EDM Tolerance Guide
Production-realistic tolerance and surface finish data for wire EDM and sinker EDM. What a competent shop with well-maintained equipment achieves — not theoretical best-case from a machine tool brochure.
Wire EDM Tolerances by Cut Type
Wire EDM tolerance tightens with each successive skim pass. The rough cut removes bulk material. Each skim pass after that refines the surface and tightens the dimension. More passes = tighter tolerance = better finish = more time = higher cost.
| Cut Type | Tolerance | Surface Finish | Speed | When to Use |
|---|---|---|---|---|
| Rough Cut | ±0.002" | Ra 100–200 μin | Fastest | First pass only. Never a final dimension. |
| First Skim | ±0.0005" | Ra 40–60 μin | Moderate | Most production tooling work. Good enough for die sections. |
| Second Skim | ±0.0002" | Ra 20–30 μin | Slow | High-precision tooling. Carbide punches, tight-fit inserts. |
| Third+ Skim | ±0.0001" | Ra 8–16 μin | Slowest | Mirror finish. 4–7 total passes. Gage work, optical tooling. |
Sinker EDM Tolerances
Sinker EDM tolerance depends heavily on electrode material, orbit strategy, and flushing conditions. Copper electrodes hold tighter tolerances than graphite but cost more and wear differently.
| Application | Tolerance | Surface Finish | Electrode | Notes |
|---|---|---|---|---|
| General Cavity | ±0.001" | Ra 60–125 μin | Graphite | Forging dies, large mold cavities. |
| Precision Cavity | ±0.0005" | Ra 30–60 μin | Graphite or Copper | Injection mold details, stamping die pockets. |
| Fine Detail | ±0.0002" | Ra 8–20 μin | Copper | Rib features, thin-wall cores, micro-features. |
| Micro EDM | ±0.0001" | Ra <8 μin | Copper Tungsten | Medical tooling, electronics, sub-0.005" features. |
Factors That Affect Achievable Tolerance
Material
Tool steels (D2, A2, S7) hold tolerance predictably. Carbide cuts cleanly but slowly — tolerance is equivalent or better. Aluminum tends to produce a rougher recast layer that makes the last few tenths harder to hold consistently. Stainless needs lighter power settings to avoid micro-cracking, which can affect edge quality.
Workpiece Geometry
Tall workpieces are harder to hold tolerance on because the wire deflects under cutting forces at height. At 1" thick, ±0.0001" is routine. At 6" thick, ±0.0002" is a realistic production target. At 12"+, expect ±0.0005" unless the shop has specialized fixturing and flushing strategies.
Machine Condition
Wire EDM tolerance capability degrades with machine wear. Wire guide condition, axis positioning accuracy, and thermal stability all matter. A machine that was capable of ±0.0001" when new might realistically hold ±0.0002" after 15 years. Ask the shop about their maintenance schedule and when guides were last replaced.
Temperature
EDM generates heat. The workpiece, the wire guides, and the dielectric fluid all change temperature during cutting. A shop that maintains climate-controlled dielectric temperature (typically 68±2°F) will hold tighter tolerances than one that doesn't. For work tighter than ±0.0002", ask about thermal management.
Tolerance and surface finish are directly linked in EDM — you can't get a fine finish without multiple passes, and each pass tightens the tolerance. If you only need ±0.001", don't spec a mirror finish. You'll pay for 5 extra skim passes you don't need. Conversely, if you need Ra 12 μin, know that you're also getting ±0.0001" whether you asked for it or not. Tell your shop what actually matters — dimension or finish — and let them optimize the cut sequence.
The tolerance you put on a drawing is the tolerance you pay for. We see drawings all the time where someone defaulted their CAD system to ±0.0005" on every wire EDM feature, when the actual functional requirement is ±0.001" on most dimensions and ±0.0002" on one or two. Call out the critical dimensions explicitly and leave the rest at a reasonable general tolerance. Your shop will thank you — and your quote will be lower.
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