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Review of hole-making technology for composites
Review of hole-making technology for composites
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M.S. Abdullah, A.B. Abdullah, Z. Samad School of Mechanical Engineering, Universiti Sains Malaysia, Nibong Tebal, Malaysia
1.1 Introduction Composite material can be used for a great number of applications in modern structures because of its high modulus, strength, light weight, durability, corrosion resistance, design flexibility, and so on. However, due its nonhomogeneity and high- abrasivity, machining composite material is a major problem [1–4]. In many manufacturing industries, conventional machining processes such as drilling remain the primary method of hole-making in composites. Generally in the aviation manufacturing industry, most aircraft structures, such as stabilizers, wings and fuselage, consist of a great number of varying types of holes (e.g., round, counterboring, countersinking, honing, reaming, lapping, sanding, etc.) with different diameters, depths and surface finishes [1]. Commonly these aircraft structures require assembly and therefore a number of holes need to be drilled. Most of the joints involved in the assembly are mechanical joints, such as bolted connections, rivet connections and pin connections, therefore mechanical joint efficiency is highly dependent on the quality of the holes [5]. With advances in processing techniques, the development of new hole-making technology, which is now integrated with the digital computer, has improved the efficiency and productivity of hole-making in terms of cutting time and hole quality. This chapter reviews the existing technologies in hole-making for composite panels, and the advantages and limitations of these technologies. The chapter begins with a brief introduction to hole-making technology, followed by a discussion of machining and nonmachining technologies.
1.2 Hole-making for composite laminates Hole-making technology can be divided into machining and nonmachining technology. The machining technology can be further divided into traditional and nontraditional machining. Drilling and milling are the traditional machining methods used for composites. The difference between the two methods is the approaching mechanism; for example, in drilling, the drill bit is rotated and fed the stationary workpiece and the volume of the workpiece is removed to produce a circular hole. Since drilling is a primary method of making holes [6], we will focus on this first. Many types of drilling technologies have evolved to meet the demand for composite materials. Hole-Making and Drilling Technology for Composites. https://doi.org/10.1016/B978-0-08-102397-6.00001-5 © 2019 Elsevier Ltd. All rights reserved.
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Hole-Making and Drilling Technology for Composites
Hole-Making Technology
Machining
Traditional machining
Milling
Drilling
AWJM
Nonmachining
Nontraditional machining
Punching
EDM
Laser
Fig. 1.1 Technologies in hole-making.
In nontraditional machining, methods such as Wire EDM, laser and Abrasive Water Jet Machining (AWJM) are being used. In nonmachining, to date only punching technology is being used for hole-making in composite materials. Fig. 1.1 summarizes the hole-making technologies that have been developed to date.
1.2.1 Machining Machining (i.e., drilling) remains the preferred method of hole-making in composite materials across industries, although it is obvious that drilling in composite material is not the same as drilling in metallic material [7]. Composite material is very abrasive and since the mechanical drilling operation involves direct contact between tool and workpiece, the tool suffers extreme wear and creates a great amount of heat, which induces residual stress that leads to degradation of both tool life and the quality of workpiece [2]. The mechanical drilling operation can be divided into five types: conventional drilling, high-speed drilling (HSD), grinding drilling, vibration-assisted twist drilling (VATD), and orbital drilling (OD). As an alternative to drilling, punching also shows potential for producing holes in composites.
1.2.1.1 Conventional drilling Most drilling operations use twist drill bits and other special drill bits (step drill bit, center drill bit and dagger drill bit) as the cutting tools. However, the twist drill bit is the most widely used [8]. In conventional drilling, multiple stages of drilling need to be executed before the hole reaches its specified size or diameter. If the diameter of the hole is relatively large, a pilot hole with a small diameter may have to be drilled first and then enlarged to the final size with a larger tool [9]. This is to avoid a concentration
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of high stress at the hole boundary on the workpiece material and to keep the damage to a minimum [10, 11]. The research on conventional drilling in composite laminates includes a number of experiments studying input variables, such as drilling parameters (spindle speed and feed), drill bit geometry, drill bit material, type of composite material and diameter size, and output variables, such as hole quality (delamination, surface roughness and roundness), thrust force and bearing strength of the hole [12].
1.2.1.2 High-speed drilling In a single aircraft there are thousands of holes that need to be drilled, mostly for mechanical fasteners like rivets and bolts [13]. The continuous development of cutting tools (material and geometry) has reduced cutting time and improved productivity in hole-making [12]. There are several basic requirements to take into account in HSD, namely, concentricity (related to tool material behavior when operating at different cutting speeds), tool material, coating material, flute geometry and coolant delivery [14]. HSD technology has been widely studied and employed in many areas of composite drilling. It is supposed to produce less delamination damage in a short time with single-shot drilling [15]. Similar to conventional drilling, HSD is the most promising drilling operation that leads to better performance and improves the quality of the hole. Unlike other drilling operations, HSD is carried out at very high spindle speed and results in reduction of delamination [16, 17]. However, increasing the speed literally increases the power consumption of the machine operation as well as the tooling cost due to excessive tool wear, and causes the total machining cost to become very expensive [18]. As the speed goes higher, usually 5–10 times more than conventional drilling, the rate of temperature increases making the composite laminates prone to thermal damage [19]. At the higher temperature, which exceeds the epoxy melting temperature, the heat from the friction contact between the tool and the workpiece softens the epoxy matrix making it evaporate, known as matrix burnout, and causes only the fiber to be left. This results in interlaminar delamination [20]. Not only that, it is also shortens the tool life span. Because the temperature significantly affects the hole quality and tools, coolant is used to combat temperature issues. Nowadays, most applications of HSD incorporate a high-pressure coolant flow system to avoid catastrophic thermal damage to the workpiece instead of removing chips. Yet the aerospace manufacturing industry is moving toward HSD under dry conditions with optimum cutting parameters due to economic and environmental reasons [21]. Dry drilling conditions might be a better choice for thin composite laminates because the short engagement time may limit heat buildup.
