EDM process (electrical discharge machining) is low volume production process with extreme precision (precision machining with surface finish on average Ra 0.32-18 microns) but expensive simultaneous metal surface machining & texturing therefore most commonly used for tooling for injection moulding parts within the aerospace industry due to use of lower voltage & slower cutting speeds / removal rates. 

The Electrical Discharge Machining method is commonly used together with CNC machining process. The two categories of EDM machining (electric discharge machining which EDM machines falls in is either a Wire EDM process (wire erosion) or a Die Sink EDM process. These two EDM process complex shapes on different EDM machine tools but produce similar results with the cutting tools. Wire EDM (wire erosion) is where a graphite wire electrode is fed between the supply spool causing the EDM wire to be held in tension in order to cut straight lines. A high voltage is charged for heat treatment then discharges on the thin wire as it passes though the workpiece. 

This results in thousands of sparks per second between the smallest gap (or small hole) of the metal workpiece. High accuracy is maintained though the Wire EDM process by constantly replacing the EDM damaged graphite electrode caused by electrode wear. This EDM machining process all occurs in submerged deionized water functioning as an insulator that constantly is running to flush the conductive material & recycle though a filtration system. Translation along the x-axis & y-axis of the upper thin wire guide is possible to enable cutting angles up to 30°. This all allows the wire EDM machines to cut external profiles as well as internal profiles which CNC turning & CNC milling struggles at. 

Die sink EDM occurs when both the electrode tooling workpiece is submerged in light oil acting as an insulator where the constantly running dielectric fluid allows the temperature of the workpiece to be maintained. The dielectric fluid also causes vaporized hard material to be flushed out. Bringing both the copper electrode (a conductive material) & metal workpiece within a small range apart, this causes a high voltage spark to jump onto the workpiece from the electrode and discharging them between the closest points on the electrode & workpiece which is the spark gap (known as sparking). 

Die sink EDM works without any pressure energy required with the operation & the result is even plus continuous high-quality surface finish with high accuracy. Next the electrode would descend in a spiral path fashion into the workpiece at rate dependant on the texture of surface finish that is required ensuring that vaporized material would be flashed out away from the cutting area, allowing for an efficient machine material removal finalising EDM machining process ending with a final desired shape of the part. The dielectric fluid in the EDM machine tool would also allow for a static discharge after it has been charged by the high voltage for heat treatment in the manufacturing process.