Copper Alloys

Copper alloys — the brasses, the bronzes and pure copper itself — are among the most machined metals outside the steel family, and they are machined for the properties that steel cannot give: the electrical and thermal conductivity of copper, the corrosion resistance and bearing quality of the bronzes, and the ease with which brass turns into clean, bright fittings and terminals. They are the metals of the plumber’s fitting, the electrical terminal, the bearing bush and the marine fastener, and a shop meets them constantly in bar and forging. In machining terms the family spans one of the widest ranges in the workshop, from free-cutting brass — the metal that machinability ratings are measured against, which cuts like no other — to pure copper, which is as gummy and work-hardening as any metal the shop faces. This entry sets out the alloy families and how each machines.

The copper alloy family

The family is built on copper, alloyed to set its strength, its corrosion resistance and its machinability. Pure copper is the soft, ductile, highly conductive metal of electrical and thermal work, and it is the hardest of the family to machine. Brass is copper with zinc, up to about 40 percent, which raises strength and greatly improves machinability; the brasses are the everyday machined copper alloys, headed by the free-cutting grades that carry a little lead. Bronze is traditionally copper with tin — usually with phosphorus added, as in phosphor bronze — though the name now covers strong alloys of copper with aluminium, silicon or other metals, made for bearings, springs and marine work. Between them they cover the shop’s copper work: the brass that screws and turns freely, the bronzes that carry load and resist corrosion and wear, and the pure copper that conducts, at the price of cutting like treacle.

Brass: the benchmark of machinability

Brass is the metal that other materials are measured against, and free-cutting brass is the benchmark of the machinability scales — the 100 percent rating that steels and other alloys are compared with. The reason is the little lead the free-cutting grades carry, about 3 percent: the lead breaks the chip into short, free pieces and lubricates the cut, so brass is machined at high speed and feed with light tool wear, low cutting forces and a fine surface finish, throwing short chips that clear easily and need little chip control. It turns, drills, taps and threads beautifully, holds fine detail and close tolerances, and is the natural material of the machined fitting, the terminal, the valve, the instrument part and the plumbing component that must be made in thousands. The ordinary brasses without the lead are less free-cutting but still easy work, and all of them reward sharp tools and high speeds — brass is a metal that asks to be cut fast and cleanly, and it rewards the shop with a finish that needs no further work.

Pure copper: the difficult end

Pure copper is the other face of the family, and it is difficult for the opposite reason from the hard alloys: it is too soft and ductile. Copper does not break into chips but tears and flows, forming long stringy chips that wrap round the tool, and its softness lets the cutting edge build an edge of smeared copper that blunts and spoils the cut. The metal also work-hardens where it is rubbed, so a tool that dwells hardens the surface it is cutting. The practice that machines copper is sharp, keen-edged tooling — positive rake, polished faces — with cutting fluid to lubricate the cut, feeds held high enough to shear rather than burnish, and chip breakers or interrupted cuts to control the stringy swarf. Where copper must be machined in quantity it is usually ordered in its free-machining grades, alloyed with a little tellurium or sulphur, which break the chips and turn the metal from a difficult one into easy work; for the rest, the shop accepts the gummy cut and uses the geometry and the fluid that tame it.

The bronzes

The bronzes sit between the two ends, and each bronze machines according to its alloy. Phosphor bronze — copper with tin and a little phosphorus — is stronger and springier than brass, with a fine grain and good resistance to corrosion and wear; it is the bronze of springs, electrical contacts, bellows and bearings, and it machines well with sharp tools, cutting clean chips and a good finish, harder on the tool than brass but easy, steady work. Aluminium bronze — copper with aluminium and often iron — is the strong, tough bronze of marine propellers, gears and wear plates, and it is the least forgiving of the bronzes: tougher and more abrasive to cut, it work-hardens and calls for the firm, sharp, well-cooled practice of a harder metal. Between them lie the bearing bronzes, often leaded, that carry the shafts of machines and are machined into the bushes and shells that run them. The bronzes are bought for load and corrosion rather than for conductivity, and their machining is the unremarkable, reliable work of the family.

Machining copper alloys in the shop

The shop’s rules for the family follow the spectrum of its difficulty. Brass is cut fast with sharp carbide or high-speed steel tools, dry or with a light oil, and its short chips and fine finish make it the easiest production metal there is; bronze is cut a little more carefully, with carbide where it is hard or abrasive, and coolant where the cut is heavy; pure copper is cut with the keenest of edges and plenty of lubrication, its free-machining grades used wherever quantity demands it. The carbide that cuts steel cuts all of them well, and the copper alloys reward the speeds that the hard metals forbid. One practice marks the copper shop: the swarf of brass, bronze and copper is valuable and kept apart — a chip of brass in a steel bin contaminates both, so copper alloy chips are segregated and returned as their own scrap, a small discipline that pays for the day’s coolant. From the terminal turned by the thousand to the bronze bearing that carries a shaft for years, the copper alloys are machined as this entry describes — fast at the easy end, careful at the hard, sharp and well-lubricated throughout — and they give the shop some of its cleanest and most profitable work.