded hole quality. In that way, it would be possible to improve one-shoot drilling operations carried out during the assembly of aeronautical components, enhancing the competitiveness of the process.
5. Acknowledgements The authors acknowledge the financial support to AIRBUS DEFENCE AND SPACE through the project DRILLING PROCESSES IMPROVEMENT FOR MULTI MATERIAL CFRP-AL-TI STACKS and to the Ministry of Economy and Competitiveness of Spain through the grant with reference PTA2015-10741-I.
6. References [1] Huang, X. (2009). Fabrication and properties of carbon fibers. Materials, 2(4), 2369-2403. [2] Sheikh-Ahmad, J. Y. (2009). Machining of polymer composites (pp. 164-165). New York: Springer. [3] Liu, D., Tang, Y., & Cong, W. L. (2012). A review of mechanical drilling for composite laminates. Composite Structures, 94(4), 1265-1279. [4] DeVor, R. E. (1996). An experimental investigation of the crack growth phenomenon for drilling of fiber-reinforced composite materials.
3rd International Conference on Mechanics of Composites – Bologna, Italy – July 2017
[5] Ho-Cheng, H., & Dharan, C. K. H. (1990). Delamination during drilling in composite laminates. Journal of Engineering for Industry (Transactions of the ASME), 112(3), 236-239. [6] Chen, W. C. (1997). Some experimental investigations in the drilling of carbon fiber-reinforced plastic (CFRP) composite laminates. International Journal of Machine Tools and Manufacture, 37(8), 1097-1108. [7] Gaugel, S., Sripathy, P., Haeger, A., Meinhard, D., Bernthaler, T., Lissek, F., ... & Schneider, G. (2016). A comparative study on tool wear and laminate damage in drilling of carbon-fiber reinforced polymers (CFRP). Composite Structures, 155, 173-183. [8] Masuda, M., Kuroshima, Y., & Chujo, Y. (1993). Failure of tungsten carbide-cobalt alloy tools in machining of carbon materials. Wear, 169(2), 135-140. [9] Mayuet, P., Gallo, A., Portal, A., Arroyo, P., Alvarez, M., & Marcos, M. (2013). Damaged area based study of the Break-IN and Break-OUT defects in the dry drilling of carbon fiber reinforced plastics (CFRP). Procedia Engineering, 63, 743-751. [10] Faraz, A., Biermann, D., & Weinert, K. (2009). Cutting edge rounding: An innovative tool wear criterion in drilling CFRP composite laminates. International Journal of Machine Tools and Manufacture, 49(15), 1185-1196. [11] Karpat, Y., Değer, B., & Bahtiyar, O. (2012). Drilling thick fabric woven CFRP laminates with double point angle drills. Journal of materials processing technology, 212(10), 2117-2127. [12] Heisel, U., & Pfeifroth, T. (2012). Influence of point angle on drill hole quality and machining forces when drilling CFRP. Procedia CIRP, 1, 471-476. [13] Merino-Pérez, J. L., Royer, R., Merson, E., Lockwood, A., Ayvar-Soberanis, S., & Marshall, M. B. (2016). Influence of workpiece constituents and cutting speed on the cutting forces developed in the conventional drilling of CFRP composites. Composite Structures, 140, 621-629. [14] Khashaba, U. A., El-Sonbaty, I. A., Selmy, A. I., & Megahed, A. A. (2010). Machinability analysis in drilling woven GFR/epoxy composites: Part I–Effect of machining parameters. Composites Part A: Applied Science and Manufacturing, 41(3), 391-400. [15] Rawat, S., & Attia, H. (2009). Wear mechanisms and tool life management of WC–Co drills during dry high speed drilling of woven carbon fibre composites. Wear, 267(5), 1022-1030. [16] Iliescu, D., Gehin, D., Gutierrez, M. E., & Girot, F. (2010). Modeling and tool wear in drilling of CFRP. International Journal of Machine Tools and Manufacture, 50(2), 204-213. [17] Composites forecast and consulting. 2014-2023 Global Composite Aerostructures Market Outlook Report. [18] Wang, D. H., Ramulu, M., & Arola, D. (1995). Orthogonal cutting mechanisms of graphite/epoxy composite. Part I: unidirectional laminate. International Journal of Machine Tools and Manufacture, 35(12), 1623-1638.
