Electrical discharge machining removes metal with a controlled electrical spark rather than a cutting edge. Because nothing touches the workpiece, hardness is irrelevant and there is no cutting force to deflect a delicate part. That combination makes it the answer for hardened tool steel, for very sharp internal corners, and for shapes no cutter can reach.
Wire and sinker
Wire EDM runs a fine wire through the workpiece like a bandsaw, cutting a profile through the full thickness with very high accuracy. Sinker or ram EDM pushes a shaped graphite or copper electrode into the work, burning a cavity that is the mirror of the electrode. Wire cuts profiles; sinker makes pockets, ribs and details, which is why toolrooms rely on both.
Why hardness stops mattering
The process erodes material by discharge, so the workpiece can be as hard as you like. That is why parts are routinely heat treated first and then finished by EDM, avoiding the distortion that heat treating a finished machined part would cause. For tooling this is not a convenience, it is the normal order of operations.
Sharp corners and thin features
A milling cutter always leaves a radius equal to its own radius, so a truly sharp internal corner cannot be milled. Wire EDM leaves a corner radius equal to the wire, which is very small. Sinker EDM reproduces whatever the electrode has. Thin ribs and deep narrow slots are similarly straightforward, because there is no side force to push them over.
What it costs and what to watch
It is slow, and slow is the price. Sinker work also needs electrodes, which are themselves machined, so the electrode is a job before the job. The process leaves a recast layer at the surface, which is removed by skim passes where it matters, particularly on fatigue critical parts. And it needs a conductive workpiece, which rules out ceramics and plastics.
Questions people ask about electrical discharge machining
What is EDM?
Machining by controlled electrical discharge, which erodes metal without any mechanical contact. It works on any conductive material regardless of hardness.
Wire or sinker EDM?
Wire for through profiles, contours and tight internal corners. Sinker for blind cavities, pockets and detail that has to be pushed into the work.
Can EDM cut hardened steel?
Yes, and that is one of its main uses. Parts are commonly hardened first and then finished by EDM to avoid heat treatment distortion.
What is the recast layer?
A thin re solidified surface layer left by the discharges. It is removed by finishing skim passes where surface integrity or fatigue life matters.