When EDM is the right answer and when it is an expensive detour
September 2, 2026
Most parts never need EDM. The ones that do usually need it for one feature, not for the whole job, and that distinction is worth understanding before you ask for a quote.
Electrical discharge machining removes metal with controlled sparks between an electrode and the workpiece, both submerged in dielectric fluid. There is no cutting force and no tool pressure, so hardness is irrelevant and thin walls do not deflect.
The three situations that call for it
1. The material is already hard. If your part is heat treated to 58 HRC and then needs a slot, a keyway or a profile, a cutter will not do it. Either you machine soft, heat treat and accept the distortion, or you heat treat first and EDM the critical features afterwards. On a die insert or a punch, the second route is standard.
2. The corner has to be sharp. A milling cutter leaves a radius. If your drawing shows a square internal corner and the mating part genuinely needs it, wire EDM gets you close to sharp. Before you go there, ask whether the mating part can take a relief cut instead. That question has saved a lot of buyers a lot of money.
3. The feature is deep, thin or delicate. Tall thin ribs, narrow deep slots and fine details chatter or deflect under a cutter. EDM applies no force, so the geometry holds.
Wire EDM and sinker EDM are different jobs
Wire EDM runs a thin brass wire through the part like a bandsaw blade, following a 2D profile through the full thickness. It needs a start hole or an open edge. Typical use: profiles through plate, punches, extrusion dies, gears, splines.
Sinker EDM, also called ram EDM, burns a shaped electrode down into the part to produce a cavity that matches the electrode. It can produce blind features and square internal corners. Typical use: mould cavities, deep ribs, keyways in blind holes, square blind pockets.
Sinker work carries an extra cost most buyers do not see: the electrode itself has to be machined first, usually from graphite or copper, and complex cavities need several. If you send a part with four blind square pockets, you are paying for four electrodes before anything is burned.
What EDM costs you in time
Expect slower removal rates and a longer queue. A profile that a mill would rough in minutes can take hours on wire. On the other hand, an EDM cell often runs unattended overnight, so a shop may quote a longer lead time without a proportional price increase.
Surface finish from EDM has a characteristic matte recast layer. For most parts that is acceptable. For fatigue-critical or medical parts, the recast layer may need to be removed by polishing or a finishing pass, and that has to be in the request or it will not be in the quote.
How to present an EDM part in a request
Give the shop these five things:
- Material and condition, including hardness after heat treat. “SKD11, hardened to 58 to 60 HRC” tells a shop everything it needs to route the job.
- Which features need EDM and which do not. Most parts are milled first and EDM only on the critical detail. Saying so prevents a shop from quoting the whole part on wire.
- Corner radius you can live with. If 0.15 mm is acceptable, say it. It may turn a sinker job into a wire job, which is cheaper.
- Surface finish requirement, and whether the recast layer must be removed.
- Datum strategy, because an EDM’d feature is usually located from a machined datum, and the stack between them is where tolerance is lost.
The question worth asking first
Before specifying EDM, ask your design team one thing: is the sharp corner load bearing, sealing or cosmetic? If it is cosmetic, a radius is free. If it is sealing, a groove might do the same job. If it is genuinely a clearance corner for a mating square shaft, a corner relief on the drawing turns an EDM feature into a milled feature and drops the price of the part.
Shops in Vietnam do have wire and sinker capacity, mostly in the tooling and die sector, and it is a smaller group than the general milling pool. That is fine as long as the request says clearly which features need it, because the routing is then obvious.
What EDM costs compared with milling
There is no single multiplier, but these patterns hold well enough for planning.
| Situation | Relative cost |
|---|---|
| Profile through 10 mm plate, simple shape | Wire EDM often comparable to milling |
| The same profile through 60 mm plate | Wire EDM clearly more expensive per part, and sometimes the only option |
| Square blind pocket, 20 mm deep | Sinker EDM plus electrodes, several times the cost of a radiused milled pocket |
| Keyway in a hardened blind bore | EDM is the only practical route |
| Fine detail on a hardened insert | EDM, and the alternative is remaking the part soft |
The pattern is consistent: EDM is competitive when it replaces something difficult and expensive when it replaces something easy.
Questions to ask a shop quoting EDM work
- How many electrodes does this part need, and who machines them? Electrode work is machining, and it is often the larger half of the job.
- What wire diameter do you plan to use? It sets the achievable internal radius.
- How many skim passes? Skims improve accuracy and finish and add time. The number is a real decision, not a detail.
- How is the part held and datumed? EDM features are usually located from machined datums, and that stack is where tolerance is lost.
- Will the recast layer be removed? If the part is fatigue critical, this needs a plan rather than an assumption.
If you are not sure yet whether your geometry needs EDM or a rethink, send the drawing and say so. The related guide on 3-axis, 4-axis and 5-axis covers the same decision from the milling side.
Questions we get on this
Is EDM slower than milling?
Much slower in material removal, and that is priced into the hour. It wins when the alternative is impossible rather than when the alternative is merely inconvenient, so it belongs on specific features and not on a whole part.
Can EDM cut any metal?
Any electrically conductive metal, including hardened tool steel, titanium, Inconel and carbide. It does not care about hardness, which is the whole point. It cannot cut plastics, ceramics or glass.
What is the smallest internal corner EDM can produce?
Wire EDM leaves a radius equal to half the wire diameter plus the spark gap, commonly around 0.1 to 0.15 mm with a 0.25 mm wire. Sinker EDM can go sharper still with a shaped electrode. A milling cutter cannot go below half its own diameter.
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