Coolant can be an effective way to cool a cutting tool, help expel the chip and prevent built up edge.
Built up edge occurs when the molten material adheres to the cutting tool.
Coolant may also have a negative effect on tool life. When a cutting tool is removing material it is under intense pressure and the heat generated during the cutting process may be extreme. If the coolant cannot provide a consistent flow to the cutting tool, then the cutting tool is subject to “thermal cracking.”
Thermal cracking is caused by the rapid heating and cooling of the cutting tool. A household example of thermal cracking would be to take a hot drinking glass from an oven and quickly place it in a freezer. The sudden change in temperature will cause the glass to crack and or break.
Many operations in machine shops today have the potential for thermal cracking of the cutting tool. Milling is an interrupted operation. The inserts are in and out of the cut. While in the cut, the inserts are subjected to extreme heat, and as they come out of the cut, the heat source is no longer present. When these hot inserts are exposed to coolant there tend to crack. The thermal cracks start as small microscopic ones that eventually cause premature tool failure. The same principal applies when turning an interrupted part. The cutting edge is in and out of the cut in a cyclical pattern. The sudden heating and cooling of the tool will cause thermal cracking and shorten tool life.
Cutting tool suppliers have made some advances in this area. For instance, many turning and milling inserts are coated with several thick layers of high temperature resistant coatings.
When turning in an interrupted cut, it is generally best to shut the coolant off and run the operation dry. When milling, it is also best to run the operation dry. There are a few exemptions to this rule of thumb. When machining gummy materials such as aluminum, low carbon steel, some stainless and exotic materials, it may be better to keep the coolant on to prevent insert failure from built up edge.
John Mitchell is general manager, Tungaloy America