Electrical Discharge Machining is a high precision machining process that uses electricity to cut metals. In fact, the metal being cut is never touched by a tool at all. Instead, a high-frequency electrical discharge basically disintegrates the metal to make a cut.
EDM involves molecular process that can be difficult to envision, so to help you understand how it works, check out this video of a lightning bolt cleaving a tree in half.
While this is not exactly how EDM works, it gives you an idea of what is happening on a molecular level. In a nutshell, EDM uses a graphite or soft metal tool as an electrode. The electrode is moved very close – within a hair’s width – to the part being machined. The tip is then loaded with electricity until dielectric breakdown – a spark – occurs. The spark’s corona causes material to vaporize, leaving a tiny cut in the surface only a few molecules deep. The process is repeated at a rate of 100,000 sparks per second and the cumulative effect is a cut through the metal piece.
To prevent the spark from simply conducting between the electrode and the surface, both are submerged in a dielectric fluid. Dielectric fluids are insulators that can rapidly quench an electrical discharge. So, in this case, once the corona forms, it is immediately quenched, but there is enough energy in the spark to allow the metal to vaporize. Check out the ETMM website for a more thorough description of the process.
EDM machines come in three varieties – wire EDM, sinker EDM and hole drill EDM.
In a sinker EDM, the electrode is a die that is moved into the part as material is removed. Wire EDM uses a thin conductive wire as the electrode, and the entire wire is moved into the tool along the cut direction. As the name implies, hole drilling EDM plunges a tubular electrode into a part to make very small and very deep holes in a part. In this case, the dielectric is inside of the electrode.
EDM is excellent for very high-precision parts and parts that must fit together perfectly, with no gaps. It is also used to machine metals that are hard to machine with traditional methods such as tungsten carbide, high strength titanium and hardened steel. The metals must be conductive to be cut with EDM.
Despite its precision and accuracy, EDM machining is also slow and expensive. Although EDM is commonly used in the aerospace and automotive industries, it is often reserved for lower-volume production where high levels of precision are critical. For most applications, CNC milling with conventional cutting tools offers a more cost-effective machining solution.