Machining Brass: Grades, Speeds, and Shop Tactics

Every machinability rating you have ever seen for a copper alloy — 20% for pure copper, 85% for tellurium copper — is a percentage of one material: C360 free-cutting brass, rated 100 by definition on the Copper Development Association scale. C360 is roughly 60% copper, balance zinc, with 2.5–3.0% lead (per ASTM B16). The lead is the whole story. It does not alloy into the copper-zinc matrix; it disperses as microscopic globules that work as a built-in chip breaker and solid lubricant. Chips fracture into short, manageable curls instead of bird-nesting around the chuck, friction at the tool face drops, and the edge stays sharp for a long time.

The practical consequences show up on the quote, not just the lathe. CNC brass parts in C360 cut at carbide speeds of 500–1,600 sfm — three to five times faster than stainless steel — come off the machine at Ra 0.4–0.8 μm without secondary finishing, and hold ±0.001 in routinely. Brass bar costs more per kilogram than aluminum or mild steel; C360 wins on total part cost because almost nothing machines faster. That is why fittings, valve components, and connector pins default to it, and why our brass CNC machining material page lists C360 first.

The same lead that makes C360 the benchmark also decides where it cannot go. In the US, the Reduction of Lead in Drinking Water Act caps lead at 0.25% weighted average on wetted surfaces of plumbing products — C360 at 2.5–3.0% does not qualify, so faucets, meter bodies, and potable-water fittings moved to lead-free alloys. In the EU, C360 currently rides RoHS Exemption 6(c), which allows up to 4% lead in copper alloys; that exemption has been extended to mid-2027, after which the practical limit drops toward 0.1%.

The leading replacement is C693 silicon brass (ECO BRASS): ~75% copper, ~3% silicon, ≤0.09% lead. Silicon forms a hard intermetallic phase that breaks chips the way lead globules do, recovering machinability to about 85% of the C360 benchmark — close enough that most shops quote it without a separate process plan. C693 is NSF/ANSI 61 compliant, resists dezincification and stress-corrosion cracking intrinsically, and is stronger than C360 (tensile in the 517–586 MPa range, overlapping some stainless steels). If you are re-specifying a plumbing or food-contact part away from C360, C693 is the first alloy to price.

Bronze shows up on the same drawings as brass — bushings, bearings, worm wheels, marine hardware — and buyers often assume machining bronze is the same job. It is not. Bronze machinability runs from about 20 to 70 against brass’s 100, and the failure mode flips depending on the alloy. C932 bearing bronze (SAE 660, 83% Cu / 7% Sn / 7% Pb / 3% Zn) is leaded like C360 and machines well at ~70 — the friendly bronze, and the default for bushings. C954 aluminum bronze (85% Cu / 11% Al / 4% Fe) rates ~60 on paper but is hard and abrasive; chip control is fine, tool wear is the problem, so plan on wear-resistant carbide, rigid setups, and honest speeds rather than brass numbers. C510 phosphor bronze rates ~20 — gummy, ductile, and stringy, machining more like pure copper than like brass.

The procurement takeaway: if a drawing calls bronze where brass would meet the functional requirement, ask why. C954’s strength and C932’s bearing properties justify their cycle times in the right application; everywhere else, brass cuts the part cost dramatically. The full three-way decision is in our brass vs bronze vs copper comparison.