1. Cauls distribute pressure
It’s not easy to get enough squeeze in the middle of a big box to force home dado or biscuit joints. Big cauls are the answer.
A caul is simply a thick, straight board. I make my cauls from stiff wood, such as hard maple, but any wood will do. The wider and thicker the caul, the less it flexes and the better it delivers pressure far from the clamps. I made a set of eight, each measuring 1-3/4 x 3 x 24 in., to have around the shop whenever I need them.
Stout cauls like these should provide plenty of pressure, but you can get extra pressure in the middle by inserting one or more shims (I use playing cards). You can also round or taper one of the caul’s edges from the middle to each end to create a crown. I do a dry run with cauls top and bottom, without shims, and place a straightedge on the cabinet to see whether the sides are flat. If one side bulges and needs more pressure in the center, I loosen the clamps, insert shims and retighten.
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