When a part is too hard to machine, too intricate to mill, or has internal corners a cutter can’t reach, EDM is usually the answer. Here’s the difference between the two main types and how to know which one your job needs.
What EDM Actually Does
EDM — Electrical Discharge Machining — removes metal with sparks, not cutting force. A precisely controlled electrical discharge jumps between a tool electrode and the workpiece through a dielectric fluid, vaporizing a tiny crater of metal thousands of times per second. The tool never touches the part.
That single fact drives everything that makes EDM special:
- Hardness doesn’t matter. If the metal conducts electricity, EDM cuts it — hardened tool steel, carbide, Inconel, titanium. You can machine a part after heat treatment, eliminating the distortion that comes from hardening a finished part.
- No cutting forces. Delicate, thin, or fragile features won’t deflect or break because nothing is pushing on them.
- Sharp internal corners. EDM can produce internal corners a rotating end mill physically cannot reach.
Wire EDM
Wire EDM uses a continuously moving brass wire — typically 0.004” to 0.012” diameter — as the electrode. Think of it as a bandsaw that cuts with sparks instead of teeth, following a programmed path through the full thickness of the material.
What It’s Good At
- Through-cut profiles — any shape that goes all the way through the plate: gears, cams, punches, dies, splines, keyways.
- Tight tolerances — routinely ±0.0002”, with excellent repeatability part to part.
- Tall, thin features — high aspect ratios that would chatter or snap under a milling cutter.
- Stacked parts — clamp a stack of plates and cut them all in one pass.
Limitations
- The wire has to pass through, so it cuts profiles, not blind pockets.
- For an internal shape, you need a start hole to thread the wire through.
- Slower than milling for simple shapes in soft material — don’t wire-EDM something a mill can knock out.
Sinker EDM
Sinker EDM (also called ram, plunge, or die-sink EDM) uses a shaped electrode — usually graphite or copper — that’s machined into the mirror image of the cavity you want. The electrode plunges into the workpiece, burning its shape into the metal. It’s how most injection mold cavities and forging dies get their detail.
What It’s Good At
- Blind cavities — pockets and detail that don’t go all the way through.
- Complex 3D shapes — the cavity matches the electrode, so intricate mold detail is reproduced exactly.
- Sharp internal corners and deep ribs — geometry impossible to mill.
- Fine surface textures — from polished to specified matte finishes.
Limitations
- You have to make the electrode first — that’s a machining job in itself, adding cost and lead time.
- Slower material removal than wire for through-features.
- Deep cavities need flushing strategy and sometimes multiple electrodes (roughing + finishing).
Wire vs Sinker: Quick Decision
| Your Part | Use |
|---|---|
| Profile cut all the way through a plate | Wire |
| Blind pocket or mold cavity | Sinker |
| Gears, splines, punches, dies | Wire |
| Sharp internal corner in a deep rib | Sinker |
| Keyway in a hardened shaft | Wire (or sinker if blind) |
| Hardened steel, carbide, Inconel | Either — both ignore hardness |
When NOT to Use EDM
EDM is precise but slow and not cheap. If a part can be milled or turned in soft material to the tolerances you need, that’s almost always faster and less expensive — see our guide on tolerances and cost before specifying a process. EDM earns its keep on hardened materials, fine detail, tight tolerances, and geometry that conventional cutters simply can’t produce. For most everyday parts, milling or turning is the right call.
A Recast Layer Note
EDM leaves a thin “recast” or heat-affected layer on the cut surface. For most parts it’s irrelevant. For high-fatigue or aerospace applications, it may need to be removed by polishing or a light finish pass — worth flagging on your drawing.
Not sure which process fits your part? Send us the drawing or model. We’ll tell you whether it’s a wire job, a sinker job, or something a mill should handle — and quote it straight.
Materials EDM Handles Well
If it conducts electricity, EDM can cut it — and it doesn’t care how hard it is. The materials where EDM routinely beats conventional machining:
- Hardened tool steels (D2, A2, S7, M2 and friends) at full hardness — cut the part after heat treat, skip the distortion.
- Tungsten carbide — effectively unmachinable by conventional cutters, routine for EDM.
- Inconel and other nickel superalloys — the same heat and work-hardening that punish end mills are a non-issue for sparks.
- Titanium — cut without the heat and fire-hazard concerns of conventional titanium machining.
- Hardened stainless and PH grades — clean cuts with no tool-pressure distortion.
Surface Finish Control
EDM finish is set by how aggressively you spark: high-energy settings remove metal fast but leave a coarser surface, while low-energy finishing passes produce mirror-smooth results. Most precision jobs use a roughing pass for speed followed by one or more finishing passes (called “skim cuts” on a wire machine) to dial in both dimension and finish. The trade-off is always time versus finish — tell us the surface you need and we’ll plan the passes accordingly.