## 3 Experimental Results and Discussion
### Effect of Cutting Direction and Joint Spacing on Cutting Performance
#### 3.2.1 Effect of Cutting Direction  
Figures 13 and 14 show the plots of the mean values and standard deviations of \(FC_{m}\) and \(FC_{p}\), respectively, for intact rock specimens and for different cutting directions in jointed rock mass specimens. Both \(FC_{m}\) and \(FC_{p}\) were lower in jointed rock mass compared to intact rock. Additionally, when the cutting direction was parallel to the joint plane (\(\alpha=0\)), the values were lower than when the cutting direction was perpendicular to the joint plane (\(\alpha=90^{\circ}\)). This suggests that the influence of the joint plane is greater when the cutting direction is parallel to the joint plane compared to when it is perpendicular. However, it should be noted that when the cutting depth exceeds 0.2 times the joint spacing (\(p\geq 0.2J_{s}\)), the \(FC_{m}\) for cutting parallel to the joint plane is greater than the \(FC_{m}\) for cutting perpendicular to the joint plane. As seen in Figure 13a, in the jointed rock mass specimen with a joint spacing of 30 mm, when the cutting depth exceeded 6 mm, the \(FC_{m}\) for cutting parallel was greater than the \(FC_{m}\) for cutting perpendicular to the joint plane. A similar trend was observed in the jointed rock mass specimen with a joint spacing of 60 mm at a cutting depth of 12 mm. Figure 15 shows the plot of the mean values and standard deviations of \(SE\) for intact rock specimens and for different cutting directions in jointed rock mass specimens. \(SE\) was higher in intact rock specimens compared to jointed rock mass specimens, and it was lower in cutting parallel to the joint plane (\(\alpha=0\)) compared to cutting perpendicular to the joint plane (\(\alpha=90^{\circ}\)). However, similar to \(FC_{m}\), when the cutting depth exceeds 0.2 times the joint spacing, \(SE\) was greater for cutting parallel to the joint plane compared to cutting perpendicular to the joint plane. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \(\alpha\) (Deg.) & \(p\) (mm) & \(s\) (mm) & \(s\)/\(p\) & \(s\)/\(J_{s}\) & \(FC_{m}\) (kN) & \(FC_{p}\) (kN) & \begin{tabular}{c} \(SE\) \\ (MJ/m\({}^{3}\)) \\ \end{tabular}  & \(s\)/\(p_{\text{opt}}\) \\ \hline \multirow{9}{*}{90} & 12 & 12 & 1 & 0.13 & 5.60 & 14.49 & 43.7 & 2.99 \\  & & 18 & 1.5 & 0.2 & 5.69 & 16.05 & 31.9 & \\  & & 24 & 2 & 0.27 & 7.75 & 18.39 & 34.0 & \\  & & 36 & 3 & 0.4 & 9.15 & 20.72 & 32.2 & \\  & & 48 & 4 & 0.53 Figure 14: Variation in \(FC_{p}\) with \(\alpha\): (**a**) \(J_{s}=30\) mm; (**b**) \(J_{s}=60\) mm; and (**c**) \(J_{s}=90\) mm. To analyze this in detail, the ratios of \(FC_{m}\) and \(SE\) in the perpendicular cutting direction (\(FC_{m90}\) and \(SE_{90}\)) to those in the parallel cutting direction (\(FC_{m0}\) and \(SE_{0}\)) were plotted against the ratio of line spacing to joint spacing (\(s/J_{s}\)), as shown in Figure 16. When the cutting depth is smaller than 0.2 times the joint spacing, the relative ratio of \(FC_{m}\) was mostly greater than 1. On the other hand, when the cutting depth exceeds 0.2 times the joint spacing, the relative ratio of \(FC_{m}\) was mostly less than 1. Notably, for s/Js greater than 0.4, the relative ratio of \(FC_{m}\) consistently fell below 1 (Figure 16a). A similar trend was observed for the relative ratio of \(SE\), which also became less than 1 when the cutting depth exceeded 0.2 times the joint spacing and \(s/J_{s}\) was greater than 0.4 (Figure 16b). Figure 16: Plot of the relative ratio of (**a**) \(FC_{m}\) and (**b**) \(SE\) to \(s/J_{s}\). As shown in Figure 17, when the cutting depth exceeded 0.2 times the joint spacing and \(s/J_{s}\) was greater than 0.4, only a single cut was performed between the joint planes. This suggests that when the cutting depth exceeds 0.2 times the joint spacing, and as the line spacing increases beyond 0.4 times the joint spacing, the interaction between cuts is progressively hindered by the joint planes. Eventually, this causes a transition to the unrelieved mode, leading to a decrease in cutting efficiency.