Fundamental Study into the Mechanics of Material Removal in Rock Cutting

The objective of this work was to understand the mechanics of material removal during rock cutting. The exact nature of the failure of the rock material at the tool tip was investigated using a single cutting tooth test rig coupled with high speed photography, for various rock-like specimens. Linear cutting tests were performed using a tungsten carbide tipped orthogonal cutting tool with three different rake angles on low and high strength simulated rocks. Statistical analysis together with high speed video analysis were supported by numerical simulation, performed using a commercially available code called ELFEN; a hybrid finite-discrete element software package.
The material removal process was modelled by studying the cutting and thrust forces in relation to the high speed videos, specific cutting energy and the chip removal process. Although considerable amounts of published work are available, which describe the mechanism of material removal while machining rocks, no systematic, dedicated research investigating the material behaviour at the extreme cutting edge has been carried out, in particular, at the microscale level. The material behaviour at the extreme cutting edge contributes to the mechanism of the material removal. Compared with its counterpart such as metal cutting, which is a highly established and well understood domain, the heterogeneous nature of rocks renders it difficult to apply a particular study simply based on one variety of rock.
In order to ensure repeatability and consistency in experimental data, the use of rock-like specimens was considered critical. Hence, using various concrete mixes, samples were manufactured and categorised by testing their mechanical properties, i.e. Compressive strength, Flexural strength, Young’s Modulus and Density. Linear cutting tests were performed on the samples and force readings using a tri-axial dynamometer were recorded and analysed. High speed video system incorporated in the test rig also recorded the cutting process. Specific cutting energy was calculated and correlated with other cutting parameters such as depth of cut and rake angle of the cutting tool.
Exhaustive statistical analysis was carried out to understand the influence of specific cutting parameters and mechanical properties on the cutting process. Finally, numerical simulation of the cutting process was modelled using ELFEN, a Finite-Discrete Element coupled code. This yielded the important results as to the stress encountered in the specimen in the immediate vicinity of the cutting tool and also insights into the fracture initiation and propagation.
The influence of cutting parameters on the measured cutting force and thrust force showed the effect of material strength, cutting tool geometry and depth of cut were important. General observations showed the increase of cutting force as the depth of cut increases. Specific energy was found to decrease as the depth of cut increased. The formation of the crushed zone was studied using the high speed video camera and found to play an important role in the cutting force component; as the crushed zone built up the cutting force was found to increase until failure. Numerical simulations also showed the formation of crushed zone and the state of stress at the tool tip.