## 4 Discussion: Analysis of Factors Affecting Drilling Forces
### Influence of Rock Mass Structural Characteristics on Drill Bit Forces  
The structural characteristics of rock masses play a crucial role in controlling the deformation and failure of rock engineering, ultimately determining the overall stability of the rock mass. Therefore, investigating the impact of these structural characteristics on drill bit forces holds significant importance. In this section, the author primarily considers three influencing factors related to rock mass structure: the dip angle of structural planes, their thickness, and the presence of soft-hard interlayers between rock strata. Each of these factors will be separately discussed in terms of their influence on thrust force and torque during the rock drilling process. The finite element mesh model containing the rock mass structural surface is shown in Figure 15. Among them, the inclination angle of the rock mass structural plane is 45\({}^{\circ}\) and 90\({}^{\circ}\), respectively, and the thicknesses of the rock mass structural plane are 1 mm, 3 mm and 5 mm, respectively. The 75# drill bit is also used for drilling. The process of drilling and rock breaking is controlled by setting the drilling speed and rotation speed. The drilling speed and rotation speed are set to 2 mm/s and 200 rpm, respectively. The drilling time is 2.5 s. The physical and mechanical parameters of the rock mass whose basic quality level is Level II in Table 1 are selected, the structural plane is replaced by solid units with a lower rock mass quality level, and the physical and mechanical parameters of the Level IV rock mass are selected. The inclination angles of the rock mass structural planes are 45\({}^{\circ}\) and 90\({}^{\circ}\), respectively, and the large principal stress cloud diagrams with a structural plane thickness of 3 mm are shown in Figure 16. The maximum stress values appear at the contact point between the drill bit and the rock, mainly distributed at the bottom of the well and around the wellbore; the minimum stress values appear at the structural surface and around the boundary. The stress values in the two areas of the 90\({}^{\circ}\) structural plane are symmetrically distributed, and the stress values in the lower area of the 45\({}^{\circ}\) structural plane are slightly larger than those in the upper area. This is due to the extrusion of the upper rock mass on the lower rock mass on the structural plane during the drilling process. Figure 14: Curve of torque and thrust force on different drill sizes. Figure 17 displays the relationship curves between drill pressure and torque and the thickness of structural planes, with structural plane inclinations of 45\({}^{\circ}\) and 90\({}^{\circ}\). A thickness of 0 mm indicates rock mass without structural planes. As observed from the graph, both drill pressure and torque decrease significantly as the thickness of the structural planes increases, indicating that the presence of structural planes in the rock mass has a substantial impact on drill pressure and torque. As shown in Figure 17a, for structural planes inclined at 45\({}^{\circ}\), drill pressure and torque initially experience a sharp reduction as the thickness of the structural planes increases. This behavior can be attributed to the tilted orientation of the structural planes, causing a relatively larger contact area with the drill bit when the structural planes are initially encountered. However, as the drilling depth increases, the contact point between the drill bit and the structural planes gradually shifts away from the center until the drill completely penetrates the structural planes, resulting in a less pronounced reduction. As shown in Figure 17b, when the inclination angle of the structural surface is 90\({}^{\circ}\), the thrust force and torque decrease in a relatively stable manner as the thickness of the structural surface increases.