Fig. 6

Cutting forces in sandstone. The cutting forces exerted on the tool during the tests are shown in a as a scatter plot for Rorschach sandstone under untreated (violet squares) and after-treatment (green circles) conditions. Symbol sizes indicate the wear flat measured at the tool tip (see legend). Here, the curves obtained from bi-linear regressions of the data are shown as dashed lines. In b, the compressive stresses, exerted on the tool by the rock are calculated using the measured force values, the contact surface area under the stress state and a friction coefficient of 0.65 (Roshan et al. 2017). In b, the scattered data distributions, representing the strength of the rock, are summarized by boxes, where the central mark indicates the median value, the bottom and top edges of the box the 25th and 75th percentiles, respectively, and the error bars represent the standard deviation range