1.2.1.3 Grinding drilling Grinding drilling, also known as core drilling, is one of the drilling operations that best reduces delamination damage. Grinding drilling focuses on the periphery of the hole. There is no chisel edge acting on the predrilling like with a normal drill bit since the center of the drill bit is hollow. The absence of chisel edge reduces thrust force and hence delamination [22, 23]. It was found that increasing the number of teeth of the cutting tool can reduce the cutting force. The easiest way to achieve an unlimited
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Hole-Making and Drilling Technology for Composites
number of teeth is by coating the cutting tool with a certain grit size of material. Tsai and Hocheng [24] proposed coating it with diamond material. Diamond material has an extremely high thermal conductivity, which can remove heat from the cutting edge and extend tool life. The research suggests that diamond material is preferable because it provides high abrasive wear resistance. The different grain size of coated material also influences the surface quality of the hole and the heat distribution over the matrix of the hole boundary. The result of the investigation shows that increasing the grain size results in lower thermal load and allows the heat to dissipate more efficiently. However, it was shown that finer grains result in better surface quality of the hole [25]. There are different types of geometry for core drill bits and each of them serves different purposes. Fig. 1.2 shows different designs of core drill bits. An improvement of the cutting mechanism in micro-core drilling has been made by introducing a shear mechanism at the cutting edge of the core drills using novel tool design (defined cutting edge using polycrystalline diamond (PCD)). The conventional core drills (randomly distributed micro grains) use an electroplated diamond abrasive micro-core drill that produces an abrasive/rubbing action that results in random cutting edge geometry (negative rake angle, protrusions, densities). This deficiency of random cutting edge geometry is not a good solution for machining parameters. According to the research, a novel micro-core drill reduces drilling force and temperature by 36% and 11%, respectively, compared with conventional core drills. In addition to these findings, the evaluation of the shearing action of the novel micro-core drill found it produces holes with superior edge definition and surface quality [26]. Fig. 1.3 shows a conceptual image of a laser-generated PCD core drill.
1.2.1.4 Vibration-assisted twist drilling VATD is another branch of vibration cutting that uses vibration in the drilling process. There are three directional modes of vibration that occur in the drilling process, namely, axial, lateral and torsional. The drill moves in these three directions when it is run on the surface of the workpiece. When it comes to composite laminates, the typical damage that has been recognized as the major damage when drilling is d elamination [27].
Fig. 1.2 Different design types of core drill bits (A) abrasive tools [25] and (B) hollow grinding [12].
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Fig. 1.3 (A) Conceptual design of laser generated PCD core drill and (B) SEM image of laser generated tooth profile in PCD and profilometric measurement image of its geometry [26].
Most of the aviation industry uses conventional drilling, which employs continuous axial motion of the drill bit toward the workpiece. This continuous motion of the drill generates heat by the friction between the tool and workpiece, and the temperature rises as the drilling progresses. The VATD process uses a small-amplitude, low/high- frequency tool superimposed with conventional feed in an axial motion with controllable intermittent intervals. This allows frequent separation and contact between the tool and the chips, which reduces the contact area and leads to a decrease in frictional force [28]. Compared with conventional drilling, VATD has unique characteristics such as impacting, separating, changing speed and changing angle during the drilling process [29]. In general, this process interrupts continuous contact between the tool and workpiece and exhibits great potential in improving the cutting ability of a chisel edge and restraining the skidding of a chisel edge, reducing surface roughness, thrust force, and delamination, and extending the tool life while maintaining process productivity [30, 31]. It has been reported in some research that thrust force reduces by around 40% in VATD compared to conventional drilling. Therefore VATD is used to reduce delamination in drilling composite laminates [32].
1.2.1.5 Orbital drilling OD has great potential as an alternative to conventional drilling for minimizing damage associated with the drilling of composite laminates. OD is particularly effective for hole-making operations in laminate materials such as carbon fiber reinforced polymers (CFRPs) used in aerospace applications, which need precise dimensional accuracy and tight tolerances [33, 34]. The working principles of OD can be described in three basic motions: spindle rotation, feed and orbital rotation. Combining these motions creates spiral/helical rotation of the cutting tool. As the cutting tool spins in
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Hole-Making and Drilling Technology for Composites
its own axial direction, it simultaneously moves offset (in a lateral direction) to the desired hole diameter. By calculating the desired offset, a single cutting tool can be used to drill any diameter larger than the tool’s diameter [35]. Thus it essentially reduces the need for multiple tools to drill a single hole and eliminates the time needed for tool changeover. Moreover, since the tool diameter is substantially smaller than the diameter of the hole, it reduces the risk of the tool sticking out during the drilling process. During the OD process the tool is in partial contact with the workpiece, and this action enables the performance of heat extraction to become more efficient. Additionally, in the normal mechanism of other mechanical drilling the tool move straight concentric in an axial motion to the laminate, which results in high pressure at the center of the machine hole. However, in the OD process the tool moves in a helical feed motion, which reduces the pressure in the center of the hole while machining. This, in turn, reduces the matrix resin from burnout in the heat-affected zone [36]. These advantages of OD significantly decrease total cycle time thus increasing productivity and profitability [37]. Fig. 1.4 shows the working mechanism of the OD. The cutting tool material commonly used in OD is PCD and carbide end mills.
1.2.1.6 Milling Milling potential is well known and there are a number of publications describing its advantages. Use of circular milling began in 1995 by Park et al. [39]. Usually, circular milling will combine with other machining processes to achieve better results. For example, Schulze et al. [40] combined circular milling and spiral milling and studied the parameters of cutting velocity, tool feed, depth of cut and tool inclination angle. Regarding all parameters, feed rate is the most influential on delamination, where increases in feed rate will result in worse delamination or greater damage. Also in their research, they introduced wobble milling (Fig. 1.5) and found that this innovation will result in less damage compared to the combined milling process. Prior to that, Ali et al. [41] found that milling performed better compared to drilling in terms of
Fig. 1.4 (A) Tool motion in orbital drilling; (B) and (C) deflection of the last ply in conventional drilling (CD) and orbital drilling (OD), respectively [38].
