Mechanisms underlying lateral force applied to a PDC cutter on a PDC drill bit during rotary rock breaking

ObjectiveTo improve the rock-breaking efficiency and durability of drill bits in the theoretical research on the optimal design of drill bits, the key is to systematically reveal both the rock-breaking mechanisms of polycrystalline diamond compact (PDC) cutters and the characteristics of forces applied to them. MethodsExisting studies on rock breaking using a single PDC cutter generally adopt a linear or approximately linear cutting mode while neglecting the impacts of the radius of gyration. To address this issue, this study conducted experiments on cylindrical granite samples using a laboratory experimental setting for single-cutter rock breaking. With cutting depth and the radius of gyration as variables, this study systematically investigated the rotary rock-breaking process under small radii of gyration using a PDC cutter on a scale of a drill bit diameter of 215.9 mm. Accordingly, the impacts of the radius of gyration on both the rock-breaking process of a PDC cutter and the applied forces were identified. Furthermore, the conventional method for calculating mechanical specific energy was optimized by indirectly considering the impacts of the lateral force on rock-breaking efficiency. Based on this, the impacts of the radius of gyration on the rock-breaking efficiency of a PDC cutter were quantitatively evaluated. Results and Conclusions The results indicate that the radius of gyration exerted a significant impact on the lateral and normal forces applied to the PDC cutter. Specifically, a smaller radius of gyration corresponded to a larger lateral force, which increased the normal force in turn. The morphological analysis of rock cutting traces reveals that during the rotary rock breaking under a small radius of gyration, rock spalling on the outer edge of cutting traces created large cuttings and crushed pits. Consequently, only the inner cutting edge of the PDC cutter came into contact with rocks, inducing an amplification effect of lateral force. The analytical results of the energy consumption in rock breaking show that a smaller radius of gyration was associated with a higher energy consumption of the PDC cutter. For a PDC drill bit, increasing the conical surface angle at the bit center can reduce the rotational curvature differences between the cutting edges on both sides of the PDC cutter. This helps reduce the adverse impacts of the radius of gyration on the rock-breaking efficiency. The results of this study account partially for the limited rock-breaking efficiency and durability of PDC cutter at the center of a drill bit, providing a theoretical reference for the optimal design of PDC bits.