Electrical discharge machining (EDM) removes material with sparks: a succession of small electrical discharges between an electrode and the part, immersed in an insulating liquid (oil or deionised water), locally vaporises the material. There is only one condition: the part must conduct electricity. Because the tool does not touch the part, there is no cutting force.
Two variants dominate. Wire EDM cuts a contour like a very fine saw: a brass wire a few tenths of a millimetre in diameter runs continuously, and tilting the head allows tapered shapes. Die-sinking EDM uses a graphite or copper electrode, the negative of the cavity to be produced, which it sinks into the part. The wire is generally threaded through a small hole made by fast-hole EDM drilling. The process reaches tolerances of a few microns, but it is slower than conventional material removal and leaves a thin recast layer on the surface, which sometimes has to be removed.
It is used for very hard materials (hardened steel, tungsten carbide), for sharp internal corners, for narrow slots and cavities that a milling cutter cannot reach, and for thin parts that cutting forces would distort. It is the technique behind punches, dies, mould inserts and cutting templates. It complements machining by milling and grinding, and fits within the CNC machining capability.
In a quote request. State the material and its hardness, the thickness to be cut, the tolerances and roughness required, the minimum internal corner radii (the wire radius sets a limit) and whether the EDM is wire or die-sinking. Say whether the recast layer is acceptable or must be removed, particularly for parts subject to fatigue. The guide to the price of CNC machining explains why a slow process weighs on the quote.