The Experimental and Theoretical Evaluation of an Indirect Cooling System for Machining

Cutting fluids have been used in machining processes for many years to decrease the temperature during machining by spraying the coolant into the machining zone directly on the cutting tool and the part. This has the effect of decreasing the tool temperature, which increases tool life and improves the part quality. These benefits come with significant drawbacks. Cutting fluids are environmentally unfriendly, costly, and potentially toxic. An alternative that has been evaluated in this paper is an internal cooling system (ICS) for lathe turning, which cools the cutting tool using a very small amount of an inert, cryogenic working fluid routed through a microchannel heat exchanger (MHX) that is mounted beneath the cutting tool insert. The working fluid absorbs the heat generated during the machining process after which it is harmlessly vented to the environment. This indirect cooling technique results in an environmentally friendly machining process that uses no cutting fluids, enables increased processing speed, and reduces manufacturing costs. An approximate heat transfer model was developed and used to predict the tool life as a function of the tool cooling approach for various speeds. Machining experiments were completed to validate the heat transfer model and confirm that the ICS can significantly improve tool life relative to conventional flood cooling. The validated model was then used to evaluate alternative cooling approaches using the ICS. It was found that the use of a cryogenic working fluid can significantly improve tool life at all cutting speeds but that the latent heat capacity of the working fluid should exceed the expected maximum heat transfer rate into the tool. This work established that the ICS approach is an effective means to increase tool life without the disadvantages associated with external cryogenic cooling methods.