- Tolerance and Ra are set by the number of skim cuts, not just the wire
- Rough + one skim = good accuracy; more skims = finer Ra and tighter size
- Taper cutting trades accuracy for angle — state it or expect it straight
- Recast layer and white layer are real, and can be removed by finishing
Accuracy is bought with skim cuts
A single rough pass erodes fast but leaves a rough, slightly oversize surface. Each skim pass re-cuts the same path with lower power, shaving toward the final size and smoothing the surface. This is why the same machine can hold ±0.002 mm or ±0.01 mm — the difference is how many passes you pay for.
Surface finish, in real terms
Wire EDM leaves a matte, layered surface with a Ra that falls with each skim cut. A rough pass might leave Ra 3.0 µm and up; skim cuts take it below Ra 1.0 µm. The finish is non-directional and burr-free, but it is not a polished surface — if the part needs a mirror finish, plan a post-process.
Taper and the vertical edge
Wire EDM normally cuts a straight vertical wall, but it can also cut a taper. Taper changes the effective accuracy because the wire is deflected across the thickness, so a tapered slot is looser than a straight one. If you need a taper, call it out; if you need a straight wall, state the perpendicularity.
The recast layer, and why it matters
The discharge melts a microscopic layer at the cut surface, which resolidifies as a 'recast' or 'white' layer. On most parts it is irrelevant; on fatigue-critical or medical surfaces it can be a stress raiser. We can remove or limit it with finish passes or a follow-on process, and we will tell you when it matters for your part, rather than discovering it in testing.
Reading the numbers on a report
A wire-EDM inspection report should show size on the critical features, perpendicularity or taper, and Ra. If a shop hands you only a dimensional sheet with no finish and no taper data, ask — the surface and the wall are half the value of the process.
What Ra can wire EDM achieve?
A rough pass is above Ra 3 µm; multiple skim cuts bring it below Ra 1 µm. State the Ra and the tolerance together, because they are bought with the same skim passes.
Does wire EDM leave burrs?
No — the process erodes rather than shears, so there is no burr. Sharp edges may still want a light break for handling, but the cut itself is clean.
State the size, the Ra and the taper you need, and we will quote the skim cuts that buy them.
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Continue reading: Wire EDM Machining Services · Wire EDM vs CNC Milling · Machining Shop
Standards referenced on this page
- ASTM B209/B209M-21 — Standard Specification for Aluminum-Alloy Sheet and Plate
- ISO 9001 — Quality management systems — Requirements
- ASTM E8/E8M-21 — Standard Test Methods for Tension Testing of Metallic Materials