Influence of indexation and impact energy on bit/rock interface law in percussive drilling: an experimental study

This paper presents the results of a laboratory study aimed at assessing the influence of indexation (the ratio of the bit angular velocity over the activation frequency) and impact energy on the performance of percussive drilling. A block of Kuru grey granite is subjected to sequential impacts with a Ø33mm seven-button drill bit driven by a drop machine. The force versus displacement curve is obtained through a strain gauge located on the impacting tup and the volume of removed rock is captured by taking a rubber specimen of the drilling bottom-hole, cleaned by air-flushing, after each impact. These two measurements enable to investigate the link between the penetration of the bit after each impact and the average rate of penetration of the bit after multiple impacts. Understanding and quantifying the dependence of these relationships on indexation and impact energy will lead to the formulation of more realistic bit/rock interface laws that implicitly account for repeated impacts of the bit on the rock.