Dealing with pressure

Sometimes, such as when bending thin strips around a form to make a curved part, you need a lot of clamping force; but most times you don’t. For example, to glue up a panel from several boards with properly jointed edges that come together with no gaps, you need about 100–150 pounds per square inch (psi) for soft woods, and 175–250 psi for hardwoods. Almost any bar-type clamp can easily apply that much pressure—over one square inch.

But the force generated by a clamp spreads over the surface area of the joint. A panel glue-up of 1"-thick boards 25" long has a surface area of 25 square inches. To achieve 150 psi over 25 square inches, you need 3,750 pounds of clamping force. If you apply 420 psi per clamp, nine clamps achieve that pressure over the entire joint. With a bit more pressure per clamp, about 470 psi, you can use only eight clamps.

So how can you know what 420 psi or more feels like? To give an idea, we clamped a hydraulic pressure gauge against scraps of four different wood species to see how much pressure it took to dent the edge, above, right.

Because those forces can dent soft woods, either use a clamping caul of harder wood between the clamps and the workpiece, or plan on trimming the panel edges to remove any dents. Or, rather than twist clamps harder, add more clamps.

In this unscientific test, 380 psi created a significant dent in a pine board. 420 psi made a shallower dent in poplar. 1,000 psi made a barely visible dent in hard maple. And 1,200 psi had no visible effect on white oak, but it did twist the C-clamp, right.