When wire EDM is the only answer: sharp corners in hardened dies
Every rotating cutter leaves a radius equal to its own radius. That is geometry, not a limitation of a particular machine. So when a drawing shows a square internal corner in a hardened die, one of two things happens: the corner gets a radius it was not supposed to have, or the feature goes on a wire EDM.
Knowing which of those you are asking for is worth a lot on a quote.
The corner radius problem
A 6 mm end mill leaves a 3 mm internal corner radius. To get a 0.5 mm corner you need a 1 mm cutter — and a 1 mm carbide end mill in 60 HRC D2, reaching into a 15 mm deep pocket, will snap. The depth-to-diameter ratio needed simply does not survive the material.
So in hardened steel the practical floor for a milled internal corner is roughly:
| Pocket depth | Smallest sane corner radius |
|---|---|
| Under 5 mm | ~0.8 mm |
| 5-15 mm | ~1.5 mm |
| 15-30 mm | ~3 mm |
| Over 30 mm | ~5 mm, or EDM |
Below those, you are on a wire.
What wire EDM does differently
The wire is a thin brass or coated-brass filament, typically 0.25 mm, that erodes material with an electrical discharge in deionised water. Three consequences follow:
Hardness does not matter. Erosion is thermal, not mechanical. 62 HRC D2 cuts at the same rate as annealed mild steel. This is why EDM is everywhere in die work.
There is no cutting force. Thin ribs, fine details and delicate sections that would deflect under a milling cutter come out straight.
Internal corners go down to the wire radius. With a 0.25 mm wire plus a spark gap you get a corner around 0.15 mm. For genuinely sharp corners, shops drop to a 0.1 mm wire — slower again, but it gets there.
The trade is that wire EDM only makes ruled surfaces: the wire is a straight line, so it sweeps a surface you could make by moving a straightedge. Tapers and transitions between two profiles are possible on a machine with independent upper and lower heads. A blended 3D cavity is not.
The costs nobody mentions in the RFQ
A start hole. For any closed internal shape, the wire has to be threaded through a hole. In hardened steel that hole is drilled by a separate EDM drilling machine before the wire job starts. It is an extra setup and an extra queue.
Cutting speed. Wire EDM in tool steel runs in the range of 10-30 mm² per minute for a rough pass. A 20 mm thick die with 400 mm of profile is hours, not minutes — and skim passes for finish and accuracy add more.
The recast layer. The spark melts material and the flush re-solidifies a thin layer on the surface, typically 5-25 µm depending on settings. It is harder and more brittle than the parent metal and carries tensile stress. On a punch or die edge that sees impact, that layer is where cracks start. Removing it means skim passes at reduced power, or stoning and polishing afterwards — either way, more time.
Designing around it
If you can live without the sharp corner, say so on the drawing. A note reading “internal corners R1.5 acceptable unless otherwise marked” can take a wire EDM operation off the part entirely.
If you need clearance rather than a true sharp corner — which is often the real requirement, for a mating part to seat properly — specify a corner relief: a small drilled or milled undercut at the corner that gives the mating part somewhere to go. Dog-bone and T-bone reliefs are the standard forms. They mill easily and they solve the functional problem without the EDM.
Where the sharp corner is genuinely the function — a blanking die that has to produce a square hole in the strip — then it is EDM, and it is worth planning the whole part around that so the wire operation happens once rather than three times.
What to put on the RFQ
Mark which corners are functional and which are incidental. Mark whether the recast layer is acceptable or has to be removed, and say why — a shop that knows the edge carries impact load will quote skim passes; one that does not will quote a single rough cut and hand you a part that cracks in service.
And send the model with the drawing. Corner geometry is exactly the thing that gets lost when a shop rebuilds your part from a PDF. See the RFQ checklist for the rest of what belongs in the package, or send the drawing and we will flag the features that force an EDM route.