1 Introduction

Tool-chip interfacial friction plays a critical role in the mechanics of chip formation in machining and influences major machining performance measures, especially cutting forces, tool-wear and chip-form. Unfortunately, traditional Coulomb friction does not apply to a high strain-rate, high temperature plastic deformation process such as machining. Fundamental knowledge of the frictional conditions at the tool-chip interface is therefore necessary for predicting machining performance. Considerable early significant work on tool-chip interfacial friction has been reported by several researchers ([FIN 56], [ZOR 63], [WAL 64], [MAT 81]). Challen and Oxley proposed an asperity deformation model using slip-line fields to characterise the tool-chip friction in machining [CHA 84].

One of the major drawbacks in attaining complete predictive capability of models for machining has been the inability to characterise the actual tribological conditions at the tool-chip interface with the assumption of highly approximate boundary conditions. This paper especially highlights the fact that some of the traditional results from 2-D plane-strain conditions cannot be directly superimposed to solve complex 3-D machining problems due to the inherent variability of friction in 3-D processes. The paper discusses frictional effects in cyclic chip formation. A large variety of cases always involve curled chip formation; if the chips break at regular intervals into concise chip-forms, the process is very cyclic in repeatability. Non-uniquely repeatable, yet cyclic chip formation is...