What is EDM? Wire and sinker electrical discharge machining
What EDM is, how wire EDM and sinker EDM differ, what each is good at, and when to spec EDM instead of conventional machining.
EDM (electrical discharge machining) removes metal with controlled electrical sparks instead of a cutting tool, so it cuts any conductive metal regardless of hardness and leaves sharp internal corners a cutter cannot. Wire EDM cuts a profile right through the part with a thin charged wire; sinker EDM burns a shaped cavity with a custom electrode. It is slower and pricier than milling, so you use it for what it uniquely does: hardened steel, fine features, and sharp inside corners, not whole parts.
How it works
The part and an electrode (a wire, or a shaped block) sit in a dielectric fluid. A voltage builds until it arcs across the tiny gap, and each spark vaporizes a minute amount of metal. There is no mechanical cutting force, so the process does not care how hard the material is, only that it conducts. That is the whole point: you can cut fully hardened tool steel to a tenth without it fighting back.
Wire EDM vs sinker EDM
| Wire EDM | Sinker (ram) EDM | |
|---|---|---|
| Tool | A thin brass wire | A machined graphite or copper electrode |
| Cuts | A profile through the whole part | A blind cavity or shaped detail |
| Priced by | Cut length and thickness | Electrode work and burn time |
| Great for | Dies, punches, fine slots, hardened profiles | Mould cavities, sharp internal corners, keyways |
| Leaves | A clean, straight-walled kerf | A precise shaped pocket with sharp corners |
What EDM is uniquely good at
- Hardened material: cut after heat treat, so there is no distortion to chase.
- Sharp internal corners: a cutter leaves a radius equal to its own; a wire leaves a corner as sharp as the wire is thin.
- Fine, delicate features: thin ribs, small slots and tight profiles with no tool pressure to deflect them.
- Tool and die work: dies, punches, extrusion and mould details.
When not to use EDM
For a whole prismatic part, EDM is slow and expensive. Mill it, and reserve EDM for the one or two features that need it (the keyway, the sharp corner, the hardened detail). A good shop quotes the part as milling plus a little EDM, not EDM for everything.
What to send for a quote
The part, the material and its hardness, and which features need EDM (sharp corners, fine slots, post-hardening work). A STEP model plus a drawing that flags the critical features. Request quotes from Canadian machine and tool-and-die shops with wire and sinker EDM, including AS9100 shops for aerospace tooling and parts.
FAQ
What does EDM stand for?
Electrical discharge machining. It removes metal with controlled electrical sparks in a dielectric fluid, rather than with a cutting tool.
What is the difference between wire EDM and sinker EDM?
Wire EDM cuts a profile all the way through the part with a thin charged wire. Sinker (ram) EDM burns a shaped cavity into the part using a custom-machined electrode.
Can EDM cut hardened steel?
Yes. Because it cuts with sparks and not force, EDM cuts any conductive metal regardless of hardness, which is why it is used after heat treat on tool steel.
Is EDM more expensive than machining?
Per hour, yes, and it is slower, so it is used for the features that need it rather than whole parts: sharp inside corners, hardened material, and fine details.
What materials can be EDM machined?
Any electrically conductive material: tool steels, stainless, titanium, carbide, Inconel and other superalloys. Non-conductive materials cannot be EDM cut.
Last reviewed: October 2026. Related reading: what is tool and die and what is CNC machining.
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