EDM & Wire Cutting
Sinker and wire EDM: hardened materials, tight corners and burr-free fine features
Electrical discharge machining erodes material with controlled sparks — a wire (wire EDM) or a shaped electrode (sinker EDM) — in deionized water or oil. It machines any conductive material regardless of hardness, with no cutting force and no burrs, making it the standard for hardened tooling, carbide, fine slots, tight internal corners and features no end mill can reach.
1. Wire EDM
A brass/zinc wire 0.1–0.3 mm diameter erodes along a programmed path, cutting through the workpiece. Accuracy ±0.005–0.01 mm, surface Ra 0.2–0.8 µm, and a minimum internal corner radius equal to the wire radius — far tighter than any milling cutter.
Wire EDM cuts hardened steel, carbide, titanium and exotic alloys with zero burr and zero force: delicate tabs, fine slots (0.1 mm+), and tool steel die details are routine. It is slower per hour than milling, so it is used where milling cannot go.
2. Sinker EDM
A CNC-machined electrode (graphite or copper) is sunk into the workpiece to burn the inverse shape: cavities, deep ribs, ribs in hardened molds, and blind features with sharp corners. It also forms threaded holes (with orbiting electrodes) that no tap can cut in hardened steel.
Sinker EDM achieves ±0.01–0.02 mm with fine finishes (Ra 0.4 µm polished). Electrode wear means multiple electrodes for deep cavities; cost scales with cavity volume and finish requirement.
3. Design rules for EDM
Specify EDM only for: hardened material, sharp internal corners (≤ 0.25 mm radius), burr-free edges, thin slots (< 0.5 mm) and deep ribs. Everywhere else, milling is faster and cheaper.
Add a minimum of 0.1–0.2 mm stock for EDM on pre-machined surfaces; note the spark gap (~0.02–0.05 mm) that the process compensates automatically. Avoid EDM on aluminum/copper (milling wins) — EDM shines on steel, carbide and hard alloys.
4. Cost and lead time
EDM is billed per hour with a material/accuracy multiplier: wire EDM ~2–3× CNC hourly, sinker more for electrode work. Small EDM features add $20–100 per feature; full EDM-cut profiles are quoted by cutting length.
Lead time adds 2–5 days over standard machining for most parts; hardened-steel EDM inserts for molds are typically 7–14 days.
Quick reference
| Metric | Wire EDM | Sinker EDM |
|---|---|---|
| Best for | Through cuts, slots, profiles | Cavities, blind features, threads |
| Accuracy | ±0.005–0.01 mm | ±0.01–0.02 mm |
| Surface | Ra 0.2–0.8 µm | Ra 0.4 µm (polished) |
| Min corner | = wire radius (0.05–0.15 mm) | Electrode radius |
| Materials | Any conductive | Any conductive |
| Cost index | 2–3× CNC | 3–5× CNC |
Sources & references
- ASTM E407 (metallographic)
- ISO 1101 — GD&T for EDM parts
- Xometry Learning Center — EDM Guide (public engineering reference)