How Copper EDM Electrodes Are Finished and Why It Matters
September 26, 2026
An EDM electrode is a machined copper part whose surface becomes, roughly, the surface of the cavity it burns. Finishing the electrode is therefore part of finishing the mould. This is what the options cost and when they are needed.
Roughing against finishing electrodes
Roughing electrodes remove most of the material at high current. Their face texture matters little and they wear more. A machined finish is normal.
Finishing electrodes define the final cavity surface at low current. Their face matters. Tool marks, smears and micro burrs on the electrode become marks in the cavity. These are the electrodes that get finished, and they are ordered in multiples because they wear on fine detail.
Ask the EDM shop how many of each it wants before you order one. See copper EDM electrodes.
Finishing options
As machined, done properly. Sharp tooling, real feed, no built-up edge. For roughing electrodes and many finishing electrodes on ordinary cavities, a clean machined face is enough. The point is clean, not rough. See copper surface finish.
Light hand polish. Removes tool marks and micro burrs from working faces. Common on finishing electrodes for visible mould surfaces. Hand work, priced per electrode.
Mirror polish. For optical and cosmetic cavity surfaces. Skilled hand work, expensive, and only on the faces that need it.
Electropolishing. Uniform bright finish, rounds micro burrs. Removes material, so the gap offset has to allow for it. Less common on electrodes than on vacuum parts, but used.
Detail and wear
Fine ribs, sharp corners and thin features carry high current density and erode first. That is a wear problem, not a finish problem, and the answers are:
- More finishing electrodes for detailed cavities
- Copper tungsten for the finest detail, at a considerable cost premium
- Generous internal radii on the electrode wherever the cavity design allows
When a copper EDM electrode beats a graphite one covers material choice.
The gap offset again
Every finishing operation that removes material changes the electrode size. Polishing removes a little, electropolishing removes more. If the gap offset was applied in the model, the finishing has to be accounted for. Say on the drawing what the finished dimension is meant to be and let the shop plan the allowance.
What to write
Finishing electrode, [n] off.
Working faces marked W: light hand polish, no visible tool marks.
Non working faces: as machined.
Gap offset [x] mm per side already applied.
Final dimensions after polishing.
Shank per [holder system].For roughing electrodes, replace the polish line with “as machined”. The design rules for copper parts apply to the electrode geometry itself.
Frequently asked questions
Does the electrode finish copy onto the workpiece?
To a degree. The EDM process produces its own surface texture, but defects and roughness on the electrode face show up in the cavity, especially on finishing burns. A finishing electrode needs a better face than a roughing one.
Should I polish a copper electrode?
Finishing electrodes for mould cavities often are. Roughing electrodes almost never are. Polishing is hand work on copper and it belongs in the quote as a line, not as an assumption.
Why do fine details wear first?
Sharp corners and thin ribs carry higher current density during the burn and erode faster. Copper resists this better than graphite in fine detail, and copper tungsten better still. Plan extra finishing electrodes for detailed cavities.
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