CHAPTER 5 INVESTIGATION ON DRILLING CHARACTERISTICS OF HYBRID COMPOSITES

Transcription

1 CHAPTER 5 INVESTIGATION ON DRILLING CHARACTERISTICS OF HYBRID COMPOSITES 5.1 Introduction This chapter presents the experimental work carried out with different cutting parameters in drilling to evaluate drilling characteristics of unfilled and particulate filled glass fabric reinforced epoxy composites. The experimental results of thrust force, torque, delamination factor and surface roughness with discussions are presented. The analyses of the results are carried out using effect graphs and drill holes surface integrity with SEM micrographs. Taguchi method is used to optimize the thrust force and torque are selected as quality character factors to optimize drilling parameters such as step angle, stage ratio, feed rate and cutting speed for glass epoxy (G-E) and alumina filled G-E composites. 5.2 Drilling of Composites using HSS Drill Effect of Cutting Parameters on Thrust Force Figure 5.1 shows the variation of thrust force with feed rate for two cutting speeds during drilling of unfilled and particulate filled G-E composite using HSS drill. From figure 5.1 (a-b) it can be observed that the thrust force significantly increased with increasing feed rate and cutting speed. This fact was due to the increasing the cross-sectional area of the undeformed chip. The presence of ceramic filler in composites increases the hardness and cutting resistance of the material, also may result in wear of cutting edges of the drill during drilling. Therefore the thrust force was increased with increasing cutting speed. Similar results were reported by Khashaba et al. [80]. 106

2 Thrust Force (N) Thrust Force (N) a Feed rate (mm/rev) G-E SiO2-G-E Al2O3-G-E b Feed rate (mm/rev) G-E SiO2-G-E Al2O3-G-E Figure 5.1 Variation of thrust force with feed rate for unfilled and particulate filled G-E composites using HSS drill: (a) m/min and (b) m/min. Further, it can be observed that during drilling of alumina filled G-E composites using HSS drill, the thrust force decreases for both cutting speeds (15.08 and m/min) and alumina filled G-E composite shows the less thrust force. The minimum thrust force for alumina filled G-E composite is N which is nearly 52.73% less than that of unfilled G-E composite and 32.94% less than that of silica filled G-E composite under same test condition Effect of Cutting Parameters on Torque The evaluation of the torque with the feed rate for different cutting speeds is shown in Figure 5.2 (a-b). It can be observed that for all the cutting speeds the torque increases near linearly with increasing feed rate for unfilled, silica and alumina filled G-E composites. In addition it can be observed that during drilling of alumina filled G-E composites using HSS drill, the torque decreases for both cutting speeds (15.08 and m/min) compared with unfilled and silica filled G-E composites. 107

3 Torque (N-m) Torque (N-m) a Feed rate (mm/rev) G-E SiO2-G-E Al2O3-G-E b Feed rate (mm/rev) G-E SiO2-G-E Al2O3-G-E Figure 5.2 Variation of torque with feed rate for unfilled and particulate filled G-E composites using HSS drill: (a) m/min and (b) m/min. The minimum torque obtained for alumina filled G-E composite is N-m which is nearly 71.47% less than that of unfilled G-E composite and 30.98% less than that of silica filled G-E composite under same test condition. Velayudham and Krishnamurthy [68] reported that there is an insignificant change in torque with increase in cutting speed. As the cutting speed is increased for particulate filled G-E composites, generation of heat is also increased. This generated heat accumulates and stagnates around the tool edge which leads to softening of the polymer matrix, thereby resulting into reduced level of torque in the particulate filled G-E composites Effect of Cutting Parameters on Delamination The evaluation of the delamination factor (F d ) with different cutting parameters is shown in Figure 5.3 (a-b). The influence of cutting conditions on delamination factor for both unfilled and particulate filled G-E composites showed the increasing trend. It can be observed that the delamination factor increases near linearly with increasing the feed rate and cutting speed. Khashaba et al. [80] studied the influence of cutting speed and feed rate on 108

