Clamps

Pressure is published, so the count is arithmetic

"Use plenty of clamps" is not advice. The people who make the glue publish a pressure for the glue line, in pounds per square inch, and a pressure times an area is a force — which is a number of clamps once you know what one clamp pulls.

The surprise in the table is the direction. Dense hardwood needs more pressure than softwood, not less. Pressure is not there to squeeze glue out; it is there to bring two surfaces into full contact against the stiffness of the wood resisting it, and a stiff board resists more.

A worked glue-up

Take a 36" edge joint in three-quarter stock — one glue line of a table top. The glue line is 36" long and 3/4" wide, so its area is 27 square inches, and at the dense-hardwood pressure that is a force most people would not guess.

Divided by a typical 600 lb F-style clamp, it comes out at more clamps than a lot of shops own, and considerably more than most glue-ups get. That is the honest answer, and it is why a panel can look fully clamped and still have a line you can see light through.

What one clamp pulls is not on the box

A 3/4" pipe clamp, an F-style and a parallel-jaw clamp differ by a factor of two or more, and the figure depends partly on how hard the person turning the handle is turning it. Makers publish it inconsistently where they publish it at all.

So the calculator asks for it rather than inventing one. If you have no figure, F-style clamps are commonly taken as 450–600 lb and pipe clamps as 500–1,000 lb hand-tight — but those are ranges to start from and check, not specifications.

Cauls do what more clamps cannot

Clamps pull hardest where they are. Across a wide panel, a pair of cauls — stiff battens laid across the boards and clamped at their ends, often with a slight crown planed into them — spread that pressure into the middle where no bar clamp reaches, and keep the panel flat while it dries.

They are the cheapest thing in this article: two offcuts and a bit of wax paper so they do not become part of the panel.

What this page does not tell you

• —The published pressures are Franklin's, for their wood glues. Other adhesives differ — polyurethane and epoxy have their own figures, and epoxy in particular wants far less pressure, because squeezing an epoxy joint dry starves it.
• —Clamp force is a field on the calculator rather than a constant here, because it genuinely varies by clamp and by hand. Where this site cannot check a figure against a primary source, it asks for the number rather than inventing one.
• —None of this rescues a badly jointed edge. If glue is squeezing out along the whole line and the joint still will not close, the problem is the jointing, not the clamping.