Abstract. The investigation provides a method of efficiency evaluation among several kinds of tools employed to cut different types of rocks. A ballistic test stand incorporated with a modified Hopkinson bar was used to simulate a percussive drilling tool. An Apple IIe microcomputer was employed for high speed data acquisition and numerical analysis. A newly developed method for remote monitoring the rock-tool interaction, including contact force, bit penetration and energy transmission is presented in the paper. Theoretical predictions and experimental results appear in faily good agreement.
INTRODUCTION
Rock fragmentation and comminution through the impact of a hard metallic object is an ancient art, which still remains the most universal rock-cutting technique. Percussive drilling machines were introduced more than one hundred years ago and have been extensively used for rock drilling. In percussive drilling (Fig. 1), the kinetic energy of a slowly moving, pneumatically-driven piston is transformed into the energy of a strain pulse traveling along a transmission rod. The rod is used as a wave guide to transfer the strain pulse to the tip of the tool in contact with the target. Upon arrival of the pulse to the tool bit, penetration and fragmentation are induced simultaneously. To model the phenomena involved, a one-dimensional theory of elastic wave propagation has been applied to percussive drilling by Takaoka and Hayamizu (1956), Fairhurst and Kim (1958), Fischer (1959), Arndt (1959), and Simon (1964). Further, comprehensive studies have been conducted by Hustrulid and Fairhurst (1971, 1972), and Lundberg (1973). A simulation of stress wave energy transfer to rock based on the input data of incident wave and rock-bit interaction assumption was investigated with a microcomputer by Lundberg (1982). The efficiency of a percussive drilling tool is in general, determined based on: (i) the energy extraction and wave generation from this piston, and (ii) the conversion of the strain pulse energy to destructive work through the action of the drill bit on the rock. Digital computers were used by Simon (1962), and Dutta (1968) to predict the stress wave forms generated by any shape of piston. Good agreement is generally obtained between the measured and anmlytical stress wave forms if the abrupt changes in the shape predicted by the elastic wave theory are smoothed out. Rock-bit interaction is a complicated phenomena. The energy required for crater formation and the force needed for bit penetration are major issues frequently raised in the design of percussive drilling tools. The property of rock and the geometry of bit are two important factors affecting the tool performance. Hustrulid and Fairhurst (1971) have carried out a theoretical and experimental study of percussive drilling. A formula was obtained for predicting the penetration rate of the tool if the amount of fracture energy per unit volume of rock broken was known for the drill bit and the rock type associated with operational parameters (e.g., blow energy and blow frequency).
The rock-bit interaction can be characterized in terms of the force-penetration relationship. Obvious differences exist between static and dynamic loading in rock penetration as indicated by Haimson and Fairhurst (1970) and Goldsmith and Wu (1981).
