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.
Down-the-hole (DTH) percussive drilling is a widely used technology to complete boreholes in hard rock formations. This technology consists in applying repetitive impacts on a continuously rotating drill bit (Lindqvist, 1984; Hustrulid and Fairhurst, 1971). At each impact, compressive waves travel the drill bit down to the rock material, which enable the bit penetration by indentation of the drill bit buttons into the rock material. Crushing of the rock occurs right under the indenter and consumes most of the energy provided to the drill bit (Saksala et al., 2014). However, most of the material removal is achieved by the formation of larger rock fragments from lateral cracks. Indeed, this so called chipping process creates craters that are larger than the bit button marks i.e. rock is fragmented and removed under and between the buttons (Lindqvist and Lai, 1983). This process requires (i) a minimum amount of impact energy (Saksala et al., 2014) and (ii) a rotation of the drill bit between each impact. This work is motivated by the recognition that despite numerous experimental and theoretical studies (Hustrulid and Fairhust, 1971; Lundberg, 1973a, 1973b; Kou et al., 1995), the dependence of the penetration rate on indexation and impact energy remains poorly understood. A subsidiary issue that we address is the link between the penetration of the bit after each impact and the average penetration rate of the bit after multiple impacts: due to the chipping of the rock, the observed rate of penetration (ROP) obtained during percussive drilling (PD) is larger than what could have been inferred from the penetration obtained with a single impact test made in laboratory. This relationship between the bit ROP and the button penetration depends on the indexation between two percussive activations and presumably also on the impact energy. Understanding the relations between average bit ROP and single impact bit penetration, and quantifying the dependence of these relationships on indexation and impact energy will lead to the formulation of more realistic bit/rock interface (BRI) laws that implicitly account for repeated impacts of the bit on the rock. Realistic BRI laws are critical for modeling the dynamical response of DTH percussive drilling systems (Depouhon, 2014; Depouhon et al., 2014).