The milling operation is an inherently intermittent process. This causes the temperatures generated at the cutting edge to constantly fluctuate between various levels of hot (~1000°C) and cold.
The effect of cutting fluid
The temperature variations are exacerbated when the cutting edge goes into and out of the cut. The cutting edge is thus subjected to thermal shocks and cyclic stresses that can result in cracking and, in the worst case, can lead to a premature end of the tools effective life.
The hotter the cutting zone is, the more unsuitable the use of cutting fluid becomes.
In finishing operations, the application of cutting fluid will not reduce the tool life as much as in roughing, due to the reduced level of heat generation.
Thermal cracks on the cutting edge
Dry milling extends the life of the cutting edge. Temperature variations do take place but remain within the design scope of the carbide grade. Rough milling operations should always be run dry.
Milling with cutting fluid
There are some exceptions for when cutting fluid should be used:
- Finishing of stainless steel and aluminium – to prevent smearing of metal particles into the surface texture
- Milling of heat resistant alloys at low cutting speeds – to lubricate and to cool down the component
- Milling in cast iron – to dampen and flush away dust for environmental, health and component quality reasons
- Milling of thin-walled components – to prevent geometrical distortion
- Micro-lubrication systems i.e. compressed air with a small amount of special oil, can be applied to assist chip evacuation in deep cavities
In micro-lubrication systems the amount of “oil mist” is only a few ml of oil per hour and is evacuated via the normal, filtered ventilating system.
Coupious flow through
If milling has to be performed wet, cutting fluid should be applied copiously.