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Abstract
Mechanical excavation by the point attack bit of the long-wall shearers is of key importance in modern mining sites. However, serious wear, heavy airborne dust and excessive power consumption are three major problems in rock cutting using current conical bits. Regarding the above defects and enlightened by the Vicker's indentation tests on brittle materials, a pyramidal bit was put forward in order to replace the cone. Then, two stages of practical work, edge chipping and linear cutting, were set up to investigate the availability and further improvement of the pyramidal bit.
An edge chipping test can be used to simulate the rock cutting process due to the similarity of crack systems. Such experiments were then carried out to comparatively explore the cutting performances based on different crack mechanism, fine grain generations, chip sizes, cutting forces and total energy consumption. Results demonstrated that the pyramidal bit generates less radiated cracks, larger size chips and less energy consumption. Moreover, it was also found that the 2D crack trajectory of the chip from the side view is close to a straight line. The straight trajectory is angled with the force axis and this angle was found to be linearly correlated with the attack angle. In addition, the chipping force curves were found to be well quadratically fitted. The minimum peak force induced by the pyramidal bit was noted to exist at the orientation of diagonals with and to the free surface. Besides, the best-fit power relationship between the peak force and depth of cut was close to 1.3 and the power values of the total chipping energy and depth of cut were found to be in a range from 1.85 to 2.29.
Then, a model describing edge chipping in relation to the peak chipping force, material properties, attack angle and depth of cut was derived in line with the assumptions from the observations in the edge chipping tests. It was discussed that the trajectory of maximum shear stress can be the most likely explanation of the straight crack path observed in chipping experiments. The derivation of toughness based on this derivation provides a convenient way of predicting the rock toughness. Specifically, the power relationship between peak chipping force, total energy and depth of cut are theoretically explained.
Linear cutting tests were carried out on Helidon sandstone and Harcourt granite for comparing the cutting and wear performances of all bit configurations. The initial pyramidal bit profile was improved by modifying the top and bottom cutting angles, side surface and setting the optimum cutting orientation, in terms of the result analyses in the cutting tests. Then, cutting performances were investigated and compared in relation to fracture mechanism, crush zone size, fragment size distribution, excavated rock mass, cutting force and specific energy. Results show the higher cutting efficiency of the improved bit configuration is dependent on the larger fraction of big fragments, much lower mean cutting force and specific energy. Fractal analysis quantitatively validates the cutting efficiency by dealing with the larger fracture surface area and 3D multivariate analyses provide a comprehensive view of the double factor effect. A power relationship was also found between the mean cutting force and depth of cut although the force amplitude is different from that in edge chipping, which further derived the power relationship between specific energy and depth of cut. Besides, PCD bits possessed much higher wear resistance than that of WC bits, which further demonstrates the availability of the potential application of pure PCD material in hard rock cutting.