Experimental Investigation of Cutting Parameters Effects on the Surface Roughness and Tools Wear during the Drilling of Fiber Reinforced Composite Materials

Optimization of the drilling parameters of the composite material is the key objective of this research, enhancing the surface roughness and minimizing the tool wear. In contrary to the other research, optimizing the machining parameters for a specific composite material for the mass productions, machining parameters are optimized for GFRP (Glass Fiber Reinforced Polymer), CFRP (Carbon Fiber Reinforced Polymer) and KFRP (Kevlar Fiber Reinforced Polymer) for the job shop production. In this research, the machining parameters are optimized for the enhanced surface roughness and minimum tool wear by varying the three types of composite material and three levels of the cutting speed. Nine experiments were performed, which were repeated twice in random manner to eliminate the biasness of the results. In these experiments, PVD (Physical Vapor Deposition) coated carbide inserts are used with the same geometry. Seventeen holes were machined in a single experiment, which surface roughness is measured by cutting the composite plate from middle of the hole and using the Countermatic surface roughness meter at different locations. Average surface roughness is determining for each set of varying parameters and plotted to observe the set of parameters for the minimum surface roughness. It has been observed that the minimum surface roughness are observed at; 1500 rpm in GFRP, 2000 rpm in CFRP and at 2500 rpm in KFRP. Finally, the wear patterns are also observed on the drill inserts using SEM (Scanning Electron Microscope) and it has been found that no prominent wear has been observed in the drill inserts, whereas, prominent depletion of coating are found at the higher cutting speed.


INTRODUCTION
C omposite materials are the combination of two or more materials, combined to provide better mechanical properties than the individual ones.
In the past, this technology was found in making composite bricks from straws and clay. The force behind the increased application of composites is the ability to tailor the materials to the requirements and the superior properties which are gained after the combination of

Experimental Investigation of Cutting Parameters Effects on the Surface Roughness and Tools Wear during the Drilling of Fiber Reinforced Composite Materials
two more materials. High specific stiffness and strength along with the greater dimensional stability under a wide range of atmospheric conditions including temperature which is essential in exotic applications, such as; aircraft, high speed cars, wind turbines, satellite bodies, equipment casing in rockets, high pressure fuel tanks, bullet proof armors, helmets etc. Composite materials have high strength to weight ratio as compared to conventional materials, hence used a lot in aerospace industry. Moreover, sensors can also be embedded inside the body to monitor the stresses on the critical areas of the aircraft and other applications [1]. Composite material is usually made of matrix and reinforcement materials that are mixed in certain proportions depending on the requirement. Two or more constituent materials have their own properties which exhibit desired properties when combined together which is the most important aspect of composite due to its tailor ability.
Reinforcements such as fibers, particles and whiskers have great strength and stiffness. When these materials are combined with weaker matrix (usually known as matric material), a composite with lesser properties than the actual fibers are however, produced but the overall composite properties in general will have considerable difference to that of conventional matrix materials. The most common type of matrix materials used in the composite materials are; epoxy, polyester, and phenol formaldehyde resins, whereas, glass, carbon, Kevlar or organic long fibers are majorly used to achieve the favorable mechanical and especially thermal properties [2].
Hole and some feature in composite based parts are usually performed after the curing of composite material, since it is difficult to embed some minute and small feature into the cavity, which may also result in developing critical stress in the parts. Due to heterogeneity of composite materials, the machining of composites is the prime concern. Drilling is one of the main machining operations probably most widely applied to polymeric matrix composites owing to the need to assemble components, produced mainly as laminates, through riveting and bolting. For example, 100,000 holes are required in a small engine aircraft and millions of holes are necessary for a larger one [3]. Composite  also reported to be depended upon the fiber width [8].
In another study, AFRP (Aramid Fibre Reinforced Polymer) was drilled with different machining parameters to study the effect of these parameters on the tool life. Conclusion of the research was vague and defines that the larger rake angle with constant edge radii reduces the cutting force. It does not show the increased tool life and productivity of the process [9]. In a research, a prehole drilling process is performed chisel length of drill is also varied to observe the drill delamination [10]. In another research, drilling on GFRP's, manufactured with hand layup process, was performed to develop a relationship between the cutting velocity and feed rate under the constant thrust force, cutting pressure and surface roughness [11]. In a comprehensive review, machining parameters and influence of these on the thrust force and torque are studied. In addition, quality of the holes and delamination damage are also discussed [12]. A group of researchers combined the stack of titanium, CFRPs and aluminum to study the quality of holes produced using different machining parameters. They concluded that the delamination was least in the CRFP as compared to the standalone machining [13]. Numerical Similar research was performed using the coated and uncoated tool to study the same effects, and the obvious result was found that is the coated drills are better than the uncoated ones [18]. Another interesting work has been performed by a group of researchers performing gundrilling process on a thick composite material plate. In this research, critical zone in the plate thickness was found which cause the delamination in the drill tool [19]. Another research on thick epoxy based composite was performed, and it was analyzed

Experimental Investigation of Cutting Parameters Effects on the Surface Roughness and Tools Wear during the Drilling of Fiber Reinforced Composite Materials
of vibrations on the surface roughness has been neglected which is very minor since the machine, spindle and tool are designed for the machining of aerospace grade metals. Hence the vibration in the system due to epoxy based composites are minors.

EXPERIMENTAL SETUP
As mentioned earlier, the core objective of the research is to determine the optimal set of drilling parameters for the  [20].  After drilling the holes in the composite materials, the workpieces were cut in half so that half of the drilled hole can be seen as the front view is shown in Fig. 2.
Wilson Wolpert's Conturo Matic T2 Surface roughness meter is shown in Fig. 3

RESULTS AND ANALYSIS
Surface roughness values are measured and the data is analyzed. Roughness value for each material is plotted against spindle speed for drilling the samples in order to correlate these two parameters. It also leads to the result about optimal value of spindle speed that will bring about the better surface finish of the drilled work pieces.  (Fig. 6). From Fig. 6, it can also be observed that the highest surface roughness after the machining was observed in the KFRP, which is mainly due to the thin and soft nature of the Kevlar fibers, whereas, the least surface roughness is measured in the GFRP, due to the brittleness of glass fiber.
Figs. 714 presents the comparison among the different images of drilling inserts obtained after the machining of the holes, whereas, a fresh drill insert is shown in Fig. 7 to compare the image with the used inserts.   It can be observed from the images that the drill inserts which machined the CFRP at 2000rpm had their coating depleted but also the builtup edges are formed at the face of the tool causing the lowest surface roughness at this spindle speed in the CRFP workpiece (Figs. 10-11).
While machining CFRP at 2500rpm, it has been observed that the builtup edges were not formed but the PVD coating was depleted highly at the cutting edges, whereas, flank was severally depleted by the rubbing action against the CFRP at high cutting speed. The CFRP material also stuck with the relief surface while machining with 2000rpm.
In Kevlar, it was observed that the cutting edge and face was not affected by the drilling process with having spindle speed of 1500 and 2000 (Figs. 12-13), however, the flank coating was depleted due to the rubbing action ( Fig. 12). Although, the sever depletion of PVD coating was observed while machining the KFRP at 2500rpm (   Fig. 14). It can be seen from the figure that the face and flank are naked from the PVD coating but the composite material was not at all sticken with any of the face.

CONCLUSION
The objective of this study is to evaluate the effects of machining parameters of drilling, with the same tool and cutting parameters on three composite materials (GFRP, CFRP and KFRP) and then selection of optimal values for drilling achieving better surface finish. Samples of 8 mm thickness are machined (drilled) using 13