S0263-8223(17)31782-8 http://dx.doi.org/10.1016/j.compstruct.2017.06.043 COST 8630
To appear in:
Composite Structures
Received Date: Accepted Date:
5 June 2017 19 June 2017
Please cite this article as: Pérez, J.F., Cantero, J.L., Álvarez, J.D., Miguélez, M.H., Influence of cutting parameters on tool wear and hole quality in composite aerospace components drilling, Composite Structures (2017), doi: http:// dx.doi.org/10.1016/j.compstruct.2017.06.043
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
3rd International Conference on Mechanics of Composites – Bologna, Italy – July 2017
Influence of cutting parameters on tool wear and hole quality in composite aerospace components drilling J. Fernández Pérez (1) *, J. L. Cantero (1), J. Díaz Álvarez (2), M. H. Miguélez (1) (1)
Mechanical Engineering Department, Universidad Carlos III de Madrid, Av. de la Universidad 30, Leganés, Madrid (2) Bioengineering and Aerospace Engineering Department, Universidad Carlos III de Madrid, Av. de la Universidad 30, Leganés, Madrid *Corresponding author: [email protected]
ABSTRACT Composite Fiber Reinforced Plastics (CFRP) are characterized by their outstanding mechanical properties combined with reduced density and good resistance to corrosion and fatigue which make them suitable for aerospace components. During assembly procedures, one shoot drilling operations, usually including countersinking cycle, are required to minimize positional errors, enhance tight tolerances and reduce process time. Countersink drill bits were tested on CFRP test specimens, representative of aircraft components. Along testing, tool wear was monitored with an optical microscope to track its evolution and determine the dominant wear mechanism. On the other hand, hole quality was evaluated since tool life criterion is based on the assessment of machined surface quality. The influence of cutting speed and feed was analyzed with the objective of looking for extended tool life and more productive cutting parameters. The information gathered from monitoring tool wear and inspecting hole quality can be used for the enhancement of CFRP drilling and the improvement of the manufacturing process competitiveness, in terms of production cost and time. Keywords: Composites drilling, cutting parameters, quality inspection, tool wear
1. Introduction The employment of composite materials by aerospace industry has stood out in the last decade due to their outstanding mechanical properties, combined with reduced density and good resistance to corrosion and fatigue, until today when they shape over 50% of the structural weight of some aircraft. In particular, Composite Fiber Reinforced Plastics (CFRP) have high specific strength and superior fracture toughness [1]. Even though composite materials are characterized by its manufacturing process, which allows to produce near net shape, machining is unavoidable for assembly purposes, being drilling the most common operation. The different components are stacked together to perform the operation in single shot, obtaining the required tolerances of the hole and removing the reaming cycle. Usually, it is included countersinking to avoid interferences of the head of the rivet with the aerodynamic shape. This minimize positional errors, enhance tight tolerances and reduce process time. Although kinematics of composites machining process remains the same as in metal machining [2], there are several differences due to the fact that composites are inhomogeneous and anisotropic materials made from two constituents: the reinforcements which use to be brittle and the matrix which, for its part, tends to be more ductile. Hence, machining process is based on intermittent fractures and bouncing cutting forces. The drilling operation is produced by two different material removal processes, related with conventional drill bits geometry. The main cutting edges eliminates most of the material, analogously to an orthogonal cutting, which generates the bulk of the torque or cutting force. On the other hand, the chisel edge behaves as a blunt edge with a high rake angle which work as a punching process and producing most of the thrust force [3]. Regarding chip formation process, it is caused by multiple cutting edges which encounter the fibers at difference orientations despite of the material is made of
3rd International Conference on Mechanics of Composites – Bologna, Italy – July 2017
unidirectional layers [4]. Furthermore, the coefficients of thermal expansion of matrix and fibers can be quite dissimilar which may reduce the dimensional accuracy of the hole and the quality of machined surface. During composites machining, induced damage on the workpiece has to be taken into consideration since several failure modes can be produced, such as, fiber pullout, delamination, surface damage or burning being delamination most critical for the structural integrity and long term reliability of the component [5]. It is characterized by the separation of the layers caused by the low interlaminate strength of the composite structure and excessive cutting forces which are enhanced by tool wear, inadequate cutting speeds and feeds or improper tool geometry [6]. The level of tool wear is another important feature that have to be monitored on CFRP drilling operations. Most of the wear occurs at the cutting edge corner due to higher cutting speeds, although in the rest of the edges it has to be taken into account [7]. In some cases, the wear may not uniform