Look at the colour of the iron filings to determine the condition of the process

In CNC machining, with normal dry cutting, it only makes sense that almost all chips cut out of steel should appear purple. As the cutting temperature increases, the rule of thumb for chip colour change during dry cutting is: silvery white → pale yellow → dark yellow → vivid red → dark blue → blue → blue-grey → off-white → purple-black. The temperature change can rise from about 200°C to over 500°C. The process of chip colour change is also the process of converting most of the work consumed in the cutting process into cutting heat, which can also be seen as the process of tool loss (sharpness - dulling - violent dulling - scrap) (when there is no chip tumour) Note that the cutting temperature we usually refer to is the average temperature.

Chip colour in relation to cutting temperature.

Silvery white: <200℃ or less    

Light yellow: ≈220°C

dark blue: ≈ 300°C      

Light grey: ≈ 400°C      

Deep purple-black: >500°C        

Chip colour is blue or blue-violet is more reasonable, if silver white or yellow, it is not fully efficient, if the blue and grey is too large cutting amount. The use of high-speed steel tools, then the chip for the silver white and yellowish is appropriate, if the chip blue to reduce the speed or feed.