Atmospheric pressure makes an excellent clamp

Vacuum workholding, and the arithmetic behind how much force it gives.

A vacuum fixture holds a part still by removing air from underneath it, letting the ordinary pressure of the surrounding atmosphere press evenly down across the part's whole supported area, which supplies more than enough force to keep most light or delicate parts still without a single localised clamping point anywhere on them.

A kilogram on every square centimetre

Air presses on every exposed surface, normally invisibly, because it presses equally from every direction and the pushes cancel. Remove the air from one side of a surface and the push on the other side becomes a real, usable force. A vacuum fixture holds a workpiece on a flat plate perforated with small holes connected to a pump, and once the pump draws the air out from beneath the part, atmospheric pressure above pushes it down against the plate.

That pressure works out to about one kilogram of force on every square centimetre. A manifold block 5 cm by 8 cm offers 40 square centimetres, so with most of the air removed beneath it, the block is held down by something like 30 to 40 kilograms of force, far more than the light cutting forces of trimming or drilling plastic, and no jaw touches the workpiece anywhere.

A rubber sink plunger pressed onto a smooth tile shows the same thing at kitchen scale. Once most of the air is squeezed from under the cup, the plunger clings there on its own, held by the surrounding air pressing on the outside of the cup with nothing equal pushing back from underneath. A vacuum fixture scales that up across a whole workpiece.

A mechanical clamp pushes hardest exactly where its jaw touches, the local concentration of force that bends a delicate part. A vacuum fixture spreads its total holding force over the entire area sitting over the plate, so the pressure at any one point is gentle. The broad, even push is what suits vacuum holding to thin or soft parts.

The seal sets the hold

The holding force depends on how completely the space beneath the part is sealed from the surrounding air, since any leak lets air bleed back underneath and erodes the pressure difference. A part with a slightly warped or rough underside can leak enough around its edges to lose most of the hold while the pump works perfectly. Vacuum fixtures are therefore usually built with a soft gasket around the part's outline, or machined to match the part's underside closely. A bigger pump cannot fix a bad seal, since it only removes air as fast as the leak lets it back in.

Small parts and sideways loads

Because the force scales directly with sealed area, a small bracket or narrow strip offers too little surface for a meaningful hold however good the vacuum. The force also acts straight down, offering little resistance to a cutting load trying to push the part sideways across the plate, so vacuum fixtures are usually paired with a light physical stop or edge locator to take up sideways loads.

Within those limits, a part held by vacuum can be machined with essentially no clamping-induced distortion, which answers the dilemma the previous article in this set raised. A bonded manifold usually offers a single flat reference face, which is exactly the feature a vacuum fixture is best suited to hold.