End milling is an interrupted type of machining. If coolant is used during interrupted machining, thermal cracking can occur. This is because the cutting edge is continually heated and rapidly cooled during the cutting cycle. Therefore, when machining carbon steel, alloy steel, pre-hardened and hardened steel, air blow is recommended.
The graph shows tool wear when machining carbon steel under the different cutting conditions of dry, air blow and water-soluble coolant. The amount of tool wear when using air blow was found to have the lowest, followed by dry cutting. However, with water-soluble coolant, abnormal damage occurred due the development of thermal cracks.
It is believed that the effect of air blow is effective because it helps to avoid chip packing and sudden chipping of the cutting edge, as well as providing sufficient cooling.
When machining difficult-to-cut materials such as stainless and heat resistant steels, nickel-base heat-resistant and titanium alloys, the use of coolant is recommended even for interrupted cutting. This is because of the extra heat generated when cutting these materials and to avoid welding and softening of the cutting edge.