Air flow and chip removal in minimum quantity lubrication drilling

Chip removal is a significant problem in high throughput minimum quantity lubrication (MQL) drilling, due to increased feed rates and the lack water-based coolant to flush chips. Chip removal difficulties in MQL drilling may drive the use of increased air pressures at a significant cost and energy penalty. This paper summarizes basic studies of air flow and chip removal for MQL drilling, intended to provide a physical basis for process and tooling design with minimal use of high air pressures. Air flow measurements on drill blanks show that the air flow rate depends primarily on the drill coolant hole diameter and input air pressure. Air speed measurements, made using a novel plate impingement test, show that the average air speed increases with increasing coolant hole diameter, but not with increasing pressure. An energy-based analysis of chip removal shows the effects of drill and process design parameters on chip removal. Trends predicted by this analysis are verified through vertical drilling experiments to measure chip removal efficiency. It is shown that the coolant hole diameter has a large influence on air speed and chip removal efficiency, so that drills with the largest practical coolant hole diameter should be tested before increased air pressure is adopted. It is also shown that increasing the spindle speed improves chip evacuation for helical drills. Recommendations for drilling process design and chip removal testing are also discussed.