What Sinker EDM Is For

Designers learn early that an inside corner cannot be sharp, because a milling cutter is round. So they round every corner, widen every slot and design out any feature a cutter cannot reach. Often that is the right call. Sometimes the feature is the point of the part, and there is another way to make it.

Sinker EDM, short for electrical discharge machining, shapes metal by spark erosion. A shaped electrode, usually graphite or copper, is lowered toward the work in a bath of dielectric oil. Sparks jump the small gap and erode the metal, and the shape of the electrode sinks into the part. Nothing touches the part and nothing cuts it, so the hardness of the metal barely matters, and a corner can be very nearly as sharp as the electrode, which is far sharper than any cutter can leave.

Sinker EDM runs alongside milling and turning at the Uxbridge shop. Between 2020 and 2025 the shop's job records include EDM work and broken tap removal for other manufacturers. The features the shop most often sends to the EDM are internal cavities, sharp inside corners and deep ribs in mold tools, where a cutter can't reach. Before the EDM, the machinists mill away as much material as they can, so the electrode only has to finish what the mill can't reach.

What it does that a cutter cannot

• Sharp inside corners in pockets and mold cavities.
• Deep, narrow ribs and slots, common in molds, where a long thin cutter would bend or break.
• Blind keyways and other internal shapes that stop inside the part, where a broach cannot pass through.
• Features in hardened steel, cut after heat treatment, so the part has already done its moving before the feature goes in.
• Textured surfaces. The spark-eroded finish is itself a texture, and the VDI texture scale used to specify mold finishes comes from EDM surfaces.

Broken taps

A tap that snaps off in a hole is harder than the part it is stuck in. Drilling it out usually ruins the hole, and sometimes the part. EDM erodes the broken tap with a small electrode and puts no cutting force on the part, so on an expensive part the hole can often be saved and re-tapped.

Sinker and wire

Wire EDM uses a thin wire to cut profiles that go right through a part, a little like a very precise bandsaw. Sinker EDM is for shapes that stop inside the part. They solve different problems, and a drawing will usually tell you which one it needs. We run sinker EDM in-house and send wire EDM to a partner shop, and work is traded between the two shops as a job needs it.

When not to use it

EDM is slow next to milling, and the electrode has to be designed and made first, often on a milling machine. It only works on metal that conducts electricity, so it is no help on plastics. If a cutter can make the feature, milling is usually the cheaper answer, and a small corner radius the part can live with is cheaper still.

My test is whether the sharp corner or the deep rib does a job. If it does, EDM is worth paying for. If it is there because the CAD made it easy to draw, round it off and save the money.

If a feature on your drawing has been quoted as impossible, ask whether anyone considered EDM.

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