E-Cu57 is high-purity electrolytic tough pitch (ETP) copper, refined electrolytically to a minimum 99.90% copper content. Unlike the copper-zinc and copper-aluminum alloys elsewhere in this library, there's essentially no alloying here — E-Cu57 is prized specifically because it isn't an alloy. That purity is what gives it the highest electrical and thermal conductivity of any commercially available copper grade, which is the entire reason it exists: busbars, electrical conductors, terminals, commutators, and any application where conductivity is the primary design driver over strength or machinability.

The "tough pitch" designation refers to a small, controlled residual oxygen content (typically 0.02-0.04%), present as fine copper oxide inclusions distributed through the metal. That oxygen is intentional — it's what gives ETP copper its good hot- and cold-working characteristics compared with oxygen-free copper — but it also means E-Cu57 is unsuitable for reducing atmospheres or hydrogen-containing environments at elevated temperature, where hydrogen embrittlement ("steam embrittlement") can occur as hydrogen reacts with the internal oxide.

High-purity copper is one of the more counterintuitive materials to machine on this site: it's very soft, yet it's genuinely difficult to get a good finish and consistent chip control on. With no zinc, lead, or other alloying element to promote chip fracture or add stiffness to the matrix, E-Cu57 is gummy and prone to smearing rather than shearing cleanly — built-up edge forms readily on anything but a sharp, well-supported tool, and poor BUE control is the single biggest cause of bad surface finish and dimensional inconsistency on this material.

Sharp tooling with a high positive rake is essential — more so than on any brass in this library — to shear the material rather than push and smear it. Chip control is genuinely difficult: pure copper doesn't fracture the way an alloyed material does, so chips tend to be long, ribbon-like, and prone to wrapping around the tool or workpiece unless geometry and feed are managed carefully. Positive rake, generous clearance angles, and a keen, polished edge all help; coolant assists with both heat removal (copper's high thermal conductivity moves heat into the workpiece and chip quickly, which helps, but built-up material can still overheat locally) and chip evacuation. Light, high-speed passes with a sharp tool consistently outperform heavier cuts with a dull or poorly supported edge on this material.