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Posted on How Much Pressure Is TOO Much?

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2899 posts in 2485 days

#1 posted 06-14-2012 06:54 PM

I posted this another thread (and probably the thread that spawned this one). In college, our strength in materials professor was an avid woodworker. In demonstrating mechanical bonds, we used wood, glue, and pressure as our example. The class was polled first and the unanimous answer was “there isn’t a limit on pressure, hammer the clamps home!” We should have known by the Dr.’s smug grin that we were in for a surprise.

First we tested the clamps. 36” jorgensen bar clamps. We found that with minimal deflection we could apply between 300 and 500 PSI of pressure. This also depended on the hand strength of the operator.

We then tested on some oak and titebond II. We did 3 glue ups, light, moderate, and “drive it home” clamping pressure. They were glued, clamped for the recommended time on the glue bottle, unclamped, and left to cure for 3 days
We then put the pieces on a 20 ton press with an anvil right at the glue joint

First test Light pressure (`100PSI) – Instant fail. The glue joint came apart easily
Second test – moderate pressure (190-200 PSI) – unbreakable. The joint held but the wood splintered off around it
Third test – Max pressure (`400PSI) The joint failed. Not as cleanly as the 100PSI, but it did separate

There are a lot of other forces at work when you clamp wood together. Pressure makes heat, glue escapes, glue sets up before it can be pressed into the wood fibers, etc. This is what happened under “too much” pressure. We had a glue-to-glue joint. The glue did not penetrate deeply enough into the wood grain before setting up.

As with all experiments, this doesn’t exactly simulate real world conditions, however it was enough to convince me to go easy on the clamps. More clamps with less pressure is better than a few clamps hammered down.


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