Practical performance and action mechanism of ester lubricants in minimal quantity lubrication machining

Minimal quantity lubrication (MQL) machining typically uses several tens of milliliters per hour of cutting fluid: a very small quantity compared with generally several tens of thousands of milliliters per hour in the conventional flood supply of cutting fluids. MQL machining can reduce considerably the consumption of cutting fluids, so is a representative and successful environmentally friendly manufacturing operation. Specific synthetic polyol esters with high biodegradability, excellent oxidation stability, and satisfactory cutting performance were evaluated as optimal cutting fluids for MQL machining. The esters are required to work as an effective lubricant in the cutting zone in very small amounts, so the tribological actions are particularly important to improve the cutting performance. A controlled atmosphere cutting apparatus was devised to investigate the relationship between the tribological action and the practical cutting performance of the esters in MQL machining. A model ester and oxygen showed mutually enhanced adsorption activities for machining of steel by the efficient formation of an adsorption film which provided the lubricating effect and improved the cutting performance. In contrast, the presence of oxygen resulted in unfavorable cutting situations for machining of aluminum.