4 Delamination factor (F d ) Delamination factor (F d ) delamination in drilling GFR-thermoset composite structures. The results indicated that the delamination size was increased with increasing cutting speed and feed rate G-E SiO2- G-E Al2O3 -G-E G-E SiO2- G-E Al2O3 -G-E a Feed rate (mm/rev) b Feed rate (mm/rev) Figure 5.3 Variation of delamination factor with feed rate for unfilled and particulate filled G-E composites using HSS drill: (a) m/min and (b) m/min. At the interface the value of delamination factor (F d ) is higher for unfilled G-E composites in comparison with silica and alumina filled G-E composites. Further it can be observed that during drilling of alumina filled G-E composites using HSS drill, the delamination factor (F d ) decreases for both cutting speeds (15.08 and m/min) and alumina filled G-E composite shows the less delamination factor (F d ). The minimum delamination factor (F d ) for alumina filled G-E composite is which is less than that of unfilled G-E (1.126) and silica filled G-E composites (1.125) under same test condition Effect of Cutting Parameters on Surface Roughness Figure 5.4 (a-b) shows the variation of surface roughness (R a ) with feed rate and two cutting speeds for unfilled G-E, silica and alumina particulate filled G-E composites. The surface roughness was used to characterize the hole surface quality. 109

5 Surface roughness (µm) Surface roughness (µm) a Feed rate (mm/rev) G-E SiO2- G-E Al2O3 -G-E b Feed rate (mm/rev) G-E SiO2- G-E Al2O3 -G-E Figure 5.4 Variation of surface roughness with feed rate for unfilled and particulate filled G-E composites using HSS drill: (a) m/min and (b) m/min. From figure 5.4 (a-b) it can be observed that the surface roughness (Ra) increased with increasing feed rate, and decreased with increasing cutting speed, to reduce the surface roughness and to obtain a better surface finish on the machined components it is necessary to choose higher cutting speeds and at lower feed rates. Krishnaraj et al [86] reported that the value of Ra increases with feed rate and decreases with spindle speed, i.e., to get a better surface finish it is necessary to go for a high spindle speed and low feed rate. In addition, it can be observed that during drilling of alumina filled G-E composites using a HSS drill, the surface roughness (Ra) decreased for both cutting speeds (15.08 and m/min) and the various feed rates from 0.18 mm/rev to 1.40 mm/rev. The minimum (Ra) value for alumina filled G-E composite is 1.28 µm which is nearly 62.5% less than that of unfilled G-E composite and 31.25% less than that of silica filled G-E composite under same test condition. Further, the surface roughness decreases with increase in cutting speeds for particulate filled G-E composites. This is due to the fact that, as cutting speed increases, the temperature 110

6 increases at the cutting zone and drill tool edge that leads to the thermal softening of the particulate filled polymer matrix material and thus reduces the surface roughness Surface Morphology of Drilled Holes The SEM micrographs show the breakage of the fiber material and damage of the matrix material. Figure 5.5a shows the less breakage of fiber material and also ruptured matrix but as the speed and feed are increased the breakage of fibers is more as seen and also more ploughing and fiber buckling of the fibers with adhesion of matrix debris on the fiber surface as shown in Figure 5.5b. a b Figure 5.5 SEM micrographs of the unfilled G-E composite using HSS drill: (a) m/min, 0.18 mm/rev and (b) m/min, 1.40 mm/rev. The SEM micrographs show the breakage of the fiber material and damage of the matrix material. Figure 5.6a shows the less breakage of fiber material and also less damaged of matrix but as the speed and feed are increased the breakage of fibers is more as seen in Figure 5.6b due to delamination between the layers of the material. At higher cutting conditions (high speed and feed), the thrust force caused visible cracking of surface layer as seen in Figure 5.6b which resulted in deterioration of the surface, due to high speed and feed 111

7 which includes fiber fragmentation, matrix debris, large number of fiber breakage, inclined fiber fracture, severe debonding at fiber matrix interface. a b Figure 5.6 SEM micrographs of the G-E composite with SiO 2 filler using HSS drill : (a) m/min, 0.18 mm/rev and (b) m/min, 1.40 mm/rev. a b Figure 5.7 SEM micrographs of the G-E composite with Al 2 O 3 filler using HSS drill: (a) m/min, 0.18 mm/rev and (b) m/min, 1.40 mm/rev. The SEM micrographs show the breakage of the fiber material and damage of the matrix material. Figure 5.7a shows the fibers are totally misaligned and the formation of 112

