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Design Rules for Machined Copper Parts

September 21, 2026

Copper rewards a drawing that respects what the material does at the cutting edge. Most of the price difference between two similar parts comes from a handful of choices that cost nothing to make correctly at design time.

Start by deciding what the part has to be

Before any geometry rule, answer one question: does this part need to be pure copper?

If the answer is a conductivity, thermal or vacuum requirement, write that requirement on the drawing and accept pure copper machining costs. If the answer is habit, a free-machining grade will cut several times faster for a few points of conductivity. That single decision moves more money than every rule below combined. C101, C110 and tellurium copper compared covers it.

Corners and radii

Every internal corner forces a cutter diameter. A 2 mm corner radius means a 4 mm cutter at most, which means a slow feed and a lot of passes on everything that corner touches.

Give internal corners the largest radius the design tolerates. If a mating part needs a sharp corner, consider relieving the corner instead, or chamfering the mating part rather than the copper one.

Walls and ribs

Copper is soft and pushes away from the tool. Tall thin walls come out thicker at the top than at the bottom unless the shop slows down and takes spring passes.

Rules that help:

• Keep wall height to thickness ratios modest, and say which walls carry a function.
• Support long walls with a rib or a boss where the design allows.
• If a thin wall is cosmetic or a heat path rather than a fit, say so. The shop can then leave it alone rather than chase a tolerance nobody needs.

Pockets and slots

Chip evacuation is the whole problem in milled copper. A deep narrow pocket traps a long, stringy chip that re-cuts and scores the wall.

Widen slots wherever the function allows. A slot one millimetre wider can be cheaper to machine than the narrow version, even though it removes more material, because the cutter is bigger and the chips get out.

Holes

• Chamfer both ends of through holes. A chamfer cut on the machine replaces a burr removed by hand.
• Keep depth to diameter ratios sensible. Deep holes need pecking or through-tool coolant, both slower than a straight plunge.
• Flag cross holes. The burr inside a cross hole is the hardest one to reach on any copper part. If it must be clean, say so, because it may need a separate operation.
• Tolerance holes by function. Clearance holes for bolts do not need the tolerance of a locating hole. Say which is which.

Threads

Tapping is where copper parts are lost, because the tap is the last operation and a failure there scraps a part that is otherwise finished.

• Specify thread depth by the load, not by the hole depth. Threads two diameters deeper than necessary cost money and risk on every piece.
• Consider thread inserts where the joint is repeatedly assembled.
• If the part is threaded and does not need pure copper, this is the strongest argument for a free-machining grade there is.

Edges and burrs

Write the edge condition as a number. “Break all sharp edges 0.2 to 0.4 mm” is a cost a shop can estimate. “Burr free” with no further detail is an open ended hand-finishing instruction, and different shops will price it very differently, which is why quotes for the same copper part scatter so widely.

If only some edges matter, say which. Deburring every edge on a part where three of them are functional is a permanent tax on the order. There is more detail in deburring copper parts.

Surfaces that get plated or joined

Plating copies the surface underneath it. A tool mark, a smear or a scratch will still be there after tin or silver goes on, and often looks worse.

Mark the faces that are plated, brazed or used as electrical contacts, and give those a finish callout. Leave everything else as machined. See copper surface finish for realistic Ra values.

Workholding and datums

Copper marks. Jaw prints on a contact face are a real defect, and the shop needs to know where it is allowed to grip.

Add a note naming faces that must be free of clamping marks, and if possible leave a sacrificial area the shop can hold. This is a two line note that prevents an argument after first article.

A short checklist before you release the drawing

• Grade justified by a requirement, or left open with the requirement stated
• Internal corners as large as function allows
• Slots and pockets widened where possible
• Through holes chamfered both ends
• Thread depth set by load
• Edge condition given as a dimension
• Critical faces and their finish identified
• Clamping restrictions noted
• Tolerances only on features that mate

How copper behaves in milling, turning, drilling and tapping explains why each of these rules exists.

Frequently asked questions

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