Wire EDM Machining: What It Cuts and When It Beats Milling

A die insert comes off the heat treater at 58 HRC with a 0.6 mm internal corner that has to hold ±0.01 mm. The milling cutter that made the rough shape is now useless: carbide will not survive the hardness, and even if it did, a 0.6 mm corner needs a 1.2 mm cutter that would snap on the first pass. This is the point where most process routes run out of options, and where wire EDM starts.

Wire EDM removes material with a spark, not a cutting edge. There is no force on the workpiece, no tool wear in the mechanical sense, and no burr. It can cut a 1 mm wide slot through 200 mm of hardened tool steel and hold the width to a few microns. It also cannot cut a non-conductive material at all, and its corners are limited by the wire diameter in a way that surprises designers who are used to milling.

Knowing those two facts — what it can cut and how tight its corners can be — is most of what you need to decide whether a part belongs on a wire machine.

How the cut actually happens

A thin brass or coated wire, usually 0.1 to 0.3 mm in diameter, is fed continuously from a spool through the workpiece. The wire never touches the part. A deionized water dielectric floods the gap, a voltage builds until it breaks down, and a controlled discharge vaporises a small volume of metal. The wire is consumed and replaced, which is why wire EDM does not suffer from the tool-wear problems that plague small end mills.

The cut is a through-cut. The wire travels vertically — or at a programmed angle — through the full thickness of the part, so wire EDM produces prismatic geometry with straight or tapered walls. It cannot produce a blind pocket, a spherical cavity, or a feature that does not break through both faces.

The kerf is the wire diameter plus the spark gap, typically 0.15 to 0.35 mm depending on wire size. That kerf is why you cannot simply "cut a line": the toolpath is the part profile offset by half the kerf.

Conductive materials only, and what that rules out

Wire EDM works on anything that conducts electricity. That covers tool steel, stainless, carbon steel, aluminum, copper, brass, titanium, tungsten carbide, inconel and most superalloys. It also covers graphite and many sintered metals.

It does not cover plastics, ceramics, glass, composites, or most coatings. Anodized aluminum is a special case: the aluminum oxide layer is an insulator, so the cut typically starts through a bare or masked area, and thick anodic coatings can interrupt the process. If your part is hard-anodized before the wire operation, plan the sequence differently.

Corner radius is set by the wire, not by the programmer

This is the single most common misunderstanding in wire EDM design. The wire has a finite radius, so it cannot turn a sharper internal corner than its own radius plus the spark gap. With 0.25 mm wire, the smallest internal radius you can produce is about 0.13 mm. With 0.1 mm wire, it drops to roughly 0.06 mm, at a large cost in cutting speed and in the number of passes needed.

The practical rule is to design internal radii no smaller than the wire the shop will use, and to state the radius on the drawing rather than letting the CAM system choose. If you call a 0.05 mm internal radius on a profile that will be cut with 0.25 mm wire, the shop has three options: quote a slower small-wire process, quote a second operation, or send you a DFM note. All three cost you time.

Where wire EDM beats milling outright

Hardened material. Anything above about 45 HRC becomes expensive to mill and difficult to hold. Wire EDM does not care about hardness, so the sequence becomes: rough machine soft, heat treat, then wire cut the critical profile. The part keeps its heat treatment and its tolerance in one step.

Sharp internal corners. A milling cutter leaves the radius of its own diameter. A 6 mm pocket corner gets a 3 mm radius. Wire EDM gets 0.13 mm. For keyways, spline profiles, punch and die edges, and seal grooves, that difference is functional, not cosmetic.

Thin walls and delicate sections. With no cutting force, a 0.3 mm wall will not deflect. Milling the same wall requires light passes, sharp tooling and a lot of patience.

Burr-free edges. The spark leaves a matte surface and no burr. For parts that must not be deburred by hand — medical, vacuum, cleanroom hardware — that is worth real money.

Wire EDM against milling and laser cutting

What drives the price

Wire EDM is quoted on cut length, not on volume removed. A part that removes 90% of its stock can be cheaper than a part that removes 10%, because the wire only pays for the perimeter it travels.

Three factors dominate:

Thickness. Cutting speed falls with thickness. Doubling from 50 mm to 100 mm does not double the time, but it adds 70 to 90%. A 200 mm tall die block is a different cost category from a 20 mm plate.

Number of passes. A rough cut only, at ±0.02 mm and Ra 2.5 µm, is fast. Adding two skim passes to reach ±0.005 mm and Ra 0.4 µm roughly triples the time. Specify the finish you need, not the best available.

Start holes. Every internal profile needs a hole for the wire to thread. If the drawing has no start holes, the shop drills them, usually by EDM drilling or a small-hole drill, which adds cost per hole. Send the profile, the plate thickness and the finish you actually need, and the price follows from those three numbers. Request a quote and we will confirm the smallest corner radius the wire can hold for your part.

Frequently asked questions

Can wire EDM cut aluminum?

Yes, aluminum cuts quickly and cleanly. The main practical limits are the oxide layer on anodized parts and the fact that aluminum's high conductivity means the machine runs at different parameters than steel. Very thick aluminum sections cut less efficiently than steel of the same height.

Why can't wire EDM make a blind pocket?

The wire is a continuous loop that must pass through the workpiece from top to bottom. It has no end, so it cannot plunge and stop inside the material. Blind cavities are produced by sinker EDM instead, which uses a shaped electrode rather than a wire.

How tight a corner can I specify?

With 0.25 mm wire the practical minimum internal radius is 0.13 mm. Smaller wire gets you to about 0.06 mm at roughly half the cutting speed. Below that, the corner becomes a sinker EDM or laser operation.

Does wire EDM leave a heat affected zone?

Yes, a recast layer of about 1 to 5 µm on a rough cut. Skim passes remove most of it, and a multi-pass finish leaves a recast layer thin enough that most specifications accept it. For fatigue-critical or implant parts, specify the number of passes and the acceptable recast depth explicitly.