This work is an experimental investigation of the effect of geometrical parameters on the magnitude and inclination of the force acting on a sharp cutting element tracing a groove on the surface of a rock. The first part deals with the effect of cutter forward inclination (back rake angle, θ) on the magnitude and inclination of the cutting force F. Previous experiments conducted at the University of Minnesota indicate the specific energy (tangential component Fs of the force scaled by the groove cross sectional area) increases with the θ. The aim of the current research is to assess if this dependency is universal, i.e. independent of the rock being tested. In others words is the evolution of the specific energy scaled by the rock strength (for example unconfined compressive strength of the material) universal? Previous experiments show also that angle ψ between the resultant force and to the cutter normal face varies strongly with θ; strongly suggesting that ψ is not a simple interfacial friction angle between the rock and the cutting face. The objective of the current project is to assess if the angle ψ is governed (or at least affected) by the frictional properties of the material (in particular the internal friction angle of the rock being tested). The second part will focus on non-symmetric groove (non-symmetric with respect to the vertical plane containing the cutter velocity vector). Such cutting configurations generate an out of plane force components F t. In this project we aim at characterizing how this component scales with the depth of cut and aspect ratio of the groove being traced. Experimental work has been conducted on a scratching apparatus at CSIRO drilling mechanics laboratory on five sedimentary rock materials
Considerable efforts have been dedicated, since the mid 1970's and the introduction of Polycrystalline Diamond Compact, in the design of drag bits to improve drilling performance and durability [1, 2]. In particular, great attention has been given to the positioning and orientation of cutting elements (PDC cutters) on the bit face to optimise the resulting rate of penetration under given axial load (or weight-on-bit) while minimizing torque or torque fluctuation under varying weight-on-bit or minimizing the side force often referred as imbalance force (resultant force located in the cross section plane of the bit). Such optimizations have been shown to have direct consequence on the overall drilling performance such as (i) directional drilling application with mud motors where inappropriate bit design can lead to frequent loss of trajectory (tool face) and time consuming and thus costly readjustments or (ii) bit lateral vibrations or whirl with possible dramatic failure of drilling equipment [3, 4, 5].. Calculation of the total force acting on the bit relies on a detail description of the cutting structure as well as proper modelling of the force (magnitude and orientation) acting on each cutting elements. Modelling has often been built on the basis of results of laboratory single cutter experiments [6, 7, 8].