Dynamics of ultrasonic percussive drilling of hard rocks

Ultrasonic percussive drilling with diamond-coated tools has been extensively studied under laboratory conditions on rocks such as sandstone, limestone, granite and basalt, in order to investigate the applicability of this technique to downhole drilling. An experimental set-up, a programme of work and example results are presented. The studies showed that an introduction of high-frequency axial vibration significantly enhances drilling rates compared to the traditional rotary type method. It has been found out that the material removal rate (MRR) as a function of static load has at least one maximum. Looking at the time histories of the measured drilling force, strong nonlinear effects have been observed, which were explained using simple nonlinear models. Among them, pure impact and impact with dry friction oscillators were used to provide an insight into the complex dynamics of ultrasonic percussive drilling. It is postulated that the main mechanism of the MRR enhancement is associated with high amplitudes of forces generated by impacts. Novel procedures for calculating MRR are proposed, explaining an experimentally observed fall of MRR at higher static loads.