Copper is chosen for electrical and thermal conductivity, and both properties make it awkward to machine. It is soft and ductile, so it smears and forms long stringy chips rather than breaking them; and it conducts heat away from the cutting zone into the workpiece, which changes how the part behaves dimensionally during the cut.
Why pure copper is difficult
High ductility means the material tears rather than shears cleanly, producing a poor surface and a built up edge on the tool. Chips are long and stringy and wrap around the tool and the part. And its thermal conductivity carries heat into the workpiece rather than out with the chip, so the part grows during machining and shrinks afterwards, which matters on tight tolerances.
The grades that machine well
Free machining copper alloys with added tellurium or sulphur cut cleanly, break chips and hold tolerance, at a small cost in conductivity that is acceptable for most electrical hardware. Brass, which is copper with zinc, is one of the easiest materials to machine at all. Where full conductivity is required, oxygen free copper must be used and the difficulty accepted.
Tooling and technique
Very sharp tools with high positive rake and polished flutes reduce the built up edge. Higher speeds with steady feeds and no dwelling help. Copious coolant carries away heat and flushes the stringy chips. Carbide tooling is standard, and diamond tooling is used where the surface finish must be exceptional, as it is on electrical discharge machining electrodes.
What it is used for
Bus bars, terminals, connectors and electrical hardware. Heat sinks, cold plates and induction coils. Electrical discharge machining electrodes, which is a large part of the toolroom demand. Vacuum and radio frequency components. In each case the conductivity is the whole reason for the material, which is why substituting an easier alloy usually is not an option.
Questions people ask about copper machining services
Why is copper hard to machine?
It is soft and ductile so it smears rather than shearing, it forms long stringy chips, and it conducts heat into the part rather than out with the chip.
Which copper grades machine best?
Free machining grades with tellurium or sulphur additions, and brass. Both cut cleanly at a small cost in conductivity that most applications accept.
Can I substitute brass for copper?
Only where the reduced conductivity is acceptable. Where full electrical or thermal performance is the point, pure copper must be used.
What tooling does copper need?
Very sharp, high positive rake, polished flute tools with plenty of coolant. Diamond tooling is used where surface finish must be exceptional.