8 Thrust Force (N) Thrust Force (N) voids in between the matrix and the fiber is clearly observed and the drilled surface at lower speed and feed seems to have less number of cracks resulting in lower value of surface roughness. Figure 5.7b shows the breakage of fiber material and also the ploughing action of fibers due to poor bonding between fibers and matrix, inclined fiber breakage and cohesive resin fracture. 5.3 Drilling of Composites using Carbide Drill Effect of Cutting Parameters on Thrust Force a Feed rate (mm/rev) G-E SiO2-G-E Al2O3-G-E b Feed rate (mm/rev) G-E SiO2-G-E Al2O3-G-E Figure 5.8 Variation of thrust force with feed rate for unfilled and particulate filled G-E composites using carbide drill: (a) m/min and (b) m/min. Figure 5.8 (a-b) shows the variation of thrust force with feed rate against two cutting speeds viz m/min and m/min during drilling of unfilled G-E, silica and alumina filled G-E composites by using carbide drills. It can be observed from figure 5.8 (a-b), thrust force increased with increasing feed rate from 0.18 mm/rev to 1.40 mm/rev and with two cutting speeds. Velayudham and Krishnamurthy [68] reported that the thrust force increases as the feed rate is increased for carbide twist drills. In the case of conventional point geometry drill, thrust force increases steadily up to 0.1 mm/rev feed rate and beyond which 113

9 the rate of increase in thrust is quite rapid. This is mainly due to higher impact of the cutting edges against the fibres as the feed rate is increased. In addition, at higher feed rates there is an increase in self-generated feed angle which significantly reduces the effective clearance angle and thereby creates rubbing against the work material resulting in to higher thrust. Further, it can be observed that during drilling of alumina filled G-E composites using carbide drill, the thrust force decreases for both cutting speeds (15.08 and m/min) and alumina filled G-E composite shows the less thrust force. The minimum thrust force for alumina filled G-E composite is 41.27N which is nearly 83.01% less than that of unfilled G-E composites and 37.84% less than that of silica filled G-E composites under same test condition. The addition of Al 2 O 3 particulate fillers reduces the thrust force as compared to unfilled G-E and SiO 2 filled G-E composites. This is due to the fact that, Al 2 O 3 particulate filler has more hardness and Al 2 O 3 filled G-E composite has higher hardness than that of unfilled G-E and SiO 2 filled G-E composites. As a cutting speed increases during drilling of alumina filled G-E composites, the temperature increases at the cutting zone and drill tool edge that leads to the thermal softening of the particulate filled polymer matrix material and thus reduces the thrust force. On the other hand it was found that the thrust force appearing when using carbide drills were smaller than those when using HSS drills Effect of Cutting Parameters on Torque Figure 5.9 (a-b) shows the variation of torque with feed rate and different cutting speeds during drilling of unfilled for G-E and particulate filled G-E composites using carbide drill. It is evident from Figure 5.9 (a-b) alumina filled G-E composites showed the lower torque values with different cutting speeds as compared to the unfilled and SiO 2 filled G-E composites. 114

10 Torque (N-m) Torque (N-m) a Feed rate (mm/rev) G-E SiO2-G-E Al2O3-G-E b G-E SiO2-G-E Al2O3-G-E Feed rate (mm/rev) Figure 5.9 Variation of torque with feed rate for unfilled and particulate filled G-E composites using carbide drill: (a) m/min and (b) m/min. In addition it can be observed that during drilling of alumina filled G-E composites using carbide drill, the torque decreases for both cutting speeds (15.08 and m/min) and alumina filled G-E composite shows the less torque. The minimum torque for alumina filled G-E composite is N-m which is nearly 54.62% less than that of unfilled G-E composites and 25.21% less than that of silica filled G-E composites under same test condition. It is observed that there is an insignificant change in torque with increase in cutting speed. As the cutting speed is increased during drilling of particulate filled G-E composites using carbide drills, generation of heat is also increased. This generated heat accumulates and stagnates around the tool edge which leads to softening of the polymer matrix, thereby resulting into a reduced level of torque [68]. On the other hand it was found that the torque appearing when using carbide drills were smaller than those when using HSS drills. 115