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Fig. 1.5 Concept of wobble milling for hole-making in composites [40].
minimum surface roughness and minimum difference between exit and entrance hole diameters. This can be achieved at low feed rate and high cutting speed. Milling also performed on composite-metal stacking panel. For example Rahim et al. [42] found that feed rate and cutting speed is not significant to the roughness of the hole surface produced via milling on CFRP/Al stacks. In other work, Yagishita and Osawa [43] studied the CFRP/TiAl6V4 stack from the perspective of roundness and hole diameter. They found that newly developed hole-making machine according to double eccentric mechanism improved the quality of the hole. Similarly, Wang et al. [44] compared the hole-making process using helical milling of CFRP/Ti stacks and they found that tool wear will affect the cutting performance. Liu et al. [45] investigated the temperature variation of helical milling.
1.2.2 Nontraditional machining Moving toward future technology, which involves the machining of complex job profiles, hard/brittle materials, and the abrasive nature of the reinforcing phases, mechanical drilling might prove impractical for several reasons. These include extreme tool wear, incapability of machining a complex profile, poor surface finish quality and prohibitive economic costs. Thus unconventional machining methods have a huge potential over conventional/mechanical drilling for composite drilling. Generally, the unconventional machining technique is defined as a group of processes that removes excess material by various methods involving mechanical, thermal, electrical or chemical energy, or combinations of these energies, but does not use a sharp cutting tool as is necessary in traditional manufacturing processes. Among the many unconventional machining techniques, there are a few we will discuss in this chapter, namely, water jet machining (WJM), electro-discharge machining (EDM) and laser beam machining (LBM).
1.2.2.1 Abrasive water jet machining WJM has been used across industries and provides some advantages over conventional/mechanical drilling operations. Some of these advantages include no thermal
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Hole-Making and Drilling Technology for Composites
Pressurized water, p
Orifice, do
Traverse feed rate, v
Abrasive ma
Stand-off distance, s Focusing tube, df
Fig. 1.6 Process parameters in abrasive water jet machining (AWJM) [46].
effect, high machining versatility, high flexibility, small cutting force and high productivity. Also, WJM makes issues like burr formation and delamination in hole making on CFRPs almost negligible [46]. AWJM uses an erosion mechanism; a water jet of high pressure, high velocity and abrasive slurry cuts the target material by means of erosion [47]. Fig. 1.6 shows the schematic diagram of AWJM for hole-making on composite materials. There are few process parameters that affect the quality of the workpiece surface cut by AWJM, namely, hydraulic pressure, standoff distance, abrasive flow rate and types of abrasive [48]. Important quality parameters in AWJM are material removal rate (MMR), surface roughness, kerf width and tapering of the kerf. The abrasive particle used for AWJM is usually silicon carbide and aluminum oxide. The abrasive particle is embedded to the water jet purposely to increase the MMR of the process. Conventional/mechanical drilling uses friction and shearing to cut the workpiece. Alternatively, AWJM uses erosive action to cut the workpiece leaving a smooth, finished edge, less burr and no delamination. Since there is no physical contact between the cutting tool and workpiece, AWJM eliminates heat-affected zones and expensive cutting tools like diamond and others [49]. In research investigating the quality of the cutting hole in glass fiber reinforced plastic (GFRP), the author found that the quality of the hole in GFRP is highly dependent on the right choice of the cutting parameter in the cutting process [50]. Another researcher carried out the experiment using AWJM on composite laminates and found the delamination of the laminates can be reduced by reducing the jet speed. However, this also caused the piercing to deteriorate [51].
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1.2.2.2 Electro-discharge machining EDM is also known as spark erosion machining. The process basically removes the workpiece material using electric spark. There are three types of EDM machining, namely, die sinking EDM, wire EDM and hole drilling EDM. Here we will cover basic concepts of hole drilling EDM, which is also called electro-discharge drilling (EDD) [52]. EDD is a hole-making process for electrically conductive workpiece materials, which harden to the machine. In addition to CFRPs, metal matrix composites (MMCs) are also growing rapidly in the aircraft manufacturing industry. Machining of MMCs using EDD is inevitable due to the material’s high hardness and wear-resistance properties. Conventional/mechanical drilling might not be a suitable choice for making a substantial number of holes in an MMC due to rapid tool wear. MMCs are composite materials with at least two constituent parts; one being a metal necessarily, the other material may be a different metal or another material, such as a ceramic or organic compound. When at least three materials are present, it is called a hybrid composite [53]. This makes drilling MMCs more difficult because each material has different properties. The working principles of this EDD can be explained clearly based on the schematic diagram shown in Fig. 1.7. The tool (metal electrode as a cathode) and the workpiece (acting as an anode) are placed very close together with a small gap (around 0.01–0.5 mm), separated by nonconducting liquid known as dielectric. As a pulse of DC electricity reaches the electrode and the part, an intense electrical field develops in the gap and microscopic contaminants suspended in the dielectric fluid are attracted by the field and concentrate at the field’s strongest point and build a high conductivity across the gap. As the field voltage increases, the material in the conductive bridge heats up and forms the spark channel between the tool and the workpiece [54]. As a
Fig. 1.7 Schematic experimental set-up of electro-discharge drilling (EDD) [52].
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Hole-Making and Drilling Technology for Composites
result, the workpiece erodes. Drilling process set-up is attached with servo system provide tool feed to keep the gap constant. The electrode used is usually copper, graphite, tungsten and brass. For dielectric fluid, kerosene is commonly used.
1.2.2.3 Laser beam cutting technology LBM is a form of machining in which a laser is directed toward the workpiece. LBM is one of the advanced manufacturing processes capable of machining all ranges of material from metallic alloys to nonmetals [55, 56]. Therefore LBM provides a solution to a critical problem that conventional/mechanical drilling is not capable of solving. Material removal in LBM is a thermal material removal process that utilizes a high- energy, coherent light beam to melt and vaporize particles on the workpiece in the focus point [57]. The advantages of LBM, such as improved end product quality, short processing time, noncontact process, cost reduction and small heat affected zone, have led to its use in many manufacturing industries [58]. For cutting composites, the general types of lasers used are CO2 and neodymium yttrium aluminum garnet (Nd:YAG) [59]. Fig. 1.8 shows the schematic diagram of the LBM removal mechanism. Composite laminates have more than one constituent material, which makes them difficult to laser machine because the components of the composites have varied thermal conductivity [59]. This means, in fiber-reinforced plastics, the power needed for a laser to vaporize (cut) the fiber (CFRP or GFRP) is much higher than the power needed for a polymer (epoxy). For this reason, it is important to set the laser cutting parameters (laser power, cutting speed, gas pressure, etc.) on the cut quality parameters (heat-affected zone, surface roughness, kerf width, taper angle etc.) carefully before starting LBM.