due to difference mechanical properties of fibers and polymer matrix. The predominant wear mechanisms are abrasion, promoted by the abrasiveness of fibers, and chipping produced by the fluctuation of forces due to material inhomogeneity. These type of wear mechanisms are mitigated with different tool material properties: hardness is required for abrasive wear and toughness for chipping [8]. General recommendation for CFRP drill bits are sharp cutting edges and large positive rake angles to facilitate clean shaving of the fibers. Several authors have been researching about tool geometry for CFRP drilling and the associated wear produced. Mayuet et al. [9] showed that abrasion was identified to be the dominant wear mechanism on a conventional carbide geometry drill bit under different cutting conditions. The direct influence of tool wear on machining induced damage in woven materials was studied in [10] and correlations between delamination, cutting edge rounding and drilling loads were found. Karpat et al. [11] analyzed the performance of double point angle drill bits on fabric woven CFRP laminates and it was found that feed was the dominant parameter of tool wearing. The influence of the point angle was analyzed on woven CFRP in [12] showing that larger angles produce higher thrust force while the torque remains constant and that the increase of cutting speed does influence hole quality but increase thrust forces. On the other hand, the influence of cutting parameters on the axial and cutting force produced during dry drilling of woven CFRP composites was studied in [13]. Also, Khashaba et al. [14] have analyzed the effect of machining parameters on the cutting force and the quality of the workpiece on woven glass fiber-reinforced epoxy (GFRE) composites in terms of delamination size, surface roughness, and bearing strength. High speed drilling of woven graphite epoxy was investigated by Rawat and Attia in [15] and it was found chipping at the very beginning of the drilling process, followed by abrasion and adhesion of material. Finally, the behavior of coated versus uncoated carbide drills was studied by Iliescu et al. on [16]. It was shown that thrust force is more influenced by tool wear than torque and that uncoated tools have a power law dependence of tool wear with axial force while coated carbide tools have a linear dependence instead. The market for composite aerospace materials has duplicated the levels corresponding to 10-years ago, and it is expected to grow another 55% over the next following years [17]. According to Airbus data, the number of holes drilled on composite made aircraft components will be five times bigger in 2020. As a matter of fact, due to the huge number of holes required for the assembly of aeronautical components, machining operations must be designed ensuring competitive levels of productivity and cost per operation, while maintaining the required quality on the machined surface. So, the main target of this work is to study the wearing process of carbide countersink drill bits, on CFRP test specimens representative of aircraft components, with the objective of looking for extended tool life and more productive cutting parameters. For this purpose, an optical microscope was used to characterize the wear mechanism of the tools and a contact profilometer to assess the quality of the hole, as well as different measurement tools, such as calibers or inside micrometers.
3rd International Conference on Mechanics of Composites – Bologna, Italy – July 2017
2. Experimental procedure The study was focused on carbide drill bits employed on the assembly of aeronautical components made of CFRP. Wearing process was carried out in a controlled environment to replicate real production conditions. Along testing, tool wear was periodically monitored and hole quality was evaluated according to the engineering requirements that have to be controlled to ensure optimal performance of the manufacturing process, based on internal rules and standards defined by Airbus Group. In this way, it is possible to compare the behavior of several cutting parameters and analyze their influence on tool life and hole quality. 2.1 Tool geometry and workpiece specification The test specimens are made of CFRP laminates at different orientations. The upper surface of the coupon is covered by an epoxy preimpregnated expanded copper foil and in the lower part, it has a prepreg layer made from an E-glass fiber fabric preimpregnated with epoxy. The total thickness of the coupons is 9.5 mm. Regarding cutting tools, carbide countersink drill bits with diamond coating were tested. They are right hand cutting with two high positive rake angle cutting edges and 90⁰ point angle. The nominal diameter of tool studied is 4.825 mm. Figure 1 shows one of the countersink drill bits tested.
Figure 1. Countersink drill bit geometry 2.2 Experimental set up and machining conditions Machining experiments were carried out on a 3 axis CNC machine, KONDIA B500, with a numerical control Heidenhain. To position the test specimens and vacuum the chips and dust generated, a made on purpose tooling was designed and manufactured (Figure 2). The aspiration system was also intended to simulate the cutting conditions found on real production process, where the vacuum hose is positioned concentric to the cutting tool. The aspirator employed is a Nilfisk S2B with a cyclonic filter, an antistatic filter and a micro filter to retain the smallest particles.
