A theoretical and experimental study of the percussive drilling of rock Part II—force-penetration and specific energy determinations

Abstract

Force-penetration and energy per unit rock crater volume were determined ‘statically’ by repeated forced indentations into the rock, rotating (indexing) the bit equally between each indentation, so as to ‘drill’ a shallow hole in the rock. Similar dynamic tests were conducted using a ‘drop tester’ in which the bit is attached to a heavy mass and allowed to free-fall onto the rock. Indexing angles of 10°, 20°, 30° and 40° were used. The static energy per unit volume for the three rocks (Tennessee marble, Swedish West Coast granite, Charcoal Grey granite) tested was found to be much less than the dynamic. Similarly, the slope of the force-penetration curves for tests on smooth surfaces were much higher dynamically than statically. The difference was less noticeable for blows on a previously surface. An explanation based on rate of loading effects is given.