Cast Iron

Cast iron is the metal of the machine bed, the engine block and the brake disc, and it is the iron the shop casts rather than rolls: an iron alloyed with carbon above about 2 percent — more than the steels can carry — so that it pours as a liquid and is cast to shape, taking the form of the bed, the block or the housing directly from the mould. That high carbon is its whole story, because the way the carbon appears in the cooled casting decides which cast iron the shop has and how it will machine. In some the carbon forms flakes or nodules of soft graphite; in others it combines with the iron into hard carbides; and between those extremes the metal runs from some of the freest-cutting work in the shop to metal that only a grinder will touch. This entry sets out the cast iron family and how each kind is machined.

The family of cast irons

The cast irons are named by the shape their carbon takes, and the family is read from that. Grey iron is the commonest: its carbon forms flakes of graphite, which give the metal its grey fracture, its excellent damping of vibration and its ease of cutting, at the price of low strength and ductility — the iron of machine bases, housings, brake discs and pulleys, strong in compression and stable under load. Ductile iron (also called nodular or spheroidal) treats the melt so the graphite forms as round nodules, giving an iron with real strength and toughness, approaching steel, that is still cast to shape — the iron of crankshafts, gears and heavy brackets. Malleable iron is white iron annealed to make its carbon workable, and compacted graphite iron sits between grey and ductile, strong yet damping, for diesel blocks and the like. The last of the family is white iron, whose carbon is combined into hard carbides rather than graphite: it is very hard, very wear-resistant and essentially unmachinable by cutting. The family is thus a spectrum of machinability, and the shop reads the casting’s name before it sets the cut.

Machining grey iron

Grey iron is some of the easiest metal in the shop to cut, and its ease is itself the product of its carbon. The graphite flakes act as a built-in chip breaker and a lubricant: the metal parts from the tool in short, powdery chips that clear easily, the tool runs cool and lightly loaded, and fine surfaces are cut at speeds and feeds that would destroy a tool on steel. The practice that machines grey iron well is less about force than about dust and edges: the free graphite makes a fine black dust that is abrasive to the machine’s ways and unpleasant to breathe, so the work is cut dry or with the dust controlled, and the machine is kept clean. The casting’s skin is the real obstacle — the outer surface of a casting carries scale, sand and hard spots from the mould — so the first cut goes beneath the skin with a robust edge, and the interruptions of scale and the occasional hard inclusion call for carbide with a tough edge rather than a keen, fragile one. Grey iron is the production metal of the cast-iron shop: cut firmly beneath the skin, at high speed, with carbide, it gives the smooth, stable machined surfaces that the castings of this family are famous for.

Machining ductile iron

Ductile iron is machined like a strong steel rather than like grey iron, and the difference is its toughness. Its graphite nodules do not break the chip as freely as grey iron’s flakes, so the metal cuts with stronger, more continuous chips and loads the tool more heavily; it is tougher and less forgiving of a dull edge, and the shop cuts it a little slower, with sharper, more positive tooling and a good supply of coolant, treating it as the strong metal it is. It still machines well — far better than steel of the same strength, for the graphite continues to lubricate — and its combination of castability, strength and machinability is exactly why it is chosen for the crankshaft, the gear and the heavy bracket that must carry load and still be made as a casting. The cutting is the work of a strong, well-sharpened carbide edge at a moderate speed; the metal rewards the respect and punishes nothing but neglect.

White iron and the hard end

At the far end of the family, white iron is the metal that cutting tools cannot touch, and the shop treats it as a grinding job. Its carbon is locked up as hard carbides throughout the metal, giving it immense hardness and wear resistance — the white iron of mill liners, pump impellers and wear plates — and a cutting edge of any ordinary tool material is blunted the moment it meets that structure. White iron is therefore machined only by the abrasive processes: ground on the grinding machines of the shop, or cut by EDM where a shape must be made, and the “machining” of a white iron part is really the finishing of a casting whose form was fixed in the mould. Between white and grey sit the compacted graphite and malleable irons, machined as the nearer of their neighbours. The shop that knows its cast irons knows where on this spectrum the casting lies, and machines it accordingly.

Cast iron in the machine shop

Cast iron comes to the machine shop in two ways, and both matter to this wiki. It comes as the work: the casting that is machined into a finished part, located on its datum faces and held in workholding while its critical surfaces are cut, drilled and finished on the machining centre. And it comes as the machine: the grey iron of the machine’s own bed, column and slides, chosen for exactly the damping and stability that this entry has described, and machined in the foundry to the accuracy the machine needs. For the machinist the castings of the family are among the most tractable of work — grey iron cut fast and dry beneath its skin, ductile iron cut like a strong steel, white iron ground — and the family shows, as clearly as any in the materials group, how the same base metal, treated differently, becomes everything from the freest-cutting job in the shop to the one that only a grinder may touch.