11 Delamination factor (F d ) Delamination factor (F d ) Effect of Cutting Parameters on Delamination The evaluation of the delamination factor (F d ) with cutting parameters is shown in Figure 5.10 (a-b). The influence of cutting conditions on delamination factor for both unfilled and particulate filled G-E composites showed the increasing trend. a Feed rate (mm/rev) G-E SiO2- G-E Al2O3 -G-E b G-E SiO2- G-E Al2O3 -G-E Feed rate (mm/rev) Figure 5.10 Variation of delamination factor with feed rate for unfilled and particulate filled G-E composites using carbide drill: (a) m/min and (b) m/min. The obtained results show that the delamination factor increases with the feed rate and cutting speed. When drilling woven GFRP material with carbide twist drill, it can be seen that the lower the feed rate, the smaller the delaminated area on both hole entrance and hole exit. However, increasing of cutting speed increases delamination factor at both hole entrance and hole exit. The damage caused at hole exit is bigger than at hole entrance with influence of each cutting parameters Birhan Işık and Ergün Ekici [92]. Further, it can be observed that during drilling of alumina filled G-E composites using carbide drill, the delamination factor (F d ) decreases for both cutting speeds (15.08 and m/min) and alumina filled G-E composite shows the less delamination factor (F d ). The minimum delamination factor (F d ) for alumina filled G-E composite is which is less 116

12 Surface roughness (µm) Surface roughness (µm) than that of unfilled (1.0067) and silica filled (1.0063) G-E composites under same test condition. On the other hand carbide drill holes showed lesser damage factor compared to HSS drill holes Effect of Cutting Parameters on Surface Roughness Figure 5.11 (a-b) shows the variation of surface roughness (Ra) with feed rate and for two different cutting speeds for unfilled G-E, alumina and silica particulate filled G-E composites. a Feed rate (mm/rev) G-E SiO2- G-E Al2O3 -G-E b Feed rate (mm/rev) G-E SiO2- G-E Al2O3 -G-E Figure 5.11 Variation of surface roughness with feed rate for unfilled and particulate filled G-E composites using carbide drill: (a) m/min and (b) m/min. From Figure 5.11 (a-b) it can be observed that surface roughness (Ra) increased with feed rate, and decreases with spindle speed and to get a better surface finish it is necessary to select higher spindle speeds and lower feed rates. We can infer that the value of (Ra) increases with feed rate and decreases with spindle speed, i.e., to get a better surface finish it is necessary to go for a high spindle speed and low feed rate [86]. In addition it can be observed that alumina filled G-E composite surface roughness (Ra) decreases for both cutting speeds (15.08 and m/min) and alumina filled G-E composite shows the less surface 117

13 roughness. The minimum Ra value for alumina filled G-E composite is 1.66 µm which is nearly 68.2% less than that of unfilled G-E composites and % less than that of silica filled G-E composites under same test condition. On the other hand carbide drill holes showed the lower value of surface roughness compared to HSS drill holes Surface Morphology of Drilled Holes The SEM micrographs show the breakage of the fiber material and damage of the matrix material. Figure 5.12a shows that the longitudinal fibers are pulled up and are oriented and more breakage of matrix material such as cohesive bonding of matrix and fiber at interface, more transverse fiber breakage, matrix debris, voids and fiber fragmentation but as the speed and feed are increased the breakage of fibers as well as matrix is increased and also chopping of fibers is being seen in Figure 5.12b. a b Figure 5.12 SEM micrographs of the unfilled G-E composite using carbide drill: (a) m/min, 0.18 mm/rev and (b) m/min, 1.40 mm/rev. 118

14 a b Figure 5.13 SEM micrographs of the G-E composite with SiO 2 filler using carbide drill: (a) m/min, 0.18 mm/rev and (b) m/min, 1.40 mm/rev. Figure 5.13a shows the less breakage of fiber material and more damage to the matrix material at lower speed and feed the drilled work surface showed the similar surface features such as cohesive bonding of matrix and fiber at interface, more transverse fiber breakage, matrix debris, voids and fiber fragmentation and as the speed and feed are increased the breakage of fibers and matrix is more as seen in Figure 5.13b. a b Figure 5.14 SEM micrographs of the G-E composite with Al 2 O 3 filler using carbide drill: (a) m/min, 0.18 mm/rev and (b) m/min, 1.40 mm/rev. 119

