# Clamping pressure question



## 60Grit (Mar 29, 2009)

So according to Titebond, the required pressure for gluing hardwoods together is between 175 and 250 psi. I am designing and fabricating some clamps for clamping a surface area of 540 sq.in. If I say that an equivalent point force (say i want a clamping pressure of 225 psi), it would be 121500 lb. Here's my question:

If my clamping area is just a fraction, say 1/2 the size, wouldn't that necessarily mean that i need a higher clamping pressure applied through the surface area of my clamp faces….does that make sense?


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## 60Grit (Mar 29, 2009)

oh…...nm, i just read what i typed and answered my own question


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## Ger21 (Oct 29, 2009)

I've always been curious about those Titebond numbers. Do you really think you need 7000 lbs to glue two 3" x 12" boards together? Really??

They also say "enough to bring joints tightly together". 10 psi will bring joints tightly together. I use Titebond Cold Press veneer glue in my vacuum bag. The absolute maximum you can ever get in a vacuum is 14.7psi. I think 10-12psi is far more common when vacuum veneering. The Cold Press Glue recommends 100-250psi, but I've used several gallons at 10 psi with no problems.

I guess what I'm saying is you don't need 225 psi to clamp boards with good fitting joints.


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## a1Jim (Aug 9, 2008)

If you needed that kind of pressure everyone's joinery except large factories would fail.


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## Ger21 (Oct 29, 2009)

I'm also curious as to how you plan on making clamps that can apply 60 tons of pressure.  Or do you only need 30 tons now??


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## CharlieM1958 (Nov 7, 2006)

I'm from the school that says "tight enough for a little squeeze out is tight enough". Too tight and you'll squeeze *too much* glue out of the joint.


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## childress (Sep 14, 2008)

It all depends on the specific gravity of the wood and grain being clamped. In hardwoods, flatsawn boards require twice as much force as quarter sawn boards. And yes Ger21 it can take that much pressure. This is to get an ideal glue joint. You can join two boards together with less pressure, but the joint won't be as strong and will have the *possibility* of failing over time.

60grit. 540 sq.in. is a huge glue joint. If you are laminating mutiple boards together, you only have to calculate one glue joint. This is because the pressure (if clamped right) is spread equally throughout the lamination. Oh, it's also impossible to squeeze out *too much* glue from a joint with hand clamps. try it.


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## Tony_S (Dec 16, 2009)

There's WAY too much 'Rocket science' flying around here! LOL!

Ive glued up, literally thousands of softwood and hardwood 'plates' to be used for stair treads, and trained many people to do so. One of the most common questions, is 'how much clamp pressure'. Basically, I tell them to apply just enough pressure to squeeze the glue out of the joint to the point that…no more glue will squeeze out. Tighten till you THINK it's tight enough…wait 5 or ten seconds and give the clamp another small turn. Did more glue squeeze out? If it did, you didn't have enough pressure. It doesn't take long to get the hang/feel of it.
I also agree with childress…with today's modern glues, it is IMPOSSIBLE to squeeze too much glue from a joint. But with that said, if you've done the millwork *properly* to begin with(one of the most critical aspects of glue up)...TOO MUCH pressure should never become an issue, because it doesn't take A LOT of pressure to close up a properly milled joint.


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## 60Grit (Mar 29, 2009)

LOL, you are right…There is a lot of rocket science going on, but this is going to be a big investment for me, so therefore, in order to make clamps that, where the clamping faces do not bow is important. I can only figure this out by knowing the forces acting in the system

-To childress

there will be multiple faces, each face having 540 sq in

Thanks for all the input guys. Will report back later today.


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## KayBee (Jul 6, 2009)

You know, I think they mean 175-250 psi applied with the clamps. It has nothing to do with the size of the project. Kinda like all those clamp tests where they crank down on a meter to see how much psi they can get from each clamp. That's my thought anyway.


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## CharlieM1958 (Nov 7, 2006)

After reading a couple of comments from guys with a lot more experience than me who disagree with my statement that too much clamp pressure can starve the joint of glue, I was prompted to do a little internet research.

