In order to make clear the mechanism by which material is removed in ultrasonic machining, the following observations and discussions are described.
(1) Appearance of a chip removed is like that of a plate, the diameter of which is larger than ten times of its depth. Chips, the diameters of which are larger than that of an abrasive particle are often found.
(2) The size of a trace left on the work surface is almost similar to that of a chip removed in view of form, diameter and depth.
(3) Surface roughness which is expected from thickness of chip removed and depth of trace left is obtained.
(4) Residual strain under the trace left is very small in depth.
(5) Machining depth per one cycle is far smaller compared with thickness of chip removed, depth of trace left and surface roughness.
By comparing mean machining volume per one cycle with calculated volume from the above observation and effective number of active abrasive particles, the following conclusion is obtained. If we observe one paticular chip, it is considered that the chip is not removed until the strain originates and grows enough up to a crack by repeated impacts between the horn tip and the work surface through one particle in a chance and another one in after chance, while cavitations which permeate easily into the micro crack may help the growing up of the strain and the tearing-off of the chip.