Clamping pressure: How much pressure is enough?

[Dan Barr]

Just wanting to start a discussion and feel out the limits to this question.

How much clamping pressure is enough, too much, and too little?

I know that it all depends. obviously, 500 psi on a jewelry box drawer is not possible and not clamping altogether is also a big mistake.

thanks in advance. :)

v/r

dan

[glenn bradley]

A search here on 'clamping pressure' yields this thread:

http://www.sawmillcreek.org/showthread.php?t=28953

And this one with a poll:

http://www.sawmillcreek.org/showthread.php?t=57355

P.s. Overtightening leads to starved joints, DAMHIKT. In the immortal words of Sam Maloof "Not so tight, leave some room for the glue".

[Ed Brady]

The December issue of fine woodworking has an informative article on this subject with a helpful table to help calculate pressure.

[Keith Starosta]

The December issue of fine woodworking has an informative article on this subject with a helpful table to help calculate pressure.

That is a really nice article. I was amazed when the K-Body clamp ended up at the bottom of the charts on the relative max. PSI applied. Very interesting...

- Keith

[Jason Beam]

If the boards break, it was too much.

There is no such thing as overclamping otherwise.

[Lee Schierer]

This is something you'll gain a feel for as you do it more than anything else. Appying just enough glue instead of too much or too little glue is an aquired skill.

You need to apply enough pressure to hold the parts together. The key here is that your parts should fit tight without clamping. Clamping warped or bent parts into position with clamp will only lead to problems later on.

Apply your thin film of glue to one side of the joint (some poeople do both sides, but you only need to do one). When the joint is clamped you should see small beads of glue form every 1/8 inch along the joint form on the edges of the joint as you tighten the clamps. These should be small beads that do not run. If they run you applied too much glue. If you don't see the beads you didn't apply enough glue. The worst condition is a glue starved joint. Glue starvation is caused by too little glue, not too much pressure. Excess glue just makes a mess that you will have to clean up before finishing. If you are crushing the wood then that is too much pressure. With that said, you should also put wood strips or blocks between the finished piece and the clamp face to eliminate clamp marks unless you plan on jointing the edge again after glue up.

[Brett Baldwin]

To add to Lee's points, the main reason to put glue on both sides of the joint is to avoid the glue starvation. When you only apply it to one side, you are counting on even pressure to put glue on the unglued side before it gets squeezed out the sides. Putting half your total glue on each side lets the glue soak into the wood before the pressure is applied and tightens up the pores.

That article said that for most woods and joints, not enough pressure is applied. The recommendation was generally to put more clamps on the work so that each clamp doesn't have its clamping pressure spread out as much and you won't have to put as much force on each clamping site.