1.2.3 Nonmachining methods Other than machining, punching is another hole-making technology used to produce holes in composite panels.
Fig. 1.8 Laser cutting material removal mechanism [55].
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Puncher Composite panel
Die
Slug
Fig. 1.9 Principles of the punching process on composite panel [61].
1.2.3.1 Punching Based on preliminary study, the punching technique shows potential for hole-making on laminated composites [5, 60–63]. Punching is a forming process that uses punch and die to form holes via shearing. In composites punching, the mechanism for cutting holes is different from that of metal material since the composite is brittle. There are several cutting parameters affecting the punching of composites, namely, clearance, tool geometry, speed or stroke rate, blank holder force, sheet thickness, blank layout, material type, punch-die alignment and friction [64]. Punching in composites might result in poor surface quality due to the nonhomogeneity, multiphase structure and anisotropic nature of the composite [65]. Lambiase and Durante [66] investigated the influence of process parameters (punch-die clearance) on quality of punched holes (delamination factor, bearing strength) compared to drilled holes on thin GFRP laminates. The results showed that the punching force increases when punch-die clearance is decreased. They also noticed that the delamination factor increases when the punchdie clearance increases. They concluded that punching results in a higher delamination factor and lower bearing strength. One of the main advantages of punching is that it does not produce peel-up edge; in drilling, both peel-up and push-down edges occur. Furthermore the process is faster and produces less wear compared to drilling. The basic principles of punching are illustrated in Fig. 1.9.
1.3 Conclusions Hole-making technology can be divided into two main technologies: machining and nonmachining methods. The machining method can be further divided into traditional and nontraditional machining. Capability of the technologies is evaluated based on several parameters, for example, spindle speed and feed, drill bit diameter, drill bit geometry, drill bit material and type of composite material. In punching, for example, die clearance and puncher profiles are among the most influential parameters. All these parameters are important for ensuring high quality of the produced holes.
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Hole-Making and Drilling Technology for Composites
In hole-making, quality depends on the cutting method or tools used. In drilling, for example, wear is the main problem. In addition, other important factors such as productivity and technology cost also need to be taken into consideration.
Acknowledgments The authors want to acknowledge Universiti Sains Malaysia for their sponsorship through Research University Grant (Acc. No. 1001/PMEKANIK/814270) and to Mr. Fakruruzi Fadzil who helps in conducting the experiments.
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[33] Q. Zhao, X. Qin, C. Ji, Y. Li, D. Sun, Y. Jin, Tool life and hole surface integrity studies for hole-making of Ti6Al4V alloy, Int. J. Adv. Manuf. Technol. 79 (5–8) (2015) 1017–1026. [34] H. Yagishita, J. Osawa, Highly accurate hole making technology of Ti6Al4V by orbital drilling: effect of oil mist, Proc. Manuf. 5 (2016) 195–204. [35] R. Iyer, P. Koshy, E. Ng, Helical milling: an enabling technology for hard machining precision holes in AISI D2 tool steel, Int. J. Mach. Tools Manuf. 47 (2) (2007) 205–210. [36]. Vanysek, P., Us, I.L., Ricco, H.S. United States Patent, US 2008/0141517 A1, 2008. [37] I. Sultana, Z. Shi, M.H. Attia, V. Thomson, Surface integrity of holes machined by orbital drilling of composites with single layer diamond tools, Proc. CIRP 45 (2016) 23–26. [38] A. Sadek, M. Meshreki, M.H. Attia, Characterization and optimization of orbital drilling of woven carbon fiber reinforced epoxy laminates, CIRP Ann. Manuf. Technol. 61 (1) (2012) 123–126. [39] K.Y. Park, J.H. Choi, D.G. Lee, Delamination-free and high efficiency drilling of carbon fiber reinforced plastics, J. Compos. Mater. 29 (1995) 1988–2002. [40] V. Schulze, C. Becke, K. Weidenmann, S. Dietrich, Machining strategies for hole making in composites with minimal workpiece damage by directing the process forces inwards, J. Mater. Process. Technol. 211 (2011) 329–338. [41] H.M. Ali, A. Iqbal, L. Liang, A comparative study on the use of drilling and milling processes in hole making of GFRP composite, Sadhana 38 (4) (2013) 743–760. [42] E.A. Rahim, Z. Mohid, M.R. Hamzah, A.F. Yusuf, N.A. Rahman, Helical milling of CFRP with aluminum stack, Appl. Mech. Mater. 465–466 (2014) 1075–1079. [43] H. Yagishita, J. Osawa, Hole making machine based on double eccentric mechanism for CFRP/TiAl6V4 stacks, Proc. Manuf. 1 (2015) 747–755. [44] H. Wang, X. Qin, H. Li, Y. Tan, A comparative study on helical milling of CFRP/Ti stacks and its individual layers, Int. J. Adv. Manuf. Technol. 86 (2016) 1973–1983. [45] J. Liu, G. Chen, C.H. Ji, X.D. Qin, H. Li, C.Z. Ren, An investigation of workpiece temperature variation of helical milling for carbon fiber reinforced plastics (CFRP), Int. J. Mach. Tools Manuf. 86 (2014) 89–103. [46] A. Alberdi, A. Suárez, T. Artaza, G.A. Escobar-Palafox, K. Ridgway, Composite cutting with abrasive water jet, Proc. Eng. 63 (2013) 421–429. [47] P.D. Unde, M.D. Gayakwad, R.S. Ghadge, Abrasive water jet machining of composite materials—a review, Int. J. Innov. Res. Sci. Eng. Technol. 3 (4) (2014) 6–8. [48] H. Ali, A. Iqbal, M. Hashemipour, Experimental analysis of hole making in GFRP composite using abrasive water jet cutting technology, Appl. Mech. Mater. 325–326 (2013) 1392–1398. [49] P.D. Unde, M.D. Gayakwad, N.G. Patil, R.S. Pawade, D.G. Thakur, P.K. Brahmankar, Experimental investigations into abrasive waterjet machining of carbon fiber reinforced plastic, J. Compos. (2015) 971596. [50] K. Thongkaew, J. Wang, G.H. Yeoh, An investigation of hole machining process on a carbon-fiber reinforced plastic sheet by abrasive waterjet, Adv. Mater. Res. 1136 (2016) 113–118. [51] H.M.A. Ibraheem, A. Iqbal, M. Hashemipour, Numerical optimization of hole making in GFRP composite using abrasive water jet machining process, J. Chin. Inst. Eng. 38 (1) (2015) 66–76. [52] A. Singh, P. Kumar, I. Singh, Electric discharge drilling of metal matrix composites with different tool geometries, Proc. Inst. Mech. Eng. B J. Eng. Manuf. 227 (8) (2013) 1245–1249. [53] A. Singh, P. Kumar, I. Singh, Process optimization for electro-discharge drilling of metal matrix composites, Proc. Eng. 64 (2013) 1157–1165.
