The Role of Fracture Toughness in the Cutting of Wood

When chips in wood cutting are formed by through thickness shear along a major shear plane emanating from the tip of the tool to the free surface of the timber, the Ernst-Merchant theory (e.g. Ernst and Merchant, 1944), originally propounded for continuous-chip cutting in ductile metals, is often employed for analysis of wood cutting forces. The original rigid-plastic theory considers that the forces in cutting are determined by two work components, viz: (i) work required permanently to deform the chip and (ii) work against friction. Despite all the improvements to the basic Ernst-Merchant model (secondary shear giving chip curl, the effect of work hardening, temperature and rate effects, and so on) and improved modelling of friction, it remains a fact that such algebraic analyses are not able to predict some well-established features of the mechanics of cutting. For example, the theory predicts that the primary shear plane angle is independent of the metal being cut whereas experiments show that the angles vary not only with friction but also with the metal and mechanical properties. Again, when experimental cutting forces are plotted against the uncut chip thickness (depth of cut), there is invariably a positive force intercept at zero uncut chip thickness: the theory predicts that force vs uncut chip thickness plots should pass through the origin of coordinates. The same experimental facts, not agreeing with the theory, are also found when cutting other materials such as plastics and wood.