# Clamp presesure for gluing



## jrcharvey (Dec 26, 2015)

Hi,

My dad and I have been having a debate on how much pressure is really needed for gluing joints. I've been making a bench top by gluing 2x4s together on their narrow side. I told him that I've been using Irwin Quick Grip 600lb clamps to hold it together while the glue dries. He says that I should be using pipe clamps because the Irwins don't provide enough gluing pressure. Who's right?


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## Rebelwork (Jan 15, 2012)

Pipe clamps...


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## woodchux (Jul 6, 2014)

Your "bench top" project should use pipe clamps for best results. Be safe.


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## Tony B (Jul 30, 2008)

I dont know how much pressure in PSI you should use but I'm sure you can look it up.
From a practical side, you should use enough to make it tight and glue to ooze out. How tight is tight? Hard to describe. You dont want to use all of your might or even close to that for tightening or you will force the glue out. Also, if you need to use all of your might to clamp a warped piece in place, better rethink using that board. The glue is normally applied at about .005 to .009" thick. When clamping, it will squeezed out to about .001 to .002" including some glue that actually gets absorbed by the wood. Thats is less the 1/2 thickness of a piece of paper. 
Keep in mind that wood glues are meant to adhere 2 pieces of wood together. The strength is in the adhesion and not in the thickness of the glue itself. Hard to explain so feel free to shoot this full of holes.


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## Rebelwork (Jan 15, 2012)

Tony B said:


> I dont know how much pressure in PSI you should use but I'm sure you can look it up.
> From a practical side, you should use enough to make it tight and glue to ooze out. How tight is tight? Hard to describe. You dont want to use all of your might or even close to that for tightening or you will force the glue out. Also, if you need to use all of your might to clamp a warped piece in place, better rethink using that board. The glue is normally applied at about .005 to .009" thick. When clamping, it will squeezed out to about .001 to .002" including some glue that actually gets absorbed by the wood. Thats is less the 1/2 thickness of a piece of paper.
> Keep in mind that wood glues are meant to adhere 2 pieces of wood together. The strength is in the adhesion and not in the thickness of the glue itself. Hard to explain so feel free to shoot this full of holes.


"Force the glue out"


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## woodnthings (Jan 24, 2009)

*over tightening the clamps....*

Extreme pressure is not needed IF the edges are straight and square:
https://www.youtube.com/watch?list=UUI7MD1EPPkEFAuVQuLx_3rw&v=5cy1wmzGC28

Clamping pressure developed by types of clamps:
http://www.woodmagazine.com/woodworking-tips/techniques/skills/take-it-easy-with-clamping-pressure/

Quoting the article above:
_However, let's assume that most joints fall short of perfect, and benefit from enough force to push them into complete contact. Dale Zimmerman of Franklin International, maker of Titebond woodworking glues, recommends 100 to 150 pounds per square inch (psi) for clamping softwoods and 175–250 psi for hardwoods. When we tested one-handed bar clamps (issue 139), we found that they provided pressure just into the softwood range or a bit less. Squeeze those clamps as hard as you can. But R. Bruce Hoadley, author of the book Understanding Wood, reports that other kinds of clamps, including the bottom three pictured at right, can produce far more pressure than needed. So don't go beyond "snug" when tightening those clamps. 
*The maximum recommended clamping pressure for most joints is 250 psi. Putting all your muscle into many common clamp styles generates excess pressure that could force out most of the glue and produce a weak bond.*_ 

Different type of wood and different grains of the same wood require different pressures:
http://www.finewoodworking.com/2010...ferent-woods-need-different-clamping-pressure

Just so you know..... :wink2:


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## GeorgeC (Jul 30, 2008)

Good straight wood the quick clamps are sufficient. If you need to pull wood together then pipe clamps may be required.

"Snug"(for want of a better word) is what you want. In my experience less experienced persons tend to use too much pressure when clamping.

When I made my bench top of 2x4's I did not bother with glue. My bench was not free standing as it was fastened to the wall. I screwed the first 2x4 to wall studs in 3 places. Then screwed the additional boards to each other. Then covered the whole thing with a sheet of hardboard. A sheet of Formica would also be good.

George


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## Steve Neul (Sep 2, 2011)

If the joint is well fitted it doesn't take very much pressure. And like others have said you can actually squeeze too much glue out with too much pressure and weaken the joint.


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## epicfail48 (Mar 27, 2014)

Those quick grip clamps are only useful for holding something together while you grab an actual clamp. It doesn't take a whole lot of pressure to glue a joint, but I've yet to find a quick grip clamp that applies enough


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## Rebelwork (Jan 15, 2012)

Steve Neul said:


> If the joint is well fitted it doesn't take very much pressure. And like others have said you can actually squeeze too much glue out with too much pressure and weaken the joint.


I've never seen where you can squeeze all the glue out of cabinetry. You should be able to all the pressure you want till the glue no longer comes out. 

