# Investigation of Rock-Breaking Mechanisms Based on the Adaptive Matching Method for Drilling Loads
## 2 Methodology
### Loading Method for Adaptive Matching of Drilling Loads  
There are two methods for downhole load adaptive adjustment, which indirectly regulate the cutting depth of a PDC cutter while intruding into bottomhole rock or reducing applied external loads during rock breaking.  
Cutting depth control: By monitoring the axial invasion depth of the PDC drill bit into the bottomhole rock during rock breaking, when the cutting depth exceeds a predefined threshold, the magnitude of the applied axial force is automatically reduced by some amount. This reduction subsequently decreases the single-pass cutting depth.  
External load control on cutters: During the process of rock breaking in the bottomhole with PDC drill bits, three-dimensional reaction forces are monitored. If the reaction force in the cutting direction surpasses a set threshold, the cutting speed of the PDC cutter gradually decreases, leading to a reduction in rock-breaking efficiency. At this stage, lowering the external axial force causes the single-penetration depth to decrease.  
During drilling in strongly heterogeneous formations or soft-hard interbedded formations, there will be a high contacting force suffered in the PDC cutters of the drill bit, resulting in greater stress while cutting a piece of a large particle with certain high strength. Therefore, this paper proposes a kind of adaptive matching drilling load method based on the above second load adjustment method. The axial load acting on the PDC cutter (similar with the weight on bit, or WOB) can be adjusted automatically to release the contacting force of the drill bit by reducing the cutting depth in the bottomhole. Figure 1 shows the working principle and the 3D design structure of a kind of tool using a downhole load-adjusting method according to the loading states of the axial load.  
Figure 1(a) indicates the loading principle of automatic downhole load adjustment. This mechanism operates as follows: when the drilling force exceeds a predefined threshold and leads to significant load or torque fluctuations in the process of rock breaking by the drill bit at the bottomhole, the spiral guide rod rotation device integrated within the tool will automatically activate. This action lifts the drill bit by a specified axial distance, thereby reducing both the cutting depth and the reaction force exerted on the drill bit.  
A tool implementing this working principle was structurally designed as shown in Figure 1(b), of which the detailed component configuration is presented in Figure 2. As can be seen in Figure 2, it consists primarily of a housing, upper connector (box), lower connector (pin), spiral guide rod, and disc spring assembly. Here, the terms \"box\" and \"pin\" are alternatively used to denote the upper and lower connectors, respectively.  
The implementation of this tool enables automatic and prompt mitigation of the downhole load acting on the drill bit during drilling. This dynamic adjustment method can lead to a reduction in the contact force exerted by the PDC cutters integrated into the drill bit, thereby minimizing the risk of bottomhole drilling failures caused by excessive impact loading.  
Figure 1: Loading principle and design structure of the adaptive matching tool.