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Clamping cauls are indispensable for gluing up casework. A caul is simply a stiff piece of wood used to apply pressure where a clamp can’t reach.
Sooner or later, you’re going to run into a situation like this; you’re clamping a wide shelf, for example, far from the end of the cabinet’s side. You can’t reach anything but the outer edges with clamps. The only way to apply pressure at the center of the shelf is with a caul. Obviously, it’s best to make your cauls before you’re in the middle of a glue-up.
I make my cauls with a very slight crown on one edge to ensure pressure at the center. Here’s how. I use stock that’s at least 1″ thick and 2″ wide. Set the tablesaw’s fence to the same distance as the caul’s width. Tape a shim, 1/32″ thick for each 18″ of length on one end of the caul. Run the caul through the saw with the shim side against the fence, non-shim end first. Flip the caul end-for-end and tape the shims to the other end and repeat.
You’ll end up with a crown that’s barely noticeable. That’s exactly what you want; just enough crown to apply pressure in the middle, and not enough to prevent pressure at both ends. I use a sharpie to clearly mark which side has the crown. –Howard Hirsch
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