Cutting forces in the linear rock cutting process

Abstract

Several calculation models have been developed to predict the cutting forces in the linear rock cutting process. Rock failure can occur in different manners: brittle tensile, brittle shear and ductile. Each failure mechanism has a corresponding calculation model with specific cutting geometries and assumptions to predict the cutting forces as accurate as possible. However, several researches conducted in the past show that the predicted forces can differ considerably from the forces measured in the experiments. In this research the author proposes expansions to the existingmodels to increase the accuracy of the force predictions and thereby contribute to a more (cost) efficient cutting process. The calculation models considered in this research are based on two main assumptions: the cutting process is 3D and the tip of the cutting tool is sharp. In a 2D cutting process the width of the cut is equal to the width of the pick point and stays constant throughout the cut. During experiments the sideways outbreaking of rock pieces is observed which can increase the width of the cut tremendously and therefore increase the actual cutting forces. This sideways outbreaking makes the rock cutting process in fact a 3D problem. The assumption of a sharp cutting tool results in another inaccuracy in the force predictions. A sharp cutting tool will penetrate the rock more easily and create a clean cut. If the sharpness of the cutting tool decreases, for example due to wear and tear during the cutting process, a crushed zone will appear in front of the cutting tool, while a wear flat arise at the tip of the cutting tool. These phenomena result in extra forces that has to be overcome by the cutting force. For large cutting depths these effects might be negligible, but for small cutting depths these forces need to be considered. In some applications in the offshore environment only small cutting depths are encountered, such as the vertical drilling for monopile installation or the scraping techniques used in the oil and gas industry. An expansion to include these phenomena in the force prediction models would therefore be a widely applicable concept. After conducting experiments on sandstone samples the magnitudes of the 2D force prediction models, the forces due to the 3D effects and the indentation forces are established. Afterwards the empirical relations of these terms are composed based on the rock characteristics and cutting geometry. The results of this research show that the proposed expansions increase the accuracy of the cutting force predictions tremendously. It also shows that for cutting at a very small depth the forces predicted by the 2D models are negligible compared to the 3D expansion and the indentation expansion, but the 2D models are necessary to calculate the angle and length of the shear plane. However, if the cutting depth increases the indentation force converges to a constant value and the influence of the 3D expansion decreases as well, while the forces in the 2D models increase. The author therefore suggests to keep the 2D effects into the equation to keep the total formula broadly applicable.