Sinker EDM vs Wire EDM

Both processes use electrical discharge to remove material. The difference is geometry — wire cuts profiles, sinker creates cavities.

How they differ mechanically

Wire EDM uses a thin, continuously-fed wire as the electrode. The wire travels vertically through the workpiece, cutting a 2D profile (with optional taper). It can only cut shapes where the wire can enter and exit the material — through-profiles, not blind cavities.

Sinker EDM (also called ram EDM or plunge EDM) uses a shaped electrode — typically graphite or copper — that plunges into the workpiece. The electrode is a negative of the desired cavity shape. As it sinks into the material, spark erosion creates the cavity. Sinker can create blind holes, complex 3D pockets, and internal features that wire can't reach.

Tolerance and surface finish

Wire EDM: ±0.0001" achievable, ±0.0002" routine. Surface finish to Ra 4μin with multiple skim passes. The finish is consistent across the cut — uniform from top to bottom of the workpiece.

Sinker EDM: ±0.0002-0.0005" typical. Surface finish ranges from Ra 4μin (fine finishing with low power settings) to Ra 125μin (rough cavity). The finish depends on power settings and electrode wear — deeper cavities show more variation from top to bottom.

Electrode cost

Wire EDM's "electrode" (the wire) is cheap — $6-12/lb, consumed continuously. No custom electrodes needed. Setup for a new part means programming the profile and threading the wire — 15-45 minutes.

Sinker EDM requires a custom electrode for each unique cavity shape. Graphite electrodes are machined on a CNC mill — a simple shape takes 30 minutes, a complex 3D cavity electrode takes 2-8 hours. Copper electrodes cost more to machine but wear slower. Electrode cost is often 30-50% of the total job cost for sinker work.

For repeat production, electrode cost amortizes across volume. For one-off work, the electrode is a significant upfront cost that makes sinker EDM expensive for prototype quantities.

When to use each

The simplest rule: if the wire can pass through the workpiece, use wire EDM. If the feature is a blind cavity or the wire can't enter/exit, use sinker EDM. Many tooling jobs use both — wire cuts the punch profiles, sinker creates the die cavities.

Cost comparison

Wire EDM: $55-125/hour, no electrode cost, fast setup.

Sinker EDM: $65-135/hour, plus electrode cost ($50-500+ per electrode), longer setup.

For through-cut profiles, wire EDM is almost always cheaper. For blind cavities, sinker EDM is the only option — cost comparison is irrelevant because wire can't do the job.