Investigation of Insert-Hole Interference Fit Tolerance Influence on DTH Drilling Bit Performance

DTH drilling is a reliable method for drilling in medium to hard geological structures. Considering the country's need for different types of drilling bits in various industries, with development of knowledge and production of them, it can limit the vast importation of these products. In this study, process of bit/rock collision taking into account interaction between the insert/body and stresses resulting from interference fit is simulated to determine proper tolerance for fitting, which is the most important factor limiting the life of drilling bit. Simulations were carried out taking account rotary motion of bit in addition to impact in single-insert condition with different angles of collision and also in a 3.5 inch bit with 13 inserts. Mechanical properties of specimens made of DIN1.6580 steel, used for manufacturing of bit body, were obtained through experiments. Then, in order to validate and also obtain appropriate mesh size, results of pin-hub interference fit simulations with selected parameters were compared to Lame's analytical method. Results show that interference tolerance and speed and angle of impact have great influence on stresses and process forces. In case of 1.2% of interference, stresses in bit body are about to reach material strength. In the case of 0.9% and 0.6%, certain amount of plastic strain is also created in the body. The results of this study is greatly helpful in choosing fitting tolerance of inserts into bit body during manufacturing process.