Brass Milling: The Free-Machining Benchmark and Its Grades

Free-machining brass C360 contains roughly 2-3 percent lead, which forms tiny dispersed particles that break the chip into small fragments and lubricate the cut. The result is the benchmark for the entire machinability scale: C360 is rated 100 percent, the reference every other metal is compared against. Chips break and clear effortlessly, tool wear is minimal, finishes come out bright and clean with little effort, and cutting speeds can be very high. A shop can run C360 fast and largely unattended, which is why it dominates high-volume turned and milled components like fittings, valves, and fasteners. The one modern caveat is lead. Regulatory pressure, especially around potable-water and food-contact parts under low-lead drinking-water rules, has pushed some applications to low-lead or lead-free brasses such as C272 or bismuth-bearing alloys. Those grades machine well but not as effortlessly as C360, so when a part is going into plumbing or anything regulated, the buyer should confirm whether leaded C360 is permitted before defaulting to it. For unregulated industrial parts, C360 remains the easiest and cheapest brass to mill.

C260 cartridge brass is a 70/30 copper-zinc alloy with no lead, chosen for its excellent cold formability and ductility rather than machinability. It is the alloy for deep-drawn and formed parts, ammunition cases, and components that get bent or spun. Because it lacks the chip-breaking lead, it machines more like a gummy copper alloy than free-cutting brass, producing stringier chips and more burrs, so milling C260 is noticeably harder and slower than C360. Choose it when the part needs forming, not when it just needs machining. Naval brass (C464) adds about 1 percent tin to a 60/40 copper-zinc base, which improves resistance to dezincification and seawater corrosion. It is the marine-hardware and saltwater-fitting alloy, used for valve stems, fasteners, and propeller-shaft components. Its machinability is moderate, better than C260 but well below C360, and it is specified for corrosion survival rather than ease of cutting. The grade logic for brass is straightforward: C360 when you just need to machine it, C260 when you need to form it, and naval brass when it has to survive seawater.

Brass holds tight tolerances easily and predictably; C360 in particular machines so cleanly that +/-0.001 in is routine and fine finishes come straight off the tool with minimal deburring. The dimensional stability and clean chip make brass one of the most repeatable materials in a production run, which is part of why it is favored for high-volume precision parts. Naval brass and C260 hold tolerance well too but require more attention to burrs and finish. On cost, brass material is more expensive than carbon steel or aluminum per pound because of the copper content, but the machining is so fast and tool-friendly, especially for C360, that the total part cost on high-volume work is often very competitive. The fast cycle times and long tool life offset the material price. Lead times are generally short because brass machines quickly and rarely bottlenecks on tooling. Finishing is often minimal since brass has an attractive natural surface, though parts may be plated (nickel or chrome) for appearance or corrosion, or left bright. For marine naval-brass parts, expect the material premium and slower machining to raise cost versus C360.