3rd International Conference on Mechanics of Composites – Bologna, Italy – July 2017
Figure 2. Tooling designed to position the test specimens and vacuum dust generated In order to analyze the influence of cutting parameters on tool wear and hole quality, it was proposed a reference condition and then, the cutting speed and the feed were increased a 20% with respect to that reference (Table 1). Notice that machining was performed on dry conditions. Table 1. Cutting conditions employed: 1 Reference case, 2 Increase cutting speed, 3 Increase feed, 4 Increase cutting speed + feed Cutting conditions 1 2 3 feed [mm/rev] 0.10 0.10 0.12 Vc [m/min] 50.0 60.0 50.0 2.3 Inspection equipment and damage evaluation Parameter
4 0.12 60.0
The inspection equipment employed to monitor tool wear and hole quality was an optical microscope OLYMPUS SZ40 and a contact profilometer MARWIN XCR 20, respectively. Furthermore, different measurement tools were employed, such as calibers or inside micrometers. Figure 3 shows one of the profilometer probes measuring a countersink hole profile.
Figure 3. MARWIN XCR 20 profilometer equipment measuring a test specimen For a given application with specific cutting parameters, each tool has a certified life that ensures the quality of the produced hole will fulfill the following engineering requirements:
Hole geometry. Diameter must comply some acceptable limits which depends on the required tolerance of the hole. Also, the tool has to ensure capability indexes, to guarantee the stability of the process Countersink geometry. It is controlled by the countersink angle and the transition radius which is the part of the fastener between the head and the body. Hole integrity. It is evaluated as a function of surface finish roughness and the absence of machining induced damage.
Hence, the useful life of the tool is determined by one of the previously defined parameters that no longer comply with the required quality to guarantee optimal performance of the manufacturing process. In the case of CFRP drilling, delamination is usually the criteria defining end of tool life. It is caused by the low interlaminate strength of the material and large axial forces promoted by inadequate
3rd International Conference on Mechanics of Composites – Bologna, Italy – July 2017
cutting parameters and level of wear. Under proper conditions, tool life is reached gradually because of progressive wear (main driver of machining induced damage), so by monitoring machining process it may be possible to increment tool life or find optimum cutting parameters [2]. The quantification of the delamination damage extension was done with the concept of delamination factor which is defined as the quotient of maximum diameter damaged over the nominal diameter of the hole.
3. Results For each cutting condition, two test were carried out to guarantee the repeatability of the experiment. In this case, the tool life criterion is based on the assessment of the hole quality with respect to the requirements defined in Section 2.3. 3.1 Tool wear characterization Worn tools were inspected and discontinuous flakes on the main cutting edges were observed. So, main wear mechanism found was chipping. It is produced by the presence of highly abrasive fibers and the fluctuation of forces during the drilling process which promotes stress concentration regions weakening the cutting edge. Eventually, after some level of wear, the sharp edge is lost and the cutting geometry of the tool is modified towards negative rake angles, which significantly modifies material removal process producing higher machining induced damage. From this point, wearing process stabilizes, which smooths the breaks and rounds the edges [18]. Once the main wear mechanism was identified, tools were inspected at different levels of wear to analyze its evolution and compare the behavior of the four cutting parameters tested. Figure 4 shows the condition of the main cutting edge of the reference case tool after drilling a total distance of 4400 mm, in comparison with the new cutting edge.
Figure 4. Comparison of cutting edge condition between new (left) and worn tool after drilling a total distance of 4400 mm (right). Reference cutting conditions (case 1). Noticeable differences can be observed between the condition of the new and worn tool for the reference case in Figure 4. Chipping phenomena starts promoting abrasive wear and modifying the cutting geometry. Same behavior was produced for the rest of cutting conditions, although the level of wear found was slightly dissimilar. Table 2 shows the total cutting time and the equivalent linear distance covered by each cutting edge for the tests performed. For condition 2, the cutting speed was increased while maintaining constant the feed in terms of mm/rev, which implies having shorter cutting time since the overall feed increases and, despite of that, it was found almost the same wear as in the case 1. On the other hand, cutting condition 3, with higher feed and same cutting speed as in the reference case, has a smaller edge linear distance covered and a shorter drilling time and shows a smaller level of wear with respect to previous condition.
