# C Clamp Curiosity



## drsurfrat (Aug 17, 2020)

I came across a statement that most glues need about 250 psi for good adhesion, too much more will starve the joint. and too little will not take advantage of thin wall stress conditions. Then I thought, well, how much am I applying? I crank down kinda hard and make sure the glue squeezes out. Not exactly quantitative.

Punchlines:

-It turns out that when I glue up 3/4" boards, I crank down clamps about 10" apart, and that gives just about 250 psi.

-My 4" clamp seems to have about 1500 lbs force when I crank it down (not quite 20 ft.lbs)

-My 8" clamp with 'better' threads gets about 2400 lbs.

-I can only get my squeezable Irwin pistol grip clamps to 120 lbs!

*This is not a scientific report.* Please don't assume anything here is exact - it's not. And don't necessarily change your method of clamping. This is only a one-event 'experiment' and not statistically legitimate to generalize. And this is an opinion forum, so take it as that - opinion.

This, like most questions, leads to more. Is 250 psi really a good pressure? (it's on the internet, it must be true) For which glues? How do I measure how strong a glue joint is? Does wood species really matter?

…

*Nerdy details*
I set up the measurement like the pictures.



















[c clamp pics]

A big, 300 lb spring scale, heavy chain and two different C clamps on my beater workbench. I don't have a load cell, so the clamps pull against the spring scale. I used a 10 lb sledge for a torque weight (I don't have a torque wrench) and hooked it up to pull the clamp lever. I used two clamps, a 4" Wilton and an 8" KC Pro. Clamp screws were liberally oiled. I hand cranked up to what I thought was close, then would turn the crank another 180 (to horizontal) and have the sledge pull. When the sledge wouldn't turn the crank, that was the limit. Then I could calculate the torque at the limit (10 lbs at 2 5/16" = 0.19 ft.lbs) and correlate to the spring scale reading.

A reminder: weight and force are the same units, pressure is a force over a certain area. A 10 lb weight on a 1" square block gives 10 psi; a 10 lb weight on a 5 sure inch block will only give 2 psi.










[Data]

This is a static system, so flexing is not relevant; a 10 lb weight on a spring is still 10 lbs., and I am measuring weight after everything has settled.

Friction *is* relevant. Some of the work to crank the clamp goes only into fighting the friction. And as tightening force goes up. so does friction. I didn't do anything to model, calculate or eliminate that, it just is part of the measurement.

For this case, a bigger dia screw alone won't matter much, and neither will the thread pitch, but together they do. The ratio defines the mechanical advantage: the slope of the thread. I had two different thread angles, 6.5 deg and 4.5 deg. The 4.5 is a shallower angle, and therefore a greater mechanical advantage.

What I decided to call 'climb' is the opposite of the usual roof pitch proportion. Steeper roof pitches have a larger number. Here, shallower pitch has a higher number because they have greater mechanical advantage. To checkout this is all reasonable, I corrected the force by the screw by the climb ratio (12:9), and the twisting force vaguely matches.

A larger diameter screw might indirectly affect clamp pressure; it may reduce friction because the sliding faces will be larger. This means that clamp pressure might be higher for a given torque on the screw with bigger diameter.

Pipe clamps should follow the same logic, and be climb ratio dependent

Pistol handle clamp will not follow this analysis, since there is no screw. I used the same setup to see what would happen. I could get 120 lbs total with my hand strength. That is about 10 times *less* force. That surprised me.

…

*How is this useful?* I think it tells me that 8-12 inch spacing with pipe clamps will get me a good glue bond. With around 1500 lbs for force, I can distribute this over the gluing surface for a typical 3/4" board edge to get 250 psi. (1500 lbs / 250 psi = 6 square inches) 3/4" by 8" spacing is 6 square inches. This assumes that there is even distribution of the clamping force, of course. I have clamped boards together with the pistol grip clamps, and it seems to work, so to try to get exactly 250 psi would be silly.

…

*Anyone got a torque wrench?*

I'd like to get other people's applied torque. If you do have a torque wrench, put it on some bolt that is really solid, put your thumb on the bit end and grab the handle closeup. Note what value you can get when you twist it the same as you would for clamping - for right and left hand, clockwise and counter clockwise (and tell me if you have arthritis). Please don't fudge it to show how manly you are. 









