Free machining brass is the benchmark against which the machinability of every other metal is rated, and it earns that position: it cuts at very high speeds, breaks chips cleanly, produces an excellent surface straight off the tool and wears cutting edges very slowly. When a part can be brass, machining it is close to the cheapest metal cutting there is.
Why it cuts so well
Free machining grades contain lead or, increasingly, a lead free chip breaking addition, which makes the chip fracture into short pieces instead of forming a continuous ribbon. That means chips clear themselves, cutting forces stay low, heat leaves with the chip and the surface finish is excellent. Tool life is measured in a way that barely resembles machining steel.
Tool geometry matters more than tool material
Brass is soft enough that almost any tool material cuts it, but the rake angle does not want to be aggressive: a high positive rake tool can dig in and grab, particularly when drilling, pulling itself into the work. Neutral or slightly negative rake tooling, and drills with the cutting edge dubbed back, are the standard answer and prevent the tool self feeding.
Lead and the shift away from it
Traditional free machining brass contains lead, which is what makes the chips break. Regulation of lead in drinking water fittings and in electrical and electronic equipment has driven a shift to lead free machinable brasses using silicon or bismuth. They machine well but not identically, so a substitution needs a trial rather than an assumption, and it changes the speeds and feeds.
Where brass is used
Plumbing and gas fittings, valve bodies, electrical connectors and terminals, instrument parts, decorative hardware and marine fittings. In each case the combination of corrosion resistance, machinability and, for electrical work, conductivity is what selects it. Where full conductivity is required, copper is used instead and the machining becomes considerably harder.
Questions people ask about brass milling
Why is brass so easy to machine?
Free machining grades break chips into short pieces rather than forming ribbons, so chips clear themselves, forces and heat stay low and tool life is very long.
What tool geometry does brass need?
Neutral or slightly negative rake, and drills with the edge dubbed back. Aggressive positive rake tooling can grab and pull itself into the work.
Is lead free brass as machinable?
Close, but not identical. Silicon and bismuth grades machine well and need their own speeds and feeds, so a substitution needs a trial rather than an assumption.
Brass or copper?
Brass wherever its conductivity is sufficient, because it machines far more easily. Copper only where full electrical or thermal conductivity is the requirement.