3. Experimental investigations on CFRP drilling process
3.2. Investigation on thrust force according to the fiber volume fraction
Thrust force is the major force component affects the quality of drilled hole, and it is the force generated in feed direction of the drilling process. 33, 94, 136, 166 As shown figure 3-1, parameters of the drilling quality and productivity controlled by the several process parameters. Machine tool performance, composite material properties, drilling tool, and cutting conditions are the major factors.
In this work, fiber volume fraction, and fiber orientation of the composite materials were considered. 3 types of uni-directional CFRP were chosen in this experiment. Fiber volume fraction contributes to the thermo-mechanical properties of composite materials. Drilling tool was fixed to twist drill bit which has 9mm of diameter with 135° of point angle, and It was manufactured by the Kennametal Inc. Drilling experiment was conducted according to the varying spindle speed and feed. The magnitude of the force was measured with the dynamometer (9257B) manufactured by Kistler Inc. Sampling rate of the dynamometer was 1000hz for the measurement. Temperature elevation due to drilling process was also measured with the thermocouples equipped to the CFRP composite materials as shown in figure 3-2.
Software and DAQ manufactured by National Instrument Inc. were used for the temperature measurement. Machining center manufactured by Hermle Inc. was utilized for the drilling experiment.
Detailed summaries of the equipment are expressed in the table 3-1, and table 3-2. Spindle speeds for
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the drilling experiment were from 3000RPM to 9000RPM which are the adequate speed in general use
Figure 3-2. Experimental setup for drilling test
18 of drilling process.
Three different unidirectional carbon fiber composite materials with varying volume fraction, namely USN 150(66%), USN 150(57%) and USN 150(49%) (SK chemicals) was used for the experiments. Each sample was cured using autoclave at 125℃ for 90 minutes under 5bar pressure. The detailed material properties for the three CFRP composites are displayed in the table 3-1. The material properties of the CFRP composite were calculated using the rule of mixture. Each of the composite material was tested for the four set of speed and feed rate, varying each parameter one at a time. Hence, in total 48 experiments were conducted i.e. 16 experiments for each type of CFRP sample. The experimental thrust force results obtained were analyzed in this section.
The effect of increase in volume fraction on maximum thrust force is shown in the figure 3-5.
under different speeds and feed rates. It can be seen form the Fig 3.4 to Fig 3.6 that when the feed rate was increased from 0.02 mm/rev to 0.08mm/rev, a steep increase in the thrust force was observed. It
Figure 3-3. Optical microscopic image for drill bit
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can be attributed to the increase in the amount of material removed per unit time with the increase in the feed rate which might have resulted in the load on the cutting lip. This trend is similar for all the three CFRP material under four different speeds of 3000 rpm, 5000rpm, 7000rpm and 9000rpm. It can be seen from the respective figures (Fig 3.4 to Fig 3.6) that the thrust force is highest for the specimen USN 150E (Vf=67%) as compared to specimens USN 150Y (Vf= 49%) and USN 150B (Vf=57%).
This might have occurred because of the increase in the overall strength of the material due to the presence of high content of fiber. Hence the force requirement for deforming and removing the material also got increased. It can also be seen from the results shown in figures (Fig 3.4 to Fig 3.6) that when the fiber volume fraction of the specimens was increased from 49% to 57%, the rise in the force is not linear. In fact, the differences in the thrust force between the two specimens were very minimal and no specific trend was observed. This deviation might have resulted because of the nonlinear change in the material properties like strength and modulus, when the fiber volume fraction was increased from 50%
to 70%. It was observed from the work of Amuthakkannan et al. that when the fiber volume fraction got increased from 48% to 75%, the non-linearity in the change in the modulus of the fiber composite material becomes more prominent. 167 This nonlinear change in the strength was also reported in the work of karam where this change was attributed to the misorientation and inhomogenous fiber spread.
168 This nonlinear change in the material property might have resulted in the nonlinear rise in the thrust force during drilling. Hence when the fiber volume fraction gets increased within very narrow range (49% to 57%), the variation in the force generation might be very minimal and nonlinear. The experimental thrust forces values obtained were then compared with the predicted mean thrust force of the model at the end moment of stage 2.
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Table 3-1. Material properties of CFRP according to the volume fraction of fiber
Table 3-2. Drill bit geometrical parameters
USN 150(66%) USN 150(57%) USN 150(49%)
Content of fiber % 75 67 60
Fiber volume fraction % (Vf) 66 57 49
Young’s modulus, E1 (GPa) 160 138 121
Young’s modulus, E3, E2 (GPa) 10.13 8.02 6.8
Shear strength, τ1 (Mpa) 99 89 79
Shear strength, τ2 (Mpa) 75 63 54
Poission’s ratio, υ13 0.28 0.31 0.33
Glass transition temperature (℃) 126 Decomposition temperature (℃) 405
Diameter of drill (mm) 9
Chisel edge thickness (mm) 0.49
Point angle (degree) 135
Clearance angle (degree) 11
Helix angle (degree) 30
21 Table 3-3. Machining parameters for drilling test
Cutting condition Spindle Speed (rpm) Feed (mm/rev)
1 3000 0.02
2 5000 0.02
3 7000 0.02
4 9000 0.02
5 3000 0.04
6 5000 0.04
7 7000 0.04
8 9000 0.04
9 3000 0.06
10 5000 0.06
11 7000 0.06
12 9000 0.06
13 3000 0.08
14 5000 0.08
15 7000 0.08
16 9000 0.08
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Figure 3-5. Thrust force according to the feed and fiber volume fraction at spindle speed of 5000RPM
Figure 3-4. Thrust force according to the feed and fiber volume fraction at spindle speed of 3000RPM
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Figure 3-6. Thrust force according to the feed and fiber volume fraction at spindle speed of 7000RPM
Figure 3-7. Thrust force according to the feed and fiber volume fraction at spindle speed of 9000RPM
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