[torque pic]

Some internet sources:
Titebond oughta know, they are a glue manufacturer:
http://www.titebond.com/product/glues/e8d40b45-0ab3-49f7-8a9c-b53970f736af
up to 250 psi

Woodmagazine:
https://www.woodmagazine.com/woodworking-tips/techniques/skills/take-it-easy-with-clamping-pressure
all says that my numbers are pretty good

Jorgensen - pony notes, no actual numbers for desired clamp pressure
https://ponyjorgensen.com/blogs/clamping-pressure-techniques/

Jorgensen - pony clamp specs:
https://ponyjorgensen.com/products/c-clamps/

MANY comments about glue squeeze-out starving the joint, but no actual proof or numbers.


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## Unknowncraftsman (Jun 23, 2013)

Interesting way of testing. 
I've found it's impossible to squeeze all the glue out of a good mating joint if I wet both sides. Starved glue joints are from not enough glue on the wood not from over clamping. 
I do believe over clamping can cause other problems crushing wood fibers and racking a box out of square. 
We could go on and on about glues and wood not every wood glues up the same . Example open grain oaks and closed grain Maple.
Good Luck


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## EEngineer (Jul 4, 2008)

"I came across a statement that most glues need about 250 psi for good adhesion, too much more will starve the joint. and too little will not take advantage of thin wall stress conditions."

Source, please?

For years now, I have only used clamps initially to hold glued joints until I can brad-nail them. I set a few 18 gauge brads to hold the joints then remove the clamps and move onto the next joint. I suspect that brads do not hold the same force as the initial clamping. Have I been doing it wrong all these years?

On small projects where I can afford the luxury of allowing the clamps to stay in place until the glue is set, I sometimes do that. I haven't noticed any difference in the strengths of the glue joints.

Just as a side-note, I HATE squeezable clamps of any type. I find that they just do not apply enough force to keep things from shifting while I work with them. I prefer bar clamps with a screw tightener - even the cheapest HF bar clamps work better than the best squeeze-type clamps. It doesn't surprise me that your tests show so much less force with these clamps.


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## ChefHDAN (Aug 7, 2010)

> Just as a side-note, I HATE squeezable clamps of any type. I find that they just do not apply enough force to keep things from shifting while I work with them.
> - EEngineer


Completely agree, I started with the black & blue squeeze ones and still have many, but seem to really only use them as an absolute last resort, or for situations where they are used more to secure something than gluing, up a project, think angle square when gluing a box. or holding a stop block along the fence in the crosscut sled.


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## MadMark (Jun 3, 2014)

*Clamp Spacing:*
Clamping force fans out at a 45° angle from the clamp location. Assuming the join is centered this means the clamps should be spaced the width of the glue up apart starting 1/2 of the width from the end.

More clamps than this don't help the join and less clamps means the midspaces aren't getting the same pressure. Wider joins space the clamps further apart, not closer. Thin pieces (like the side of a musical instrument) need tons of closely spaced clamps.

PVA glue likes to be spread thinly, if you have glue dripping out of the joint you're just wasting glue. A small (~1/16") amount of squeezeout evenly along the seam should be the goal.

A ton of pressure isn't needed. PVA glue bonds well in a "rub joint" where the two pieces are rubbed back and forth to spread the glue, held with hand pressure for 15S or so to let the glue start to grab, and set, unclamped, from there.

*Starvation:*
Glue starvation is a myth. Industrial clamping systems clamp with more force but don't starve the joints. If an industrial clamp driven tight with an air tool doesn't cause glue starvation no F-clamp you have is going to do it.

Glue starvation is caused by not putting even coverage on the seam. Voids are "starvation" or more properly omissions.

The biggest gluing error is incomplete coverage. Voids cause weak points where failures start. Simply running a bead of glue down the center doesn't suffice. The glue needs to be spread evenly over the full width. Smearing it with a finger works - sort of. Better to use a small piece of lexan riding the edge at a small angle will spread evenly and let you see as you spread.


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## Newbie17 (Feb 20, 2017)

I LOVE squeezable clamps! I don't use them for holding mating boards together though (except small molding pieces and decorative additions). They are useful for a number of purposes in my shop. I only make large furniture, so a lot of clamps are usually necessary.

