159 CHAPTER 6 RESULTS AND DISCUSSION Composite materials are widely used in different fields due to their excellent properties. CFRP composite plates are used in many applications such as aerospace, defense, biomedical, sports, automobiles, structural members and so on, especially in light weight applications that require a high stiffness and rigidity. The varying structures of these applications require different stages of manufacturing in order to bring it to a near-net shape. Drilling is one such manufacturing process that joins different structures rigidly. However, drilling induces various damages in CFRP composites that need to be minimized. Hence a detailed study on drilling of CFRP composites is required. In the present study, the process parameters and tool parameters that influence the damage on the drilled holes are to be analyzed so that a proper selection of these parameters will minimize the damage encountered during drilling operation. 6.1 INTRODUCTION In the present study, CFRP composite plates are manufactured through hand lay-up technique and the drilling experiments are carried out on a CNC machine under varying levels of the process parameters viz. spindle speed and feed rate. Since, it is evident from the literature survey that the tool parameters including the tool material and tool geometry plays a vital role in producing good quality holes, three different tool materials are chosen viz. HSS, carbide and PCD coated drills each with three different point angles as, 118 and 135. Three levels of spindle speed and feed rate are
160 chosen and experiments were carried out based on Taguchi s L 27 orthogonal array of experiments and is carried out thrice for repeatibility. There are several responses that characterize the quality of holes including thrust force, torque, delamination, eccentricity, surface roughness and so on. In the present study, six responses are observed to study the performance of drilling operation to achieve good quality holes. The responses include thrust force, torque, entry-delamination, exit-delamination, eccentricity and surface roughness. The results and discussions based on this experimental study are presented in this chapter. CFRP composite structures are widely used in a variety of applications owing to their superior mechanical properties and hence machining of CFRP is of prime importance. In spite of the fact that in general, composites are produced to near-net shape, an additional machining operation is often required. Of various additional machining operations, drilling is indispensable and is used to fasten structures of different assemblies. However, drilling CFRP composites is a challenging operation, as it faces numerous problems such as delamination at the drill entry and drill exit, spalling, fuzzing, fiber pull-out and matrix cracking. Zhang et al (1) investigated the formation of exit defects in CFRP composite plates and characterized their features in terms of drilling conditions. It was found that spalling and fuzzing are the major mechanisms of exit defects. The degree of spalling may be minimized by arranging aluminium or bakelite plate under the CFRP composite plate subjected to drilling. In addition, it is necessary to control the thrust force, especially at the stage when the chisel edge is penetrating the exit surface of a CFRP composite plate. In order to reduce the damage and improve the quality of holes, it is necessary to investigate the mechanism of drilling in CFRP composite plates.
161 6.2 MECHANISM OF DRILLING IN CFRP COMPOSITES Drilling of composite materials is an important operation. Maintaining proper surface finish in the drilled holes is an important concern. In addition, delamination associated in drilling is to be reduced and is a prime concern in machining of CFRP composites. The delamination in drilling can be reduced by reducing thrust force and torque in drilling of composites. In CFRP composites drilling, selection of tool materials is very important for any machining applications. The tool materials are required to have high hardness to withstand high temperatures, high toughness for the impact forces on tool in interrupted operations, good wear resistance for increased tool life, low cost for maximizing production and chemical stability or inertness to avoid adverse reactions. The CFRP composite plates used as work piece for drilling has high strength-to-weight ratio and stiffness-to-weight ratio, high toughness and generally combination of properties superior than metals, ceramics or polymers. To suit the challenging needs of the CFRP composites, there are a variety of tool materials available viz. carbon and medium alloy steels, HSS, cast cobalt alloys, carbides, coated tools, diamond tools, and so on. Carbon and medium alloy steels are the oldest of tool materials. They are inexpensive and can be easily shaped and sharpened. But hardness and wear resistance are not sufficient enough for drilling CFRP composite structures. Also they are limited to low speed operations. HSS tool materials are commonly used alloy steel for various machining operations owing to its high hardness at various depths and good wear resistance. They are suitable for high positive rake angle tools. It possesses high toughness to suit the demands of CFRP composite machining. Cast cobalt alloys are commonly known as stellite tools. It has high wear resistance and hardness. However it is less tough than HSS and sensitive
162 to impact forces and hence less suitable than HSS for interrupted cutting operations. It is found to be good for deep boring and continuous turning applications where it is better suited than HSS. Most of the HSS and cast alloys have very low high temperature hardness and low life for high speed machining. Hence carbides are preferred that has high temperature hardness, low thermal expansion and high modulus of elasticity. Two groups of carbides are used for machining operations, viz. tungsten carbide and titanium carbide. Tungsten carbides are used for machining non ferrous abrasive metal and cast iron. Tungsten carbide particles are bonded in cobalt matrix. The amount of cobalt present affects properties of carbide tools. As cobalt content increases, the hardness and wear resistance increases. Titanium carbide has higher wear resistance than tungsten carbide but poorer toughness and is used for very high speed operations. Hence tungsten carbides are preferred in the present study. Coated tools have lower friction, high resistance to cracks and wear, high cutting speeds, low cost and longer tool life. They have higher strength and toughness but generally abrasive and sometimes chemically reactive with tool materials. A variety of coating materials are used Titanium Nitride, Titanium carbide, Titanium carbonitride, Aluminium oxide and polycrystalline diamond. In the present study, polycrystalline diamond coated tools are preferred because of significant improvement in tool life, improved hole quality, sharper cutting edge, reduced thermal impact, optimum thermal conductivity of the cutting tool material, minimized risk of delamination and dry machining conditions. Though diamond tools have superior characteristics, owing to its high cost, it is not commonly preferred tool for machining applications. Thus, for drilling CFRP composite plates in the present study the following three tool materials are used:
163 HSS drills Carbide drills PCD coated drills Twist drills are most commonly used drills as the chip comes out from it automatically. Holes drilled by this drill are accurate and of good finishing. They are available in almost all sizes. Hocheng and Tsao (3) presented a comprehensive analysis of delamination in use of various drill types, such as saw drill, core drill and step drill. The critical thrust force at the onset of delamination is predicted and compared with the twist drill. It was found that the performance was almost equal to that of twist drills. Tsao and Hocheng (4) made a Taguchi analysis of delamination associated with various drill bits in drilling of composite material. The delamination factors calculated for twist drill, saw drill and candle stick drill shows that there is an improvement in saw and candle stick drills, but the improvement is not much superior. Also, comparatively saw drills are applicable to thin plates only. Hence owing to the simplicity, twist drills are used in this experimental study for drilling of CFRP composite plates. The various drill angles in a twist drill are point angle, rake angle, chisel edge angle and clearance angle. Point angle is the included angle between the cutting edges. This angle varies according to the hardness of the material to be drilled. The point angle commonly used for drilling is 118. Twist drills are made with different helix angle. Rake angle is angle of the flute (helix angle). The helix angle determines the rake angle at the cutting edge. The web/chisel edge angle is the angle between the chisel edge and the cutting lip. The clearance angle is to prevent the friction of the tool behind the cutting edge. This will help in the penetration of the cutting edge into the material. If the clearance is high, the cutting edge will be weak and if the clearance is too small, the drill will not cut. In the present study, point angle is varied as a tool parameter.
164 Chen (1997) observed that the torque decreases with increasing point angle because the tool orthogonal rake angle at each point on the primary cutting edge increases with increasing point angle. On the contrary, the thrust force increases with increasing point angle. Therefore, in order to decrease the thrust force, the smaller point angle is a good choice for drilling of CFRP composite materials. Durao et al (2010b) evaluated five different drilling tool geometry including a twist drill for drilling reinforced composites and identified that delamination factor is found to be minimum for holes drilled with twist drill. The other drill types used in the study are twist drill with 85 point angle, Brad drill, Dagger drill and customized step drills. Velayudham and Krishnamurthy (7) studied the effect of point geometry and their influence on thrust and delamination in drilling of polymeric composites. Observations on thrust indicate a critical feed rate of 0.01 mm/rev and above for which there is a rapid increase in thrust force. Thus in the present experimental study, the twist drills with three different point angles are used to study the effect of low and high point angles as follows: 118 135 The drilling of CFRP composite material involves six different stages. The various stages of the drilling operation is shown in Figure 6.1. The six different stages in drilling of CFRP composites is explained as follows: 1. In stage 1 shown in Figure 6.1(a), the drill tool just touches the work piece. The drilling is to be carried out in the axial direction. This plane of the work piece is referred as entry side of the drill tool.
165 2. In stage 2 shown in Figure 6.1(b), the tip of the drill tool pierces into the work piece. 3. In stage 3 shown in Figure 6.1(c), the drill tool advances into the work piece to produce a hole and thereby breaks the adjacent layers of the CFRP composite plates. (a) Stage 1 (b) Stage 2 (c) Stage 3 (d) Stage 4 (e) Stage 5 (f) Stage 6 Figure 6.1 Six stages of drilling operation
166 4. In stage 4 shown in Figure 6.1(d), the drill tool touches the last layer of the CFRP composite plate and is about to come out of the plate. 5. In stage 5 shown in Figure 6.1(e), the drill tip has come out of the work piece and a hole is produced in the CFRP composite plate. This plane of the work piece is referred as exit side of the drill tool. 6. In stage 6 shown in Figure 6.1(f), the drill tool advances into the work piece and reaming takes place. In order to reduce the damage, the mechanism of drilling is to be studied in detail as it is a major concern in drilling of CFRP composite plate. The delamination damage is characterized by the separation of adjacent plies caused by an external action. It depends not only on fiber nature but also on resin type and respective adjacent properties of plies. Delamination in drilling is a consequence of the indentation force exerted by the drill chisel edge stationary center that acts more as a pierce than as a drill. Khashaba et al (7) explained two mechanisms of delamination associated with drilling viz. peel-up at the entrance and push-out at the exit. Peel-up occurs as the drill enters the laminate and is shown schematically in Figure 6.2. After the cutting edge of the drill makes contact with the laminate, the cutting force acting in the peripheral direction is the driving force for delamination. It generates a peeling force in the axial direction through the slope of the drill flute that results in separating the laminas from each other forming a delamination zone at the top surface of the laminate. This mechanism is known as peel-up at the entrance delamination. The mechanism of push-out delamination is shown in Figure 6.3. The laminate under the drill tends to be drawn away from the upper plies,
167 Figure 6.2 Peel-up at entrance (Persson et al, 1997) breaking the inter-laminar bond in the region around the hole. As the drill approaches the end of the laminate, the uncut thickness becomes smaller and the resistance to deformation decreases. At some point, loading exceeds the inter-laminar bond strength and delamination occurs. Durao et al (8a) identified that Push-out delamination caused by this piercing action can be reduced if thrust force during drilling is minimized. Figure 6.3 Mechanism of Push-out delamination (Hocheng and Dharan, 1990) In the present study, CFRP drilling is carried out by using different tool materials and point angles. Table 6.1 shows the drilled holes using point angle drills made of HSS, carbide and PCD coated drills at three different feed rates. It is observed that the drilled holes at lower feed rates produces better quality of holes for all drill tool materials. On comparing the holes drilled with different drill tool materials, it is observed that PCD coated and carbide drills gives better results than HSS drills.
