Lesson 6.5CutChapter 6 · Cutting and non-traditional machining

Wire EDM

Sparks from a thin moving wire cut through-profiles

Wire EDM (electrical discharge machining) cuts metal with a thin brass wire that never touches it: thousands of tiny electric sparks per second jump from the wire and erode the metal away. It makes the precise openings in punching and stamping dies, gear and spline profiles, and fine slots in hardened steel. Because the wire does no pushing, hardness does not matter, and accuracy reaches a few thousandths of a millimeter.

Photo: Ricard Polls, Public domain, via Wikimedia Commons

Wire EDM: front section through the wire, plus top views of the path and the spark gapThe large view is a front section through the wire; the two small panels look from above. Circled numbers match the steps below.

How it works

Feed the wire. A brass wire, typically about 0.25 mm thick, unwinds from a spool and runs through the part at a steady speed, so fresh wire always does the cutting.
Guide it. Precision guides (often diamond) above and below the part hold the wire straight and taut. Tilting the guides relative to each other cuts tapered walls.
Spark. A pulse generator applies short voltage pulses between the wire (negative) and the part (positive). Sparks jump the tiny gap, each melting and vaporizing a microscopic crater.
Flush. The part sits in deionized water (water with its minerals removed so it barely conducts). It insulates the gap until each spark, cools the cut and flushes away debris.
Follow the outline. The CNC table moves the part so the wire traces the contour, starting from the edge or from a drilled start hole; used wire goes to a bin. When the loop closes, the inner piece drops free.

For the best accuracy, the machine usually follows the first cut with one to several lighter skim cuts that smooth the wall and correct its size.

Key facts

Materials: Any electrically conductive material, especially hard ones: hardened tool steel, carbide, titanium, nickel alloys, copper, graphite
Typical parts: Punches and die openings, gears and splines, extrusion dies, medical and aerospace parts, fine slots
Part size: Through-cuts in plates up to about 300 mm thick; wire from about 0.02 to 0.3 mm
Precision: Very high: down to a few micrometers (thousandths of a millimeter) with skim cuts
Surface finish: Fine, matte, slightly frosted surface; very smooth after skim cuts
Batch size it suits: One-offs and small series: tooling, prototypes, high-value parts
Tooling and machine cost: No part-specific tooling; wire is used up; machines cost from tens to hundreds of thousands of dollars
Speed: Slow: often minutes to hours per part; machines run unattended overnight

Strengths and limits

Strengths

Very high accuracy and sharp inside corners
Hardness does not matter, so parts can be cut after hardening
No cutting force: thin, delicate walls do not bend
Thick plates and tapered walls in one setup

Limits

Slow compared with milling or laser
Only electrically conductive materials
Only through-cuts: the wire must pass all the way through
Wire is consumed; the surface layer is slightly altered by heat

Where it's used

Tool and die makingPunch and die sets, stamping and extrusion dies
AerospaceTurbine disc slots, fir-tree roots, nickel-alloy parts
MedicalSurgical tools and implant parts in titanium
MachineryGears, splines, keyways in hardened shafts and hubs
ElectronicsLead-frame dies and fine connector tooling

How to recognize it

A uniform matte, frosted gray surface with no tool marks or directional lines.
Very sharp inside corners with only a tiny radius, about half the wire thickness.
Perfectly parallel or precisely tapered walls in thick, hard parts such as die blocks.
Sometimes a tiny witness mark where the cut started and ended.

Compare with

Sinker EDM6.6the same sparks, but from a shaped electrode that sinks into blind cavities instead of a wire cutting through.Laser cutting6.2much faster in sheet, but far less precise and limited in thickness.Milling5.2faster and makes 3D shapes, but struggles with hardened steel and sharp inside corners.

Watch it

RAPID DIRECT · An animated look at the spark gap and the process

wireedmguy · An operator explains the wire, guides and water

ReliableEDM · Typical parts and why shops choose wire EDM

TITANS of CNC MACHINING · A real shop machine at work, and a slug falling out

See real photos

Wikipedia: Electrical discharge machining — overview with a wire EDM section (wire, deionized water, guides) and history
Wikimedia Commons photo search

Remember

The wire never touches the part: sparks across a tiny gap erode the metal.
Hardness does not matter, and accuracy reaches a few micrometers.
Slow and limited to conductive through-cuts, so it is kept for precise, hard or high-value parts.