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What Tolerances Are Realistic on Machined Copper

September 21, 2026

Copper is soft, it conducts heat into itself, and it deflects under clamping pressure. All three work against tight tolerances. Here is what is realistic, why, and how to tolerance a drawing so the money goes where it matters.

Realistic bands

Typical values for the material rather than a commitment from any shop. The shop quoting your geometry confirms what it can hold.

Feature	Pure copper (C101, C110)	Free-machining (C145, C147)
General dimensions	±0.1 mm comfortably	±0.05 mm comfortably
Machined features, normal care	±0.025 mm	±0.015 mm
Single critical feature, finishing pass	±0.01 mm achievable	±0.01 mm achievable
Tighter than ±0.01 mm	Specialist, temperature controlled	Specialist, temperature controlled
Flatness on a machined face	Depends on size and thickness, call it out separately	Same

Two notes on that table. Achievable on one feature is not the same as achievable everywhere, and the cost difference between the rows is large.

Why a soft metal drifts

Heat. In most metals a good share of the cutting heat leaves with the chip. Copper takes it into the part. Thermal expansion for copper is around 17 micrometres per metre per degree, so a 200 mm part that has warmed 5 degrees is roughly 17 micrometres longer while it is warm than it will be on the inspection bench. On a tight part, when you measure matters as much as how.

Clamping. Copper yields at pressures that would not trouble steel. A part clamped hard enough to be secure can deform, get machined flat in that deformed state and spring back out of tolerance when released. This is why shops use soft jaws, sleeves, vacuum plates and light multi-point clamping, and why those add setup time.

Tool pressure. Thin walls and tall features push away from the cutter. The wall ends up thicker where the tool deflected and the shop has to take spring passes to correct it.

Stress release. Rolled plate and drawn bar carry internal stress. Machining a lot of material off one side lets that stress move the part. On flat parts with tight flatness requirements this is the usual culprit, and the answer is either a stress relief step or machining in stages.

What this means for inspection

If your part is tight, agree the inspection conditions rather than assuming them.

• Is the part measured after it has cooled and stabilised, and for how long?
• Is the room temperature controlled, and to what?
• Which features are inspected, at what frequency, and with what equipment?
• Is a first article report required, and to which standard?

A measurement dispute on a copper part usually turns out to be a temperature dispute. Writing two lines into the drawing prevents it.

How to tolerance the drawing

Tolerance function, not habit. Go through the part and mark which dimensions actually mate, locate or seal. Those get real tolerances. Everything else gets the general block tolerance.

Use the general tolerance block properly. A sensible general block plus three tight dimensions is far cheaper than a tight general block with no exceptions, and it tells the shop where to spend its attention.

Separate form from size. Flatness, parallelism and perpendicularity are different operations from hitting a dimension. Call them out only where they matter, typically on a contact or sealing face.

Say what the feature does. A note like “contact face, flatness critical for thermal interface” lets a shop propose a cheaper route that still works. A bare number does not.

Consider the grade. If you need tighter bands and the application allows it, moving to a free-machining grade is often cheaper than paying pure copper to hold the same number. See C101, C110 and tellurium copper compared.

Remember plating. Electroplated layers add thickness. State whether dimensions are before or after plating, every time.

The two questions to ask a shop

First, what tolerance do you hold on pure copper, rather than on aluminum. Shops quote capability from their comfortable material and copper is not it.

Second, how do you inspect it and at what temperature. The answer tells you whether they have hit this problem before.

Design rules for machined copper parts covers the geometry side of the same problem.

Frequently asked questions

Keep reading

• When a Machined Copper Part Needs Annealing and How to Ask for It
• How Copper EDM Electrodes Are Finished and Why It Matters