168 Table 6.1 Typical drilled holes using point angle drill at mid-level speed Feed rate in mm/min HSS drill Carbide drill PCD coated drill Table 6.2 shows the drilled holes using 118 point angle drills made of all the three different materials and Table 6.3 shows that of 135 point angle used in the present study. On comparing these drilled holes with that of Table 6.1, it is observed that low point angle drills gives better results. Therefore, low point angle and low feed rate are desirable for producing good quality of drilled holes.
169 Table 6.2 Typical drilled holes using 118 point angle drill at mid-level speed Feed rate in mm/min HSS drill Carbide drill PCD coated drill Figure 6.4 and Figure 6.5 shows a typical thrust force and torque observed from Kistler dynamometer. The six stages of drilling are represented in these figures and the drilling operations are explained below: 1. In the first stage of drilling, the drill tool touches the CFRP composite plate. The thrust force and torque are maintained at their initial level.
170 Table 6.3 Feed rate in mm/min Typical drilled holes using 135 point angle drill at mid-level speed HSS drill Carbide drill PCD coated drill 2. In the second stage of drilling, the drill tool pierces the top lamina of the CFRP composite plate. The thrust force increases rapidly as the drill tool removes the material. The torque too increases but the increase is not much significant. 3. In the third stage of drilling, the drill tool breaks the bottom ply. Hence maximum value of thrust force and torque is observed in this stage. 4. In the fourth stage of drilling, the drill just comes out of the CFRP composite plate and the material removal occurs. Hence there is a decrease of thrust force and torque observed in this stage.
171 Thrust force, N 90 80 70 60 50 40 30 20 10 0-10 3 2 4 5 6 1 8 9 10 11 12 13 Time [s] Figure 6.4 A typical Thrust force obtained from Kistler dynamometer Torque, Nm 1.2 1.0 0.8 0.6 0.4 0.2 Zoom on 3 4 2 5 6 1-0 4-0.2 5 6 7 8 9 10 11 12 1-0.4-0.6 Time [s] Figure 6.5 A typical Torque obtained from Kistler dynamometer 5. In the fifth stage of drilling, the tip of the drill tool comes out of the CFRP composite plate. The thrust force and torque gradually decreases. 6. In the sixth stage, reaming takes place and hence thrust force drops down to almost zero.
172 The typical defects encountered in drilling of CFRP composite plates are shown in Figure 6.6. Figure 6.6(a) shows delamination at the entry side of drill tool and Figure 6.6(b) shows delamination at the exit side of drill tool. It is seen that there is no definite round hole that is obtained. There is a removal of material that takes place around the circumference of the hole. Figure 6.6(c) shows uncut fibers that are protruding and remain in the work piece material after drilling. The image also shows the fiber pull-out in the drilled holes. Figure 6.6(d) shows the exit defects, viz. spalling and fuzzing which co-exists in the drilled holes and is developed due to the chisel edge action phase and cutting edge action phase. Delamination at the drill entrance Delamination at the drill exit 20X 20X (a) (b) Uncut fibers Fuzzing Fiber pull-out Spalling (c) 20X (d) 20X Figure 6.6 Damages observed in drilling of CFRP composite plates
173 Figure 6.7 shows a typical scanned electron microscope (SEM) image on the surface of a drilled hole in CFRP composite plate. The fiber pull-out and excessive resin are observed in this SEM image. A higher magnification SEM image shown in Figure 6.8 shows the improper distribution of resin observed in the surface of a drilled hole. Figure 6.9 shows the typical voids observed and Figure 6.10 shows the carbon fiber dust particles on the surface of a typical drilled hole in the CFRP composite plate. Figure 6.11 shows matrix cracking observed in the drilled hole due to excessive force. Any defects that lead to the rejection of the parts represent an expensive loss. Delamination is one of the major damage that affects the use of composite materials for structural applications. For example, in the aircraft industry, drilling associated delamination accounts for 60% of all part rejections during final assembly of aircraft (Stone and Krishnamurthy, 3). The economic impact of this rejection is significant considering the value associated with the part when it reaches the assembly stage. The quality of the drilled holes such as roughness of its wall surface, axial straightness and Fiber pull-out Excessive Resin Figure 6.7 SEM images with fiber pull out and excessive resin
174 Improper distribution of Resin and fiber Figure 6.8 Improper distribution of resin Void Figure 6.9 Typical Voids observed roundness of the hole can cause high stresses on the rivet, which will lead to its failure. Stress concentration, delamination and micro-cracking associated with machined holes significantly reduce the composites performance.
