Wire EDM: Cutting Hardened Parts Without Tool Pressure

Wire EDM cuts metal with an electrically charged wire instead of a rotating tool. It shapes parts that are too hard, too thin, or too sharp-cornered to mill, holding a tight profile in fully hardened steel while putting no cutting force on the part. This guide covers how it works, how it compares with sinker EDM, the tolerances to expect, and when another process fits the part better. At Aizaki Vietnam, wire EDM runs on a SEIBU M500S alongside our turning, milling, and grinding lines.

How does wire EDM cut metal?

Wire EDM removes metal by electrical discharge. A thin brass wire carries a charge as it feeds off a spool, the part and the wire sit submerged in dielectric fluid a few microns apart, and thousands of controlled sparks a second erode tiny craters along a programmed path. The wire never touches the work.

Because no mechanical force reaches the part, hardened metal is no harder to cut than soft. The process erodes annealed steel and fully hardened tool steel at the same rate, responding only to how well the metal conducts. The wire is usually about 0.25 mm, and the machine traces it through the stock like a bandsaw following a line.

The part features that point to wire EDM

Four features send a part to the wire.

Sharp internal corners: A milling cutter leaves a radius equal to its own size; a wire turns a corner down to roughly its own radius, near 0.1 mm inside.

Thin walls and slender webs: With no cutting pressure, a wall that would chatter or deflect under an end mill holds its shape.

Hardened material: Parts that must be heat treated for wear or strength can be cut after hardening, and the finished geometry never has to chase the distortion that heat treatment introduces.

Tall, straight profiles held tight: The wire produces a consistent cut from top to bottom. Gears, cam profiles, punch and die details, and precision slots tend to land here.

Wire EDM vs sinker EDM: what's the difference?

Wire EDM cuts a profile all the way through the part with a moving wire, producing through-features like contours, slots, tapers, and openings. Sinker EDM plunges a shaped electrode into the work to burn a blind cavity that mirrors the electrode, which suits mold cavities and blind pockets a wire cannot reach. It is also called ram or die-sink EDM.

Aizaki Vietnam runs wire EDM. For blind-cavity work, we machine it conventionally where the geometry allows, or point you to the right route during quoting.

What tolerance and surface finish can wire EDM hold?

On the SEIBU M500S, positional accuracy of ±0.005 mm (about ±0.0002 in) is typically achievable under controlled conditions, within a 500 × 350 × 310 mm work envelope. Surface roughness reaches roughly Ra 0.8 to 1.6 µm, depending on the material and the number of passes.

The number of skim passes sets the surface result. The first pass cuts the profile; each pass after it takes a lighter cut that refines the surface and tightens the size. We check every critical dimension after cutting, most often on our ACCRETECH AXCEL RDS coordinate measuring machine, which resolves to 0.01 µm, fine enough to verify micron-level profile features with margin to spare.

A knife edge, cut on the wire

One of our recurring wire EDM parts is a hardened knife edge that acts as the pivot inside a precision measuring instrument. Its edge angle and profile set the lever ratio the instrument depends on. An error of a few microns at the edge changes every reading the instrument takes.

The part is hardened first, then cut on the wire. The profile is fixed in its final hardened state, with no post-hardening distortion to chase. After cutting, the bearing edge is finished and the profile verified on the CMM before the lot ships. We run this one as a monthly reorder at roughly 200 pieces per drawing, and the two-hundredth part has to hold the same profile as the first.

When is milling or grinding the better choice?

When a rotating tool can reach the feature in machinable material, milling is faster and cheaper than wire EDM, and we quote it that way. For a cylindrical outside diameter or a bore that needs a fine ground finish, grinding is the right process.

Our grinding lines cover it: the SHIGIYA GPL-30B for cylindrical work, a KENT surface grinder, and a PINTAI internal grinder for bores from Ø1 to Ø45 mm. General ground surfaces hold ±0.005 mm, and lapped reference faces reach ±0.001 mm under controlled conditions.

Wire EDM at Aizaki Vietnam

Our wire EDM services run on the SEIBU M500S in tool steel, hardened alloy steel, and stainless grades including SUS304 and SUS316L, within a 500 × 350 × 310 mm envelope. Every part moves through the same quality system as the rest of the floor: first-article inspection on new drawings, in-process checks, and final verification on the AXCEL RDS and XYZAX SVF NEX coordinate measuring machines, the KEYENCE IM-7000 vision system, and an ACCRETECH SURFCOM surface tester. Each shipment carries a Certificate of Conformance and dimensional data. We work from pilot lots through recurring production under ISO 9001:2015 and ISO 14001:2015.

Send us a drawing and we will tell you whether the wire fits the part.

FREQUENTLY ASKED QUESTIONS

Q: When should you use wire EDM instead of milling? A: Use wire EDM when the part is hardened, has sharp internal corners a cutter cannot reach, has thin sections that would deflect under cutting load, or needs a profile held tight through heat treatment. For open geometry in machinable material, milling is faster and cheaper.

Q: What is the difference between wire EDM and sinker EDM? A: Wire EDM cuts a profile all the way through the part with a moving wire. Sinker EDM burns a blind cavity into the part with a shaped electrode. Aizaki Vietnam runs wire EDM.

Q: Can you cut hardened steel with wire EDM? A: Yes. Because the process removes material by electrical discharge with no mechanical contact, part hardness does not affect it. Hardened tool steel and annealed steel cut at the same rate.

Q: What materials can be cut with wire EDM? A: Any electrically conductive metal. At Aizaki Vietnam that includes tool steel, hardened alloy steel, and stainless grades such as SUS304 and SUS316L. Non-conductive materials like most ceramics and plastics cannot be cut by wire EDM.

Q: What does wire EDM cost compared with milling? A: Per part, wire EDM usually costs more and runs slower than milling, because it removes material spark by spark. It earns its cost on features milling cannot produce: hardened material, sharp internal corners, and thin sections. Where a cutter can reach the feature, we quote milling.

Q: What tolerances can you hold, and do you offer wire EDM services in Vietnam? A: On our SEIBU M500S at our Dong Nai facility, positional accuracy of ±0.005 mm (about ±0.0002 in) is typically achievable under controlled conditions, verified on our CMM. We machine parts for export to the US, Europe, and Japan.

Evaluating a precision partner outside China, or qualifying a new supplier in Asia? Send a drawing to sales@aizakivn.com for a quoted assessment.