3rd International Conference on Mechanics of Composites – Bologna, Italy – July 2017
Finally, in condition 4, the cutting speed and the feed were increased in the same proportion, hence the drilling time is further reduced, and the tool wear obtained was even smaller. Regarding the countersink cutting edge, it was observed a more uniform abrasive wear, so the differences between cutting conditions are more subtle which complicates the quantification of the wear extension. In general, flank is the main mechanism, although chipping is also appearing. Table 2. Summary of test characteristics. 1 Reference case, 2 Increase cutting speed, 3 Increase feed, 4 Increase cutting speed + feed Cutting condition 1 2 3 4
Distance drilled (mm)
Cutting time (min)
Edge linear distance (m)
2500
7.7
382.8
4400
13.4
670.8
2500
6.4
382.8
4400
11.2
670.8
2500
6.4
319.0
4400
11.2
559.0
2500
5.3
319.2
4400
9.3
559.2
Vc (m/min)
Vf (mm/rev)
Feed (mm/min)
50.0
0.10
330
60.0
0.10
396
50.0
0.12
396
60.0
0.12
475
3.2 Hole quality assessment After characterizing the type of wear on the tools, the quality of the holes on the test specimens was analyzed. Hole diameter was measured with an inside micrometer and transition radius, countersink angle and surface finish with the profilometer equipment. Machining induced damage was evaluated by visual inspection and with the optical microscope. Figure 5 shows the differences between the conditions of the holes made with a new tool and with a tool after drilling a total distance of 4400 mm.
Figure 5. Comparison of the exit surface between holes made by a new and a worn tool with reference cutting parameters Diameter and countersink geometry were monitored along the tool life and it was verified that these parameters remain around the nominal values in the four cutting conditions. Roughness was also measured on the hole and on the countersink surface and it was found out that it is still under typical quality demands which used to be below 4 μm. Main problem arises from machining induced damage at the exit of the hole, experiencing severe delamination and flaking. It can be seen in Figure 5 that quality delivered by new tool is very good without any evidence of machining induced damage, but holes made with tools after drilling a distance of 4400 mm show severe delamination at the exit surface. Notice, that delamination is not axisymmetric, instead, it is produced in preference directions which are linked to fiber directions of the
3rd International Conference on Mechanics of Composites – Bologna, Italy – July 2017
exit layer. [7] Since the quality of the hole at the beginning of the tool life is quite good, the machining induced damage produced is directly related with the wear mechanism of the tool. Hence, the delamination at the exit of the hole starts appearing when wear deforms the cutting geometry of the tool edges towards negative rake angles, which is the main difference between the four conditions. In the reference case, delamination appears after drilling a distance of 1400 mm. Regarding the tests at higher cutting speed, delamination is produced with an 85% of the thickness, and at the condition of higher feed, with a 130%. On the other hand, the cutting condition which increases the feed and the cutting speed produced the first delamination with a 160% of the reference distance. From this point, wear stabilizes and delamination increases progressively. In the reference cutting condition it was produced a critical delamination factor, which defines the end of tool life, after drilling a total distance of 4400 mm. For the rest of cutting conditions, the distance drilled at which critical delamination factor was produced follows the same proportion as the appearance of first delamination explained in previous paragraph.
4. Conclusions The main objective of this work was to analyze the influence of cutting parameters on CFRP drilling process and assess its influence on tool wear and hole quality. For this purpose, it was defined a reference test and then, the cutting speed and the feed were increased individually. Finally, both were incremented in the same proportion towards more productive cutting parameters. Each test was performed two times to ensure the repeatability and enhance the reliability of the results. The tool wear mechanism observed in the four cutting conditions was very similar being chipping main phenomena followed by a progressive abrasive wear on the main cutting edges. Regarding the countersink edge, it was observed a more uniform and smooth abrasive wear. The quality of the holes was similar in all cases with the only difference of the machining induced damage. It was observed that the initiation of the delamination at the exit of the hole is linked to the level of wear at which the cutting geometry is modified from a sharp edge towards negative rake angles. Furthermore, it was found that the condition of higher cutting speed and feed showed a reduced level of wear, delaying the appearance of delamination and, in consequence, increasing the amount of distance drilled before reaching the critical delamination factor which defines the tool life. The results fulfill a twofold objective: reducing the cycle time by employing more productive cutting parameters and increasing the tool life due to a reduced level of wear on the tool, always ensuring the demanded hole quality. In that way, it would be possible to improve one-shoot drilling operations carried out during the assembly of aeronautical components, enhancing the competitiveness of the process.
