The drawing calls for an internal corner radius of 0.1 mm in a die made from hardened tool steel at 60 HRC. A CNC end mill physically cannot reach that corner - its cutter has a radius of its own. The material is too hard to cut economically, and any clamping force risks distorting the part.
This is the moment engineers reach for the process that cuts metal with lightning instead of blades: wire cut EDM (wire electrical discharge machining, or WEDM).
This guide explains what wire EDM is, how it works, what tolerances and finishes you can realistically expect, which materials it handles, and - most importantly - when it is the right process instead of CNC milling or laser cutting.
The short version: Wire EDM uses a thin, electrically charged wire to erode conductive material with thousands of controlled sparks per second - a non-contact process with no cutting force, no burrs, and no tool-radius limits. It is the go-to process for hardened steels, sharp internal corners, fine contours, and stress-free precision parts. It is slower and more expensive per part than milling or laser, so its sweet spot is high-value, high-precision, low-to-medium-volume work.
What is wire EDM?
Wire electrical discharge machining is a non-contact thermal process. A thin metal wire - typically 0.1–0.25 mm in diameter - is fed continuously through the workpiece along a CNC-programmed path. The wire and the workpiece act as two electrodes, separated by a microscopic gap filled with deionized water (the dielectric fluid).
When a voltage is applied, the gap breaks down and a spark jumps between wire and workpiece. Each spark melts and vaporizes a tiny crater of material. Thousands of these sparks fire every second, and the wire slowly advances, cutting a narrow slot - like an ultra-precise "electric saw" that never physically touches the metal.
Because the wire moves through the full thickness of the part, wire EDM is fundamentally a through-cutting process: it cuts profiles, contours, slots and shaped holes in flat or near-flat parts. Internal features need a start hole drilled first, so the wire can be threaded through.
Key characteristics at a glance
- No cutting force. The material is eroded, not pushed - thin walls and delicate features don't distort.
- No burrs. The spark melts the edge cleanly; parts come off the machine ready for final inspection.
- No hardness limit. It cuts anything electrically conductive, including hardened steel at 60–65 HRC that would destroy conventional tooling.
- Sharp corners. Internal corner radii can go down to roughly 0.05–0.1 mm - far beyond what any milling cutter can produce.
- Conductive materials only. If the material doesn't conduct electricity (wood, plastics, ceramics), wire EDM can't touch it.
What tolerances and finishes can you actually get?
Realistic, achievable numbers from production wire EDM:
| Parameter | Typical value | Notes |
|---|---|---|
| Positioning tolerance | ±0.003–0.01 mm | Precision machines with good thermal control reach the tighter end |
| Internal corner radius | 0.05–0.1 mm | Dictated by wire diameter |
| Surface finish | Ra 0.2–1.6 µm | Rough cut is coarser; skim (trim) passes bring it down |
| Kerf (cut width) | 0.15–0.35 mm | Slightly wider than the wire itself |
| Max thickness | Up to ~300 mm | Practical limits depend on machine and wire type |
Two things worth knowing before you put a tolerance on the drawing:
- Surface integrity. EDM leaves a thin recast (white) layer on the cut surface - a few microns thick. For most parts it's harmless; for fatigue-critical aerospace or medical parts, specify skim passes or a post-EDM operation to remove it.
- Speed vs. finish trade-off. The same cut can be done fast and rough, or slow and fine. A typical job runs a rough pass plus one or two skim passes - slower, but that's where the mirror-like edges come from. Specify how many passes you're paying for.
Wire EDM vs. CNC milling vs. laser cutting
| Decision factor | Wire EDM | CNC milling | Laser cutting |
|---|---|---|---|
| Typical tolerance | ±0.003–0.01 mm | ±0.01–0.025 mm | ±0.05–0.2 mm (thermal) |
| Hardened steel (>50–55 HRC) | Excellent | Difficult, slow | Poor (heat-affected zone) |
| Internal sharp corners | Down to ~0.05–0.1 mm | Limited by cutter radius | Limited by beam/kerf |
| Mechanical stress on part | Virtually none | Cutting forces can distort thin parts | Minimal (but heat input) |
| Burr | None | Common, needs deburring | Some dross on edges |
| Through-profiles, slots, contours | Excellent | Good | Good (thinner stock) |
| 3D surfaces, pockets, cavities | No | Excellent | No |
| Speed / cost per part | Slow, higher per-part cost | Fast at volume | Fastest for thin sheet |
| Best volume range | Prototype, low-to-medium, high-value | Medium to high volume | Medium to high volume |
Use wire EDM when: the material is hardened and must hold tolerance after heat treatment; internal corners are sharper than a cutter can make; the part is thin-walled or stress-sensitive; you need burr-free edges; or you're making dies, punches, molds, or precision gear prototypes.
Use CNC milling when: you need 3D surfaces, pockets, cavities, threads, or open geometries; you're machining softer metals and plastics at volume; or the feature is simply faster and cheaper to mill.
Use laser when: you're cutting thin sheet at high speed and volume, and the material and edge quality requirements fit laser's capabilities.
They are complements, not competitors. In real production, a part often sees all three: milled for the body, laser-cut for a blank, and wire EDM for the precision contour.
What materials does wire EDM cut?
