Study on Electron Beam Machining: Beam Defocusing Phenomenon in Drilling of Alumina Green Sheet

This paper deals with a high speed drilling of alumina green sheet with a fine focused electron beam. Some experiments to drill many holes in a 16 × 16 mm2 area by means of single pulse beam scanning with beam deflection were performed and the following results were obtained. (1) The diameter increase is observed during machining when the time interval of beam pulses is short. This diameter increase is due to upward focal movement of an electron beam focused on the workpiece surface by the influence of the gas which is generated by the thermal degradation of organic binding material in the green sheet. (2) The increment in diameter decreases logarithmicly according to the increase in time interval of beam pulses and also decreases somewhat according to the increase in distance between holes. (3) The diameter increase can be compensated by a dynamic focus control which cancels the actual focal movement in a machining.