Abstract This paper considers some important findings which resulted from an investigation on the mechanism of heat generation during metal-cutting operations. It has been found that many changes during the cutting of metals are due to the change in tool-chip interface temperature through its influence on tool-chip friction. The paper is divided into two parts, the first of which pertains to the cutting forces and cutting temperatures observed during conventional turning and orthogonal cutting under otherwise identical conditions. The higher heat-dissipating capacity of the tool in orthogonal cutting operations was found to be fundamentally responsible for the observed differences. In the second part of the paper, the role of tool-chip contact area on interface temperatures is further investigated. This study has been of value in the interpretation of tool-chip interface temperature data and other phenomena heretofore inexplicable.