Gluing Question

[Darrin Johnson]

First post, sorry if it is in the wrong place.

I am gluing up 72 inches of 5/4 QS white oak for a desktop. I thought I'd do some reading and came accross the "Get serious about clamping" article in FWW from Nov last year. If I am to believe what it says:

The recommended clamping pressure is:
Glue surface (sq inches) X Required clamping pressue / Force by each clamp= # of clamps needed.

According to the table, red Oak needs flatsawn face needs 900 lbs/sq inch and QS face needs 450. I assume White oak will need the same.

Doing the math, I come up with the need for 81 clamps for FS joints and roughly 40 clamps for QS joints assuming pipe clamps can exert 1000PSI.

This seems like a crazy # of clamps. The article seems to imply that QS boards joined together is a "flatsawn joint" which would imply that I need 90!!! clamps in 72 inches.

Any help is appreciated. Feel free to point out any stupidity on my part.

thx,
dj

[jack duren]

You need only the amount of clamps to make a good joint using as much pressure as to close the joint..

[Darrin Johnson]

Thanks for the reply. That was my plan. reading can be dangerous!

dj

[Peter Quinn]

I read that article and have been scratching my head ever since? What does all that math mean. What about all the joints I have used that have performed flawlessly for years? I work in in millwork shop and have never seen ANYONE ever come close to the number of clamps suggested in that article. Even with cumaru and hard exotics I space the clamps 6"-8" or better, never a problem. Too much pressure and all the glue squeezes out making a weak joint.

I'd say one clamp at each end, two per foot there after is a very conservative and safe bet. That would cut your number to 14 clamps, no? I always clamp up panels and tops with the clamps alternating one over/one under. Keeps things flat.

[Roger Everett]

I recall reading that article, and thinking it was BS. Using a bit of common sence, not every thing a person reads is always the gospel, even if it comes from FWW.

Roger

[Darrin Johnson]

I would like to see a comparison of failure rates/point of failure with various clamping pressures and number of clamps used. 90 clamps in 72 inches clearly isn't possible or practicle, which makes me question the "whole enchilada"/article.

dj

[Brian Effinger]

I'm by no means an expert, but I think the article oversimplified things too much. I believe there is more involved in the equation - such as width of the boards, and how far away from the glue line the pressure point is.

[Karl Brogger]

I typically use one clamp every foot or so. I use a stick inbetween the clamp and the piece to protect it and spread out the pressure a bit. If there is a spot that needs more pressure, I add another clamp.

[Gene Michael]

Recently built a shop cabinet with a 25 x 37 x3/4" top glued up from red oak scraps between 3/4" and 1 1/4" wide. Used 5 clamps, alternating top and bottom to help keep it flat. So far, it hasn't come apart. Also built a 24" x24 x 5/4" stove cover for an RV almopst 10 years ago. It's been bounced, dropped, jarred and vibrated. Would have used 3 or 4 clamps on it.

You can see the shop cabinet in the SMC thread, what to do with scraps and judge for yourself.

[Paul Gatti]

There is no way you need that many clamps. Use good calls that have the least amount of flex. You only need enough clamps to close the joints completely with no gaps along the joint. A good rule of thumb is you want to see glue squeeze out along the length of the joint.

[John Schreiber]

It seems that you may need to use that much pressure if you want to get the maximum strength out of your joint, BUT you don't need to have your joint stronger than the wood on either side of it, so a more normal amount of pressure is just fine. It also makes a difference how well the wood is prepared. If it's really flat, no pressure goes into forcing the pieces together.

[Lee Schierer]

As the saying goes, you never have enough clamps.

I have always placed clamps every 4-8 inches apart and alternate top and bottom to keep the boards flat. With most hand screw clamps you cannot apply too much pressure to squeeze all the glue out of the joint, assuming you put in enough glue to start with, so don't worry about too much pressure other than crushing your wood.

The primary concern for optimum joint strength is a good fit. The boards should fit tight without any pressure. Bowed or cupped edges can be drawn in with clamps, but that will build stress into your glue up that will relieve itself by the joint failing, the wood failing or twisting and warping. These failurs may occur immediately or wait several years before they occur. If you can see daylight through the joint before applying clamping pressure the joint strength will be less than a properly fitted joint.

[Chris Padilla]

I think Bessey sponsored that article! ;) :D

[Darrin Johnson]

I think Bessey sponsored that article! ;) :D

hah hah! :D

[Ted Shrader]

Darrin -

Welcome to the Creek. A great place to hang out.
As the saying goes, you never have enough clamps. THAT IS A FACT!!!

I remember that article and thought at the time they misplaced a decimal point in their calculations.

Just make sure the edges of your pieces are straight and square. Alternate the clamps top/bottom of the glue up and you will be all set. When you have a little bit of glue squeeze out, that is just right.

Regards,
Ted

[Howard Acheson]

I haven't read the article you refer to but the US Forest Services Wood Products Lab and Titebond recommends 200-250 psi if you are using a yellow pva adhesive. What issue is it you are looking at?

As to the calculation. If you are edge gluing 3/4" thick boards to make a panel or tabletop, the you want to multiply the lenght of the glue joint by the thickness. For a 24" long board, the formular is 3/4" x 24" x 200 psi. The result is that you need 3,600 pounds of force. 3/4" pipe clamps are rated at 2000 psi if the clamps are fully tighted. Therefore, you could get by with two clamps properly positioned but three would be better. Parallel clamps like Bessey's or Jorgies only exert about 1000 psi so twice as many would be needed.

Remember, it's the square inches of the edges, not the face and the number of boards in the glue up is not a factor. In other words, glueing two boards or five board requires the same number of clamps.

It is not satisfactory to just close up the joint if you want a strong and glueline minimized joint. You want to exert the psi recommended for the adhesive.