Abstract: The joint-use of ultrasonic vibration and cutting fluid in metal machining is able to decrease cutting-force and surface roughness. Though the phenomenon has been observed and well utilized, its mechanism has not been clear. The mechanism is revealed in order to establish foundation for the research and development of the next generation of precision and ultra-precision machining technologies. The joint-use of ultrasonic vibration and cutting fluid changes the way in which a cutting tool exerts cutting forces, friction force and compression force, on a workpiece:when cutting fluid is not applied, the friction force direction exerted on a cutting-chip surface by a cutting tool are away from its cutting edge; but when cutting fluid is applied, the friction force direction is towards to the cutting edge. The friction force whose directions away from the cutting edge leads to an increase in shear angle, which is equivalent to the effect that a sharpened cutting tool bring about; the pressure distribution of cutting-fluid is on a narrower zone surrounding the cutting edge than the zone of the cutting force exerted by cutting edge alone without cutting fluid, which is equivalent to the effect that a cutting edge with a reduced radius can do; the pressure distribution of cutting fluid in front of the cutting tip demote the formation of micro cracks. Because the above mentioned advantages can also be brought about by a shaped cutting tool with its cutting edge's radius reduced, the effects of the joint-use of ultrasonic vibration and cutting fluid is called as non-physical sharpening.

Key words: cutting fluid, friction force, non-physical sharpening, shear angle, surface roughness, tool sharpening, ultrasonic assisted cutting