A buyer sends the same drawing to three shops. The print calls for a sharp internal corner on a hardened insert. All three say yes. All three deliver a corner with a visible radius, out of print. Nobody lied, exactly. They just never mentioned the one number that decided the outcome before the machine ever started: the diameter of their wire.
The geometry nobody can negotiate
When a wire EDM turns an internal corner, it leaves a radius of roughly half the wire diameter. Not because of skill, settings or machine brand. The wire is round; the corner it cuts can't be sharper than the wire's own radius plus the spark gap. Run the numbers and the market sorts itself out fast:
| Wire diameter | Sharpest internal corner | Who runs it |
|---|---|---|
| 0.25 mm | ≈ R0.13 mm | Common general-purpose setup |
| 0.18 mm | ≈ R0.09 mm | Most precision shops in Mexico |
| 0.13 mm | ≈ R0.07 mm | Our Saturno wire EDM |
If your drawing tolerates R0.15 mm, almost anyone can make your part. If it says R0.08 mm or "sharp", the wire decides who's telling the truth.
Where those corners actually matter
Sharp internal corners aren't a styling choice. On a punch or die insert, the tool's corner becomes the product's corner on every hit, millions of times. On mating components, corner radii set the real clearance. On mold inserts, a corner that should be crisp shows up as a soft edge on every molded part. The corner is the function; that's why the print calls it out.
And these parts are almost always hardened, D2 or H13 at HRC 60 plus, which removes milling from the conversation entirely. A cutter small enough for the corner won't survive the material. The spark doesn't care: wire EDM cuts hardened steel with no contact force, which is also why thin, fragile geometry keeps its shape.
The sequence that makes it work
Thin wire is a finishing instrument, not a hog-out tool. The efficient route roughs the blank soft on a machining center, heat treats to final hardness, then wires the critical profile on the hardened part. Done in one plant, the part keeps one set of datums from bar stock to inspection, and the corner lands where the CAD says it should.
What to check before you send the PO
One question saves the lot: "what wire diameter will my part run on?" If the answer is 0.18 or 0.25 mm and your print needs R0.08, the rejection is already scheduled; it just hasn't happened yet. If the answer is 0.13 mm, ask for a sample cut. We're happy to be tested that way; it's the fastest way to prove a capability most shops can only describe.
What internal corner radius can wire EDM achieve?
Roughly half the wire diameter. With 0.25 mm wire you get about R0.13 mm; with 0.18 mm wire, about R0.09 mm; with 0.13 mm wire, close to R0.07 mm. Operator skill cannot beat that geometry.
Why do punches and die inserts need sharp internal corners?
Because the punch and die define the stamped part. A rounded corner on the tool becomes a rounded corner on every part it produces, and on mating tool sets the radii decide clearance and burr behavior.
Can milling produce a sharp internal corner?
No. An end mill always leaves a radius equal to its own, and tiny cutters deflect and break in hardened material. For corners below about R1 mm in hard steel, EDM is the practical route.
Does thinner wire cut slower?
Somewhat, yes. Thin wire carries less current, so roughing big open profiles with it wastes time. The smart sequence cuts the bulk with standard logic and reserves the 0.13 mm wire for the geometry that needs it.