Assessment of the thermal effects on chip surfaces

Cermets not only possess the advantage of high hot hardness, but they are also chemically stable, which effects high wear resistance as well as good surface quality of the workpiece. However, owing to their disadvantages of high brittleness and low thermal resistance, there have been many attempts to improve their service life by means of the disposition of wear-resistant coatings. Various techniques have been used to evaluate the cutting performance of all of these coatings. A common method to evaluate the temperature at the tool–chip contact zone is done by using a tool–work thermocouple, an embedded thermocouple technique or by infra-red scanning. These techniques are difficult to be implemented, especially for low thermal conductivity and brittle tool materials such as cermets. This paper describes the chip-colour approach to analyse the effects of the thermal behaviour of coated cermet tools. This approach is useful, in particular, to overcome the limitations of the other methods described earlier. It was observed that different colours will be produced in the chip when using different tool inserts. With experiment results, the obtained, a chart with chip colours and lightness as a function of the cutting temperature was plotted. The cutting temperature for different cutting conditions can then be assessed by the lightness and the colour of the chip.