With the increase of cutting temperature, the color of chips during dry cutting changes as follows: silver-white→light-yellow→dark-yellow→magenta→dark-blue→blue→blue-gray→gray-white→dark purple. The temperature change can rise from about 200 ℃ to over 500 ℃.
The process of chips color change is the process in which most of the work consumed in the cutting process is converted into cutting heat, and it can also be regarded as the process of tool wear without built-up edge (sharp – blunt – severe blunt – scrap). Note that what we usually call the cutting temperature refers to the average temperature.
A customer for CNC cutting in Chile said that “the metal color change is really neat and pretty useful stuff. I use it a lot to help identify what type of metal-wear chips are.”
The relationship between chip color and cutting temperature as below:
|Color of Chips||Cutting Temperature|
|Silver white||< 200℃|
|Light yellow||about 220℃|
|Dark blue||about 300℃|
|Light gray||about 400℃|
Meanwhile, feed and tool geometry has a big impact on chip shape and color. Chip shape and color will also change as the tool wears. The processing state is more reasonable when the chip color is blue or blue-purple. If the chips are silvery white or yellow, it means that CNC machining is not fully efficient. If the chip is blue-gray, it means that the cutting amount is too large.
The huge caveat is that this depends on cutting tool material. When using high-speed steel tools, the chips should be silvery-white and slightly yellow. If your chips are blue while running HSS, you’re gonna have a bad time, what you need is to reduce the speed or feed.
Of course, depending on the color change to determine whether the processing parameters are reasonable is only one of the methods. Have you got it?