A theory of percussive drill bit penetration

Abstract The author has developed a theory of percussive bit penetration. The theory is in substantial agreement with his experimental results. In developing the theory the author has assumed a mathematical model which is based on some of his experimental observations. The mathematical force-penetration relationships deduced by the author have been compared with experimental results of the author himself and also with the results obtained by Reichmuth, and Gnirk and Cheatham. It is concluded that (a) the force-penetration curve for each bit-rock combination reflects the characteristics of both the rock type and the bit. (b) During the crushing phase, rock in contact with the bit becomes pulverized to a certain depth and subsequently becomes compact under increasing force. The size of this zone is a function of bit geometry, penetration and angle of internal friction of the rock. (c) The friction between the pulverized wedge and the confining solid rock has a great influence on the chipping force. (d) The angle of fracture, profile and shape of the crater are in substantial conformity with the theoretical prediction and from the theory it is also possible to predict the forces for given depths of penetration.