Machining
EDM vs CNC Milling
EDM erodes conductive material with controlled spark discharges through dielectric fluid, using a wire or a shaped electrode with no cutting force. CNC milling removes material with rotating cutters under real tool pressure. The core trade: milling is far faster and cheaper per cm3; EDM ignores hardness and reaches geometry no cutter can.
| EDM | CNC Milling | |
|---|---|---|
| Removal rate | Wire 100 to 300 mm2/min, sinker 50 to 400 mm3/min | 50 to 500+ cm3/min in aluminum |
| Hardness limit | None if conductive, 65 HRC same speed | Above 50 HRC slow, ceramic or CBN tooling |
| Tolerance | ±0.003 to 0.01 mm wire EDM | ±0.01 to 0.05 mm typical |
| Internal corners | Sharp, wire radius 0.05 to 0.15 mm | Radius equals cutter radius, 0.5 mm+ practical |
| Deep narrow features | Ribs and slots to 20:1 depth ratio | Chatter past 4:1 to 8:1 L/D |
| Surface | Ra 0.2 to 3.2 um, recast layer 2 to 20 um | Ra 0.4 to 1.6 um, clean burnished surface |
| Cutting force on part | Zero, thin walls unstressed | Real force, thin walls deflect |
Choose EDM when
- Hardened tool steel and carbide at 55 to 65 HRC, post heat treat
- Sharp internal corners, deep ribs, and slots past 8:1 depth
- Fragile or thin-wall features where cutting force would deflect them
Choose CNC Milling when
- Everything a rotating cutter can reach: 10 to 100x cheaper per cm3
- 3D contours, pockets, and holes in materials under 50 HRC
- Nonconductive materials and short lead times without electrode prep
The verdict
Mill first, EDM last. Rough and semi-finish everything before heat treat on the mill, then send only what milling cannot do to EDM: hardened cavities, sharp corners, deep ribs, and wire-cut punch and die profiles. Shops that EDM millable features are paying $80 to $120 per hour for nothing.
Cost comparison
A 3-axis VMC costs $80,000 to $250,000 and removes aluminum at 100+ cm3/min, so milled features cost dollars each. Wire EDM machines run $100,000 to $300,000, cut at 100 to 300 mm2/min, and consume $3 to $8/hr in wire; sinker work adds graphite electrodes at $50 to $500 each. The crossover is capability, not volume: mill anything a cutter reaches, and reserve EDM's $80 to $120/hr for hardened steel, sharp corners, and deep ribs where milling scraps parts or breaks tools.
Common questions
Has hard milling replaced sinker EDM in mold shops?
Partly. Modern carbide and CBN cutters mill 50 to 60 HRC cavity steel directly, skipping electrode manufacture, and win on open 3D geometry. Sinker EDM still owns deep narrow ribs, sharp internal corners, textured surfaces, and features past about 8:1 depth where any cutter chatters or breaks.
Does the EDM recast layer matter?
For fatigue-critical or sealing surfaces, yes. Spark erosion leaves a 2 to 20 um recast layer with microcracks that can seed fatigue failures. Fine finish skim passes cut it under 5 um, and aerospace parts often specify post-EDM polishing, etching, or peening to remove it entirely.