175 The quality of drilling holes is tested by measuring or calculating several responses that are related with the drilling parameters. The chosen responses may differ in their characteristics such as lower-the-better or higher-the-better. A detailed study of these drilling parameters on the various responses would lead to proper selection of the drilling parameters that in turn produce holes of good quality. Machined fibers stick to the surface Figure 6.10 Typical dust particles observed on the machined surface Matrix cracking Figure 6.11 Matrix cracking observed on the machined surface
176 6.3 PARAMETRIC INFLUENCE IN DRILLING OF CFRP COMPOSITES In the present study, drilling experiments are carried out at three levels of drilling parameters as detailed in Table 3.2. The drill tools used in the present study are of 6 mm diameter and made of HSS, carbide and PCD coated materials. The responses chosen in the present study includes eccentricity and surface roughness apart from the essential responses viz. thrust force, torque and delamination. Each of these responses has lower-the-better characteristics. A good quality hole can be obtained with a low value of each of these responses. However, it is not possible to obtain the lowest values for each of the response for any particular experiment. Hence the optimum drilling conditions are to be identified at which the combination of low values of these responses is comparatively better than other drilling conditions. In the present study, grey fuzzy approach is used as optimization technique to obtain the optimum drilling conditions. 6.3.1 Effect of drilling parameters on thrust force Thrust force has been widely cited as the main cause of delamination. Hence a lower value of thrust force would result in a good quality of hole with less amount of delamination. A sample of thrust force measurements obtained from Kistler dynamometer is shown in Figure 6.12 for various drill tools for a typical drilling condition. It is observed that thrust force varies according to six stages of drilling operation explained earlier in section 6.2. Similarly the values are observed for all experimental conditions with three drill tool materials and are tabulated in Table 3.9 to Table 3.11. The thrust force obtained under different drilling conditions is plotted with respect to spindle speed, feed rate and point angle. Figure 6.13
177 Zoom on Thrust force, N 80 60 40 20 0 6.0 6.2 6.4 6.6 6.8 7.0 7.2 7.4 7.6 7.8 8.0-20 Time [s] (a) HSS drills Thrust force, N Zoom on 90 80 70 60 50 40 30 20 10 0-106.0 6.2 6.4 6.6 6.8 7.0 7.2 7.4 7.6 7.8 8.0 Time [s] (b) Carbide drills Thrust force, N Zoom on 90 80 70 60 50 40 30 20 10 0-108.8 9.2 9.6 10.0 10.4 10.8 11.2 11.6 12.0 12.4 12.8 13.2 Time [s] (c) PCD coated drills Figure 6.12 Typical thrust force plots observed in Kistler dynamometer for v=0 rpm, = and f= mm/min
178 shows the variation of thrust force with respect to spindle speed with the other two drilling parameters viz. feed rate and point angle maintained at their mid values. The variations are plotted for the experiments conducted with the three drill tool materials. The variations show that thrust force is at a lower value for high spindle speed condition. This is due to the fact that there is an increase of heat produced at high spindle speed that assists drilling operation. A low coefficient of thermal conduction and low transition temperature of plastics in combination with increase of heat leads to a low value of thrust force at high speed of 0 rpm in the present study. An increasing cutting speed will certainly increase production rate. It is also observed that the experiments conducted with PCD coated drill tool have produced lowest thrust force as compared to carbide and HSS drills. Figure 6.14 shows the variation of thrust force with point angle with spindle speed and feed rate maintained at their mid values. The variations are plotted for three drill tool materials HSS, carbide and PCD coated. The variation shows that thrust force increases with increasing point angle. This increase in thrust force with respect to point angle is due to the fact that as the point angle increases an elevation in the cutting edge angle observed. Therefore, in order to decrease the thrust force, the smaller point angle is a good choice for the drilling of CFRP composite materials. It is also observed that the experiments conducted with PCD coated and carbide drill tools have lower values of thrust force as compared to HSS drills. Figure 6.15 shows the variation of thrust force with feed rate with spindle speed and point angle maintained at their mid values. Thrust force increases with increasing feed rate because of the fact that the shear area is elevated. In addition, as the feed rate increases, self generated feed helix angle
179 250 Thrust force in N 150 50 HSS Carbide PCD coated 0 Point angle = 118 ; Feed rate = mm/min 0 0 Spindle speed in rpm Figure 6.13 Variation of thrust force with spindle speed 250 Thrust force in N 150 50 HSS Carbide PCD coated 0 Spindle speed= rpm; Feed rate= mm/min 118 135 Point angle in degrees Figure 6.14 Variation of thrust force with point angle Figure 6.15 Variation of thrust force with feed rate
