## 6 Conclusions  
In this paper, a finite element model of heterogeneous granite was constructed with the Drucker-Prager criterion employed for rock failure. The accuracy of the rock model was validated through uniaxial compression simulations. The impact of different combinations of axe, triangular prism, and cylindrical PDC cutters on rock-breaking efficiency was analyzed, with optimization of cutter spacing parameters under the optimal scheme using the Box-Behnken approach. The rock-breaking efficiency of the PDC drill bit with the optimal cutter arrangement was compared with that of single triangular prism and axe  
Figure 24: Effect of using a bit: **(a)** Before using the bit with a mixed arrangement cutter; **(b)** After using the bit with a mixed arrangement cutter. Figure 25: Data from the actual drilling process of the bit. drill bits. The feasibility of the simulation results was verified through field experiments. The following conclusions can be drawn:  
1. Due to the distinct shape variations between the axe, triangular prism, and cylindrical PDC cutters, the cylindrical PDC cutter predominantly crushes rocks in a shearing manner. On the other hand, the protruding ridge structure of the axe and triangular cutters exhibits a compressive effect during the rock-breaking process, thus enhancing its efficiency. The crushing effect of the axe cutter is notably more significant, raising the maximum stress on the rocks by 8.9% compared to the triangular prism cutter. This results in a stronger assault on the rocks, making it well-suited for front-row placement in drill bit cutter arrangements. Although the crushing effect of the triangular prism cutter does not match that of the axe cutter, its symmetric structure ensures stability during the cutting process, making it fit for the rear layout of the cutter. The rock-breaking performance of the cylindrical cutter is found to be the least efficient, suggesting its placement in the rear of the drill bit blade. 2. Different combinations of cutter arrangements have a significant impact on rock-breaking efficiency and stability. Therefore, it is necessary to make a rational selection of the arrangement scheme. When designing a mixed arrangement of cutters for a drill bit, it is recommended to place the axe cutters in the leading row of the blade while alternating the triangular prism and axe cutters in the rear row of the blade to fully utilize the benefits of the mixed arrangement of cutters in the drill bit. 3. The spacing between the cutters has a significant impact on the stability of the rock-breaking process. When designing a mixed arrangement of cutters for a drill bit, it is recommended to set the longitudinal distances between the leading row of axe cutters and the rear row of triangular prism cutters and axe cutters at 10 mm and 10.06 mm, respectively. The transverse distances between the centers of the rear row of triangular prism cutters and axe cutters and the centers of the leading row of axe cutters should be set at 7 mm to ensure that the mixed arrangement design maximizes drilling efficiency. 4. The numerical simulations and field drilling results indicate that compared to PDC drill bits with only axe or triangular prism cutters, the use of mixed arrangement cutters designs facilitates easier penetration of rocks, provides a more stable ROP during the rock-breaking process, and achieves higher efficiency. The utilization of PDC drill bits with mixed-arrangement cutters in complex drilling environments can effectively enhance the ROP and save drilling costs. 5. Although the numerical simulation method calculates the difference in the efficiency of the drill bit in breaking the rock in agreement with the field experiments, the calculation of the ROP of the drill bit is much different from reality. This is one of the limitations of numerical simulation of drill bit rock-breaking, but the obtained comparative simulation results are informative and can provide support for actual drilling.