Tony and Childress, I defer to your real-world experience. But if the idea of glue starvation from too much clamping pressure is an old wives tale, it is certainly one that is repeated often. I found articles in Fine Woodworking, American Woodworker, and numerous other sources that warned against squeezing too much glue out of a joint.


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## childress (Sep 14, 2008)

Well then, I guess there is no one right answer here. Tony is right when saying "if you've done the mill work properly to begin with". This is *the* most important aspect of a glue up. If you have to apply more pressure to get the joint together, then something is wrong.

Charlie, I have also read an articlein FWW that stated flatsawn maple requires 1200psi to glue and that is the least amount of pressure needed. It continued on to say that starvation can only occur with hydraulic clamps…never with hand clamps. So, it seems to depend on the author and thier experience.

Doing lamination's on a regular basis for the cutting boards I sell, I have tested this theory. Sometimes, when gluing a large maple block, I don't have enough room for all the clamps needed to achieve the optimum pressure. But I have not had any joint failures. I guess time will tell on those. Also, with smaller boards, I have gone to or above the optimum clamping pressure, and guess what. No starved joints which lead to failure. I have come to the conclusion that with todays glues and proper milling (jointing or planing the material the same day as glue up) you can follow Tony's technique.

Not trying to confuse any one with "rocket science". Just think that it's important to covey what I've learned from a technical standpoint. One of my downfalls


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## 60Grit (Mar 29, 2009)

So say i have a clamp with a surface area of 100 sq inches and apply 225 psi on a glue face of 25 sq inches, then the actual pressure on the glue face is 900 psi. That just seems way too high. That means, that if your clamps surface is a consistent 100 sq inches and applies a pressure at 225 psi then the smaller the glue face surface area, the more pressure that is going to be on that glue face area. Meaning pressure will increase on the glue face as surface area on the glue face decreases. It just seems like it has to be based on the surface area of your glue face. Kaybee, i'm glad you brought that up though. I didnt even think about that. I'm going to keep thinking about it because i'm not convinced i'm right

nice responses guys. They are greatly appreciated!


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## CharlieM1958 (Nov 7, 2006)

I go back to what Tony said about too much rocket science.

In researching the question of glue starvation, I read quite a few articles on clamping pressure. None of them got into psi to any extent. The one thing I read over and over is that there should be some squeeze-out, and it should be even. If you are getting beads in one area, but not in another, you either didn't apply the glue evenly, or you need another clamp where there is no squeeze-out. (That's how I judge if my clamps are tight enough, and I've never had a glue joint fail.)

I think we can all agree that you shouldn't need a degree in physics and a torque wrench to make a successful flue-up.


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## 60Grit (Mar 29, 2009)

you are right , ....my engineering intuition is getting the better of me. I should just over engineer the clamp so it can provide godly amounts of force, but only apply as much as needed using the technique you mentioned. I needed to know so that i could design the clamps so that the clamp faces, which stretch the entire length of the glue face will apply an even distributed force. If they cannot resist bending then my clamping force will be uneven. And i have to design the clamps in a certain budget range so i can afford to buy the wood


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## Fuzzy (Jun 25, 2007)

Here's my simple logic on it .. right or wrong.

It is pretty much accepted that you cannot glue glue to glue. If you have a piece of furniture with a failed joint, it is accepted wisdom that the old glue must be removed for this reason.

Taken a step further, the ideal glue joint is one molecule thick .. that is, the bonding molecule is in contact with both surfaces. Once glue is applied sufficiently to wet both surfaces, I don't believe it's even possible for the types of clamps most of us own to "starve" the joint .. it would take an enormous amount of pressure to squeeze that last molecule out of place. I clamp until I can't clamp anymore, UNLESS it's a smallish surface, and I fear that more pressure might disfigure the stock.