180 increases (Shaw, 1992) which reduce the effective clearance angle. This leads to rubbing of cutting wedge with work material and corresponding increase in thrust force. It is also observed that the experiments conducted with PCD coated drill tool produces minimum thrust force as compared to carbide and HSS drills. From the above discussion, it is concluded that a high speed of 0 rpm, low point angle of and a low feed rate of mm/min is the best combination to obtain a minimum thrust force. 6.3.2 Effect of drilling parameters on torque The moment of the cutting force in the z-direction commonly referred as toque also influences the quality of the hole produced. A sample of torque observed from Kistler dynamometer for various drilling conditions is shown in Figure 6.16. It correlates with various stages of drilling as explained in section 6.2. The variation of torque with spindle speed while point angle and feed rate are held at their mid values is shown in Figure 6.17. In the experiments conducted using HSS drill tools, torque decreases consistently with an increase in spindle speed. This is due to the fact that there is a possible softening of the work material due to the increase in temperature at higher spindle speeds. At this higher temperature, the softened work piece acts as a solid lubricant that reduces the friction with the drill tool and thereby decreases the resistance to drilling. Thus a lower value of torque is produced at higher cutting speeds. However, a fluctuation in torque is observed and the lowest value of torque is observed in the experiments using carbide drills. Figure 6.18 shows the variation of torque with point angle while spindle speed and feed rate are maintained at their mid values. In these experiments, the torque (tangential force) decreases with increasing point angle because the tool orthogonal rake angle at each point on the primary cutting edge increases with increasing point angle. On the contrary, thrust force increases with increasing point angle. This decrease in torque is
181 Torque, Nm Torque, Nm Zoom on 7 6 5 4 3 2 1 0 4-1 5 6 7 8 9 10 11 12 Time [s] Zoom on 1.2 (a) HSS drills 1.0 0.8 0.6 0.4 0.2-0 4-0.2 5 6 7 8 9 10 11 1-0.4-0.6 Time [s] 1.6 1.2 Zoom on (b) Carbide drills Torque, Nm 0.8 0.4 0 8 10 12 14 16 18 20 22-0.4-0.8 Time [s] (c) PCD coated drills Figure 6.16 Typical plots of torque observed in Kistler dynamometer with v=0 rpm, = and f= mm/min
182 Figure 6.17 Variation of torque with spindle speed Figure 6.18 Variation of torque with point angle prominent in HSS drills as compared to PCD coated and carbide drills and it is also observed that the three drill tool materials produce low value of torque with only a small variation at a point angle of 135 and the lowest value of torque is observed for the experiments conducted using carbide drills.
183 Figure 6.19 shows the variation of torque with feed rate while spindle speed and point angle are fixed at their mid values. Similar to the observations made in thrust force variations with respect to feed rate, minimum value of torque is produced at lower values of feed rate for all the drill tools used in the experiments and the lowest value of torque is observed for the experiments conducted using carbide drills. Thrust force and torque are influenced to a larger extent by feed rate as compared to spindle speed and point angle. It is also observed that the experiments conducted with carbide drill tools produce low values of torque followed by PCD coated drills and then by HSS drills. Thus a combination of high spindle speed, high point angle and low feed rate produces minimum torque thereby producing minimum damage in the drilled holes. Though the process parameters remain at the same level for a minimum value of thrust and torque, the point angle requirement to produce a minimum torque differs. Figure 6.19 Variation of Torque with feed rate
184 6.4 OBSERVATION OF DRILLED HOLES The superior and unique properties of CFRP composite materials lead to numerous applications in various fields such as aerospace, defense, biomedical, transport, structural applications, etc. Though near-net shape is possible, various machining operations are required to meet out the close dimensional accuracy of the assembly requirements. Drilling is one of the essential operations that join different parts together. Drilling CFRP composite materials poses great challenge in the form of various damages of which delamination is a major damage. In the present study, the CFRP composite plates are drilled using HSS, carbide and PCD coated drills under three levels of spindle speed, point angle and feed rate conditions. The images of the drilled holes using HSS drills are shown in Table 6.4. The drilled holes show certain imperfections like fiber pull-out and delamination at the entry and exit planes. The delamination is due to thrust force in the axial direction through the drill tool. It is observed that the variations in the drilling parameters have a marked influence on the drilled holes. It is seen that high spindle speed, low point angle and low feed rate provides good quality of holes as compared to other drilling conditions. In order to have a clear idea on the quality of the drilled holes, a study of various responses are included, viz. delamination at the entry and exit of the drill tool, eccentricity and surface roughness. Table 6.5 shows the images of the drilled holes using carbide drills and Table 6.6 shows that of PCD coated drills. It is seen that the quality of the drilled holes are better in the case of holes drilled using PCD coated drills than that of carbide and HSS drills. Delamination is one of the major damage caused in drilling of CFRP composites. Delamination occurs at the entry and the exit sides of the CFRP composite plate. Delamination causes separation of layers from each other. This results in loss of stiffness, strength and expected
185 Table 6.4 Images of drilled holes using HSS drill tool Point Spindle angle Speed in degrees in rpm Feed rate in mm/min 0 0 0 118 0 0 135 0
186 Table 6.5 Images of drilled holes using carbide drill tool Point Spindle angle Speed in degrees in rpm Feed rate in mm/min 0 0 0 118 0 0 135 0
187 Table 6.6 Images of drilled holes using PCD coated drill tool Point Spindle angle Speed in degrees in rpm Feed rate in mm/min 0 0 0 118 0 0 135 0
