Swiss Machining Copper: C101, C110 and Tellurium Copper

C101 (oxygen-free electronic, OFE) and C110 (electrolytic tough pitch, ETP) are essentially pure copper, chosen for maximum electrical and thermal conductivity (C101 hits around 101 percent IACS). That purity is exactly what makes them hard to machine: the metal is soft, extremely ductile, and tears rather than shears cleanly, producing long, stringy, smeary chips that wrap the tool and the part. On a Swiss lathe running small slender parts, this is a constant battle with bird-nesting swarf and built-up edge. Winning that battle takes sharp, highly polished, high-positive-rake tooling that slices the metal cleanly rather than pushing it, generous high-pressure coolant to flush chips before they nest, and feed rates tuned to force chip breakage. Even then, pure copper rarely produces the clean broken chips you get from brass. Surface finish can actually be excellent because the soft metal burnishes, but holding tight tolerance is complicated by the material's tendency to deflect and smear. Many shops will openly tell a buyer that if the application can tolerate a slight conductivity hit, switching to tellurium copper is worth far more than any tooling trick.

Tellurium copper (C145) adds about 0.5 percent tellurium, which forms tiny dispersed particles that break the chip cleanly, raising machinability to roughly 85 to 90 percent of free-cutting brass while retaining around 90 to 95 percent IACS conductivity. For the vast majority of machined copper parts, electrical connectors, contacts, terminals, and electrode bodies, that small conductivity sacrifice is well worth the enormous gain in machinability, cycle time, and surface quality. It is, frankly, the alloy most copper Swiss parts should be made from unless the spec absolutely demands maximum conductivity. The cases where you cannot substitute are narrow but real: applications requiring the very highest conductivity or those sensitive to tellurium for metallurgical or end-use reasons (certain ultra-high-purity, vacuum, or welding applications) must stay with C101 or C110. When a print calls for C101 on a part that is clearly a connector or terminal, it is worth confirming whether maximum conductivity is genuinely required or whether the designer simply specified the most familiar copper. That single question often saves a buyer real money and a shop real grief.

Copper tarnishes and oxidizes readily, so most machined copper parts are plated, commonly with nickel, tin, silver, or gold depending on the electrical application, both to prevent oxidation and to improve solderability or contact resistance. Plating buildup affects dimensions, so tight features and threads are sized to accommodate it, and masking adds cost. Bare copper parts shipped without plating need protective packaging and oil to slow tarnish. The softness that complicates machining also makes copper easy to dent, scratch, and deform in handling, so deburring and post-process care matter; a part can machine perfectly and then get marred in a tumbler. Burrs themselves are a challenge because soft copper rolls a burr rather than breaking it, so deburring strategy is part of the process plan. For high-conductivity parts, any contamination or embedded abrasive from finishing can matter electrically, so process cleanliness is taken seriously. Buyers should specify the plating and the acceptable surface condition clearly, because copper's appearance and conductivity both degrade quickly if the downstream steps are not planned.