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[54] H. Hocheng, W.T. Lei, A.S. Hsu, Preliminary study of material removal in electrical- discharge machining of SiC/AI, J. Mater. Process. Technol. 63 (1997) 813–818. [55] P. Parandoush, A. Hossain, A review of modeling and simulation of laser beam machining, Int. J. Mach. Tools Manuf. 85 (2014) 135–145. [56] P. Patel, P. Gohil, S. Rajpurohit, Laser machining of polymer matrix composites: scope, limitation and application, Int. J. Eng. Trends Technol. 4 (6) (2013) 2391–2399. [57] M.H. Industries, High-speed & high-quality laser drilling technology using a prism rotator, Mitsubishi Heavy Ind. Tech. Rev. 52 (1) (2015) 106–109. [58] A.K. Dubey, V. Yadava, Laser beam machining—a review, Int. J. Mach. Tools Manuf. 48 (6) (2008) 609–628. [59] J. Meijer, Laser beam machining (LBM), state of the art and new opportunities, J. Mater. Process. Technol. 149 (1–3) (2004) 2–17. [60] H.Y. Chan, A.B. Abdullah, Z. Samad, M.S.M. Zain, Precision punching on laminates composite panel: effect of dual-stages puncher, Int. J. Mater. Eng. Innov. 6 (4) (2015) 288–296. [61] M.S.M. Zain, A.B. Abdullah, Z. Samad, Effect of puncher profile on the precision of punched holes on composite panels, Int. J. Adv. Manuf. Technol. 89 (9–12) (2017) 3331–3336. [62] A.B. Abdullah, M.S.M. Zain, Z. Samad, Delamination assessment of punched holes on laminated composite panels based on the profile measurement technique, Int. J. Adv. Manuf. Technol. 93 (1–4) (2017) 993–1000. [63] N.A. Ghaffar, A.B. Abdullah, Z. Samad, Precision hole making on laminate composite: a tool Wear analysis and comparison between drilling and punching, Int. J. Mater. Eng. Innov. 7 (2) (2016) 143–157. [64] E. Al-momani, I. Rawabdeh, An application of finite element method and design of experiments in the optimization of sheet metal blanking process, Jordan J. Mech. Indust. Eng. 2 (1) (2008) 53–63. [65] K.K. Panchagnula, K. Palaniyandi, Drilling on fiber reinforced polymer/nanopolymer composite laminates: a review. J. Mater. Res. Technol. (2018). https://doi.org/10.1016/j. jmrt.2017.06.003. [66] F. Lambiase, M. Durante, Mechanical behavior of punched holes produced on thin glass fiber reinforced plastic laminates, Compos. Struct. 173 (2017) 25–34.
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M.S. Abdullah, A.B. Abdullah, Z. Samad School of Mechanical Engineering, Universiti Sains Malaysia, Nibong Tebal, Malaysia
1.1 Introduction Composite material can be used for a great number of applications in modern structures because of its high modulus, strength, light weight, durability, corrosion resistance, design flexibility, and so on. However, due its nonhomogeneity and high- abrasivity, machining composite material is a major problem [1–4]. In many manufacturing industries, conventional machining processes such as drilling remain the primary method of hole-making in composites. Generally in the aviation manufacturing industry, most aircraft structures, such as stabilizers, wings and fuselage, consist of a great number of varying types of holes (e.g., round, counterboring, countersinking, honing, reaming, lapping, sanding, etc.) with different diameters, depths and surface finishes [1]. Commonly these aircraft structures require assembly and therefore a number of holes need to be drilled. Most of the joints involved in the assembly are mechanical joints, such as bolted connections, rivet connections and pin connections, therefore mechanical joint efficiency is highly dependent on the quality of the holes [5]. With advances in processing techniques, the development of new hole-making technology, which is now integrated with the digital computer, has improved the efficiency and productivity of hole-making in terms of cutting time and hole quality. This chapter reviews the existing technologies in hole-making for composite panels, and the advantages and limitations of these technologies. The chapter begins with a brief introduction to hole-making technology, followed by a discussion of machining and nonmachining technologies.
1.2 Hole-making for composite laminates Hole-making technology can be divided into machining and nonmachining technology. The machining technology can be further divided into traditional and nontraditional machining. Drilling and milling are the traditional machining methods used for composites. The difference between the two methods is the approaching mechanism; for example, in drilling, the drill bit is rotated and fed the stationary workpiece and the volume of the workpiece is removed to produce a circular hole. Since drilling is a primary method of making holes [6], we will focus on this first. Many types of drilling technologies have evolved to meet the demand for composite materials. Hole-Making and Drilling Technology for Composites. https://doi.org/10.1016/B978-0-08-102397-6.00001-5 © 2019 Elsevier Ltd. All rights reserved.
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Hole-Making and Drilling Technology for Composites
Hole-Making Technology
Machining
Traditional machining
Milling
Drilling
AWJM
Nonmachining
Nontraditional machining
Punching
EDM
Laser
Fig. 1.1 Technologies in hole-making.
In nontraditional machining, methods such as Wire EDM, laser and Abrasive Water Jet Machining (AWJM) are being used. In nonmachining, to date only punching technology is being used for hole-making in composite materials. Fig. 1.1 summarizes the hole-making technologies that have been developed to date.
