Magnetic Workholding

Magnetic workholding holds ferromagnetic work by magnetism, and it is the answer to the parts the vice and the clamps hold badly: the flat steel part that must be machined on its top face and its edges, with nothing covering the work. A magnetic chuck — the plate of magnets on the machine table — holds the part by magnetic attraction, drawing it down on to the chuck over its whole area, so that the top and the sides of the part are entirely clear of the workholding. It is the classic workholder of the surface grinder, where steel blocks and plates are held by magnetism while they are ground flat, and it is used on milling machines and EDM machines too, wherever a flat ferrous part must be held firmly with free access. This entry, with the vacuum entry beside it, sets out the two fieldless workholders of the workholding group: where vacuum holds the non-magnetic sheet by suction, magnetism holds the steel part by attraction.

What magnetism holds

A magnetic chuck holds the parts that the magnet can grip, and the grip is a matter of the material. Magnetic workholding works on ferromagnetic materials — the iron and the steel that a magnet attracts: the carbon steels, the tool steels, the cast iron and the ferritic steels of the shop. It does not hold the non-ferrous and non-magnetic metals: aluminium, copper and brass are untouched by the magnet, and the austenitic stainless steels — the common 304 and 316 grades — are not magnetic and will not hold. The hold also needs contact: the chuck’s magnet pulls through the face of the part, so the part must lie on the chuck with a clean, flat underside, its face in full contact with the poles of the chuck; a part standing on three high spots, or with chips beneath it, is held at those points only and will shift. For the flat steel part that meets those conditions, the magnetic hold is even, firm and immediate — a lever on the chuck’s side and the part is held, no jaws, no clamps, no time spent tightening.

Permanent and electromagnetic chucks

The chucks that make the hold are of two families, and the difference is the source of the magnetism. The permanent magnet chuck holds with permanent magnets built into its face, and it is switched on and off mechanically: turning the chuck’s handle rotates an internal assembly that either directs the magnetic flux up through the poles to the part — on — or diverts it within the chuck so that the face is neutral — off. Because its magnetism comes from permanent magnets, it needs no power to hold: a permanent magnet chuck holds a part indefinitely with the electricity off, which is why it is the safe, everyday chuck of the toolroom. The electromagnetic chuck holds instead by an electric coil beneath its face: current through the coil magnetises the chuck, and the pull can be adjusted by the current, from a light hold on a thin part to a firm grip on a thick one. Its power is its risk, for an electromagnetic chuck holds only while the current flows, so the two are often combined — a permanent-magnet chuck that is switched by an electric pulse, holding by magnetism alone once set.

Where magnetic workholding is used

The magnetic chuck earns its place on the machines that finish flat steel, and its uses are distinctive. Its home is the surface grinder: the flat steel part is laid on the magnetic chuck, the magnet is switched on, and the grinding wheel passes over the part, grinding its top face flat while the magnet holds it down across its whole area — the way the blocks, plates and hardened steel parts of the shop are ground flat and true. On the milling machine, the magnetic chuck holds the thin steel plate or the flat blank for facing and light milling, the work held over its whole surface rather than at its edges, so the top is clear for the cutter to pass across and the edges are clear for it to reach. Because the magnet holds the part over its full area, the thin steel part is held without distortion — drawn flat against the chuck instead of squeezed at its edges — which is why magnetic workholding, like its vacuum cousin, is a workholder of the thin and the flat as well as of the solid.

The limits of the magnet

Magnetic workholding has its limits, and the machinist works round them. The pull of the magnet is vertical — it draws the part down on to the chuck — so it holds best against forces that press down or lift up, and less well against a strong sideways push: a heavy milling cut that drives the part sideways can slide it across the chuck, which is why magnetic workholding is the workholder of the light finishing cut rather than the brute roughing, and why stops are often set against the part’s edges when side forces are expected. The part must be ferromagnetic and flat, as described above, which rules out the non-magnetic metals and the rough, warped or dirty part. And the magnet leaves its mark of a kind: the part is left slightly magnetised when it comes off the chuck, and a steel part that must be clean and free of magnetism — a part that will hold swarf, or that must not attract debris in service — is passed through a demagnetiser after machining, which cancels the residual magnetism before the part goes on. Set within those limits, the magnetic chuck holds steel work quickly, evenly and without a clamp in the way.

The magnetic chuck in the shop

In the shop the magnetic chuck is the workholder that holds steel by the square inch, and it complements the rest of the workholding family. The vice and the clamps grip a part from its edges; the magnetic chuck holds it from beneath, over its whole face, leaving the top and the sides clear for the tool — the difference that matters when the part’s whole upper surface and its edges must be machined in one setup. It is fast — a lever, and the steel plate is held or released, no tightening, no jaws to wind — and it is repeatable, holding every part the same way. The ferrous, the flat and the finished steel work of the shop — the blocks ground on the surface grinder, the plates faced on the mill, the hardened die and the tool-steel part — is held by the magnet that grips steel without marking it and without covering it, the invisible hand of the workholding group that holds the steel part firmly while the machine works its face.