Large Project Clamping: Tips for Even Pressure & Strong Joints
George VondriskaDescription
The Challenges of Clamping Large Projects
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.
The Art of Even Clamping: Introducing Cauls
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.
Achieving Center Pressure: The Shim Technique
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!
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I've got glue in the joinery that's holding the leg to the rail. And next thing I want to do is get clamps on here. Now, if all I do is clamp outside here and outside here, certainly I can draw the outsides of this big slab down. But am I closing it in the middle? Well, a caul will help. There's a maple two by two. So this will help a lot, clamp, clamp. I'm probably getting pressure across the middle. Here's a way to guarantee that we get pressure across the middle, and that is to add a couple of playing cards. I'm gonna put those in the center. Keep the caul lined up with the leg. Now, what'll happen, because of the rigidity of that maple, when I draw these outsides down, because we basically added a hump in the center, I know I'm getting pressure on the outside, 'cause I'm making contact out there. And the cards are pushing down in the middle to make sure that we're closing on the center of the joint as well. So it's very simple. We're just using those playing cards like a shim. But it's a great and effective way to make sure we're closing that joint all the way down its length. Same thing on the other leg. And this table is gonna look great when it comes out of clamps.
Thanks, George! Quick question here: aren't you worried about the top moving here? The joinery and glue seem to prevent any wood movement. I'd be worried about the top cracking/splitting over the seasons.