1.2.1 Machining Machining (i.e., drilling) remains the preferred method of hole-making in composite materials across industries, although it is obvious that drilling in composite material is not the same as drilling in metallic material [7]. Composite material is very abrasive and since the mechanical drilling operation involves direct contact between tool and workpiece, the tool suffers extreme wear and creates a great amount of heat, which induces residual stress that leads to degradation of both tool life and the quality of workpiece [2]. The mechanical drilling operation can be divided into five types: conventional drilling, high-speed drilling (HSD), grinding drilling, vibration-assisted twist drilling (VATD), and orbital drilling (OD). As an alternative to drilling, punching also shows potential for producing holes in composites.
1.2.1.1 Conventional drilling Most drilling operations use twist drill bits and other special drill bits (step drill bit, center drill bit and dagger drill bit) as the cutting tools. However, the twist drill bit is the most widely used [8]. In conventional drilling, multiple stages of drilling need to be executed before the hole reaches its specified size or diameter. If the diameter of the hole is relatively large, a pilot hole with a small diameter may have to be drilled first and then enlarged to the final size with a larger tool [9]. This is to avoid a concentration
Review of hole-making technology for composites3
of high stress at the hole boundary on the workpiece material and to keep the damage to a minimum [10, 11]. The research on conventional drilling in composite laminates includes a number of experiments studying input variables, such as drilling parameters (spindle speed and feed), drill bit geometry, drill bit material, type of composite material and diameter size, and output variables, such as hole quality (delamination, surface roughness and roundness), thrust force and bearing strength of the hole [12].
1.2.1.2 High-speed drilling In a single aircraft there are thousands of holes that need to be drilled, mostly for mechanical fasteners like rivets and bolts [13]. The continuous development of cutting tools (material and geometry) has reduced cutting time and improved productivity in hole-making [12]. There are several basic requirements to take into account in HSD, namely, concentricity (related to tool material behavior when operating at different cutting speeds), tool material, coating material, flute geometry and coolant delivery [14]. HSD technology has been widely studied and employed in many areas of composite drilling. It is supposed to produce less delamination damage in a short time with single-shot drilling [15]. Similar to conventional drilling, HSD is the most promising drilling operation that leads to better performance and improves the quality of the hole. Unlike other drilling operations, HSD is carried out at very high spindle speed and results in reduction of delamination [16, 17]. However, increasing the speed literally increases the power consumption of the machine operation as well as the tooling cost due to excessive tool wear, and causes the total machining cost to become very expensive [18]. As the speed goes higher, usually 5–10 times more than conventional drilling, the rate of temperature increases making the composite laminates prone to thermal damage [19]. At the higher temperature, which exceeds the epoxy melting temperature, the heat from the friction contact between the tool and the workpiece softens the epoxy matrix making it evaporate, known as matrix burnout, and causes only the fiber to be left. This results in interlaminar delamination [20]. Not only that, it is also shortens the tool life span. Because the temperature significantly affects the hole quality and tools, coolant is used to combat temperature issues. Nowadays, most applications of HSD incorporate a high-pressure coolant flow system to avoid catastrophic thermal damage to the workpiece instead of removing chips. Yet the aerospace manufacturing industry is moving toward HSD under dry conditions with optimum cutting parameters due to economic and environmental reasons [21]. Dry drilling conditions might be a better choice for thin composite laminates because the short engagement time may limit heat buildup.
1.2.1.3 Grinding drilling Grinding drilling, also known as core drilling, is one of the drilling operations that best reduces delamination damage. Grinding drilling focuses on the periphery of the hole. There is no chisel edge acting on the predrilling like with a normal drill bit since the center of the drill bit is hollow. The absence of chisel edge reduces thrust force and hence delamination [22, 23]. It was found that increasing the number of teeth of the cutting tool can reduce the cutting force. The easiest way to achieve an unlimited
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Hole-Making and Drilling Technology for Composites
number of teeth is by coating the cutting tool with a certain grit size of material. Tsai and Hocheng [24] proposed coating it with diamond material. Diamond material has an extremely high thermal conductivity, which can remove heat from the cutting edge and extend tool life. The research suggests that diamond material is preferable because it provides high abrasive wear resistance. The different grain size of coated material also influences the surface quality of the hole and the heat distribution over the matrix of the hole boundary. The result of the investigation shows that increasing the grain size results in lower thermal load and allows the heat to dissipate more efficiently. However, it was shown that finer grains result in better surface quality of the hole [25]. There are different types of geometry for core drill bits and each of them serves different purposes. Fig. 1.2 shows different designs of core drill bits. An improvement of the cutting mechanism in micro-core drilling has been made by introducing a shear mechanism at the cutting edge of the core drills using novel tool design (defined cutting edge using polycrystalline diamond (PCD)). The conventional core drills (randomly distributed micro grains) use an electroplated diamond abrasive micro-core drill that produces an abrasive/rubbing action that results in random cutting edge geometry (negative rake angle, protrusions, densities). This deficiency of random cutting edge geometry is not a good solution for machining parameters. According to the research, a novel micro-core drill reduces drilling force and temperature by 36% and 11%, respectively, compared with conventional core drills. In addition to these findings, the evaluation of the shearing action of the novel micro-core drill found it produces holes with superior edge definition and surface quality [26]. Fig. 1.3 shows a conceptual image of a laser-generated PCD core drill.
1.2.1.4 Vibration-assisted twist drilling VATD is another branch of vibration cutting that uses vibration in the drilling process. There are three directional modes of vibration that occur in the drilling process, namely, axial, lateral and torsional. The drill moves in these three directions when it is run on the surface of the workpiece. When it comes to composite laminates, the typical damage that has been recognized as the major damage when drilling is d elamination [27].
Fig. 1.2 Different design types of core drill bits (A) abrasive tools [25] and (B) hollow grinding [12].
Review of hole-making technology for composites5
Fig. 1.3 (A) Conceptual design of laser generated PCD core drill and (B) SEM image of laser generated tooth profile in PCD and profilometric measurement image of its geometry [26].
