Electric-discharge machining (EDM) or spark machining tips.
ELECTRONICS NOW Feb 1993 (v.64#2) pg. 79
Added Info ELECTRONICS NOW May 1993 (v.64#5) pg. 78
Experimental "electric discharge" machine cuts holes and cavities in metal by means of electric sparks. Est. cost: $5.
POPULAR SCIENCE Mar 1968 (v.192#3) pg. 149