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## Kindlingmaker (Sep 29, 2008)

At 540 Sq in and you used vaccum to bond the boards together (1 psi = 2 Hg) and you pulled full sea level vaccum then you would be appling a full 8,100 lbs of force to bonding surface.


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## Kindlingmaker (Sep 29, 2008)

...oh, and of course full vaccum will not crush an uncracked egg…


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## RedShirt013 (May 17, 2008)

Why go and dispute what Titebond said. The 175 psi to 225 psi is likely the pressure required for an optimal glue joint, how you can get the absolute stronest joint regardless of its end use. What a lot of you suggesting, that you don't need that much pressure, you probably got a satisfactory glue joint, good enough for the intended purpose, but probably not the strongest it couldn've been. Lots of full size press can generate 200+ psi over the area of a large panel, so it's rather pointless to be comparing that with what you get with hand clamps unless it's over a small gluing area. If whatever pressure you use is good enough then stick with what works.

Maybe FWW should do a test of joint strength at different clamping preesure to resolve this issue

60grit, however you clamp design is, you still need to consider how the clamp is distributing the pressure over a large area. Assuming you are using multiple cylinders, directly at that point would be the highest pressure while the area between cylinders will be the lowest. And by your example, if you are pressing a 25sf piece with a 100sf area platen, the platen will deflect if any cylinder is applying force above an open space. The easist way around that is to put filler pieces in any open spaces inside the clamp, then you won't be concentrating all the clamping force on the small gluing piece.


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## christherookie (Jan 22, 2010)

If two flatsawn boards are traveling in opposite directions…


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## Ger21 (Oct 29, 2009)

If you have 100 square inches of clamping area, then to apply 225 psi, the clamp needs to be able to apply 22,500 lbs of pressure. If you applied that 22,500 lbs to 25 square inches, then yes, you'd apply 900 psi to the 25 square inches. But how do you plan on getting 22,500 lbs of force? And if you can do it, why won't the clamping force be variable, or adjustable?


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## 60Grit (Mar 29, 2009)

The 22500 is an equivalent force, which does not truly exist at one point, so that's why we use psi, so its that i need to apply an equivalent force, which is done by said clamp and all its mightiness. Does that clear things up, or am i misreading something.

I'm quite confused about what you are asking, honestly. All i'm saying is that the 225 psi recommended by titebond refers to what the recommended pressure is for the surface area of your glue face and not the recommended clamping pressure applied by the clamp. If it was of your recommended clamp pressure, then that would mean that the surface area of the glue face is arbitrary and doesnt matter, which all of you know is not the case, and that the larger the surface area of the glue face, the tighter we need our clamps.

In my example above i only had one variable, which was the surface area. Everything else, which in our case was just the clamping pressure, was constant. So if surface area of the glue face decreases, then pressure on that face would increase as surface area approached zero square inches IF the recommended clamping pressure was a constant one. This is why i'm saying that 225psi is not the recommended pressure applied by the clamp and it is related to the pressure that needs to be over your entire surface area of the glue face.

So if 225 psi refers to the pressure recommended over the 25 square inch glue face, then my clamp which has a glue face of 100 square inches only needs to apply around 5600 lbs of equivalent force which is 56 psi of pressure applied by the clamp, which is much more reasonable.

my clamps will be applying an equivalent load of 121500 pounds, which is 337 psi of pressure in order to get a 225 psi on my glue faces which have a surface area of 540 inches. My clamps total surface area is 360 square inches. Four clamps, and position as mentioned earlier is key. Now i just need to design clamps that wont deform under these types of circumstances, bowing outward in the middle, which would make pressure uneven. So yes, this is all probably very unnecessary, but like yall, you work the way that makes you happy….this is how i like to work : / I hope that came out right. ....the project is not just the wood, but also the clamps. The feedback has been great though and i agree with everyone on their own feel for technique, because that's what i do as well, this was just a case where i had no clue what i was doing and I needed clamps that could do the job. Thanks guys, led me all the way to my answer, which i am content with.