188 life. Hence it is necessary to minimize this form of damage. The extent of the damage is determined by its drilling conditions. An optimal drilling condition will result in minimum value of delamination factor. In the present study, damage in the form of delamination is quantified in the form of delamination factor as given in Equation (3.1). The delamination factor is calculated for the damage at the entry and exit side of the CFRP composite plate. The maximum diameter of the drilled hole is measured using Coreldraw software that covers the damage area. A circle is drawn covering the damage in the surrounding area and the diameter of the circle is noted from the Coreldraw software. Thus the maximum diameter of the drilled hole is obtained and using Equation (3.1) the delamination factor is calculated for each drilled hole. The delamination factors, listed in section 3.7 is calculated by taking the average value of the delamination factors of three holes drilled under same drilling conditions. It is observed that the exitdelamination is more compared to that of entry-delamination for the same drilling conditions. It is seen from these images that higher the spindle speed, lower the feed rate and lower the point angle, the delamination decreases. On comparison of the drilled holes, it is identified that the delamination is less for the holes drilled with PCD coated drills as compared to the holes drilled with carbide and HSS drills. 6.4.1 Effect of drilling parameters on entry-delamination The delamination is very much dependent on the induced thrust force during drilling and varies almost linearly while drilling CFRP composite plates. The delamination factor has a tendency to reduce with increased spindle speed, thus justifying high speed drilling. Figure 6.20 shows the variation of entry-delamination with the spindle speed while maintaining point angle and feed rate at their mid values. In these experiments, it is found
189 that entry-delamination reduces with increasing spindle speed. This behavior can be explained by the fact that increasing spindle speed leads to higher temperatures, which promotes the softening of matrix and thus diminishing the delamination. It is also observed that the experiments conducted using PCD coated drill tools produces minimum value of entry-delamination as compared to that of HSS and carbide drills. Figure 6.21 shows the variation of entry-delamination with point angle while spindle speed and feed rate are maintained at their mid values. In these experiments, it is observed that the entry-delamination factor is small at lower point angles. This is due to the fact that at low point angle the thrust force reduces, which in turn minimizes the delamination damage. As the trend of thrust force with respect to point angle increases, the same is expected in the case of entry-delamination too. However, fluctuations of entrydelamination factors are observed for an increasing point angle. It is also observed that the experiments conducted using PCD coated drills produce minimum value of entry-delamination factor. Figure 6.22 shows the variation of entry-delamination with feed rate while spindle speed and point angle are at their mid values. The variations of entry-delamination factor follow almost similar pattern as that of thrust force with respect to feed rate. The entry-delamination factor shows increasing trend with an increase in feed rate. In general, at high feed rates the built-up edge is normally formed on tool due to more heat generation, resulting into high tool wear and hence corresponding increase in delamination. However, the exact tendency of delamination behavior with variation of one factor is also governed by the constant values of other factors. It is observed that a low value of entry-delamination factor is obtained for experiments conducted using PCD coated drills. Thus it is concluded that a high spindle speed and low feed rate produces minimum entry-delamination irrespective of point angle variations. It is preferable to use PCD coated drills to obtain minimum value of entry-delamination.
190 Figure 6.20 Variation of entry-delamination with spindle speed Figure 6.21 Variation of entry-delamination with point angle Figure 6.22 Variation of entry-delamination with feed rate
191 6.4.2 Effect of drilling parameters on exit-delamination Generally, the values of exit-delamination factors are higher than that of entry-delamination factors for the experiments with the same drilling conditions. However, the trend graph remains the same for the delamination at entry and exit sides of the CFRP plates. Exit defects are characterized by spalling and fuzzing (Zhang et al, 1). The spalling defect is due to chisel and cutting edge actions and fuzzing is due to the included angle between the fiber directions at the surface layer. Figure 6.23 shows the variation of exit-delamination factor with spindle speed while feed rate and point angle are maintained at their mid values. It is observed that there is a decrease in the exit-delamination factor as the spindle speed increase that leads to a decrease in thrust force, reducing the damage. It is also observed that the experiments conducted using PCD coated drills produce minimum value of exit-delamination factor. Figure 6.23 Variation of exit-delamination with spindle speed
192 Figure 6.24 shows the variation of exit-delamination with point angle as the spindle speed and feed rate are maintained at their mid values. It is apparent that the exit-delamination is due to the bottom ply or plies of a laminate peel away from the rest of the laminate and is attributed to the force of the drill, which pushes the layers apart rather than cutting through them. Thus a pointed drill bit can prevent delamination and fiber break-out because of the lower thrust forces on the final plies of the laminate. The variation in the plot shows that the exit-delamination factor increases with point angle. It is further observed from these experiments that PCD coated drills produces minimum exit-delamination factor. Figure 6.24 Variation of exit-delamination with point angle In order to improve the hole quality at the drill exit, the feed rate needs to be decreased during the drilling process. When drilling a CFRP plate with a HSS drill, it has been found that the spalling damage increases with the feed rate but decreases with the spindle speed. However, the effect of the feed rate is often greater than that of the spindle speed. In addition, to reduce the spalling, it is necessary to control the thrust force, especially at the stage when the chisel edge is penetrating the exit surface of a CFRP plate.
