Manufacturing services · Chapter C · Method 18

Sinker EDM

A shaped graphite or copper electrode is plunged into the workpiece under dielectric fluid, eroding its form into hardened steel.

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Spark-erosion machine with electrode head over the work tank. Representative process photo · Ricard1990 · CC BY-SA 3.0 · Wikimedia Commons

How it works

Sinker EDM, step by step.

A labelled schematic of the process (not to scale), and the four steps every job goes through.

Labelled diagram showing how sinker edm works — Schematic · not to scale. Drawn by TCI Group.

STEP 01

Electrode

Machine the electrode as the negative of the form.

STEP 02

Set up

Align the electrode over the workpiece in oil.

STEP 03

Erode

Sparks reproduce the electrode shape.

STEP 04

Finish

Change to a finishing electrode for texture.

Reference figures

What to expect.

Typical industry values to help you plan. We agree the figures for your part on the drawing.

AccuracyVery highElectrode dependent

TexturesVDI 3400Spark-eroded grades

Blind FormsYesRibs, cavities, logos

MaterialConductive onlyHardened steel

Materials	Tool steels (hardened), carbide; graphite or copper electrodes.
Finishes	VDI 3400 EDM textures from fine to coarse; can be polished afterwards.
Design guidance	Use for deep narrow ribs and sharp internal features in moulds. Plan electrodes early; they are a tool in their own right. Specify VDI grade for textured surfaces.
Best suited to	Injection and LSR mould cavities, deep ribs, textured surfaces and logos.
Watch-outs	Slow; electrodes add cost and wear during erosion.

Other methods in CNC machining

Previous: Wire EDM Next: Sheet metal fabrication

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Reference figures are typical published industry values for planning, not a guarantee; final tolerances, materials and finishes are agreed in writing for each project.