Vacuum Workholding

Workholding|Process Desk|Published · Sep 7, 2026

Vacuum workholding holds work by suction, and it is the workholding method for the parts that cannot be clamped or held in a vice: the thin sheet, the flat plate, the non-magnetic and the delicate part. A vacuum chuck or table carries the part on a surface that is sealed beneath it, a vacuum pump draws the air out from under the part, and the pressure of the atmosphere presses the part down on to the chuck — holding it firmly and evenly across its whole area, no jaws, no clamps, no marks. Where clamps and vices grip a part from its edges, vacuum workholding holds it by air pressure spread over the whole face, and it is the method of the sheet and the plate that a clamp would distort or a jaw would scar. This entry sets out how it works, the parts it suits and its limits.

How a vacuum holds

The holding force of a vacuum chuck is the pressure of the atmosphere, and the principle is simple. The chuck’s face is machined with a pattern of grooves and a seal — an o-ring or gasket that runs round the area of the part; the part is laid on the seal, and a vacuum pump draws the air from the enclosed space through ports in the chuck, so that the pressure under the part falls well below atmospheric. The atmosphere above the part then presses it down with the difference in pressure, and the holding force is that pressure multiplied by the sealed area of the part. Because the air pressure is at most about one bar, the hold is limited — roughly a kilogram of force for every square centimetre of sealed area, and less if the vacuum is not perfect — which is why vacuum workholding holds best over a large area: a big flat plate is held firmly, while a small part needs most of its face to be within the seal. The hold is even and gentle, spread over the whole surface, which is the quality that no jaw or clamp can match.

The vacuum chuck and table

The hardware that makes the vacuum is a chuck or a table, and its design is about seals and ports. A vacuum chuck is a plate — often mounted on a fixture plate or a sub-plate — with a machined face of grooves and a groove for a seal that outlines the part; the shop machines the seal layout to match the part, and often machines several seal areas on one chuck so that several parts are held and machined at once. A vacuum table is the larger form, its face covered with a grid of grooves and a movable or replaceable seal that can be set to outline parts of different sizes. Below the face, the ports, the manifold and the reservoir connect the chuck to a vacuum pump, and a gauge or a pressure switch tells the machinist that the part is held — a vacuum below the working level means the seal has failed and the part will move. The pump runs throughout the job, and a valve releases the vacuum when the part is done, so it lifts off the chuck as the air returns beneath it.

The parts vacuum suits

Vacuum workholding is chosen for the parts that defeat the other methods, and the list is a clear one. Thin, flat parts come first: the sheet and the plate so thin that a vice or a clamp would bow or distort them, but which a vacuum holds flat against the chuck over their whole area — the aluminium sheet, the aluminium-alloy plate, the panel that must be machined true without springing. Non-magnetic and non-ferrous parts follow: the austenitic stainless steel, the copper, the plastic, the composite and the carbon-fibre part, which a magnetic chuck cannot hold at all, are held perfectly by vacuum. And finished, polished or delicate parts — the part whose face must not be marked or scratched — are held on the clean face of the chuck with no jaws to scar them, which is why vacuum workholding is the method of the aerospace skin, the instrument panel and the thin plate machined in the flat.

The limits of the vacuum

A vacuum hold has its limits, and a shop works within them. The part must seal: its underside must be flat and clean enough to sit on the seal, for a warped part, a rough sawn face or a chip under the edge lets air in and the vacuum falls — so the work is usually machined flat on one side, or is stock sheet, before it is vacuum-held. The part must be solid under the seal: a hole or a cut-out that breaks the sealed area admits air, so a part is vacuum-held while its features are cut, with the seal running outside the region being machined, and through-cuts are planned so that the sealed area is not opened until the part is nearly done. And the hold is gentle rather than brute: a vacuum holds a large flat part against light and moderate cuts, but not against the heavy roughing that a jawed workholder takes — so vacuum workholding is the method of the finishing cut and the light pass on sheet and plate, its cuts sized to the hold. When those limits are respected, the vacuum holds the part firmly, evenly and without mark.

Setting up a vacuum job

The setup of a vacuum job is quick, and its order is methodical. The chuck face and the seal are clean — a chip under the seal or in a groove is a leak, and a leak is a part that moves — and the part is placed on the seal, located by pins or edge stops against the locating principles this group follows. The vacuum is switched on, and the gauge is watched: the needle must fall to the working level and stay there, telling the machinist the part is sealed and held, before the first cut begins. The part is then machined, its clamps replaced by the vacuum that holds it evenly; and when the job is done the valve opens, the air returns, and the finished part lifts from the chuck, ready for the next blank to be laid on the same seal. The changeover is a matter of seconds and a new seal layout, which is why vacuum workholding also suits the short run of thin flat parts.