Turning Copper: Gummy Metal, Clean Threads, and the Tellurium Trick

Pure coppers like C101 (oxygen-free, OFHC) and C110 (electrolytic tough pitch, ETP) are prized for conductivity, not machinability. Their high ductility means the chip wants to deform and tear rather than shear cleanly, producing long stringy chips, built-up edge, and a smeared, dragged surface finish. Pushing harder makes it worse, because more heat and pressure just feed the built-up-edge cycle. The technique that works is the opposite of brute force: very sharp tools with high positive rake and polished flutes, light-to-moderate depth of cut, high surface speeds (copper tolerates 300 to 1,000+ SFM because it conducts heat away beautifully), and generous coolant or cutting oil to keep the chip moving and prevent welding. Honed sharp edges shear the ductile metal cleanly instead of tearing it. Diamond (PCD) tooling gives the best finishes on pure copper because it stays sharp and resists the copper's tendency to adhere. Chip control remains a daily reality. Those long stringy chips from C101 and C110 will wrap the part and the chuck, so chipbreaking geometry, peck strategies, and well-aimed coolant matter. There is a real tension here: aggressive chipbreakers help with chips but can degrade finish on a soft metal, so finding the balance for a given part takes setup time.

Tellurium copper (C145) adds about 0.5% tellurium, which forms a fine dispersion that breaks chips and lubricates the cut, raising machinability to around 85% of free-cutting brass, dramatically better than pure copper's roughly 20%. Crucially, it does this while retaining about 90 to 95% of pure copper's electrical and thermal conductivity, so for the vast majority of conductive parts you lose almost nothing electrically and gain enormously in the shop. This is why tellurium copper is the default for high-volume turned conductive parts: electrical contacts, connector bodies, welding-gun components, terminals, and fittings made on screw machines and Swiss lathes. Chips break cleanly, finishes are good, tool life is long, and cycle times approach those of brass. If your application can tolerate the small conductivity reduction and does not require oxygen-free purity (for example for certain ultra-high-vacuum or specialized electronic uses), tellurium copper is almost always the right specification for a turned part. The cases where you must stay with pure C101 or C110 are specific: oxygen-free C101 for ultra-high-vacuum, high-purity electronics, and brazing applications where the tellurium or oxygen content would cause problems, and where absolute maximum conductivity is required. In those cases you accept the machining difficulty as the cost of the purity requirement, and you machine with sharp PCD or honed carbide tooling and patience.

Turned copper holds ±0.001 in and tighter, but copper's softness introduces practical limits that harder metals do not have. The material deflects and burrs easily, so thin walls and slender features distort under chuck pressure and cutting load, and burrs form readily at edges and require deburring. Workholding needs to be gentle: over-clamping a soft copper part in hard jaws will deform it, so soft jaws, collets, and light clamping protect roundness. Surface finish depends heavily on tooling and grade. Tellurium copper finishes cleanly with standard sharp carbide; pure copper needs honed or PCD edges to reach a good finish and will smear with anything less. For sealing or contact surfaces requiring fine finishes, PCD tooling on copper produces excellent results, often better than 16 Ra. Copper's high thermal expansion (about 9.4 µin/in/°F, higher than steel) means tight-tolerance parts grow noticeably with temperature, so dimensional inspection should account for part and coolant temperature. The good news is copper's excellent thermal conductivity keeps the part from developing large local hot spots during machining, so it stays more uniform than a low-conductivity metal. For conductivity-critical parts, also remember that surface contamination and the machining process itself can affect contact resistance, so handling and cleaning after turning may be specified.