Sometimes a quick clamp is the only way to clamp something while working by myself since they are light weight and can apply pressure quickly.

Having said that, I have 4 or 5 times as many parallel clamps. If I were to say a type of clamp I hate, it would have to be pipe clamps. I finally gave up on them and bought longer parallel clamps.

Switching gears to the glue: the only failed glue joints I've had is with the Festool Domino system. Sometimes I can pull the Dominos out with pliers. Not sure what the issue could be, but after switching to dowels, I don't have problems anymore. The DowelMax system is awesome.


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## drsurfrat (Aug 17, 2020)

I added some notes to the original post


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## stevepeterson (Dec 17, 2009)

> "I came across a statement that most glues need about 250 psi for good adhesion, too much more will starve the joint. and too little will not take advantage of thin wall stress conditions."
> 
> Source, please?


Here is a snippet from the Titebond web site at http://www.titebond.com/product/glues/e8d40b45-0ab3-49f7-8a9c-b53970f736af
REQUIRED CLAMPING PRESSURE:
Enough to bring joints tightly together (generally, 100-150 psi for softwoods, 125-175 psi for medium woods and 175-250 psi for hardwoods)

What they seem to ignore is that simply gluing wood together with no clamps produces a fairly strong glue joint. It is not a scientific test, but a simple estimate that 1psi probably produces a glue joint that is between 25% to 50% of the "required clamping pressure" strength. Any reasonable pressure will produce something closer to the max.

The "required clamping pressure" statement might be better stated as "ideal clamping pressure for maximum strength". The glue strength does not drop to zero even if you are significantly outside the "required" range.


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## bigblockyeti (Sep 9, 2013)

This has to be making some massive assumptions about the roughness average and porosity of the wood being glued. I've just hand squeezed some smaller, very smooth blocks together to generate an initial tight joint then left it with nothing trying to motivate the joint apart, the wood has always fails before the glued joint with proper preparation. I do believe it may be slightly stronger if clamped to significant pressure but I would bet only by a few percent.


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## jdh122 (Sep 8, 2010)

I agree that squeeze clamps are not strong enough for edge gluing wood, with one exception. The aluminum ones are pretty impressive clamps (I have two at 1 foot long, really work well). I bought mine at Princess Auto (Canadian equivalent to Harbour Freight, they sell a lot of junky tools, but these are identical to the ones sold by Lee Valley, and very stout):

https://www.leevalley.com/en-ca/shop/tools/hand-tools/clamps/bar/60584-aluminum-squeezer-spreader-clamp?item=17F9124

https://www.princessauto.com/en/12-in-aluminum-adjustable-bar-clamp-spreader/product/PA0008344277


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## Foghorn (Jan 30, 2020)

Plenty of edge and other joints put together with "rub joints" and no clamps that are plenty strong without any clamping pressure. I like my clamps but sometimes a rub joint is the easiest way to do something. https://www.wwgoa.com/video/gluing-wood-using-a-rub-joint-004312/#:~:text=When%20gluing%20wood%20together%20a%20rub%20joint%20can,strong%20enough%20to%20hold%20even%20under%20severe%20force.


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## therealSteveN (Oct 29, 2016)

> "I came across a statement that most glues need about 250 psi for good adhesion, too much more will starve the joint. and too little will not take advantage of thin wall stress conditions."
> 
> Source, please?
> 
> ...


In this case my source for you is Wood magazine, but it's after an interview of Dale Zimmerman of Franklin International, maker of Titebond. He recommends 100 to 150 pounds per square inch (psi) for clamping softwoods and 175-250 psi for hardwoods.

SOURCE: https://www.woodmagazine.com/woodworking-tips/techniques/skills/take-it-easy-with-clamping-pressure

OR

You can call or write Franklin, and talk to a scientist, or just one of the product guys for very specific info. They love it when people reach out with questions.

Here is a form to ask directly. They call it the Wood Adhesives Bonding Questionnaire. Give them as complete a question as you can, and be ready for a really complete answer.

http://www.franklinadhesivesandpolymers.com/Wood-Adhesives-US/Wood-Adhesives/wood-adhesives-bonding-questionnaire

I figure they make it, they should be the best at telling about what works best.