15 The SEM micrographs show the breakage of the fiber material and damage of the matrix material. Figure 5.14a shows the breakage of matrix material and also less damaged fibers but as the speed and feed are increasing the breakage of fibers as well as orientation of the fibers is seen with damaged matrix as seen in Figure 5.14b. Further it is observed from Figure 5.14 (a-b) Al 2 O 3 particulate filled G-E composites with carbide drill very less breakage of the fiber material and damage of the matrix material compared to G-E and silica filled composites with same test/cutting conditions shown in the Figures 5.12 (a-b) and 5.13 (a-b). 5.4 Drilling of Composites using Carbide Step Drill by Taguchi Method Taguchi method is a scientifically disciplined mechanism for evaluating and implementing improvements in products, processes, materials, equipment, and facilities. These improvements are aimed at improving the desired characteristics and simultaneously reducing the number of defects by studying the key variables controlling the process and optimizing the procedures or design to yield the best results. Taguchi addresses quality in two main areas, which are, off-line and on-line quality control (QC). Both of these are cost sensitive in decisions that are made with respect to activities in each. Off-line QC refers to the improvement of quality in product and process development stages. On-line QC refers to the monitoring of current manufacturing processes to verify the quality levels produced. In this study the prediction of thrust force and torque were selected as quality character factors to optimize drilling parameters such as step angle (A), stage ratio (B), feed rate (C), and spindle speed (D). A major concern in drilling of composite materials is the thrust force and torque these occur during machining. In order to assess the influence of the factors on the thrust and torque response for the step drill in drilling of unfilled and particulate filled G-E composites, 120

16 the means and signal-to-noise (S/N) for each control factor on the average responses and the noises are measures of the influence on the deviations from the average responses, which accounts for the sensitiveness of the experiment output to the noise factors. In this study the S/N ratio chosen was according to the criterion the smaller-the-better in order to minimize the response. The S/N ratio of the smaller-the-better can be expressed as follows S/N = -10 Log 1 2 y n (5.1) Where n is the number of observations and y is the observed data. In this study the prediction of thrust force and torque were selected as a quality character factor to optimize drilling parameters such as step angle (A), stage ratio (B), feed rate (C), and spindle speed (D). L 27 (3 13 ) orthogonal array is chosen due to its capability to check the iterations among the factors. Various factors and levels in drilling to obtain the optimum drilling conditions for thrust force and torque are as shown in Table 5.1. From the four factors and three levels an orthogonal array L 27 (3 13 ) (Table 5.2) is being chosen for one set of experiment due to its capability to check the interactions among the factors A, B, C, and D. Table 5.1 Factors and levels in drilling. SYMBOL FACTOR LEVEL 1 LEVEL 2 LEVEL 3 A Step angle ( 0 ) B Stage ratio (mm/mm) C Feed rate (mm/rev) D Cutting speed (m/min)

17 Table 5.2 L 27 (3 13 ) Orthogonal array of Taguchi Trial A B A B A B C A C A C B C D A D B C B D C D 122

18 5.4.1 Analysis of the Thrust force for G-E composites Using the concept of Taguchi Table 5.3 presents the thrust force measurements taken for G-E composites drilled in L 27 experimental setups. Table 5.3 Experimental results for the step drill in drilling of G-E composites. Test No. Step angle ( 0 ) Stage ratio (mm/mm) Feed rate (mm/rev) Cutting Speed (m/min) Thrust force (N) S/N Ratio (db)

19 Figure 5.15 Main effects plot for S/N ratios of thrust force with G-E composites. Table 5.4 Response table for S/N ratios of thrust force with G-E composites. Level Step Angle ( ) Stage Ratio Feed rate (mm/rev) Cutting Speed (m/min) Delta Rank After analyzing the main effects plot (Figure 5.15) and response table (Table 5.4) for S/N ratios it can be observed that the optimal cutting conditions to obtain the lower thrust 124