193 Figure 6.25 shows the variation of exit-delamination factor with feed rate while spindle speed and point angle maintained at their mid values. As expected, it is observed from these experiments that exit-delamination factor increases as the feed rate increases. It is also observed that the experiments conducted with PCD coated tool produces minimum values of exit-delamination factors as compared to carbide. Thus it is concluded that a low feed rate with high speed combination in drilling CFRP composite plates produces minimum exitdelamination and it is preferable to use PCD coated drills. Owing to the cost and manufacturing complexities, carbide drills may also be preferred as their values of exit-delamination factors are almost equal to that of PCD coated drills. 6.4.3 Effect of drilling parameters on eccentricity Eccentricity is measured using UMM in the present study. The research work with regard to eccentricity of the drilled holes is not reported so far. As defined in section 3.6.4 eccentricity is the degree of the displacement Figure 6.25 Variation of exit-delamination with feed rate
194 of the geometric center of the concentric circles at two different planes in a drilled hole. The degree of the displacement of the concentric circles is expressed in mm. Figure 6.26 shows the variation of eccentricity with spindle speed while feed rate and point angle are maintained at their mid values. It is observed from these experiments that eccentricity is low for high speed conditions. This is attributed to the fact that at low speeds the drill bit rotates slowly and hence the holes are being drilled with more vibrations and thereby the displacement of the geometric centers in the drilled holes is greater as compared to that at high spindle speeds. It is also observed that the experiments conducted using PCD coated drills produces minimum value of eccentricity as compared to HSS and carbide drills. Figure 6.27 shows the variation of eccentricity with point angle while the spindle speed and feed rate are maintained at their mid values. The less obtuse is the point angle, smaller is the eccentricity and hence lesser is the damage of drilled holes. This is due to the fact that narrower is the point angle, the sharper is the drill tip and hence the area of contact is less and the thrust force is concentrated in a smaller region and hence the eccentricity is less, and thereby the damage on the holes reduces. Also, it is observed that PCD coated drilled holes has lower eccentricity followed by carbide and HSS drills in drilling of CFRP composite plates. Figure 6.28 shows the variation of eccentricity with feed rate while the spindle speed and point angle are maintained at their mid values. It is observed that lower the feed rate, lower is the eccentricity. At higher feed rate, the displacement between the two planes inside the drilled holes is greater than that at lower feeds. It is also observed that PCD coated drills produce lower values of eccentricity compared to carbide and HSS drills. Thus it is concluded that high spindle speed, smaller point angle and low feed rates are used for drilling CFRP composite plates with a lower value of eccentricity and thereby produce good quality hole.
195 Figure 6.26 Variation of eccentricity with spindle speed Figure 6.27 Variation of eccentricity with point angle Eccentricity in mm 0.09 0.08 0.07 0.06 0.05 0.04 0.03 0.02 HSS Carbide PCD coated 0.01 0 Spindle speed= rpm; Point angle = 118 Feed rate in mm/min Figure 6.28 Variation of eccentricity with feed rate
196 6.4.4 Effect of drilling parameters on surface roughness Surface roughness of the drilled holes is a measure of indicating smooth surface of holes. Due to inter-laminar stress during drilling of unidirectional CFRP composite plate the fibers tend to pull out in the drilled holes which is measured by surface roughness measuring machine. A typical surface roughness measurements obtained from surface measuring equipment is shown in Figure 6.29 to Figure 6.31. The roughness value Ra is indicated in the figure according to the F-profiles and P-profiles displayed. Figure 6.29 F-Profile and P-Profiles (x-axis 0.2 mm) observed in surface roughness measuring equipment with v=0 rpm, = and f= mm/min using HSS drills Figure 6.30 F-Profile and P-Profiles (x-axis 0.2 mm) observed in surface roughness measuring equipment with v=0 rpm, =118 and f= mm/min using carbide drills
197 Figure 6.31 F-Profile and P-Profiles (x-axis 0.2 mm) observed in surface roughness measuring equipment with v=0 rpm, =118 and f= mm/min using PCD coated drills Figure 6.32 shows the variation of surface roughness with spindle speed while feed rate and point angle are kept at their mid values. It is observed from these experiments that higher the spindle speed, lower is the surface roughness and the values of surface roughness in microns are lower for PCD coated drills as compared to that of carbide and HSS drills. Figure 6.33 shows the variation of surface roughness with point angle while the spindle speed and feed rate are maintained at their mid values. The effect of point angle is less pronounced in the case of PCD coated drill experiments as compared to that of carbide and HSS drills. It is observed that minimum value of surface roughness is obtained for a lower value of point angle because the smaller is the contact area of the drill bit, lower is the thrust force and hence better surface finish. Figure 6.34 shows the variation of surface roughness with feed rate while the spindle speed and point angle are maintained at their mid values. It is observed form these experiments that surface roughness increases with increasing feed rate. The increase in feed rate increases thrust force in drilling of CFRP composite plates that increases the load on the drill tool which in turn induces wear on the tool leading to increased surface roughness.
198 Figure 6.32 Variation of surface roughness with spindle speed Figure 6.33 Variation of surface roughness with point angle Figure 6.34 Variation of surface roughness with feed rate
199 Among the drill tools used, PCD coated drill tools produces a lower value of surface roughness as compared to HSS drills. However, the values of surface roughness obtained using PCD coated drills are almost nearer to that of carbide drills. Thus it is concluded that higher spindle speed and lower feed rate produces lower value of surface roughness producing a good quality of hole. The study of parametric influence on various responses indicate a good quality of hole can be produced with a combination of higher spindle speed and lower feed rates in drilling of CFRP composite plates. PCD coated drills produce lower values of thrust force, delamination, eccentricity and surface roughness as compared to other drill tools used in this experimental study. 6.5 SEM IMAGES ILLUSTRATING SURFACE ROUGHNESS Surface roughness is measured in terms of R a values in the present study. However, the images obtained using SEM would give a clear pictorial view of the surface roughness. The SEM images are observed with various degrees of magnification and the defects such as fiber pull out and excessive resin areas is shown in Figure 6.35 in drilling of CFRP composite plates. The optimal and extreme conditions pertaining to surface roughness values are identified for each drill type. The optimal and extreme R a values obtained using HSS drills are shown in Table 3.9. 19 th and 6 th experiments correspond to R a values of 1.23 microns and 3.43 microns respectively. Similarly the optimal and extreme R a values for carbide and PCD coated drills can be obtained from Table 3.10 and 3.11. For carbide drills, the optimal and extreme values are 0.81 microns and 2.83 microns and for PCD coated drills, 0.49 microns and 1.76 microns respectively. Figure 6.36 shows the SEM images illustrating surface finish the drilled holes at different conditions.
