Optimal design of the coring bit cutting edge in percussive/vibratory drilling

The objective of this study is to develop an analytical method for the optimization of the design of the coring bits of vibratory drills with cutting teeth that percussively penetrate into brittle material. This optimization is intended to improve the drill rate of the Ultrasonic Driller and Corer (USDC) that was developed by scientists from JPL and engineers from Cybersonics, Inc. The USDC was developed for possible in-situ sample and other extraterrestrial applications in future NASA missions. A theoretical investigation was undertaken to study the rock-bit penetration characteristics and derive the analytical formulation of the specific energy for a coring bit with wedge-shape cutting teeth, based on two analytical models from rock fracture mechanics. It was found that there exists an optimal spacing/depth ratio for the cutting teeth, and the optimal number of cutting teeth on the bottom annulus of the coring bit can be designed to achieve a minimum total coring bit specific energy. Experimental tests were performed to corroborate the analytical results. It was shown that the laboratory drilling tests follow the trend predicted by the theoretical analysis. The methodology developed here can be used for the optimization of any coring/drill bit with a wedge-shape cutting teeth array in percussive/vibratory drills into brittle material.