Two faces of the same cutting tool: A tribological perspective

The rake and clearance faces are the main contact surfaces of the tool-workpiece system, over which the friction force opposes the relative movement of the chip and the newly formed machined surface, respectively. Although friction forces are mainly associated with the rake face in most published studies, the analysis of the chip formation mechanism can benefit from additional accounting of the tribological contribution from clearance face. This is particularly significant when the friction coefficient value at the rake surface is only estimated based on the resultant cutting force components and the Merchant’s circle diagram, generally returning higher values compared to experiments carried out on laboratory tribometers. This investigation aims to provide a comprehensive review of the Merchant’s circle diagram from a tribological point of view. The friction coefficient value is considered to be equal on both the rake and clearance faces and, at the same time, the resultant force angle is constant based on the linear relationship between the cutting and thrust forces as uncut chip thickness increases. Thus, the force circle diagram can be overhauled to include the friction force contribution from the clearance face and a realistic prediction of the normal and friction forces on the rake face thereby can be achieved. Specially designed apparatus and tribological tools have been used to validate the proposed analytical methodology for accurate calibration of friction models.