Reply by clin

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Posted on Glue up pressure and number of clamps?????

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1107 posts in 1736 days

#1 posted 01-18-2018 04:54 AM

As an example, Titebond II clamping pressure, according to Titebond, is 100-150 PSI for softwoods, 125-175 PSI for medium woods, and 175-250 PSI for hardwoods.

While the clamping pressures, in the article listed by AlaskaGuy, may seem really large, the PSI generated can be much less if the area being glued is very large. So if you’re edge gluing boards that are 1” thick and 10” long, you have just 10 sq-in of area and still need a total of something like 1,500 lbs of pressure. Which you can get pretty easy with a handful of lighter duty clamps.

But if you were gluing up something, much larger, like laminating two boards together, something that may have 100 or more square inches, you probably need every clamp you can lay your hands on to begin to get close to what is recommended.

The width of the boards matters too, not just the length. Since the pressure radiates outward from the clamp (I ve seen diagrams that show 45ยบ but that s probably just an approximation), the wider the board, the fewer clamps you need for a given length. For example, picture gluing two 1/4 inch strips. You ll need a clamp every inch or two, or, better yet, cauls to get even pressure along the joint.

And, I agree with AG, too much pressure will starve the joint. I learned to use just enough to bring the boards together firmly.

- Rich

I think the 45 degree thing is not just a rough approximation, I think it has something to do with the way materials react. Though I’m sure it does assume a uniform material. And you’re absolutely right on to bring up using cauls. I think most tend to use them as a way to not crush our projects where the clamp contacts it, but they’re important to spread the clamping force out across the joint. But because of the 45 degrees, you actually need the cauls + stock to be as thick as your spacing between clamps to get truly even pressure.

So if you were edge gluing 6” wide boards, you don’t need cauls if the clamps are less than 6” apart. But if you were gluing something just 1” thick, you probably need cauls.

-- Clin

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