Wire EDM vs Sinker EDM: Spark Erosion for Hard Metals and Sharp Corners

Some features defeat an ordinary cutter: a square internal corner with no radius, a slot in a part already hardened to glass, a delicate rib too thin to push against. For these, the answer is EDM — electrical discharge machining — which removes metal not by cutting but by controlled sparks. Because nothing touches the work and hardness doesn’t matter, EDM does what milling and turning can’t. This guide explains how EDM works, the difference between wire and sinker EDM, and when each earns its place.

How Spark Erosion Works

EDM erodes metal with a rapid series of electrical sparks between an electrode and the workpiece, both submerged in a dielectric fluid. Each spark melts and vaporises a tiny crater of material; the fluid flushes the debris away and cools the gap. The electrode never touches the part — a precise gap is maintained — so there is no cutting force at all. The only requirement is that the workpiece conducts electricity, which means EDM cuts any conductive metal regardless of how hard it is. A part hardened to 60 HRC machines as easily as a soft one.

Wire EDM

Wire EDM uses a continuously moving thin wire (often brass, 0.1–0.3 mm) as the electrode, threaded through the part like a cheese-cutter. As the wire sparks its way along a programmed path, it cuts a precise 2D profile through the full thickness — think of it as a scroll saw that can cut hardened steel. It produces extremely accurate profiles, tight internal corners (limited only by the wire radius), and clean edges with no burr or cutting force. Wire EDM is the go-to for dies, punches, gears, thin precision profiles and any tight-tolerance shape in hardened material — see tolerances.

Sinker (Ram) EDM

Sinker EDM — also called ram or cavity EDM — uses a shaped electrode (usually graphite or copper) machined into the negative of the feature you want. The electrode is pushed (“sunk”) into the workpiece, sparking as it goes, and burns its mirror image into the metal. This is how you produce blind cavities, sharp internal corners and complex 3D shapes that a rotating tool cannot reach — the classic example being a mould cavity or a square-cornered keyway. The catch is that every shape needs its own electrode, which must itself be machined, so sinker EDM carries more setup.

Wire vs Sinker at a Glance

Strengths and Limits

EDM’s strengths are unique: it machines any conductive material at any hardness, applies zero cutting force (so it won’t deflect thin or delicate features), produces sharp internal corners and fine finishes, and leaves essentially no burr. The trade-offs are equally clear — it is slow compared with milling, works only on conductive materials (no plastics or ceramics), and sinker work needs custom electrodes. EDM also leaves a thin heat-affected “recast” layer that critical parts may need polished or removed. Because of the speed and cost, EDM is reserved for what conventional machining can’t do, not for bulk material removal.

When to Choose EDM

• The part is already hardened — EDM cuts heat-treated tool steel without re-softening it.
• You need truly sharp internal corners a rotating cutter cannot produce.
• The feature is thin, delicate or deep and would deflect or break under cutting force.
• Tolerances and finish are very tight on a hard material — dies, punches, moulds, aerospace and medical components.
• It’s a conductive metal. For non-conductive materials, other processes apply — see our materials guide.

The Bottom Line

EDM is the specialist that starts where milling and turning stop. Wire EDM cuts precise profiles clean through hardened material; sinker EDM burns sharp-cornered cavities and 3D shapes a cutter can’t reach. It is slower and limited to conductive metals, so it is chosen for the features that demand it — hardness, sharp corners, delicate geometry and tight tolerances — not for everyday stock removal. Knowing when a feature truly needs EDM keeps a part both manufacturable and affordable.

MechPart Pro runs both wire and sinker EDM alongside conventional and 5-axis machining, so the right process is matched to each feature — hardened profiles, sharp internal corners and precision cavities included. Share your drawing and our engineers will advise where EDM adds value as part of our free design-for-manufacturability review. Explore hard-to-machine metals in our tool steel and titanium pages.