Clamping up large projects can impose challenges. For example, sometimes projects require assemblies that involve clamping across a wide span where it’s difficult to apply even clamping pressure to the middle portion of the joint. Consider a wide bench, where you are attaching the top to a wide base, and you do not have any clamps with a deep enough throat to allow the clamp to apply direct pressure at the center of the bench. If you just clamp at the both ends and ignore the missing area in the center it is likely that you will not achieve a good glue joint in the middle, which could lead to a bowed top or worse, a failed joint down the line. Your first instinct might be to set some heavy weight on the middle of the joint, such as a cinder block or your next door neighbor, but it would take a LOT of weight to deliver the PSI that is needed for a wood joint.
To address this problem, there are a couple tricks that will be helpful to learn.
Cauls provide a mechanism to distribute clamping pressure across a wide surface. To use these you will apply clamping pressure on each end and the caul will span across the joint area. Using cauls for even clamp pressure will help to some extent, but you will still find that you have uneven pressure in the middle.
Shim the caul
The additional measure that you can take to apply pressure in the center of the joint is to place a shim underneath the caul in the center of the joint, which will create an increased pressure point underneath the shim, just where you need it the most. See what George uses as the perfect shim for this application!