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How Copper Behaves in Milling, Turning, Drilling and Tapping

September 21, 2026

Copper does not have one machining behaviour. It has four, one per operation, and the cost of your part depends on which of them your geometry demands. This is a practical walk through each, written for the person reading the quote rather than the person writing the program.

What stays the same in all four

Three properties follow copper through every operation.

It is ductile, so the chip stretches instead of breaking. It is thermally conductive, so heat goes into the part and the tool instead of leaving with the chip. It is adhesive, so it welds to the cutting edge under pressure and forms a built-up edge that ruins finish and makes burrs worse.

The shop’s answer to all three is the same: a sharp tool with a high positive rake and a polished flute, a real feed rate rather than a timid one, and plenty of coolant. Backing off the feed to be gentle is the classic mistake. It causes rubbing, which causes built-up edge, which causes exactly the problems the operator was trying to avoid.

Milling

Milling copper is mostly a chip evacuation problem. The cut is intermittent so heat is less punishing than in turning, but the long chips have to leave the pocket.

What this means for your part:

• Deep narrow pockets are expensive. The chips cannot get out, so they re-cut and score the wall. Widening a slot by a millimetre can change the price more than the extra material costs.
• Thin walls move. Copper is soft. A wall that would stand up in steel will push away from the cutter and come out thicker at the top than the bottom. Say which walls are critical.
• Climb milling is standard because it enters the material rather than rubbing into it.
• Internal corners want the largest radius your design allows. A tight corner forces a small cutter, which forces a slow feed on every pass.

Face milling a large flat surface, which is most of a busbar, is the easy end of this. Fin packs and deep cavities are the expensive end.

Turning

Turning copper is a continuous cut, which means a continuous chip. On pure copper it comes off as a ribbon that wraps around the part, the tool and the turret.

This is why turned copper parts are the strongest argument for a free-machining grade. Tellurium or sulphur copper breaks the chip and the whole problem goes away. Cycle time drops, the operator stops clearing nests, and the surface finish improves without a separate operation.

If the part must stay on pure copper, expect chip breaking geometry on the insert, more frequent intervention and a longer cycle. That is a real cost and it belongs in the quote.

Drilling

Drilling adds two problems. The chip has nowhere to go except back up the flute, and the heat has nowhere to go except into the drill.

• Deep holes need pecking or through-tool coolant. Both are slower than a straight plunge.
• Cross holes burr on the inside, where nobody can reach them. If you need a cross hole free of burrs, say so explicitly and expect it to cost, because it may need a secondary operation or a different order of operations.
• Hole exits burr badly. Design a chamfer on both ends where you can.

Tapping

This is where copper parts get scrapped. The tap is doing the most difficult version of everything copper does badly: a continuous cut in a confined space with nowhere for a gummy chip to go.

In pure copper, threads tear, taps load up and pick up material, and a broken tap at the last operation turns a nearly finished part into scrap. Shops know this, and it shows up as either a higher price or a suggestion to change the grade.

Practical options, cheapest first: move to C145 or C147, use a thread insert where the application allows it, increase the thread size, or reduce the thread depth to what the joint actually needs. A thread specified two diameters deeper than necessary is a free way to raise your own price.

Where this lands on your drawing

• Give internal corners a generous radius.
• Widen slots and pockets where function allows.
• Chamfer both ends of every through hole.
• Only call out the threads you need, at the depth you need.
• Name the walls and faces that are critical, so the rest can be machined efficiently.
• If the part is turned and feature heavy, consider a free-machining grade before anything else.

Why copper is difficult to machine covers the material behaviour behind all of this, and design rules for machined copper parts turns it into drawing decisions.

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