20 force for G-E composites, the feed rate (0.18 mm/rev) plays a major role during the drilling of composites followed by step angle (120 0 ), stage ratio (0.6), and cutting speed (18.85 m/min). Table 5.5 Analysis of Variance ( ANOVA) results for thrust force with G-E composites. Factors DF Seq SS Adj SS Adj MS F P P (%) Step Angle (A) Stage Ratio (B) Feed Rate (C) Cutting Speed (D) Step Angle Stage Ratio (A B) Step Angle Feed Rate (A C) Step Angle Cutting Speed (A D) Residual Error Total DF- degree of freedom, SS- sum of the squares, P- contribution factor, F- distribution factor, P (%) - percentage of contribution. From the Table 5.5 Analysis of Variance is carried out with a confidence limit of 95% or P-value of 5% or 0.05, indicating any factor with P-value equal to or less than 0.05 is significant. It is observed that the F-value shows larger value for feed rate thus feed rate having more effect on thrust force followed by step angle, stage ratio and cutting speed and using the percentage contribution of significant factors is calculated by dividing the sum of squares of the factor by the total sum of squares. Table 5.5 illustrates the results of the analysis of variance (ANOVA) for the thrust force in drilling G-E composites. In Table 5.5, the most important variables affecting the 125

21 thrust force are feed rate (factor C, P=32.97%), step angle (factor A, P=32.8%), stage ratio (factor B, P=14.93%). The feed rate, step angle, stage ratio show statistical and physical significance whereas the effect of cutting speed (factor D, P=0.17%) is less significant. The interactions of A B is significant and while A C and A D were found to be negligible. Since the spindle speed is less significant, it could be set at the highest cutting speed to obtain high rate of material removal or at the lowest cutting speed to prolong the tool life depending on the need for application Analysis of the Torque for G-E composites Using the concept of Taguchi Table 5.6 presents the Torque measurements taken for G-E composites drilled in L 27 experimental setups. Table 5.6 Experimental results for the step drill in drilling of G-E composites. Test No. Step angle ( 0 ) Stage Ratio Feed rate (mm/rev) Cutting speed (m/min) Torque (N-m) S/N Ratio(db)

22 Figure 5.16 Main effects plot for S/N ratios of torque with G-E composites. 127

23 Table 5.7 Response table for S/N ratios of torque with G-E composites. Level Step Angle ( ) Stage Ratio Feed rate (mm/rev) Cutting Speed (m/min) Delta Rank After analyzing the main effects plot for the S/N ratios (Figure 5.16) and response table for S/N ratios (Table 5.7) it can be observed that the best combination of cutting parameters to obtain the lower torque for G-E composites, the stage ratio (0.6), plays a major role during the drilling of composites followed by step angle (120 0 ), feed rate (0.18 mm/rev) and cutting speed (18.85 m/min). Table 5.8 Analysis of Variance (ANOVA) results for torque with G-E composites. Factors DF Seq SS Adj SS Adj MS F P P (%) Step Angle (A) Stage Ratio (B) Feed Rate (C) Cutting Speed (D) Step Angle Stage Ratio (A B) Step Angle Feed Rate (A C) Step Angle Cutting Speed (A D) Residual Error Total

24 DF- degree of freedom, SS- sum of the squares, P- contribution factor, F- distribution factor, P (%) - percentage of contribution. From the Table 5.8 Analysis of Variance is carried out with a confidence limit of 95% or P-value of 5% or 0.05, indicating any factor with P-value equal to or less than 0.05 is significant. It is observed that the F-value shows larger value for stage ratio thus stage ratio having more effect on torque, followed by step angle, feed rate and cutting speed and using the percentage contribution of significant factors is calculated by dividing the sum of squares of the factor by the total sum of squares. Table 5.8 illustrates the results of the analysis of variance (ANOVA) for the torque in drilling G-E composites. In Table 5.8, the most important variables affecting the torque are stage ratio (factor B, P=46.36%) has highest statistical significant followed by step angle (factor A, P=36.04%), feed rate (factor C, P=12.71%), whereas the effect of cutting speed (factor D, P=0.02%) is less significant. The interactions of A B is less significant and while A C and A D were found to be negligible. Since the cutting speed is less significant, it could be set at the highest cutting speed in the experiments because the higher cutting speed reduced heat generation during drilling process, could reduce the tool wear Analysis of the Thrust force for Alumina filled G-E composites Using the concept of Taguchi Table 5.9 presents the thrust force measurements taken for Alumina filled G-E composites drilled in L 27 experimental setup. 129

25 Table 5.9 Experimental results for step drill in drilling of alumina filled G-E composites. Test No. Step angle ( 0 ) Stage ratio (mm/mm) Feed rate (mm/rev) Cutting speed (rpm) Thrust force (N) S/N Ratio (db)

