Electrical Discharge Machining

§1Cutting with sparks

Electrical discharge machining removes metal not by a cutting edge but by a rapid succession of tiny electric sparks, each melting away a minute crater. The tool and work never touch.

An electrode — the shaped tool — is held a fraction of a millimetre above the workpiece, both submerged in an insulating (dielectric) fluid, and a voltage is applied. When the gap is small enough, the fluid breaks down and a spark jumps, melting and vaporising a speck of metal; thousands of such sparks a second erode the work into the electrode’s shape. Because nothing touches and nothing is cut mechanically, EDM does what no cutter can: it machines the hardest conductive materials — hardened tool steel, carbide — and forms intricate, deep, sharp-cornered cavities with no cutting force to break a delicate shape (§3). It works only on conductive materials, and it is slow, but for hard dies and fine detail it is unmatched.

§2How the spark erodes

Each spark is a controlled, contained melting event: a discharge across the gap heats a tiny spot to vaporisation, and the fluid flushes the debris away before the next.

The cycle repeats thousands of times a second. Voltage builds across the gap until the dielectric ionises and a spark discharges through it, concentrating enough energy on a microscopic spot to melt and boil away metal from both the work and, to a lesser degree, the electrode (§5). The discharge collapses, the dielectric flushes the molten debris out of the gap and re-insulates, and the next spark forms — often at the next-closest point, so erosion spreads evenly. The dielectric fluid does three jobs: it insulates so the spark only jumps when the gap is right, it flushes the eroded particles clear, and it cools. Control the energy of each spark and their number, and you control how fast and how finely the metal is removed (§6). It is melting by spark, not cutting by force.

§3No force, no hardness limit

The two defining advantages of EDM both follow from the tool never touching the work: there is no cutting force, and hardness is irrelevant.

Because erosion is by spark, not by a mechanical edge, no cutting force acts on the workpiece or the tool — so a slender, delicate or thin-walled feature that any cutter’s force would deflect or shatter can be machined intact, and a fragile electrode can form a deep narrow slot. And because the metal is melted, not sheared, the material’s hardness does not matter: fully hardened tool steel and cemented carbide cut no slower for being hard, which is exactly the situation that defeats conventional machining. This is why EDM owns the making of hardened dies and moulds — the part can be hardened first and then EDM’d to shape, avoiding the distortion of hardening a finished cavity. The one requirement is that the work conduct electricity; within that, hardness and delicacy cease to be obstacles.

§4The spark gap and overcut

Since the spark jumps a gap, the cavity EDM cuts is always slightly larger than the electrode — an overcut that must be allowed for by making the electrode undersize.

§5Electrode wear

The sparks erode the electrode as well as the work, so the tool slowly wears — a cost that shapes how electrodes are chosen and used.

Because each discharge removes a little metal from both electrodes, the tool gradually loses its sharp corners and length, and that wear transfers error to the cavity if uncorrected. Electrode materials are chosen to resist it: graphite and copper are the usual choices, eroding far more slowly than the steel they cut because the process can be tuned (polarity, energy) so most of the erosion falls on the workpiece rather than the tool. Even so, a roughing electrode is often followed by a fresh finishing one, and multiple electrodes may be used to complete a worn detail. The wear is the price of a contactless process — modest, controllable, but real — and managing it, by material choice and by finishing with a fresh electrode, is part of holding the cavity accurate.

§6Rate against finish, and wire EDM

As in every machining process, EDM trades speed against finish — set by the spark energy — and a second form, wire EDM, cuts profiles right through the work with a travelling wire.

The energy per spark sets the trade: high current gives big sparks that erode fast but leave a rough surface and a wide gap — roughing; low current gives small sparks that erode slowly but finish finely with a tight gap — finishing. So an EDM job is roughed hard then finished light, exactly as turning is, but with spark energy as the lever instead of feed and depth. Wire EDM is the important variant: instead of a shaped electrode plunging in, a thin travelling wire — like a bandsaw blade — is fed through the work while sparking, cutting a fine, precise profile straight through plate. It cuts hardened punch and die profiles, gears and intricate outlines to great accuracy with a very narrow kerf. Sinker EDM makes cavities with a shaped electrode; wire EDM cuts through-profiles with a wire — together covering the hard, precise, forceless machining conventional tools cannot.

§7Quick reference

The working core of the page on one card rack.