Influence of polycrystalline diamond compact bit geometric parameters on drilling performance during gas drainage borehole in soft coal seam

Polycrystalline diamond compact bit is the main rock-breaking tool of gas drainage borehole. In order to investigate the influence of geometric parameters of polycrystalline diamond bit on its drilling performance, the integrated force model of polycrystalline diamond bit pressed into rock was established. The stress analysis results show that the tooth rake angle, pressing depth, and horizontal tangential displacement are the main factors to influence the normal load and the axial load of polycrystalline diamond bit. There is an optimal tooth rake angle value of polycrystalline diamond bit cutter when it is used in the soft coal seam. The drilling experiment system was built to verify the drilling performance of drill bits with different geometric structure and different tooth rake angle through drilling the soft coal wall. The experiment results show that the drilling torque and feeding resistance of polycrystalline diamond bit are higher than that of the wing bit at the beginning of drilling the soft coal wall. With the increase in the drilling depth, the feeding resistance of wing bit will increase to exceed that of the polycrystalline diamond bit. So the polycrystalline diamond bits should be selected to drill the gas drainage borehole in soft coal seam. The polycrystalline diamond bit with bigger tooth rake angle cannot cut and break the coal rock effectively, which will reduce the drilling efficiency and the feeding speed. When the tooth rake angle is smaller, polycrystalline diamond bit cannot cut in coal wall effectively, which will increase the feeding resistance. So the polycrystalline diamond bit with tooth rake angle of 15 degrees-20 degrees is suitable for the gas drainage borehole in soft coal seam. With the increase in the rotary speed of polycrystalline diamond bit, the cutting thickness of coal rock decreases and the horizontal tangential displacement increases. When the rotary speed of polycrystalline diamond bit is 290 r/min, the mean drilling torque and mean feeding resistance of polycrystalline diamond bit reach the minimum. Therefore, in order to reach the optimum drilling performance, the working parameters of low drilling pressure and high rotary speed are usually selected during the actual gas drainage borehole in soft coal seam.