Any electrically conductive material:
- Tool steels and hardened steels (up to ~65 HRC) - the classic case: machine soft, heat-treat, then wire EDM the final contour.
- Carbide and tungsten carbide - dies, punches, and cutting tools.
- Titanium and its alloys - aerospace and medical components.
- Stainless steel, aluminum, copper, brass - precision contours and dies.
- Exotic and conductive composites - where conventional tooling struggles.
If the material conducts electricity, wire EDM can erode it - regardless of hardness. That is its superpower.
Where wire EDM earns its keep: applications
- Dies and molds. Stamping dies, extrusion dies, injection mold inserts - hardened, sharp-cornered, burr-free. The classic wire EDM territory.
- Punches and cutting tools. Precise profiles that must stay sharp and stress-free.
- Precision gears - prototypes and specials. For small batches, prototypes, or special tooth profiles, wire EDM avoids the cost of hobbing or shaping tooling entirely: the gear is simply cut from a hardened blank with the exact tooth geometry programmed in. It's the economical answer when tooling costs would be prohibitive for a handful of parts. (For production volumes, dedicated processes like gear hobbing remain the economical route - we cover both in our precision gear range.)
- Cam profiles and indexer components. Complex, precise contours - the kind of profiles found in precision motion components.
- Medical and aerospace. Thin-walled, fatigue-conscious, hard-material parts where stress-free cutting matters.
- Prototype and low-volume precision parts. When per-part value is high and setup cost of other processes can't be justified.
Wire types and machine classes
The wire is a consumable, and its choice affects speed and finish:
- Brass wire. The general-purpose standard - good balance of speed, surface quality and cost.
- Zinc-coated (coated) wire. Faster cutting, better surface finish, slightly higher cost - popular for production runs.
- Molybdenum wire. Common in medium-speed wire EDM machines - a cost-effective option widely used in Asian manufacturing.
Machine classes are usually described as high-speed, medium-speed, and low-speed wire EDM (fast, medium, and precision - the terminology comes from the Chinese machine market). Precision work belongs on low-speed machines; high-speed machines prioritize throughput for less demanding parts.
How to spec a wire EDM job (so you don't overpay)
- Provide a start hole location for every internal contour - the shop needs to thread the wire through.
- State the material and hardness. "H13 at 52 HRC" tells the shop far more than "hard steel."
- Define the corner radius realistically. If your design truly needs 0.1 mm corners, wire EDM can do it - just know it costs more than 0.5 mm corners.
- Specify the surface finish and skim passes explicitly: "Ra 0.8 µm, 1 rough + 2 skim" is a clear requirement; a bare "fine finish" is not.
- Flag fatigue-critical surfaces so the shop plans to remove the recast layer.
- Match tolerances to what you need. ±0.005 mm is achievable but slower; ±0.02 mm may be all your application requires and will cost a fraction.
When it pays to outsource wire EDM to a precision manufacturer
Wire EDM is not a "buy a machine and figure it out" process. Getting ±0.003 mm consistently needs experience, thermal control, and the right wire/parameter combinations. If your shop doesn't run EDM daily, outsourcing is usually the smarter call - and for precision mechanical parts, it should come from a manufacturer who understands the whole process chain.
At Hansheng Automation, precision is our business: precision CNC machining (to ±0.002 mm), custom parts on demand, precision gear manufacturing, castings, and OEM/ODM programs. If your part needs spark-eroded features - hardened contours, sharp internal corners, precision gear prototypes - tell our engineering team what you're making and we'll advise on the right process route and tolerance strategy.
FAQ
Is wire EDM expensive?
Per part, yes - it's slower than milling or laser. But it's often cheaper overall: no tooling costs (no hobbing cutters, no special end mills), no deburring, no scrap from distorted or chipped parts, and it can machine material that nothing else can touch economically.
Can wire EDM cut thick parts?
Yes - thicknesses up to roughly 300 mm are practical on large machines. The wire cuts through the full thickness, which is exactly why it's used for dies and molds.
Does wire EDM work on hardened steel?
It's the ideal case. Hardness doesn't slow the spark; the common workflow is machine soft → heat-treat → wire EDM the final contour.
Why is there a recast layer, and does it matter?
Each spark melts a microscopic layer that re-solidifies. It's microns thick and usually harmless; on fatigue-critical parts it should be removed with skim passes or a secondary process. Specify this if your part is aerospace or medical.
Can wire EDM replace CNC machining?
No - they're complementary. Wire EDM is a through-cutting process for contours and profiles; CNC milling handles 3D surfaces, pockets and cavities. High-value parts often need both.
What about wire EDM for gears?
For prototypes, special tooth profiles, and small batches, wire EDM is excellent - no tooling investment, and the tooth geometry is exactly as programmed. For production volumes, hobbing or shaping remains more economical.
Bottom line
Wire cut EDM is the process that wins when other processes physically cannot: hardened steel, sharp internal corners, burr-free edges, zero cutting force, tolerances down to ±0.003 mm. It's slower and costs more per part - so use it where it earns its keep: dies, punches, precision contours, and small-batch precision gears and components.
When your drawing calls for the impossible corner, that's not a dead end. It's a wire EDM job.
Need a process partner who understands precision? Talk to the Hansheng Automation team about your part - from CNC machining to precision gears, we'll help you pick the right route.