What Is Wire EDM? How It Works, Tolerances & Uses

By Owen Lin, Engineer, Tuowei Precision · November 2026

The answer to what is wire EDM sits in the name: wire electrical discharge machining cuts metal with electrical sparks instead of a cutting tool. A thin, electrically charged wire travels through the part while thousands of tiny sparks per second melt and vaporize the metal in its path. The wire never touches the workpiece.

That one fact explains the whole process. With no cutting force, hardness stops mattering and thin features do not bend. The wire cuts hardened tool steel as easily as soft steel, leaves sharp internal corners, and holds tolerances that would be very hard to reach with an end mill.

Wire EDM in Short

Short answer: Wire EDM is a non-contact machining process that cuts electrically conductive metals with a thin brass or coated wire, usually 0.1 to 0.3 mm in diameter. Controlled sparks jump between the wire and the part through deionized water, eroding a narrow slot along a CNC-programmed path. It suits hardened steels, carbide and titanium, punches, dies and precise 2D profiles.

This guide explains how the process works, what the machine and the wire are made of, what tolerances and finishes to expect, and where wire EDM is the right choice over milling or laser cutting.

What Does Wire EDM Stand For?

EDM stands for electrical discharge machining. The “wire” refers to the electrode, which in this version of the process is a continuously fed wire rather than a shaped block. You will also hear it called wire cutting, wire erosion, spark erosion or simply “wire.”

EDM as a family has three main members:

• Wire EDM goes through the full thickness of a part along a 2D or tapered path, like a very precise bandsaw that never touches the metal.
• Sinker EDM (also called ram or die-sinking EDM) plunges a shaped graphite or copper electrode into the part to form a blind cavity, such as a mold pocket.
• Hole-drilling EDM uses a rotating tube electrode to drill small, deep holes, including the start holes that wire cutting often needs.

How Does Wire EDM Work?

The physics are simple once you picture the gap. The wire and the workpiece are both connected to a power supply, but they never touch. Between them is a gap of a few hundredths of a millimeter, filled with deionized water that acts as an insulator, or dielectric.

When the voltage across that gap is high enough, the water breaks down and a spark jumps across. The spark is tiny, but its temperature is extreme, so it melts a microscopic crater in the metal. The surrounding water then flushes the molten particles away and cools the surface. The generator repeats this thousands of times per second while the CNC moves the wire along the programmed path.

The Wire EDM Process, Step by Step

Step by step, a typical job looks like this:

1. Program the path. The CAD profile is converted into a wire path, with an offset for the wire radius and the spark gap.
2. Prepare a start hole. If the cut is an internal shape, a small hole is drilled first so the wire can be threaded through the part.
3. Thread the wire. Most modern machines thread automatically, even after a wire break.
4. Rough cut. The first pass removes the slug at high power and gets close to size.
5. Skim cuts. One to several lighter passes follow the same path at lower power, improving accuracy and surface finish.
6. Remove and inspect. The slug or part is removed and measured.

Why Skim Cuts Matter

The rough cut is fast but leaves a rougher surface and a thin melted layer. Each skim cut removes a little more, at lower energy, so accuracy and finish improve with every pass. The trade-off is time. A part with one cut might be ready in an hour, while the same profile with three skims can take several times as long. Ask for skims only on the faces that need them.

What Is a Wire EDM Machine?

A wire EDM machine has four main systems:

• Wire feed: a spool feeds fresh wire past the cut and into a chopper or take-up bin. Upper and lower diamond guides hold it straight and in position.
• Generator: the power supply that shapes each spark. Modern generators control spark energy very finely, which is how they reach fine finishes.
• Dielectric system: a tank of deionized water, plus filters and resin to keep conductivity low and remove debris.
• CNC axes: X and Y move the table, while separate U and V axes shift the upper guide. Tilting the wire that way lets the machine cut tapers and parts with different top and bottom shapes.

