EDM: electrical discharge machining explained

How a spark cuts metal

An EDM machine puts a voltage across a small gap between an electrode and the workpiece, both sitting in a dielectric fluid. When the gap is small enough the fluid breaks down and a spark jumps, and the spark melts and vaporises a crater of metal. The fluid then flushes the debris away and reinsulates the gap, and the cycle repeats thousands of times a second. The shape is cut by the sum of those craters.

Two consequences follow, and both are the reason the process exists. The workpiece has to conduct electricity, so EDM is for metals and a few conductive ceramics. And nothing touches the work, so there is no cutting force to deflect a thin wall, no tool to blunt against hardened steel, and hardness is close to irrelevant to how fast the machine cuts.

Wire EDM and die-sinking EDM

Wire EDM threads a fine brass wire through the workpiece and travels it along a programmed path while unspooling it, so a fresh surface is always in the cut. It cuts a profile straight through the part, the way a bandsaw would if a bandsaw could hold tool-room tolerances. Punches, dies, extrusion tooling and close-fitting slots are wire work.

Die-sinking EDM, also called ram or sinker EDM, uses a shaped electrode, typically graphite or copper, and pushes it into the work so the cavity takes the electrode’s mirror image. It makes blind cavities, sharp internal corners that no round cutter can produce, and deep ribs. The electrode itself is usually milled on a vertical machining centre first, so a mould shop that owns a sinker owns a mill to feed it.

A third form, small-hole or fast-hole EDM, drills the start holes wire EDM needs and the cooling holes in turbine parts.

The recast layer, and other things to ask about

Every spark melts metal, and some of it resolidifies on the surface. That recast layer, with the heat-affected zone below it, is harder and more brittle than the parent metal and carries the machine’s fingerprint. Modern generators reduce it with fine finishing passes, and the drawing for a fatigue-critical part will say how much of it may remain and how it is to be removed.

Cutting rate on a wire machine is quoted in square millimetres per minute of cut face, not in parts per hour, so compare quotes against your own part height. Then ask about the generator and its finishing passes, wire consumption per hour at your settings, the filtration and deionisation system, automatic wire threading and whether it rethreads at the break point, and the accuracy of the taper head if you cut drafted dies. The safety requirements for these machines, including the fire risk of hydrocarbon dielectrics and the fume extraction that goes with them, are set out in ISO 28881.

Why an Indian tool room owns an EDM machine

Die, mould and press-tool work is the core of EDM demand in India, and it clusters where the tooling is made: Rajkot, Pune, Ahmedabad, Coimbatore and the NCR. A wire machine turns a tool room from one that can make a part into one that can make the tool that makes the part, which is a different customer and a longer contract. Which builders and dealers are selling into those clusters is followed in this site’s automation systems news from India.

It is also the machine most often bought second-hand by a young shop, which is why the questions above about the generator and the wire path matter more than the badge. The rest of the machine vocabulary sits in the entry on computer numerical control, and a shop weighing the purchase against a state subsidy should first check that it is registered as an MSME.