Most of the aviation industry uses conventional drilling, which employs continuous axial motion of the drill bit toward the workpiece. This continuous motion of the drill generates heat by the friction between the tool and workpiece, and the temperature rises as the drilling progresses. The VATD process uses a small-amplitude, low/high- frequency tool superimposed with conventional feed in an axial motion with controllable intermittent intervals. This allows frequent separation and contact between the tool and the chips, which reduces the contact area and leads to a decrease in frictional force [28]. Compared with conventional drilling, VATD has unique characteristics such as impacting, separating, changing speed and changing angle during the drilling process [29]. In general, this process interrupts continuous contact between the tool and workpiece and exhibits great potential in improving the cutting ability of a chisel edge and restraining the skidding of a chisel edge, reducing surface roughness, thrust force, and delamination, and extending the tool life while maintaining process productivity [30, 31]. It has been reported in some research that thrust force reduces by around 40% in VATD compared to conventional drilling. Therefore VATD is used to reduce delamination in drilling composite laminates [32].
1.2.1.5 Orbital drilling OD has great potential as an alternative to conventional drilling for minimizing damage associated with the drilling of composite laminates. OD is particularly effective for hole-making operations in laminate materials such as carbon fiber reinforced polymers (CFRPs) used in aerospace applications, which need precise dimensional accuracy and tight tolerances [33, 34]. The working principles of OD can be described in three basic motions: spindle rotation, feed and orbital rotation. Combining these motions creates spiral/helical rotation of the cutting tool. As the cutting tool spins in
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Hole-Making and Drilling Technology for Composites
its own axial direction, it simultaneously moves offset (in a lateral direction) to the desired hole diameter. By calculating the desired offset, a single cutting tool can be used to drill any diameter larger than the tool’s diameter [35]. Thus it essentially reduces the need for multiple tools to drill a single hole and eliminates the time needed for tool changeover. Moreover, since the tool diameter is substantially smaller than the diameter of the hole, it reduces the risk of the tool sticking out during the drilling process. During the OD process the tool is in partial contact with the workpiece, and this action enables the performance of heat extraction to become more efficient. Additionally, in the normal mechanism of other mechanical drilling the tool move straight concentric in an axial motion to the laminate, which results in high pressure at the center of the machine hole. However, in the OD process the tool moves in a helical feed motion, which reduces the pressure in the center of the hole while machining. This, in turn, reduces the matrix resin from burnout in the heat-affected zone [36]. These advantages of OD significantly decrease total cycle time thus increasing productivity and profitability [37]. Fig. 1.4 shows the working mechanism of the OD. The cutting tool material commonly used in OD is PCD and carbide end mills.
1.2.1.6 Milling Milling potential is well known and there are a number of publications describing its advantages. Use of circular milling began in 1995 by Park et al. [39]. Usually, circular milling will combine with other machining processes to achieve better results. For example, Schulze et al. [40] combined circular milling and spiral milling and studied the parameters of cutting velocity, tool feed, depth of cut and tool inclination angle. Regarding all parameters, feed rate is the most influential on delamination, where increases in feed rate will result in worse delamination or greater damage. Also in their research, they introduced wobble milling (Fig. 1.5) and found that this innovation will result in less damage compared to the combined milling process. Prior to that, Ali et al. [41] found that milling performed better compared to drilling in terms of
Fig. 1.4 (A) Tool motion in orbital drilling; (B) and (C) deflection of the last ply in conventional drilling (CD) and orbital drilling (OD), respectively [38].
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Fig. 1.5 Concept of wobble milling for hole-making in composites [40].
minimum surface roughness and minimum difference between exit and entrance hole diameters. This can be achieved at low feed rate and high cutting speed. Milling also performed on composite-metal stacking panel. For example Rahim et al. [42] found that feed rate and cutting speed is not significant to the roughness of the hole surface produced via milling on CFRP/Al stacks. In other work, Yagishita and Osawa [43] studied the CFRP/TiAl6V4 stack from the perspective of roundness and hole diameter. They found that newly developed hole-making machine according to double eccentric mechanism improved the quality of the hole. Similarly, Wang et al. [44] compared the hole-making process using helical milling of CFRP/Ti stacks and they found that tool wear will affect the cutting performance. Liu et al. [45] investigated the temperature variation of helical milling.
1.2.2 Nontraditional machining Moving toward future technology, which involves the machining of complex job profiles, hard/brittle materials, and the abrasive nature of the reinforcing phases, mechanical drilling might prove impractical for several reasons. These include extreme tool wear, incapability of machining a complex profile, poor surface finish quality and prohibitive economic costs. Thus unconventional machining methods have a huge potential over conventional/mechanical drilling for composite drilling. Generally, the unconventional machining technique is defined as a group of processes that removes excess material by various methods involving mechanical, thermal, electrical or chemical energy, or combinations of these energies, but does not use a sharp cutting tool as is necessary in traditional manufacturing processes. Among the many unconventional machining techniques, there are a few we will discuss in this chapter, namely, water jet machining (WJM), electro-discharge machining (EDM) and laser beam machining (LBM).
1.2.2.1 Abrasive water jet machining WJM has been used across industries and provides some advantages over conventional/mechanical drilling operations. Some of these advantages include no thermal
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Hole-Making and Drilling Technology for Composites
Pressurized water, p
Orifice, do
Traverse feed rate, v
Abrasive ma
Stand-off distance, s Focusing tube, df
Fig. 1.6 Process parameters in abrasive water jet machining (AWJM) [46].
effect, high machining versatility, high flexibility, small cutting force and high productivity. Also, WJM makes issues like burr formation and delamination in hole making on CFRPs almost negligible [46]. AWJM uses an erosion mechanism; a water jet of high pressure, high velocity and abrasive slurry cuts the target material by means of erosion [47]. Fig. 1.6 shows the schematic diagram of AWJM for hole-making on composite materials. There are few process parameters that affect the quality of the workpiece surface cut by AWJM, namely, hydraulic pressure, standoff distance, abrasive flow rate and types of abrasive [48]. Important quality parameters in AWJM are material removal rate (MMR), surface roughness, kerf width and tapering of the kerf. The abrasive particle used for AWJM is usually silicon carbide and aluminum oxide. The abrasive particle is embedded to the water jet purposely to increase the MMR of the process. Conventional/mechanical drilling uses friction and shearing to cut the workpiece. Alternatively, AWJM uses erosive action to cut the workpiece leaving a smooth, finished edge, less burr and no delamination. Since there is no physical contact between the cutting tool and workpiece, AWJM eliminates heat-affected zones and expensive cutting tools like diamond and others [49]. In research investigating the quality of the cutting hole in glass fiber reinforced plastic (GFRP), the author found that the quality of the hole in GFRP is highly dependent on the right choice of the cutting parameter in the cutting process [50]. Another researcher carried out the experiment using AWJM on composite laminates and found the delamination of the laminates can be reduced by reducing the jet speed. However, this also caused the piercing to deteriorate [51].
