Experimental Investigation On New Style Of Cemented Carbide Bits And Drilling In Hard Rock

Gas drainage is one of basic methods to prevent gas disasters in coal mine. but when encountered hard rock in borehole and Shimen coal drilling , the drilling bit will become lower and wear quickly, at the same time, frequent replacement of bit wastes a lot of time, which seriously affecting the efficiency of coal production. It has important significance to find a new way and new type of bit to drilling in hard rock. This paper theoretical analysis of the original three-wing rock-drill mechanism, This paper presents a new method of drilling hard rock which using abrasive water jet breaking out of the pilot hole, and then bit drilling. The new type of bit was designed .Indoor experiment contrast analysis the effect of rock broken by original bit and new type of bit .The main conclusions as follows:①The study shows rock-breaking mechanism of original bit in hard rock drilling. There existing a boss at the end of the drill hole. It needs to overcome the compressive strength of rock for tool to break the boss, at the same time, the cutting force increases. Heat dissipation is difficult because of the boss. As a result of temperature increasing, the tool strength decreases, which reducing the drilling speed.②This paper presented a new way to drilling in hard rock and designed a new type of bit, which includes three parts: the nozzle, pilot drill bit, three-wing basic bit. Abrasive water jet erosion a pilot hole to elimination the boss and pre-broken rock, further ,leading drill bit expand the center hole and broken the rock further, then the three-wing bit drill the hole. Mechanical analysis showed that the bit can change the blade stress state, reduce blade stress and increase hard rock drilling speed.③Abrasive water jet rock erosion performance is the key factors to impact drilling efficiency of new type of bits. The nozzle structure size is limited. The numerical simulation software fluent is used to optimize a fluent nozzle structure, and the jet velocity is the most preferred standard to determine the best nozzle structure: outlet diameter d2 = 3mm, the convergence angleα= 25°, aspect ratio L1/d2 = 2④Through the abrasive water jet erosion of hard rock experiments with optimized nozzle, the influence of nozzle diameter, pump pressure, abrasive concentration on the diameter of punching speed and punching is obtained. The three parameters affecting the impacting speed is nozzle diameter> pump pressure > density of grinding. Considering drilling speed and pilot drill bit blade spacing, there exists a best pump pressure 20MPa and density of grinding 7%⑤The laboratory experiments of hard rock drilling by the original bit, improved bit and new type of bit showed that the improved bit is 1.57 times as fast as the original bit, but torque increased 11.3%, and the wearing increased; new type of bit drilling speed is 3.72 times as fast as the original bit and the torque decreased 34.0%, and no significant wearing. The results show that the drilling speed by new type of bit is high and it can effectively reduce the drill wear.