Figure 6.35 SEM image of CFRP composite plate showing strong and stiff graphite fibers embedded in the tough epoxy matrix (a) SEM image of CFRP composite plate (b)sem image showing surface of drilled holes for extreme conditions using HSS drills (c)sem image showing surface of drilled holes for mid values using carbide drills (d)sem image showing surface of drilled holes for optimal conditions using PCD coated drills Figure 6.36 SEM images illustrating surface finish
201 Figure 6.36(a) shows the SEM image of plain work piece used in the present study. The image shows the structure of carbon fibers embedded in the resin matrix. Figure 6.36(b) shows the SEM image of the surface of the drilled hole under the extreme conditions using HSS drills. The extreme condition corresponds to a spindle speed of 0 rpm, a point angle of 118 and a feed rate of mm/min. The surface roughness is found to be very poor as the R a value is very high, 3.43 microns. Figure 6.36(c) shows the SEM image of the surface of the drilled hole under the mid value conditions using carbide drill. The mid value conditions corresponds to a spindle speed of rpm, a point angle of 118 and a feed rate of mm/min. The surface roughness is found to be moderate as the R a value is 1.65 microns. Similarly Figure 6.36(d) shows the SEM image of the surface of the drilled hole under the optimal conditions using PCD coated drill. The optimal conditions correspond to a spindle speed of 0 rpm, a point angle of and a feed rate of mm/min. The surface roughness is found to be low as the R a value is 0.49 microns. The SEM images reveal the actual structural conditions of the work piece and the drilled surfaces of the holes. The measured R a values correlates with the SEM image study. The drill materials that give better response are summarized in Table 6.7. Table 6.7 Responses and drill materials Drill tool HSS Carbide PCD coated Thrust force Minimum values of responses Entrydelaminatiodelamination Exit- Torque Eccentricity Surface roughness
202 6.6 ANALYSIS OF DRILLING PARAMETERS USING ANOVA AND INTERACTION PLOTS IN DRILLING OF CFRP COMPOSITES The effect of drilling parameters on each response was studied in earlier section. In addition, a statistical analysis of the data would be useful to find the influential factor, the effect of interactions, the deviation from actual value and so on. Analysis of variance, a popular statistical tool is used for data analysis in the present study for further exploration. ANOVA is to determine the parameters and combination of parameters that significantly affect the machining process. 6.6.1 Analysis of thrust force using ANOVA The ANOVA result for thrust force using HSS drills is tabulated in Table 6.8. Among the three parameters, viz. spindle speed, point angle and feed rate, feed rate is the most influential parameter as its F-value is higher as compared to v and. Table 6.8 ANOVA for thrust force using HSS drills Source Sum of Squares DoF Mean Square F-Value Percentage contribution Model 118520.28 9 13168.92 129.97 98.57 v 3813.85 1 3813.85 37.64 3.17 32018.84 1 32018.84 316.01 26.63 f 73317.69 1 73317.69 723.62 60.97 v 2 9.80 1 9.80 0.10 0.01 2 5475.26 1 5475.26 54.04 4.55 f 2 1509.55 1 1509.55 14.90 1.26 v* 14.59 1 14.59 0.14 0.01 v*f 182.60 1 182.60 1.80 0.15 *f 2178.10 1 2178.10 21.50 1.81 Residual 1722.45 17 101.32 1.43 Total 242.73 26
203 The next contributing factor is point angle which is then followed by spindle speed. And, the point angle-feed rate interaction is the most influential parameter among the three interactions. Percentage of contribution is a field on the Effects List view that expresses the sum of squares for a particular term as a percentage of the total sum of squares. Table 6.9 shows the ANOVA table for the response, thrust force, using carbide drills. The feed rate is found to be the most influential parameter in determining the thrust force using carbide drills, followed by point angle. And, the point angle-feed rate interaction is the most influential as compared to the other two interactions. Table 6.10 shows the ANOVA table for the response, thrust force, using PCD coated drills. The feed rate is found to be the most influential parameter in determining the thrust force using PCD coated drills, followed by point angle. And, the point angle-feed rate interaction is the most influential as compared to the other two interactions. Table 6.9 ANOVA for thrust force using carbide drills Source Sum of Squares DoF Mean Square F-Value Percentage contribution Model 103171.69 9 11463.52 20.62 91.61 v 8752.97 1 8752.97 15.74 7.77 17141.50 1 17141.50 30.83 15.22 f 66294.11 1 66294.11 119.24 58.86 v 2 705.39 1 705.39 1.27 0.63 2 5755.05 1 5755.05 10.35 5.11 f 2 43.67 1 43.67 0.08 0.04 v* 270.37 1 270.37 0.49 0.24 v*f.65 1.65 0.90 0.44 *f 3707.97 1 3707.97 6.67 3.29 Residual 9451.54 17 555.97 8.39 Total 112623.22 26