Review of hole-making technology for composites9
1.2.2.2 Electro-discharge machining EDM is also known as spark erosion machining. The process basically removes the workpiece material using electric spark. There are three types of EDM machining, namely, die sinking EDM, wire EDM and hole drilling EDM. Here we will cover basic concepts of hole drilling EDM, which is also called electro-discharge drilling (EDD) [52]. EDD is a hole-making process for electrically conductive workpiece materials, which harden to the machine. In addition to CFRPs, metal matrix composites (MMCs) are also growing rapidly in the aircraft manufacturing industry. Machining of MMCs using EDD is inevitable due to the material’s high hardness and wear-resistance properties. Conventional/mechanical drilling might not be a suitable choice for making a substantial number of holes in an MMC due to rapid tool wear. MMCs are composite materials with at least two constituent parts; one being a metal necessarily, the other material may be a different metal or another material, such as a ceramic or organic compound. When at least three materials are present, it is called a hybrid composite [53]. This makes drilling MMCs more difficult because each material has different properties. The working principles of this EDD can be explained clearly based on the schematic diagram shown in Fig. 1.7. The tool (metal electrode as a cathode) and the workpiece (acting as an anode) are placed very close together with a small gap (around 0.01–0.5 mm), separated by nonconducting liquid known as dielectric. As a pulse of DC electricity reaches the electrode and the part, an intense electrical field develops in the gap and microscopic contaminants suspended in the dielectric fluid are attracted by the field and concentrate at the field’s strongest point and build a high conductivity across the gap. As the field voltage increases, the material in the conductive bridge heats up and forms the spark channel between the tool and the workpiece [54]. As a
Fig. 1.7 Schematic experimental set-up of electro-discharge drilling (EDD) [52].
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Hole-Making and Drilling Technology for Composites
result, the workpiece erodes. Drilling process set-up is attached with servo system provide tool feed to keep the gap constant. The electrode used is usually copper, graphite, tungsten and brass. For dielectric fluid, kerosene is commonly used.
1.2.2.3 Laser beam cutting technology LBM is a form of machining in which a laser is directed toward the workpiece. LBM is one of the advanced manufacturing processes capable of machining all ranges of material from metallic alloys to nonmetals [55, 56]. Therefore LBM provides a solution to a critical problem that conventional/mechanical drilling is not capable of solving. Material removal in LBM is a thermal material removal process that utilizes a high- energy, coherent light beam to melt and vaporize particles on the workpiece in the focus point [57]. The advantages of LBM, such as improved end product quality, short processing time, noncontact process, cost reduction and small heat affected zone, have led to its use in many manufacturing industries [58]. For cutting composites, the general types of lasers used are CO2 and neodymium yttrium aluminum garnet (Nd:YAG) [59]. Fig. 1.8 shows the schematic diagram of the LBM removal mechanism. Composite laminates have more than one constituent material, which makes them difficult to laser machine because the components of the composites have varied thermal conductivity [59]. This means, in fiber-reinforced plastics, the power needed for a laser to vaporize (cut) the fiber (CFRP or GFRP) is much higher than the power needed for a polymer (epoxy). For this reason, it is important to set the laser cutting parameters (laser power, cutting speed, gas pressure, etc.) on the cut quality parameters (heat-affected zone, surface roughness, kerf width, taper angle etc.) carefully before starting LBM.
1.2.3 Nonmachining methods Other than machining, punching is another hole-making technology used to produce holes in composite panels.
Fig. 1.8 Laser cutting material removal mechanism [55].
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Puncher Composite panel
Die
Slug
Fig. 1.9 Principles of the punching process on composite panel [61].
1.2.3.1 Punching Based on preliminary study, the punching technique shows potential for hole-making on laminated composites [5, 60–63]. Punching is a forming process that uses punch and die to form holes via shearing. In composites punching, the mechanism for cutting holes is different from that of metal material since the composite is brittle. There are several cutting parameters affecting the punching of composites, namely, clearance, tool geometry, speed or stroke rate, blank holder force, sheet thickness, blank layout, material type, punch-die alignment and friction [64]. Punching in composites might result in poor surface quality due to the nonhomogeneity, multiphase structure and anisotropic nature of the composite [65]. Lambiase and Durante [66] investigated the influence of process parameters (punch-die clearance) on quality of punched holes (delamination factor, bearing strength) compared to drilled holes on thin GFRP laminates. The results showed that the punching force increases when punch-die clearance is decreased. They also noticed that the delamination factor increases when the punchdie clearance increases. They concluded that punching results in a higher delamination factor and lower bearing strength. One of the main advantages of punching is that it does not produce peel-up edge; in drilling, both peel-up and push-down edges occur. Furthermore the process is faster and produces less wear compared to drilling. The basic principles of punching are illustrated in Fig. 1.9.
1.3 Conclusions Hole-making technology can be divided into two main technologies: machining and nonmachining methods. The machining method can be further divided into traditional and nontraditional machining. Capability of the technologies is evaluated based on several parameters, for example, spindle speed and feed, drill bit diameter, drill bit geometry, drill bit material and type of composite material. In punching, for example, die clearance and puncher profiles are among the most influential parameters. All these parameters are important for ensuring high quality of the produced holes.
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Hole-Making and Drilling Technology for Composites
In hole-making, quality depends on the cutting method or tools used. In drilling, for example, wear is the main problem. In addition, other important factors such as productivity and technology cost also need to be taken into consideration.
Acknowledgments The authors want to acknowledge Universiti Sains Malaysia for their sponsorship through Research University Grant (Acc. No. 1001/PMEKANIK/814270) and to Mr. Fakruruzi Fadzil who helps in conducting the experiments.
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