What Surface Finish You Can Expect on Machined Copper
Updated September 26, 2026
Surface finish on copper is not simply a matter of taking a lighter cut. A soft, ductile metal cut with the wrong edge geometry will rub and smear rather than shear, and no amount of slowing down fixes that. This guide covers what is realistic, what drives it and what to write on a drawing.
Realistic Ra values
These are common expectations for machined copper rather than guarantees. The shop quoting your geometry confirms what it can hold.
| Condition | Typical Ra |
|---|---|
| As machined, general milling and turning | 1.6 micrometre |
| Dedicated finishing pass, sharp tooling | 0.8 micrometre |
| Fine finishing, controlled setup | 0.4 micrometre |
| Polished, lapped or diamond turned | Below 0.2 micrometre, a separate operation |
Two things about this table matter more than the numbers. First, each step down costs time on every part. Second, finish is per surface. Asking for Ra 0.8 on a whole part when one face needs it is one of the most common ways buyers inflate their own copper quotes.
Why copper smears
Copper is ductile and adhesive. Under pressure it welds to the cutting edge and forms a built-up edge. Once that exists, the tool is no longer the shape it was ground to, and the material in front of it gets pushed and torn rather than cut. The result is a surface that looks bright in places, dull and smeared in others, often with a fine raised burr at the edges.
The fix is counter-intuitive for anyone used to steel. You do not slow down. You use a sharper tool with a higher positive rake and a polished flute, keep a genuine feed per tooth so the edge is always shearing material rather than rubbing, and flood the cut so heat and chips both leave.
This is why the same drawing can come back with a beautiful finish from one shop and a smeared one from another at a similar price. It is a knowledge difference, not an equipment difference.
Where finish actually matters
Electrical contact faces. Contact resistance depends on the real area of metal to metal contact, which depends on flatness and finish as well as on pressure. A rough or smeared contact pad runs hotter than a clean one.
Sealing and vacuum faces. A copper gasket face in a vacuum flange depends on a controlled finish and no radial scratches. This is one of the few places where a tight Ra is genuinely required rather than specified out of caution.
RF surfaces. At high frequency, current runs in a thin layer at the surface, so surface roughness increases effective path length and loss. Waveguide and cavity surfaces are specified tightly for a real physical reason.
Anything that gets plated. The plated layer copies what is underneath. See tin, nickel and silver plating on copper busbars.
Anything that gets brazed or welded. A smeared surface holds cutting fluid and oxide in a way a cleanly sheared one does not.
Everywhere else, as machined is fine and asking for better is money spent on nothing.
Post-processing options
Deburring and edge breaking. Not a finish operation, but it is what most drawings actually need. Covered in deburring copper parts.
Brushing or graining. Gives a uniform directional appearance and hides minor tool marks. Cheap and often enough for cosmetic parts.
Vibratory tumbling. Good for quantities of small parts, rounds edges as well. Not suitable when flatness or sharp features matter.
Electropolishing. Removes a thin layer electrochemically, leaves a bright smooth surface and rounds micro burrs. Used on vacuum parts and on surfaces where cleanliness matters. It removes material, so it has to be allowed for in the tolerance.
Polishing and lapping. Manual or semi manual, for optical and sealing surfaces. Expensive per part.
Passivation and anti-tarnish. Copper oxidises in air. If the part must stay bright, it needs a protective coating or a controlled storage and packaging plan. Bare copper that ships bright will arrive dull if it travels for a month unprotected.
What to write on the drawing
- Put an Ra value only on the faces that need one, and name them.
- Say what the face does: contact, seal, plated, brazed, cosmetic. A shop that knows the function can often meet the requirement more cheaply than the number you guessed.
- Specify the edge condition separately from the surface finish. They are different operations.
- State whether the part must arrive untarnished, and whether a protective coating is acceptable.
- If a face is inspected, say how and with what.
The tarnish conversation
Copper goes brown. It is not a defect and no shop can prevent it permanently on a bare surface. If your incoming inspection rejects tarnished parts, you need either a plated finish, a lacquer, or an agreed packaging method with a sealed bag and a corrosion inhibitor. Decide this before the first shipment rather than after it.
Frequently asked questions
What Ra can a shop hold on copper?
As machined, Ra 1.6 micrometre is a normal expectation on milled and turned copper. Ra 0.8 is achievable with a dedicated finishing pass and the right tooling. Below that you are usually talking about polishing, lapping or diamond turning, which are separate operations.
Why does my copper part look streaky rather than uniformly machined?
That is usually built-up edge. Material welded itself to the cutting edge, changed the effective geometry and then smeared rather than sheared. It is a tooling and feed problem, not a polishing problem.
Will plating hide a poor machined finish?
No. Electroplated layers are thin and follow the surface underneath. A tool mark or a smear is still visible after plating and can be more obvious.
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