Many shops in China also run reciprocating “fast wire” machines, which move a molybdenum wire back and forth and reuse it many times. They cost less to run but are less accurate and leave a rougher finish than single-pass brass wire machines. When a supplier quotes the job, it is worth asking which type the part will run on.

What Is EDM Wire Made Of?

The wire is the electrode, so its material decides cutting speed, accuracy and cost.

Wire type	Material	Typical use
Plain brass	Copper-zinc alloy	General purpose; the most common choice
Coated wire	Brass or copper core with a zinc-rich coating	Faster cutting and better flushing on thick parts
Molybdenum	Moly	Reciprocating “fast wire” machines; reused many times
Tungsten	Tungsten	Very fine wires for micro features

Common diameters run from about 0.1 mm to 0.3 mm, with 0.25 mm the everyday default. Thinner wire cuts smaller corner radii but breaks more easily and cuts more slowly. On single-pass machines the wire is used once and then recycled as scrap, because spark erosion damages its surface.

Which Materials Can It Cut?

The process works on any material that conducts electricity. Hardness makes almost no difference, which is the main reason the process exists.

• Tool steels such as D2, A2 and H13, cut after hardening.
• Stainless steels, including 17-4 PH and 316.
• Titanium and nickel alloys such as Inconel.
• Tungsten carbide for punches and wear parts.
• Aluminum, copper and brass, although these are usually cheaper to mill.

It cannot cut plastics, glass, most ceramics or other insulators. Conductive composites are possible but need testing first.

Tolerances and Surface Finish

Wire cutting is one of the most accurate cutting processes in a typical machine shop. Accuracy depends on part size, thickness, the number of skim cuts, temperature control and the machine itself. These are the tolerances we work to on our wire cutting service:

Wire EDM work	Typical tolerance
Standard wire cutting	±0.005 in (±0.127 mm)
High-precision wire cutting	±0.002 in (±0.051 mm)
Small parts	±0.003 in (±0.076 mm)
Large parts	±0.010 in (±0.254 mm)
Complex features	±0.005 in (±0.127 mm)

With several skim cuts, surfaces can reach about 16 µin Ra (0.4 µm). Tolerances on drawings should follow a recognized system, such as general tolerance classes under ISO 2768-1 or geometric tolerancing under ASME Y14.5, so supplier and buyer read the drawing the same way.

How Tight Should Your Tolerances Be?

Our view: do not ask for the machine’s best tolerance on every edge. Hold the tightest values on punch and die clearances, fits and datum faces, and let the rest run at standard tolerance. Each extra skim cut adds machine time.

Advantages and Limitations

Advantages	Limitations
Cuts hardened metals without tool wear on the part	Only works on electrically conductive materials
No cutting force, so thin walls and fine features stay true	Slow compared with milling or laser cutting
Sharp internal corners, limited only by wire radius	Through-cuts only; blind pockets need sinker EDM
Very tight tolerances and fine finishes with skim cuts	Leaves a thin recast layer on the cut surface
Burr-free edges	Internal shapes need a start hole first
Cutting after heat treatment avoids distortion	Cost per part rises quickly with thickness and skim passes

The Recast Layer: A Limit Few Guides Mention

Every spark melts metal, and some of that metal re-solidifies on the surface instead of flushing away. The result is a thin recast layer, sometimes called the white layer, with a heat-affected zone underneath. It is harder and more brittle than the base metal and can contain micro-cracks.

For most parts this does not matter. For fatigue-critical aerospace parts, medical implants or highly stressed punches, it can. Skim cuts reduce the layer, and polishing or light grinding can remove it. If your part sees cyclic loads, say so on the drawing so the supplier plans for it.

