Surface Grinding, Cylindrical Grinding & Centreless Grinding
Surface, cylindrical and centreless grinding are the three great families of grinding — the abrasive finishing processes that shape metal with a grinding wheel instead of a cutting edge. Where the machining processes this wiki treats cut with a defined edge, grinding cuts with an abrasive wheel: a disc of countless hard grains — carbide-hard particles of aluminium oxide, silicon carbide or diamond — each grain a tiny cutting point, bound into the wheel and presented to the work at high speed. Grinding removes little metal, but it removes it to an accuracy and a finish that cutting rarely reaches, and it grinds metal too hard for a cutting tool — the hardened steel, the tool steel, the carbide — which is why grinding is the finishing process of the parts that must be exact. The three families are the three geometries of that work: surface grinding for flat faces, cylindrical grinding for round diameters, centreless grinding for round work in quantity. This entry sets out each process and how they differ.
How grinding cuts
Grinding is cutting on a scale too small to see, and its working follows the wheel. The grinding wheel spins fast, and each of its exposed grains takes a tiny chip from the work as it passes — the grains are the teeth of a cutter with tens of thousands of edges, each taking a chip a few micrometres thick, so the wheel removes metal in a fine, continuous stream of small cuts. The wheel is chosen for the work by its abrasive, its grain size and its bond: coarse grains and a soft bond for fast stock removal on soft metal, fine grains and a hard bond for the smooth finish on hard steel, the wheel dressed — its face trued and sharpened — as it loads and dulls. Grinding runs with coolant flooding the cut, both to carry the heat away — a dry grinding cut can burn and soften the hardened surface it is meant to finish — and to wash the swarf clear; and it finishes with spark-out, the wheel passing over the work without further feed, letting the last springs of the machine and the work settle, so that the final pass takes no cut and leaves the true size.
Surface grinding
Surface grinding makes flat surfaces, and it is the process of the flat face that must be flat and smooth. The work is held on the machine’s table — a magnetic chuck for steel parts, a vice or fixture for others — and the table carries it back and forth beneath a grinding wheel whose face sweeps the surface; with each pass the wheel is fed down a little, and the reciprocating passes grind the face down until it is flat and to size. The flatness of the surface-ground face is set by the trueness of the wheel and the table, and a good surface grinder holds a face flat and parallel to a few micrometres, with a finish that cutting cannot match — the reason surface grinding finishes the faces of gauge blocks, the seats of dies, the tops of machine tables and the flat reference surfaces that other parts are set up on. Surface grinding is the finishing process of the flat part, grinding the face that must be square, flat, smooth and true.
Cylindrical grinding
Cylindrical grinding makes round surfaces, and it finishes the outside — and the inside — of cylindrical work to the roundness and the size that turning cannot hold. In cylindrical grinding the work rotates about its axis while the grinding wheel, spinning much faster, is fed against it; the wheel and the work turn together, the wheel cutting the work’s circumference, and the wheel traverses along the length so that it grinds the whole diameter. The work is held between centres, in a chuck or on a faceplate — the grinding-machines entry describes the machine — and because both the work’s rotation and the wheel’s path are true to the axis, the ground diameter is round, straight and concentric. Cylindrical grinding finishes the journals of shafts, the bearing seats, the spindles, the hardened round parts, to the roundness and the size that a rotating part needs, and with an internal wheel it grinds bores the same way, finishing the round surfaces of the parts that must run true.
Centreless grinding
Centreless grinding makes round work without holding it at all, and it is the process of the straight round part in quantity. In centreless grinding the work is not chucked or centred: it rests on a work blade between two wheels — the grinding wheel, which does the cutting, and a smaller regulating wheel, which is angled slightly and controls the work’s rotation and its feed. The regulating wheel turns the work against the grinding wheel as it carries it through the gap, so a straight round bar or a cylindrical part is ground as it passes between the two wheels, emerging ground to size. Because the work needs no centres, no chucking and no operator between parts, centreless grinding is fast and continuous: bar after bar feeds through and is ground, making it the process of the pin, the roller, the shaft, the bearing part, produced by the thousand at a tolerance and a finish that cutting cannot match. Its limit is its simplicity — it grinds straight cylindrical parts that can pass through freely, without shoulders or features to locate — and for those parts it is the most productive grinding there is.
Choosing among the three
The three processes are chosen by the shape of the part and the number to be made, and the choice is usually clear. A flat face is surface-ground, whatever the quantity, on the reciprocating table that makes it flat. A single round part, or one with shoulders, tapers, flanges or a step that must be ground in relation to its other features, is cylindrical-ground, held and turned true to its own axis. A straight round part in quantity — the plain shaft, the pin, the roller, with no features to trouble it — is centreless-ground, fed through between the wheels at a rate no chucking could match. And in every case the parts that are ground are the parts that must be exact: the hardened and the hard, measured in hardness that defeats cutting, finished by the abrasive wheel to the tolerance and the surface that the drawing demands. Surface, cylindrical and centreless grinding between them finish the flat face, the round diameter and the high-volume round part — the three geometries of abrasive finishing, cutting with a wheel of countless tiny edges where no single edge could do the work.