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## therealSteveN (Oct 29, 2016)

> Plenty of edge and other joints put together with "rub joints" and no clamps that are plenty strong without any clamping pressure. I like my clamps but sometimes a rub joint is the easiest way to do something. https://www.wwgoa.com/video/gluing-wood-using-a-rub-joint-004312/#:~:text=When%20gluing%20wood%20together%20a%20rub%20joint%20can,strong%20enough%20to%20hold%20even%20under%20severe%20force.
> 
> - Foghorn


I love rub joints. Most of todays glues have bonding molecules that give them incredible tension, which essentially makes them stick to themselves. I think EEngineer's glue, and nailer is probably the most used clamping program in use. Thanks Norm….. Set it in the right direction, and watch it work for you. Add a clamp and it's almost no fail, provided you apply it as directed. Wait, we are American's, who reads directions…......


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## RichT (Oct 14, 2016)

Here we go again…You cannot do a rub joint with PVA glue. Yes, you can rub the two pieces together to spread the glue, but that's not a rub joint. A rub joint is done with protein glue.


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## AlaskaGuy (Jan 29, 2012)

In 45 years I don't ever recall to having a glue joint failure. Therefore, I pay little attention to all the hocus-pocus gobbledygook scientific and non-scientific writings about the subject. I just put the glue on with good coverage and clamp them with what I call moderate pressure and move on. Works for me.


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## AlaskaGuy (Jan 29, 2012)

This is what I know about rub joints.

https://www.urbandictionary.com/define.php?term=rub%20joints


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## becikeja (Sep 12, 2010)

These types of posts drive me nuts. I get lost in the details and then start thinking about them to the point I can't even enjoy what I started woodworking for, a creative escape. In 30 plus years of woodworking I've never worried too much about clamping pressure or even how many clamps I use. I put a couple of clamps on till it seems tight and move on. I see these projects with 9 million clamps on a 2 foot board, I would use maybe 4-6 tops and tighten till they felt good….. I have to admit, I have had a couple of failed joints, maybe 0.1% but nothing that could not be easily corrected. So what is my point? Don't over think it…...........


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## drsurfrat (Aug 17, 2020)

I'm sorry if you are disturbed by the post enough to comment. But it is just what the title says - curiosity. We all know that a few clamps on a glue up works just fine.

On the same note, did anyone see what their value of their "moderate pressure" is with a torque wrench? I'm still curious.


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## Foghorn (Jan 30, 2020)

> Here we go again…You cannot do a rub joint with PVA glue. Yes, you can rub the two pieces together to spread the glue, but that s not a rub joint. A rub joint is done with protein glue.
> 
> - Rich


I agree that rub joints started many years ago using protein glues long before the advent of modern PVAs etc. Rub joints do work though with PVAs certainly in my experience and many others as per the link to the Guild of American Woodworkers I posted. It was an example I gave to throw a little fly in the ointment into PSI requirements for clamping. I use and prefer/trust clamps the majority of the time. I also use my go bar deck for some tricky glue ups not just for instruments. The clamping pressure would be much less than 250 or even 100 psi on small pieces.


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## AlanWS (Aug 28, 2008)

While it's true glued boards will usually stick with less than the recommended clamping pressure, there are advantages to proper clamping. The idea is to thin out the glue line, which makes it much less visible, and can be stronger if it's consistent. Incidentally, a rub joint does the same thing to the viscous glue with less pressure.

Look at your glue lines. If you can see anything other than the grain discontinuity, you might want to try clamping harder to see whether that makes a difference. Clamping can affect strength as well.


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## Foghorn (Jan 30, 2020)

> While it s true glued boards will usually stick with less than the recommended clamping pressure, there are advantages to proper clamping. The idea is to thin out the glue line, which makes it much less visible, and can be stronger if it s consistent. Incidentally, a rub joint does the same thing to the viscous glue with less pressure.
> 
> Look at your glue lines. If you can see anything other than the grain discontinuity, you might want to try clamping harder to see whether that makes a difference. Clamping can affect strength as well.
> 
> - AlanWS


Very true. The quality of your joinery makes a big difference as well. If you "candle" your joints and can see any light at all, then a rub joint will be less than perfect. Clamps can make up for some inconsistencies. Some even purposely do spring joints.


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## drsurfrat (Aug 17, 2020)

Thank you all, I am learning more and more as i read…


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