Wire EDM vs CNC Milling vs Laser Cutting

Factor	Wire EDM	CNC milling	Laser cutting
Hard materials (over ~45 HRC)	Excellent	Difficult	Possible, with heat effects
Sharp internal corners	Yes, down to wire radius	No, limited by tool radius	Limited by kerf
Thick parts	Yes, slowly	Yes	Limited
Speed	Slow	Fast	Very fast on sheet
Typical accuracy	Very high	High	Moderate
3D features	Through-cuts and tapers only	Full 3D	2D profiles

The rule of thumb is simple. Mill it if you can, laser cut it if it is sheet, and send it to the wire machine when hardness, sharp internal corners or tight profile tolerances rule the other two out. Many parts use both: CNC machining roughs the block before heat treatment, and the wire cuts the hardened profile afterwards. For sheet parts, our guide to how laser cutting works covers the faster alternative.

Where Is Wire Cutting Used?

• Tool and die: punches, die plates, stripper plates and mold inserts.
• Aerospace: turbine parts, fir-tree slots and tight-tolerance brackets in nickel alloys and titanium.
• Medical: surgical instruments and implant components.
• Automotive: gears, splines, fuel system parts and prototype tooling.
• Electronics: fine connector tooling, lead frames and micro parts.

The process also plays a quiet role across the shop. It cuts the dies used in aluminum extrusion and many of the inserts in injection molds, so it sits behind a large share of mass-produced parts. It is one of the core processes in precision machining.

Common Mistakes When Ordering Wire-Cut Parts

• No start-hole positions on internal shapes. The supplier will choose one, possibly on a face you care about. Mark where it may go.
• Ignoring the wire radius in corners. Internal corners cannot be sharper than the wire radius plus spark gap. If a mating part has a sharp corner, add a relief.
• Sending a part that should be milled. Soft aluminum brackets with generous radii are almost always cheaper on a mill.
• Leaving out heat treatment order. If the part will be hardened, cut the profile after heat treatment. Otherwise distortion during hardening undoes the accuracy you paid for.
• Not mentioning fatigue loading. If the part sees cyclic stress, ask for extra skim passes or recast removal.
• Tight tolerances on very thick parts. Taller cuts drift more, so tolerances should loosen with thickness.

Frequently Asked Questions

What is the difference between EDM and wire EDM?

EDM is the general family of spark-erosion machining processes. Wire EDM is one type that uses a continuously fed thin wire as the electrode to cut through a part. Sinker EDM, another type, uses a shaped electrode to burn blind cavities, such as mold pockets, that do not pass through the material.

Is wire EDM faster than waterjet?

Usually not. Waterjet removes material much faster, especially on thick or soft materials, and it can cut non-conductive materials too. Wire EDM is slower but far more accurate, leaves no burrs and produces finer finishes, so it is chosen when precision matters more than speed or cost per part.

How thick a part can be wire cut?

Machines can cut parts up to about 300 mm (12 in) thick, depending on the machine’s frame and the wire used. Cutting slows considerably as thickness increases, and tolerances usually need to loosen on very tall parts because flushing and wire deflection become harder to control.

Why can’t EDM wire be reused?

Each spark erodes a little of the wire as well as the workpiece, leaving its surface pitted and its diameter uneven. Reusing it would reduce accuracy and cause wire breaks. Reciprocating fast-wire machines do reuse molybdenum wire, but they accept lower accuracy and rougher finishes in exchange.

What does wire EDM do?

It cuts precise two-dimensional and tapered profiles through conductive metal, using sparks instead of a cutting tool. Shops use it for punches, die plates, mold inserts, gears and fine slots, especially in hardened steel or carbide, where milling would struggle to hold sharp internal corners or tight tolerances.

Getting Wire EDM Parts Made at Tuowei Precision

At Tuowei Precision, wire cutting runs alongside CNC milling, turning and heat treatment, so a hardened part can be roughed, hardened and wire cut in one order. Our facilities are certified to ISO 9001:2015 and ISO 14001:2015.

For an accurate quote, send a DXF or STEP file with the material and hardness, part thickness, quantity, the faces that need skim cuts and any start-hole restrictions. Our engineers check corner radii, thickness and tolerances before quoting, and suggest where milling would save cost. Start on our wire cutting page.

About the Author

Owen Lin is an engineer at Tuowei Precision. He plans tooling and production processes and works with the quality team to keep every batch consistent.