Object:To image the cutting temperature by the chip color
The cutting temperature increases the chip temperature.
The chip temperature is decreased by the air or the cutting fluid.
The heating and cooling process generates the oxide film on the surface.
When the light is reflected on the surface, the oxide film generates the interference color.
The interfelence color is called as "temper color".
The temper color is changed by the thickness.
Thus, you can image the cutting temperature from the temper color.

However, it is difficult to understand the absolute value of the cutting temperature.
Because the oxide film thickness is affected by the chip shape, machining conditions and the cutting fluid.
Thus, you have to use this function only as a guide.

Reference:
・Yoshio FUJIMURA, Numericalization of the Temper Color by the Oxide Film Thickness -Estimation of the Cutting Temperature by the Temper Color on the Cut Chip (1st Report)-, Journal of the Japan Society of Precision Engineering, Vol.38, No.11(1972), pp.902-907(in Japanese)