Machining is accomplished by the action of rapidly occurring electrical discharges, or sparks, which erode small pieces of metal from the workpiece. The cutting tool is an electrode that is shaped to the contour of the required cut. Both the workpiece and the electrode are submerged in an electrically nonconducting (dielectric) fluid and connected to a dc power supply. Usually consisting of mineral oil or kerosene, the dielectric fluid functions as an insulator, coolant, and medium for flushing away debris from the tool and the workpiece.

 

Sent from the electrode to the workpiece at a rate of thousands per second, the sparks travel through the nonconductive fluid to reach the workpiece. The electrode vaporizes the metal without ever touching the workpiece, which is by necessity made of electrically conductive material. Electrodes are commonly made of graphite or copper tungsten; however, the range of constituent materials also includes tungsten carbide, copper, brass, and zinc alloys.

 

Although rates of metal removal are slower with electrical discharge machining than with other commercial machining methods, the slower removal produces better surface finishes. Higher rates of metal removal are known to produce rougher finishes that have a molten and resolidified (recast) structure with poor surface integrity and low fatigue properties (Kalpakjian, p. 838).

 

Certain parts can only be produced using sinker EDM, which, unlike wire EDM, does not cut all the way through the part. As its name implies, sinker EDM literally "sinks" a required shape into the workpiece.