This section describes the results and analysis of experimental data gathered during bone drilling experiments. The analysis is supported by a graphical representation of the magnitude of output variables with respect to input parameters. The analysis is performed using the ANOVA technique. Graphs showing the variation in thrust force and temperature rise is presented in Fig. 3 and Fig. 4. Experimental thrust force vs time and temperature vs time graph obtained is shown in Fig. 3 and Fig. 4. After the initial contact, the force slowly increased with time and attained a peak height when the drill tip was in full contact with the bone. Due to the high sensitivity of the measurement system and vibrations in drilling equipment, small oscillations were recorded at peak values. The force suddenly drops when the drill penetrated the cancellous area of bone.
The graph presented in Fig. 3 and Fig. 4 shows thrust force (FZ) and temperature as a function of time. The nature of thrust force in the cancellous bone and cortical bone region is also shown.
3.1 Analysis of Thrust force
1) Effect of spindle speed on thrust force: Fig. 5 shows the effect of spindle speed on thrust force at a constant feed rate. It was noted that when the spindle speed increases from 400 rpm to 2000 rpm; the thrust force decreases. It was observed that at 400 rpm spindle speed, the highest thrust force was required which is further reduced by 59.84 % at 2000 rpm. The decrease in thrust force with an increase in spindle speed is because of the decrease of mean friction coefficient at higher spindle speed [1].
2) Effect of feed rate on thrust force: Figure 6 shows the effect of a feed rate on thrust force at constant spindle speed. In this case as the feed rate increases, thrust force also increases. It is observed that at a 0.04 mm/rev feed rate lowest thrust force is required and it reaches the highest value at a 0.12 mm/rev feed rate. This can be explained by the fact that for larger values of feed rate, the proportionally greater force has to be applied in order to remove work material accumulated in front of the cutting edge.
3) Effect of point angle on thrust force: Figure 7 shows the effect of point angle on thrust force at a constant helix angle. From the results and graph, it is observed that as the point angle increases, thrust force also increases. At a point angle of 70º, the lowest thrust force was recorded and at a point angle of 110º, the highest thrust force was recorded. The smaller point angle produces a sharper tip which can easily penetrate into the material and requires less thrust force and a smaller point angle also prevents walking of the drill. The larger point angle induces higher shear deformations to the material resulting rise in thrust force.
4) Effect of the helix angle on thrust force: Figure 8 shows the effect of the helix angle on thrust force at a constant point angle. From the results and graph, it is observed that when the helix angle increases, thrust force decreases. At a helix angle of 10º, the highest thrust force was recorded and at a helix angle of 25º, the lowest thrust force was recorded. During orthopaedic treatment, bone is wet, therefore the chips produced get clogged when a small helix angle is used which increases friction and results in a higher amount of thrust force.
3.2 Analysis of Temperature
1) Effect of spindle speed on Temperature: Figure 9 represents the effect of spindle speed on the temperature at a constant feed rate. It is noted from the graph that as the spindle speed increases, temperature decreases. At a spindle speed of 400 rpm, the highest temperature was recorded and which is further reduced at a spindle speed of 2000 rpm. As spindle speed increases, thrust force decreases because of the reduced value of a mean coefficient of friction. As a result, the bone temperature decreased.
2) Effect of feed rate on Temperature: Figure 10 shows the effect of feed rate on the temperature at constant spindle speed. Temperature rise is linearly increased with the increase in feed rate. At 0.04 mm/rev feed rate, the temperature is the lowest but at 0.12 mm/rev the highest temperature was recorded. Higher feed rates cause higher thrust force due to more amount of material removal per unit time.
3) Effect of point angle on Temperature: The effect of point angle on the temperature at a constant helix angle is presented in Figure 11. From the graph, it is noticed that as the point angle increases temperature also increases. At a point angle of 70º, the lowest temperature is produced which is then increased at a point angle of 110º. A Small point angle has a more acute tip which can easily stab into the bone and less thrust force is required to drill resulting in less heat generation.
4) Effect of helix angle on Temperature: Figure 12 shows the effect of helix angle on the temperature at a constant point angle. From the graph, it is seen that as the helix angle increases, temperature decreases. At a helix angle of 10º, the highest temperature is generated and for the helix angle of 25º, the lowest temperature was recorded. Chips get clogged when a small helix angle is used. Clogged chips exert more friction while bone drilling which resulted in more heat generation in bone.
3.3 SEM analysis of bone
The drilled hole of bone is studied under the scanning electron microscope to find the micro-cracks if any. The effect of the drilling thrust force and temperature rise on bone can be compared using SEM images. It is observed that the higher thrust force causes an increase in micro-cracks in bone. However, the temperature rise causes burr formation on the drilled hole surface wall.
1) At feed rate 0.04 mm/rev and spindle speed 2000 rpm: Figure 13 (a) and (b) shows SEM images of bone drilled at the spindle speed of 2000 rpm and feed rate of 0.04 mm/rev. It is seen that microcrack is absent on the hole wall surface region because of the lower thrust force and temperature produced at this experimental analysis.
2) At feed rate 0.08 mm/rev and spindle speed 1200 rpm: Figure 14 (a) and (b) shows SEM images of bone that are drilled at a feed rate of 0.08 mm/rev and spindle speed of 1200 rpm. In this case, the microcrack is present near the hole surface region.
3)Feed rate 0.12 mm/rev and spindle speed 400 rpm: Figure 15 (a) and (b) shows SEM images of bone drilled at a feed rate of 0.12 mm/rev and spindle speed of 400 rpm. From the SEM image, we can see more micro-cracks near the drilled area and a poor surface finish of the drilled wall.
4) At helix angle 25º and point angle 70º: Figure 16 (a) and (b) shows SEM images of bone drilled at a helix angle of 25º and point angle of 70º. The hole drilled with the experimental condition is shown in fig. 20. The micro-cracks are evident at the hole wall surface region. Except for the micro crack at a few locations the remaining surface topography is fairly good.
5) At Point angle 90º and helix angle 15º (Intermediate condition): Figure 17 (a) and (b) shows SEM images of bone which are drilled at a helix angle of 15º and point angle of 90º. The output results of this experiment were; thrust force 96.9 N and temperature 38.71ºC. From the SEM image, it is observed that there are few micro-cracks near the drilled area.
6) At helix angle 10º and point angle 110º: Figure 18 (a) and (b) shows SEM images of bone that are drilled at a helix angle of 10º and point angle of 110º. The output results of this experiment are, thrust force 101.51 N and temperature of 44.29ºC. From the SEM image, we can observe that the built-up edge formation of chips around the drilled area is evident and the surface finish of the hole wall surface is poor.