5. Acknowledgements The authors acknowledge the financial support to AIRBUS DEFENCE AND SPACE through the project DRILLING PROCESSES IMPROVEMENT FOR MULTI MATERIAL CFRP-AL-TI STACKS and to the Ministry of Economy and Competitiveness of Spain through the grant with reference PTA2015-10741-I.
6. References [1] Huang, X. (2009). Fabrication and properties of carbon fibers. Materials, 2(4), 2369-2403. [2] Sheikh-Ahmad, J. Y. (2009). Machining of polymer composites (pp. 164-165). New York: Springer. [3] Liu, D., Tang, Y., & Cong, W. L. (2012). A review of mechanical drilling for composite laminates. Composite Structures, 94(4), 1265-1279. [4] DeVor, R. E. (1996). An experimental investigation of the crack growth phenomenon for drilling of fiber-reinforced composite materials.
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[5] Ho-Cheng, H., & Dharan, C. K. H. (1990). Delamination during drilling in composite laminates. Journal of Engineering for Industry (Transactions of the ASME), 112(3), 236-239. [6] Chen, W. C. (1997). Some experimental investigations in the drilling of carbon fiber-reinforced plastic (CFRP) composite laminates. International Journal of Machine Tools and Manufacture, 37(8), 1097-1108. [7] Gaugel, S., Sripathy, P., Haeger, A., Meinhard, D., Bernthaler, T., Lissek, F., ... & Schneider, G. (2016). A comparative study on tool wear and laminate damage in drilling of carbon-fiber reinforced polymers (CFRP). Composite Structures, 155, 173-183. [8] Masuda, M., Kuroshima, Y., & Chujo, Y. (1993). Failure of tungsten carbide-cobalt alloy tools in machining of carbon materials. Wear, 169(2), 135-140. [9] Mayuet, P., Gallo, A., Portal, A., Arroyo, P., Alvarez, M., & Marcos, M. (2013). Damaged area based study of the Break-IN and Break-OUT defects in the dry drilling of carbon fiber reinforced plastics (CFRP). Procedia Engineering, 63, 743-751. [10] Faraz, A., Biermann, D., & Weinert, K. (2009). Cutting edge rounding: An innovative tool wear criterion in drilling CFRP composite laminates. International Journal of Machine Tools and Manufacture, 49(15), 1185-1196. [11] Karpat, Y., Değer, B., & Bahtiyar, O. (2012). Drilling thick fabric woven CFRP laminates with double point angle drills. Journal of materials processing technology, 212(10), 2117-2127. [12] Heisel, U., & Pfeifroth, T. (2012). Influence of point angle on drill hole quality and machining forces when drilling CFRP. Procedia CIRP, 1, 471-476. [13] Merino-Pérez, J. L., Royer, R., Merson, E., Lockwood, A., Ayvar-Soberanis, S., & Marshall, M. B. (2016). Influence of workpiece constituents and cutting speed on the cutting forces developed in the conventional drilling of CFRP composites. Composite Structures, 140, 621-629. [14] Khashaba, U. A., El-Sonbaty, I. A., Selmy, A. I., & Megahed, A. A. (2010). Machinability analysis in drilling woven GFR/epoxy composites: Part I–Effect of machining parameters. Composites Part A: Applied Science and Manufacturing, 41(3), 391-400. [15] Rawat, S., & Attia, H. (2009). Wear mechanisms and tool life management of WC–Co drills during dry high speed drilling of woven carbon fibre composites. Wear, 267(5), 1022-1030. [16] Iliescu, D., Gehin, D., Gutierrez, M. E., & Girot, F. (2010). Modeling and tool wear in drilling of CFRP. International Journal of Machine Tools and Manufacture, 50(2), 204-213. [17] Composites forecast and consulting. 2014-2023 Global Composite Aerostructures Market Outlook Report. [18] Wang, D. H., Ramulu, M., & Arola, D. (1995). Orthogonal cutting mechanisms of graphite/epoxy composite. Part I: unidirectional laminate. International Journal of Machine Tools and Manufacture, 35(12), 1623-1638.