The transient beginning to machining and the transition to steady-state cutting

Knowledge of the physics behind the separation of material at the tip of the tool is of great importance for understanding the mechanisms of chip formation. How material separates along the parting line to form the chip and cut surface is still not well understood, yet it is of great importance for improving the robustness, enhancing the predictability and extending the application of currently existing finite element computer programs. This paper attempts to provide some answers to these issues by means of a combined numerical and experimental investigation of the transient beginning to machining and the transition to steady-state orthogonal metal cutting. Numerical modelling was performed by means of an updated-Lagrangian approach based on the finite element flow formulation and experiments were carried out on lead specimens under laboratory-controlled conditions. Forces and displacements are given for the initial indentation phase during which material is displaced up the rake face of the tool. Ductile damage begins to accumulate, eventually leading to separation at the tool tip. This marks the onset of a second stage during which further displacement of material along the rake face is accompanied by separation of material at the tool tip (i.e. cracking), which now continues in all subsequent deformation. The displaced material, although not yet attaining its fullest extent, now begins to take on the appearance of a continuous chip. A third stage begins when the material, which up till now has been in intimate contact with the rake face, develops curvature and leaves the tool. This does not, however, mark the beginning of steady-state cutting, because chip curl continues to increase until a steady value is attained. During this period, the contact length with the tool then reduces somewhat, before settling down to a steady value. The thrust force is a maximum at the point of greatest chip contact length. The paper demonstrates that material separation is caused by shearing rather than tension. The specific distortional energy is an appropriate criterion for evaluating ductile damage in shear and the onset of separation ahead of the cutting edge. In turn this determines the value of the fracture toughness in shear.