An experimental study is performed using both thermocouples and infrared thermovision to monitor timewise temperature variations of the tool and workpiece in orthogonal cutting. A semi-empirical formula is derived to express the temperature-time history of the tool surface, using a local element lumped conduction equation with experimental data-fitting. Infrared thermovision identifies the location of the maximum tool temperature slightly inward from the cutting edge. An optical and electronic microscope reveals the formation of micropits, the origin of flank and rake wear that originates in the region of surrounding the maximum tool temperature. It is disclosed that the progression of wears is accompanied by a consistent increase in the tool temperature which in turn accelerates the wearing process. Chip geometry is found to affect local steady-state temperatures in the cutting tool. For cast iron and copper, chip geometry may causes a zig-zag variation in the steady-state temperature with respect to the rate of material removal for a change in the feed. The study sheds light on the causes of roughness of surfaces cut with a hard tool and may thus serve as the first step in the investigation of surface machining; (C) 1997 Elsevier Science Ltd.