EDM (Electrical Discharge Machining)
EDM removes metal with controlled electrical sparks — no physical contact between tool and workpiece. A series of rapid voltage discharges (sparks) jumps between the electrode (tool) and the workpiece through a dielectric fluid, melting and vaporizing tiny particles of metal. Because there's no cutting force, EDM can machine hard materials, fragile parts, and intricate internal geometries that conventional tools cannot reach.
The Two Main Types
Sinker EDM (Ram EDM / Plunge EDM)
A shaped electrode (usually graphite or copper) is sunk into the workpiece. The electrode is the inverse of the desired cavity — like a mold punch pressing into clay. Used for mold cavities, die pockets, and internal gear forms. One electrode shape creates one cavity shape.
- Electrode material: graphite (most common, easy to machine), copper (fine finish), copper tungsten (difficult materials)
- Dielectric: hydrocarbon oil or deionized water
- Typical use: injection mold cavities, die casting dies, forging dies
Wire EDM (Wire Cut EDM)
A thin brass or copper wire (0.1–0.3 mm diameter) travels from a supply spool to a take-up spool, cutting through the workpiece like a bandsaw. The workpiece is CNC-contoured in X/Y. Wire EDM can cut extremely intricate profiles and narrow slots with no force on the part.
- Wire diameter: 0.10–0.30 mm (thinner = finer detail, slower)
- Dielectric: deionized water
- Typical use: punch and die buttons, extrusion dies, carbide punches, progressive die components
When to Choose EDM
| Scenario | Why EDM |
|---|---|
| Hardened steel (HRC 60+) after heat treat | Carbide tools can't cut it; EDM only needs electrical conductivity |
| Internal corner radius smaller than tool radius | End mills have corner radii; EDM wire can make near-zero inside corners |
| Thin or fragile walls | No cutting force means no deflection or vibration |
| Deep, narrow slots | Small wire can reach L/D ratios milling cannot |
| Carbide or ceramic parts | Grinding is slow; EDM cuts carbide efficiently |
EDM Limitations
What EDM can't do:
- Only works on electrically conductive materials (no plastics, ceramics, glass unless metalized)
- Slow material removal (mm³/min) — not for roughing large volumes
- Leaves a recast layer (white layer) on the surface — a thin, brittle, heat-affected zone that may need removal for highly stressed parts
- Creates a slight taper on tall walls (wire wanders slightly from the spark gap)
EDM vs Other Processes
| Milling | Grinding | EDM | |
|---|---|---|---|
| Material hardness | Up to HRC 50 | Any (abrasive) | Any conductive |
| Material removal rate | High | Medium | Low |
| Internal corner radius | Limited by tool | Limited by wheel | Near-zero |
| Cutting forces | Significant | Moderate | None |
| Surface integrity | Good | Good (if controlled) | Recast layer |
| Best for | General machining | Precision finishing | Hard materials, complex geometry |