## 5 Cutter Spacing Optimization and Bit Rock Breaking Simulation
### Full-Size Bit Rock Breaking Simulation
This renders the penetrating and fracturing of the granite more efficient while mitigating the stick-slip phenomenon to a considerable extent. Additionally, interpreting the visuals, it is quite perceptible that the PDC bit with a mixed cutter configuration exhibits the most prominent displacement in feed. The variance in feed between the bit with a triangular prism cutter layout and the bit with an axe cutter layout is marginal, substantiating that the bit with a mixed cutter configuration can engender a more advantageous effect. Figure 19: Drill cutting structure and finite element model. Figure 21 shows the bit footage curves of the three PDC bits during the rock fragmentation process. It is discernible from the data represented that the PDC bit equipped with a mixed cutter configuration has a notably superior bit footage, followed by the bit with a triangular prism cutter layout, while the bit with an axe cutter arrangement finishes last. While the axe cutters are more incisive, the triangular prism cutters possess a symmetric structure, and the force distribution during rock fragmentation is more balanced. This balance contributes to the stability of the drilling process; hence, efficiency is augmented. The reflected data from Figure 22 also demonstrates that due to the rigidity of the granite, the bit exhibits displacement in the opposite direction of the applied WOB during rock fragmentation, stemming from the inability to drill, which adversely impacts the drilling process. However, the PDC bit with mixed cutter configuration exhibits lesser displacement fluctuation, indicating a lower bounce propensity during drilling and the capability to  
Figure 21: Stress contour of a rock broken by a bit. Figure 20: Bit numerical simulation verification: (**a**) The rate of penetration of a bit under constant WOB; (**b**) Bit reaction at constant WOB and ROP. better maintain the stability of WOB and ROP. From the calculations conducted, the footage of the PDC bit with a mixed cutter configuration is higher by 16.6% and 16.8% when compared to the PDC bit with a triangular prism cutter and axe cutter, respectively. Figure 22b shows the instantaneous ROP fluctuation curve throughout the drilling process of the bit. The data indicates that the drill bit outfitted with a mixed cutter arrangement experiences the least velocity variance during the drilling operation, suggesting that this bit configuration penetrates granite with greater ease during drilling. This could potentially reduce drill bit stick-slip incidents, thereby enhancing overall efficiency and prolonging the lifespan of the bit. These observations are in accordance with previous analyses of bit footage. Based on calculations, the ROP exhibited by the PDC bit with a mixed cutter arrangement is elevated by 23.5% and 21.1% compared to that of the axe and triangular prism PDC bits, respectively. In addition, the numerical simulation of rock breaking by full-size drill bits shows that the triangular prism cutter, due to the symmetry of its structure, presents a better rock-breaking effect than the axe cutter in the actual drilling process. This is consistent with the results in the literature [35]. Through calculating the maximum, minimum, variance, and standard deviation of rate of penetration (ROP) for three types of drill bits, the stability of their rock-breaking performance was quantified, as shown in Table 3. The data indicated that the difference between the maximum and minimum speeds of the PDC drill bits using axe cutters, triangular prism cutters, and a mixed arrangement of cutters was 122.11 mm/s, 93.54 mm/s, and 46.72 mm/s, respectively. This implies that due to the symmetry of the structure, the ROP fluctuations of drill bits with triangular prism cutters were smaller than those with axe cutters, leading to a more stable rate of penetration. The ROP fluctuations of drill bits with a mixed cutter arrangement were even smaller, resulting in an even more stable drilling process.