Effect of Bit Configuration in Impact Penetration Tests with Button Bits

Computer simulation of percussive rock drilling requires appropriate numerical models for the interactions among the drill body, rod, rod joint, bit, and rock. In this study, the impact penetration of a bit into rock was investigated for the modeling of the bit-rock interaction. Impact penetration tests on Inada granite were carried out with six rod-bit configurations which were composed of four kinds of button bits and two kinds of rods. In the calculation of force-penetration curves from the measured rod strains, the bit model constructed from the acoustic impedance was simplified, and the empirical data correction method proposed by the authors was applied to all the rod-bit configurations. The force-penetration curves for the six rod-bit configurations showed that the bit force in loading phase was approximately proportional to the square of the penetration. The curves in unloading phase had a linear relation between the bit force and the penetration. The final penetration of each blow had a linear relation with the maximum penetration, and the measured borehole depth was proportional to the maximum penetration. The effect of rod diameter on the force-penetration curves was not obviously observed. On the other hand, the bit force corresponding to the same penetration increased and the specific energy decreased with an increase in bit diameter or in the number of button tips on the bit. These findings will contribute to the improvement of the accuracy in the simulation of percussive rock drilling.