Theoretical and practical investigations of improved hard rock cutting systems

Rock cutting machines such as roadheaders are faced with serious problems when working in hard or heterogeneous and abrasive rock, due mainly to excessive tool wear and insufficient thrust (or sometimes power). One of the most promising techniques for improving tool performance is the use of medium to high pressure water-jets (50 to 100 MPa) to assist the cutting tools. A prerequisite to understanding the factors controlling performance, and hence the potential for further improvement, is an accurate model of hard rock cutting. Research undertaken at the Centre Mines Infrastructure (CMI) of the Ecole Nationale Superieure des Mines de Paris (ENSMP) has shown that the compressive failure of brittle rock under the action of a pick is a combination of two fundamental modes: shear/compression (mode A), reasonably accurately modeled by the Coulomb criterion, and fracture propagation (mode B), which is best described by use of fracture mechanics. The dominance of one mode or the other is closely related to the geometrical shape of the zone of crushed rock surrounding the tool tip. In addition, to properly calculate the torque and thrust on a cutting head, various tool environments and factors such as interaction between neighboring cuts and deepening of existing cuts must be considered. Analysis of the mechanics of force transmission through the crushed zone to the rock and of chip formation has led to a better understanding of the influence of the water-jet on forces during cutting. Studies conducted at CMI have shown that cutting and normal force reductions observed when cutting at speeds comparable to those in practical mining excavation systems are substantially less than those reported previously from laboratory tests at low speeds. Even so, water-jet assistance offers major opportunities for improved machine performance through reduction of tool wear and enhanced interaction between neighboring cuts, resulting in less vibration and in a better penetration rate of the cutting head. Reduction of wear rate is clearly an important subject worthy of research emphasis. Diamond tip tools offer promise in this regard. 554 Additional research on enhanced tools is warranted, but it is important to ensure that testing situations reproduce in–situ conditions especially with respect to tool speed.