A semi-empirical approach to cutting force prediction for point-attack picks

In the mining and civil engineering industries point-attack picks (conical picks) are extensively employed on mechanical excavators such as roadheaders, continuous miners and longwall shearers to cut relatively harder rocks/coals compared with radial picks. They are more durable than radial picks, and can economically cut intact rocks having up to around 120 MPa uniaxial compressive strength at low abrasivity index1. A review of the literature shows that the general behaviour of point-attack picks in rock cutting2,3, wear mechanism4–7, efficiency and performance8–12, measurement of pick forces for machine design13, comparison with radial picks14, rotational properties15, and chip formation mechanism16,17 are among the topics that have received the attention of researchers. Another topic that has also received attention is the prediction of cutting forces. Knowing the magnitude of the cutting forces is an important aspect of machine design, since it allows the engineers to estimate the cutterhead torque and machine power requirements for a particular application. Perhaps due to the complexity of the required three-dimensional analysis of their cutting mechanism, only limited analytical models have been proposed for the prediction of cutting forces. Evans’s rock cutting theory18 for point-attack picks is the most widely recognized and most frequently cited in the literature. His theory attempts to give an insight into the mechanism of rock breakage under the action of a symmetrically acting point-attack pick, and enables the peak cutting force to be calculated from pick geometry, rock properties and cutting depth. Although it is one of the most practical formulas offered for calculating the peak cutting force, as emphasized by Evans and some other authors1,19, it has some deficiencies, which have recently been improved by modifications to the theory19. Both in the original and modified theories of Evans, the cutting pick is assumed to act symmetrically along the line of advance, but in practice cutting picks do not act symmetrically on the cutter heads of mechanical excavators. This could be an important point in the calculation of cutting forces. In this work, considering the current limitations of these analytical models, Evans’s modified rock cutting theory19 is empirically extended by analysing full-scale rock cutting test data. Valid for asymmetrical cutting conditions, semi-empirical equations are developed for the prediction of peak cutting force and mean cutting force in terms of pick geometry, cutting geometry, cutting depth and tensile strength of rock. Regression analysis has also been carried out to check the validity of the proposed prediction equations. A semi-empirical approach to cutting force prediction for point-attack picks