26 Figure 5.17 Main effects plot for S/N ratios of thrust force with alumina filled G-E composites. Table 5.10 Response table for S/N ratios of thrust force with alumina filled G-E composites. Level Step Angle( ) Stage Ratio Feed rate (mm/rev) Cutting Speed (m/min) Delta Rank After analyzing the main effects plot (Figure 5.17) and response table (Table 5.10) for S/N ratio, it is observed that the optimal cutting conditions to obtain the lower thrust force for alumina filled G-E composites, the step angle (120 0 ) plays a very important role 131

27 during the drilling of composites followed by feed rate (0.18 mm/rev), stage ratio (0.2) and cutting speed (23.57 m/min). Table 5.11 Analysis of Variance (ANOVA) results for thrust force with alumina filled G-E composites. Factors DF Seq SS Adj SS Adj MS F P P (%) Step Angle (A) Stage Ratio (B) Feed Rate (C) Cutting Speed (D) Step Angle Stage Ratio (A B) Step Angle Feed Rate (A C) Step Angle Cutting Speed (A D) Residual Error Total DF- degree of freedom, SS- sum of the squares, P- contribution factor, F- distribution factor, P (%) - percentage of contribution. From the Table 5.11 Analysis of Variance is carried out with a confidence limit of 95% or P-value of 5% or 0.05, indicating any factor with P-value equal to or less than 0.05 is significant. It is observed that the F-value shows larger value for step angle thus step angle having more effect on thrust force, followed by feed rate, stage ratio, and cutting speed and using the percentage contribution of significant factors is calculated by dividing the sum of squares of the factor by the total sum of squares. Table 5.11 illustrates the results of the analysis of variance (ANOVA) with the thrust force in drilling of alumina filled G-E composites. In Table 5.11, the most important variables affecting the thrust force are step angle (factor A, P=52.59%), feed rate (factor C, 132

28 P=17.48%) and stage ratio (factor B, P=14.11%). The step angle, feed rate and stage ratio show statistical and physical significance whereas the effect of cutting speed (factor D, P=0.02%) is less significant. The interactions of A B is significant and while A C and A D were found to be negligible. Since the cutting speed is less significant, it could be set at the highest spindle speed to obtain high rate of material removal or at the lowest cutting speed to prolong the tool life depending on the need for application Analysis of the Torque for Alumina filled G-E composites Using the concept of Taguchi Table 5.12 presents the thrust force measurements taken for G-E composites drilled in L 27 experimental setups. Table 5.12 Experimental results for step drill in drilling of alumina filled G-E composites. Test Step Stage Feed rate Cutting speed Torque S/N Ratio No. angle ( 0 ) Ratio (mm/rev) (m/min) (N-m) (db)

29 Figure 5.18 Main effects plot for S/N ratios of torque with alumina filled G-E composites. 134

30 Table 5.13 Response table for S/N ratios of torque with alumina filled G-E composites. Level Step Angle ( ) Stage Ratio Feed rate (mm/rev) Cutting Speed (m/min) Delta Rank After analyzing the main effects plot for the S/N ratio (Figure 5.18) and response table for S/N ratio (Table 5.13) it can be observed that the best combination of cutting parameters to obtain the lower torque for alumina filled G-E composites, the step angle (120 0 ), plays a major role during the drilling of alumina filled G-E composites followed by feed rate (0.18 mm/rev), stage ratio (0.2) and cutting speed (23.57 m/min). Table 5.14 Analysis of Variance (ANOVA) results for torque with alumina filled G-E composites. Factors DF Seq SS Adj SS Adj MS F P P (%) Step Angle (A) Stage Ratio (B) Feed Rate (C) Cutting Speed (D) Step Angle Stage Ratio (A B) Step Angle Feed Rate (A C) 0.03 Step Angle Cutting Speed (A D) 0.38 Residual Error Total DF- degree of freedom, SS- sum of the squares, P- contribution factor, F- distribution factor, P (%) - percentage of contribution. 135

