Both wire EDM and sinker EDM remove material with electrical sparks instead of a cutting tool, but they're not interchangeable. Picking the wrong process can cost you days of cycle time and an entire die block. Here's how to choose.

Wire EDM in 30 seconds

A brass wire (typically 0.010" / 0.25 mm) acts as the electrode while deionized water flushes the gap. The wire moves through the workpiece following a 2D or 4-axis profile. Think of it as an extremely accurate band saw that doesn't care how hard the steel is. Best for through-cuts, profiles, slots, and narrow keyways.

Sinker EDM in 30 seconds

A graphite or copper electrode is shaped to match the cavity you want, then plunged into the workpiece in oil dielectric while sparks erode the steel. Best for blind cavities, sharp internal corners, deep ribs, fine details, and any 3D feature you couldn't cut with a milling tool.

When to use wire EDM

Through-cut profiles in hardened tool steel (D2, A2, S7, M2)
Narrow keyways, splines, and gear forms
Punch and die clearances tighter than 0.0005"
Blanks for stamping dies after heat treat
Carbide cutting tools, EDM-friendly carbide grades
Stacks of plates that need identical profiles

When to use sinker EDM

Blind pocket cavities for plastic injection molds
Sharp internal corners (square corners with 0.005" radius)
Deep ribs in mold cores
Fine surface texture (VDI / Ra patterns) for cosmetic mold faces
Burned-in features on hardened tool steel after heat treat
Tight-tolerance bores in carbide

Surface finish reality

Wire EDM typically leaves a finish around Ra 32–63 µin in a single rough cut, dropping to Ra 8–16 µin after two skim passes. Sinker EDM finishes range from Ra 200 µin on a fast roughing burn down to Ra 4 µin on a fine finish electrode — but each finer pass adds hours of burn time and another electrode.

The cost trade-off

Wire EDM is faster per square inch of cut and doesn't need an electrode, so for through-cuts it's almost always cheaper. Sinker EDM is the only option for blind details, but every cavity needs at least one (often three) graphite or copper electrodes built first. On a complex mold, electrode build can cost as much as the burn itself.

What to send for an EDM quote

STEP file
2D drawing showing critical features and tolerances
Material spec (D2, A2, S7, etc.) and current condition (annealed or hardened)
Required surface finish (Ra or VDI number)
Quantity and any required heat treat sequence

Hybrid jobs

On a real die or mold, you'll often use both. A typical sequence: rough mill the cavity in soft steel, heat-treat to 60+ HRC, sinker EDM the sharp internal details, wire EDM the through-features and the blanking profile. Shops that own both machines can sequence the operations to minimize work-holding moves and keep your tolerances stacked correctly.

We run wire and sinker EDM in-house alongside our mills, lathes, and grinders, so die builds and mold details stay under one roof. Send your drawing and we'll quote both processes if there's a choice — with cycle time and electrode cost broken out so you can see the math.