Tree-type drilling is a new technique for drilling radial tree-type boreholes in coal seams in underground mines using water jets to break the coal. The aim is to drain gas from the coal seams in larger quantities and from larger areas than can be done by conventional gas drainage using single boreholes. The self-propelled drill bit is the most important component for this technique. The bit generates a self-propelling force to move forward, break coal, and form a borehole. This paper

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... es the relationships between the physical parameters of the forward nozzles in the bit and the diameter and shape of the borehole drilled. The effect of different physical parameters on the rock-breaking efficiency is studied by conducting drilling experiments. The results show that the size, orientation and number of the nozzles significantly affect the rock-breaking efficiency of the bit. To obtain a better rock-breaking efficiency under the experimental conditions used for this paper, the axial angle for forward nozzles should be 15˝, the radial angle 90˝, and nozzles should be arranged 2.1 mm from the center of a 12 mm drill bit. The experimental results provide a reference for the design of multi-nozzle bits for many applications such as radial jet drilling (RJD) and bent pipe cleaning. Energies 2016, 9, 249 2 of 17 drill holes are required because gas extraction from a single borehole is low and the area drained by each hole is small. To solve these problems, a new technique for gas drainage has been proposed that can create radial tree-type boreholes by utilizing water jets to break the coal and drill holes in the coal seams. As shown in Figure 1 , a whipstock is placed in a conventional gas drainage borehole (called the main borehole in this paper). Then a self-propelled bit at the end of a flexible high-pressure hose is inserted into the main borehole drill pipe, run through whipstock, and then used to drill tree-type sub-boreholes perpendicular to the main borehole. By changing the position and orientation of the whipstock, a series of radial tree-type boreholes can be completed as shown in plan A-A (Figure 1) . A detailed description of the drilling processes is provided in [14] . The self-propelled bit is without doubt the most critical piece of technology for drilling tree-type boreholes. It both breaks the rock to form the tree-type sub-borehole and also generates the self-propelling force to move forward.