31 From the Table 5.14 Analysis of Variance is carried out with a confidence limit of 95% or P-value of 5% or 0.05, indicating any factor with P-value equal to or less than 0.05 is significant. It is observed that the F-value shows larger value for step angle thus step angle having more effect on torque, followed by feed rate, stage ratio and cutting speed and using the percentage contribution of significant factors is calculated by dividing the sum of squares of the factor by the total sum of squares. Table 5.14 illustrates the results of the analysis of variance (ANOVA) for the torque in drilling alumina filled G-E composites. In Table 5.14, the most important variables affecting the torque are step angle (factor A, P=53.44%), feed rate (factor C, P=11.58%), stage ratio (factor B, P=0.71%), whereas the effect of cutting speed (factor D, P=0.10%) is less significant. The interactions of A B is significant and while A C and A D were found to be negligible. Since the cutting speed is less significant, it could be set at the highest cutting speed in the experiments because the higher cutting speed reduced heat generation during drilling process, could reduce the tool wear Correlation among thrust force, torque and cutting parameters In use of multiple linear regression analysis, the correlation between thrust force (F), torque (T) and cutting parameters in drilling of unfilled G-E and Al 2 O 3 filled G-E composites was obtained. The coefficient of determination (R 2 ) values of thrust force and torque models have very good correlations between the experimental and predicted values of cutting parameters. The empirical thrust force (F) and torque (T) equations can be expressed as follows. F G-E = f-1.47V c f V c +37 f-6.88 V c +14.8f V c (R 2 = 82.4%) (5.2) 136

32 T G-E = f V c f V c f V c f V c (R 2 = 83.6%) (5.3) F Al2O3 = f+2.38V c f V c f V c f V c (R 2 = 96.8%) (5.4) T Al2O3 = f+0.024V c f V c f V c f V c (R 2 = 88.9%) (5.5) Where, F G-E = Thrust force of unfilled G-E composites. T G-E = Torque of unfilled G-E composites. F Al2O3 = Thrust force of alumina filled G-E composites. T Al2O3 = Torque of alumina filled G-E composites. = Step angle in degrees ( ) = Stage ratio mm/mm f = Feed rate mm/ rev. V c = Cutting speed m/min Confirmation tests For the validation purpose the experiments were conducted for 3 new trials / tests, consisting of input drilling parameters which do not belong to experimental set. The cutting parameters used for the combination tests are illustrated in Table Test Table 5.15 Cutting parameters used in drilling confirmation tests Step Angle ( ) Stage Ratio (mm/mm) Feed Rate f (mm/ rev) Cutting Speed V c (m/min)

33 Table 5.16 Comparison of results obtained from experiment with regression model Test No. Experiment Model of equations ( ) Error (%) Thrust force of G-E composites (Model Equation 5.2) N N N N N N 2.84 Torque of G-E composites (Model Equation 5.3) N-m N-m N-m N-m N-m N-m 3.53 Thrust force of alumina filled G-E composites (Model Equation 5.4) N N N N N N 1.37 Torque of alumina filled G-E composites (Model Equation 5.5) N-m N-m N-m N-m N-m N-m 2.27 Table 5.16 shows the results obtained where a comparison was done between foreseen values from model developed in the present research work (equations 5.2 to 5.5); with the values obtained experimentally. From the analysis of table 5.16 we can observe that calculated error for thrust force of G-E composites (F G-E ) (max. value 4.23% and min value 2.84%), torque of G-E composites (T G-E ) (max. value 4.31% and min value 2.47%), thrust force of alumina filled G-E composites (F Al2O3 ) (max. value 3.10% and min value 1.37%), torque of alumina filled G-E composites (T Al2O3 ) (max. value 3.84% and min value 2.27%). Therefore equations 5.2 to 5.5 correlate the relationship of the thrust force and torque of unfilled and alumina filled G-E composites with the cutting parameters with reasonable degrees of approximation. 138

34 5.5 Chapter Summary This chapter has provided: The results of drilling tests for G-E and G-E composites with different particulate fillers and their comparison. The analysis of the experimental results using Taguchi method. The effect of filler loading, cutting speed and feed rate on thrust force, torque, delamination factor and surface roughness of G-E and particulate filled G-E composites. The surface morphologies of drilled holes of the composites using SEM The comparison of different particulate fillers with regard to the surface quality of the hole drilled in the composites under similar test conditions. The next chapter presents the summary of research findings and conclusions drawn from this investigation along with recommendations for potential applications and future work. 139