# Clamping pressure for glue-ups



## richgreer (Dec 25, 2009)

I just read this article on the fine woodworking website.

http://www.finewoodworking.com/item/27119/clamps-the-secrets-to-success

I am about to begin a project that will require me to glue up some flat sawn, 6/4 red oak. The gluing surface will be 24 inches long. According to this article, for optimal gluing, I need to apply 900 lbs of pressure per square inch of gluing surface. That's 30,000 pounds of pressure!

The article also notes how much pressure the average person can exert with different clamp styles. With a pipe clamp one can get about 1000 pounds of pressure. WIth an I-bar clamp you can get a about 1,300 and with all other clamps the maximum pressure is much less than the pipe clamp.

That means, for optimal gluing, I would need to use 30 pipe clamps on 24 inches. That's crazy.

Has anyone else heard of this?

If this is true (I'm not sure it is.) I will have to settle for something less than optimal gluing.

It was just yesterday that I was watching a New Yankee Workshop rerun and Norm advised not to clamp your glue joints too much because you can "squeeze all the glue out".


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## grizzman (May 10, 2009)

i think the article is bull pucky…..ive used pipe clamps for a very long time, and use just enough pressure to bring the joint together …ive never had a joint fail…so either the information is in correct or its not pertinent to what i do as a wood worker…use a good glue…and regular clamps…either bar or pipe…your work will be fine…have fun…are you referring to the pew ends you are going to make…if so you have lots of work ahead…and with some help if i remember right…good conversation and maybe some good snacks…you gotta have some snacks…lol…grizzman


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## hazbro (Mar 19, 2010)

I have found myself throwing clamp after clamp at stuff having pieces move on me during glue up. after a perfect dry fit no less.

exhibit A: http://img.photobucket.com/albums/v284/chef24pack/houseremod/drawer-glue-up.jpg


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## richgreer (Dec 25, 2009)

grizzman - I agree with you that this article is really off the mark. My standard on gluing up boards side-to-side is a clamp every 6 - 8 inches. It's worked well for me for a long time and I am not about to change.

Yes, I am beginning the pew end panel project. 870 bf of 6/4 red oak will be delivered this coming Wed. The glue-up will be slow because we only have so many acceptable clamps. I think we can glue up 4 - 5 at a time and I expect to do about 15 per day. Then the fun begins. I've been making panel cutters and jigs for every cut we will have to make.

As an FYI - We will do 3 of the straight line cuts on the TS and the other 5 with the festool plunge saw. I've made jigs to make certain that the track for the festool will be in exactly the right spot for each cut.

I've got several good people who want to volunteer to help. I think part of their motivation is they want to learn what they can from the experience. I just hope everyone keeps all their fingers.


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## lobro4 (Aug 8, 2009)

Glue works by chemical bonds-not by pressure-so how can physically increasing pressure improve that chemistry. First the glue film on the wood chemically interacts with the wood and then the glue from each edge interacts with itself. So-if the GAPS ARE CLOSED and there are not glue starved edges and you have squeeze out for the whole joint ant you keep the pressure on the clamps until the glue is cured-I am not sure how more pressure improves things.


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## Woodwrecker (Aug 11, 2008)

My old pipe clamps with just enough pressure to cause a little squeeze out do the trick every time.
That article is a bunch of hot air.


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## TimLawson (Apr 27, 2010)

(Oops should read article before passing comment on the comments). I think that the point he's making is that the stronger the wood the more likely you are to need a higher clamping pressure to get a tight joint to make any imperfections in the glue line disappear.

I think that the real issue with edge joining boards is the jointing process. That's where learning to use a jointer plane or being incredibly careful setting up your jointer is critical. I opt for a jointer plane working on paired boards. If the boards are well jointed you need enough clamping pressure to get the glue to squeeze out evenly.

He also talks about the 45 degree issue I mentioned in my first answer below.

Tim

(Original answer)
I think that the article about clamp pressure you quote is out of line. To pick up on a couple of points in replies to this thread - when clamping consider that each clamp effectively exerts pressure in a field 45 degrees either side of a line between the two clamp faces. Clamps should be spaced so that their pressure fields overlap at the glue line by an inch or two Sometimes the answer is not more clamps but to add cauls to either side of the boards to get even pressure coverage along the glue line. Same effect less clamps. Is that heresy to suggest less clamps?


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## NBeener (Sep 16, 2009)

Even though I have no talent, and no experience, I HAVE picked up a lot of "book knowledge" about woodworking



I thought that … it was a practical impossibility to squeeze a joint dry.

I found another FWW discussion that tends to support that notion.

After watching LOTS of Norm, I notice he looks for a THIN line of squeeze-out, after clamp-up. It makes him very happy.

It makes ME very happy, too.

It does seem like *lobro4* has it right, too: you're not trying to meld the two edges-just create the chemical bond. Besides, my aunt and uncle both teach at Lawrence, so …. I was giving a shout-out to *lobro4*, either way ;-)

More coffee….


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## davcefai (Feb 6, 2009)

The article referred to the ideal clamping pressure. This in no way means that a joint will fall apart if a pressure short of this is applied.

If the author performed the experiments then those are his results; the numbers are not open to negotiation!

As somebody pointed out above, the quality of the jointing may play a big part in this.

Other studies have indicated that the glue is stronger than the wood. If so then making it even stronger is an exercise in futility.


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## Eagle1 (Jan 4, 2010)

Hey Rich you could also use a few bisquits in there..


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## Dennisgrosen (Nov 14, 2009)

that kind of pressure is just hot air
and yes you can sqeeze the glue out of a straight joint
when you glue to boards together 
you don´t need that much pressure to do the work
you just need evenly spred pressure

Dennis


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## Rembo (Apr 28, 2010)

отличные струбцины ,пользуюсь такими http://radikal.ru/F/s004.radikal.ru/i208/1002/9e/4e1bb3a5891d.jpg.html


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## Catspaw (Dec 15, 2007)

If you can't close the gap by pulling it together by hand, your joint is not correct. A narrower board will flex more easily. Two narrow boards that can be drawn together by hand is fine ( and I don't mean bearing down with all your might…just a kind of arms length kind of thing.) If they are wide, you better get both joints dead straight and flat. Otherwise, even if you DO clamp them hard enough to close the joint, you're building stress into the project. Then you're squeezing too much glue out (particularly where the board originally wanted to touch) and putting stress in to pull it apart.

Dead tight joints, moderate clamp pressure. My benchmark is usually my thumb and two fingers, turning until the handle really wants to stop. I consider myself to have very strong hands.

And always remember the old adage that glue is stronger than the wood. So short of having an unwanted thick glue line, squeeze some out and leave it.

BTW, I always figure on 1 hour clamp time with titebond. I noticed your turnaround and thought it was abit conservative.


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## DanLyke (Feb 8, 2007)

Where does he come up with those recommended pressures? The Titebond web page says 100-150PSI for softwoods and 175-250PSI for hardwoods.

Seems like maybe we need to figure out a way to do some tests on this. Seems like 1200PSI, even on flat sawn sugar maple, is going to end up deforming the wood because of the clamping process.


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