## 3 Results
### Effect of the Clearance Angle on the Cutting Force  
To investigate the effect of the clearance angle, an analysis was performed on the cutting force. The mean cutting force, which is the average value of cutting force generated during cutting, and the peak cutting force, which is the maximum value of cutting force generated during cutting, were analyzed.  
Figure 9 shows the mean cutting force generated by each cutting depth and clearance angle. As shown in Figure 9, the cutting force decreases as the clearance angle increases. In particular, when the clearance angle increases from 0\({}^{\circ}\) to 5\({}^{\circ}\), the cutting force decreases significantly. When a rock of UCS 20 MPa was cut with a cutting depth of 3 mm, the mean cutting force decreased by 81% from 4.74 kN to 0.88 kN, while in the case of the UCS 50 MPa rock, it decreased by 65% from 8.66 kN to 3.02 kN (Figure 9a). At a cutting depth of 9 mm, it decreased by 67% from 14.45 kN to 4.84 kN and by 56% from 27.28 kN to 12.02 kN for UCS 20 MPa and UCS 50 MPa, respectively (Figure 9c). Thus, the sharp decrease in the mean cutting force owing to the clearance angle appears less as the depth of cut and UCS increase.  
Figure 7: Variation in the cutting force with time for each clearance angle.  
When the clearance angle was increased from 5\({}^{\circ}\) to 10\({}^{\circ}\), the mean cutting force decreased significantly. Subsequently, if it increases to 10\({}^{\circ}\) or more, the cutting force converges to a constant value. When the cutting depth is 6 mm, the average cutting force converges to about 2.5 kN and 3.5 kN for the rocks with UCS of 30 MPa and 40 MPa, respectively (Figure 9b).  
Figure 8: Variation in the cutting force with time for each rake angle.  
Figure 9: Relationship between the mean cutting force and clearance angle cutting depths (\(p\)) of: (a) 3 mm, (b) 6 mm, and (c) 9 mm.  
The peak cutting force also exhibited changes similar to those of the mean cutting force (Figure 10). For cutting depths of 3 mm, when the clearance angle is increased from 0\({}^{\circ}\) to 5\({}^{\circ}\), the peak cutting force reduces by 71% from 8.47 kN to 2.44 kN for the rock with a UCS of 20 MPa. For the same increase in the clearance angle increase and same cutting depth, the peak cutting force decreases by 47% from 11.28 kN to 5.94 kN for the rock with a UCS of 50 MPa (Figure 10a). When the cutting depth is 9 mm, it reduces by 69% from 30.8 kN to 9.65 kN and and 45% from 43.99 kN to 24.05 kN for rocks with USC of 20 MPa and 50 MPa, respectively (Figure 10c). However, the reduction was smaller than that of the average cutting force.  
Even when the clearance angle increased above 10\({}^{\circ}\), the cutting force converged to a constant value equal to the mean cutting force. When the cutting depth was 6 mm, it converged to about 5.6 kN and 6.8 kN for the rock samples of UCS 30 MPa and UCS 40 MPa, respectively (Figure 10b). The details of the mean and peak cutting forces according to the clearance angle are listed in Table 3.  
This phenomenon also seems to be caused by the friction between the cutting tool and rock. If the clearance angle is increased from 0\({}^{\circ}\) to 5\({}^{\circ}\), the contact surface with the rock is significantly reduced, resulting in a sharp decrease in cutting force. If the angle exceeds 5\({}^{\circ}\), it appears to converge to a constant value because the decrease in the contact surface is insufficient to significantly affect the frictional force.  
Figure 10: Relationship between the peak cutting force and clearance angle for cutting depths (\(p\)) of: (a) 3 mm, (b) 6 mm, and (c) 9 mm.