You can't weaken a joint unless you didn't put enough glue on to start. 

I've yet to see the proof on any forum or in the working work.

Boat making excluded...

Glue creep...........Wonder why


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## woodnthings (Jan 24, 2009)

*a reasonable test here....*

I watched this a few years ago and don't remember the test results, but the experiment puts the joint in "shear", that is twists it rather than tries to break it along the glue line. It turns out that a "gapped" joint is stronger than the rest ....?


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## Steve Neul (Sep 2, 2011)

Rebelwork said:


> I've never seen where you can squeeze all the glue out of cabinetry. You should be able to all the pressure you want till the glue no longer comes out.
> 
> You can't weaken a joint unless you didn't put enough glue on to start.
> 
> ...


It just doesn't show up right away when you squeeze too much glue out. Of course you can't squeeze all the glue out so the joint will stay together for a year or so. Wood glue works by soaking into the pores of the wood and turns to plastic. The pores don't align so if too much pressure is applied too much glue is pressed out so there is nothing linking one board with the other. You are correct you can weaken the joint by not using enough glue


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## epicfail48 (Mar 27, 2014)

Dagnabbit woodnthings, you beat me to posting that link. He's actually got the tables for the breaking strength posted on his website:
https://woodgears.ca/joint_strength/glue_methods.html

An overly clamped joint consistently measured at 20-50lbs less force required to break the joint. Funnily enough,ni posted this article the last time someone claimed that you can't over clamp a joint...


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## GeorgeC (Jul 30, 2008)

In my early years of building I was guilty of over clamping. I have a joint in my computer desk (joint about 30" from this keyboard) that failed because of over tightening a clamp.

George


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## Ron_J (Sep 22, 2014)

woodnthings said:


> I watched this a few years ago and don't remember the test results, but the experiment puts the joint in "shear", that is twists it rather than tries to break it along the glue line. It turns out that a "gapped" joint is stronger than the rest ....?
> 
> Can you squeeze all the glue out of a joint? - YouTube


I was doing a little digging to get some opinions on how much glue to use on a joint and came across this. 

I found it interesting and thought I'd resurrect it.

I'm not sure how much real world difference you would see in the loosely clamped and tightly clamped joints given some different joint types, but it's interesting non the less.

My final take is that as long as you get a little squeeze out along the joint (basically to insure that there is glue across the entire joint), you would be good to go. So it looks like I'll continue to hate cleaning up glue joints.


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## GeorgeC (Jul 30, 2008)

epicfail48 said:


> Those quick grip clamps are only useful for holding something together while you grab an actual clamp. It doesn't take a whole lot of pressure to glue a joint, but I've yet to find a quick grip clamp that applies enough



I have done many clamp jobs with only the quick clamps. All depends upon how good of a fit you have between the two parts being glued.


George


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## Larry42 (Jan 10, 2014)

Well fit, preferably freshly machined joints take very little pressure. On short parts a rubbed joint with no clamping will work fine. 
About the idea that you can over clamp a joint: I did a trial and did everything I could think of wrong. I used hard maple (supposedly difficult to glue) that had been run through the molder (had a vey smooth surface) and it had been in the shorts pile for quite some time (not freshly surfaced.) The parts were about 3" wide, 1" thick, 4" long. Titebond original glue. Slopped on liberally. Immediately put into a 20 ton hydraulic press between 1" thick steel blocks and pressed to all the force that the press could exert (40,000#s) 3,333#/sq. inch!! (Way beyond what you could get with pipe or bar clamps.) Next day machined a square notch in the ends so I could put a steel wedge in, right at the joint line. Because I used a square notch the force was on the wood, not on the glue line directly. Back to the press. Put the wedge in the notch and applied force until failure. The wood failed. No where did the glue joint fail. 

Next try was to do exactly the same but put the sample on the shelf for 6 months. Then try to split the joint. Exactly the same result.
Conclusion: I don't think you can over clamp a joint. I do think that if the joint is too sloppy the result will be less than ideal. How much is too sloppy? According to testing done at the forest products laboratory it shouldn't exceed .004" The thickness of the side of a Coke can or a piece of copy paper +-.

Now about how much glue would remain on a machined surface after clamping. The parts I used had been machined at 30 knife marks per inch assuming only one of the two knives was doing the finished cut. (The knives are ground in the head on a profile grinder.) A molder produces a rotary cut, meaning there is a very shallow hollow produced by each knife mark. Not visible at the feed rate shown. 

For production edge gluing we use industry standard practice and glue directly off the straight line rip saw. It gives an excellent gluing surface. 

Anyone else want to present their test results?

2x4s should be surfaced before gluing because of the resin that may have come to the surface. Use enough force to close the gaps that are a result of crooked material.


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## Brian T. (Dec 19, 2012)

How much of a load will the glue joint have to hold?
Compression, shearing?


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