C36000, universally known in the trade as "360 brass" or free-cutting brass, is the reference point every other material on a machinability chart gets measured against. It's a wrought copper-zinc alloy, roughly 61-63% copper and 35-37% zinc, alloyed with 2.5-3.7% lead specifically and deliberately for chip control. Unlike structural brasses where lead is a trace impurity, in C36000 the lead is a primary alloying element added at a level high enough to fundamentally change how the material cuts.

That lead content is why C36000 is formally rated at 100% machinability — it's the historical baseline that the American copper industry uses to score every other wrought copper alloy. In practical terms, this means the material can be run at high spindle speeds and aggressive feeds with minimal tool wear, predictable chip formation, and excellent surface finish, which is exactly why it dominates high-volume screw-machine work: fittings, valve stems, gears, fasteners, and precision turned components where cycle time and tool life both matter.

C36000 is the textbook case for how lead transforms machinability in copper alloys, which is exactly why it was chosen as the 100% baseline in the first place. The 2.5-3.7% lead exists as microscopic globules dispersed through the alpha-beta brass matrix rather than dissolving into it. Every one of those globules acts as a tiny internal stress riser and lubricant reservoir: as the cutting edge advances, the chip fractures cleanly at each lead particle instead of deforming plastically and dragging along the rake face.

The result is short, well-broken chips at virtually any feed and speed combination, extremely low cutting forces, and almost no tendency to form a built-up edge — the classic failure mode that plagues lead-free copper alloys and gums up the cutting edge with smeared material. Surface finishes on C36000 are consistently excellent even with simple, sharp, uncoated or lightly coated tooling, and tool life is outstanding because the material offers essentially no abrasive resistance.

Because C36000 defines the top of the scale, every other copper alloy machinability rating in this reference library is expressed relative to it. Leaded bronzes and gunmetals can approach or occasionally exceed 100% in specific operations due to their even higher lead content, while lead-free brasses, bronzes, and especially chromium copper fall well below it. When a shop needs the fastest possible cycle time on a screw machine job, 360 brass is usually the first material considered.