"I'm also curious as to how you plan on making clamps that can apply 60 tons of pressure.  Or do you only need 30 tons now??"

a ton is not a unit of pressure


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## 280305 (Sep 28, 2008)

I think that when they say clamp to 225 psi, they are really saying two things:
(1) Tighten each clamp to a maximum of 225 psi. I think that this means that the pressure inside the clamp faces is 225 psi or less.
(2) Use enough clamps properly distributed over the glue line.

Step (2) is just implied. I do not think that they are saying that if your clamps could only provide 225 psi, you would need one for each square inch of glue surface.

Have a look here. Notice the paragraph in bold at the bottom. Whether or not you believe what Charlie M. said about over-clamping, this makes it clear that they are talking about 225 psi at each clamp.

I am not sure how you plan to apply and evenly distribute such pressure, but you should probably look here. At some point, wood does crush. In the table, they use white oak as the standard, which crushes at 853 psi.

As people have pointed out, in reality you just learn what works.


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## 60Grit (Mar 29, 2009)

"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."

It is not clear they are talking about each clamp…If they are, they are wrong. It says the glue joint…....the area of which two pcs of wood are joined together. I just proved one post above that the 225 psi of pressure is about the glue face, not the clamping face pressure. The only way that statement is true is if the clamp face (platen) has the exact same surface area as the joints being glued surface area.


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## CharlieM1958 (Nov 7, 2006)

60Grit, help me out here.

Let's say you were able to design a clamp with a surface area of 100 sq. inches that distributed the pressure uniformly across the clamping surface.

Let's also say your glue surface was 100 sq. inches (same size and shape as the clamp jaws).

Now, if you apply a clamp pressure of 200 psi, you would also be applying 200 psi to the glue joint. Am I correct on that so far?


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## 280305 (Sep 28, 2008)

If you look to the right of that paragraph, you will see the clamping face pressures of common clamp types. This is what they are referring to when they write "Putting all your muscle into many common clamp styles generates excess pressure".

If you intend to apply such pressures as you describe, please record it on video for us


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## North40 (Oct 17, 2007)

My suggestion (for what it's worth) would be not to get too carried away. With a tight-fitting joint and a reasonable amount of pressure, the glue joint will be stronger than the wood. Maybe the joint could have been stronger, but what would be the point?

For example, take cope and stick joints which have very little face-grain to face-grain glue surface (and that surface receives little or no clamping pressure - all the clamping pressure is on the end-grain to side-grain part of the joint): when you break the joint, it's almost always the wood that breaks, not the glue joint.


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## 280305 (Sep 28, 2008)

Peter has the idea. If your stock is properly milled and your joints fit properly, you do not need to get into such details.


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## 60Grit (Mar 29, 2009)

Charlie, if the glue joint surface area and the clamp face area are the same size and same dimensions, meaning both are 10×10 or both 5×20, then yes, this is very valid.


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## CharlieM1958 (Nov 7, 2006)

Peter, I agree.

I'll cut to the chase of the point I was starting to make above: It seems to me that all the math about how many thousands of pounds of force are being applied is, at the least, misleading. The reason is that no one is taking into consideration the rigidity (or lack thereof) of the surfaces being clamped.

In other words, say for the sake of argument you were gluing together two boards which were absolutely rigid. (I know that is not possible, but follow me.) If those surfaces had no flex or compression at all, then tightening one small c-clamp to 200 psi should apply 200 psi to the entire glue joint, right? If there is no "give" the pressure of that one clamp would be equally distributed across the entire joint. Theoretically, you could apply 10 more clamps at the same 200 psi clamping pressure, and the pressure on the joint would still be 200 psi.

My point is that rigidity of the surfaces being glued plays a big part in how many clamps you need, and thus how many pounds of total force are ultimately being applied to the joint. So the math, the way I see it, is pretty much useless unless you factor rigidity into the equation.

Somebody tell me if my logic is totally wrong. I'm a liberal arts major.


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## 60Grit (Mar 29, 2009)

I agree, which is why you position of your clamps is key. That is why my clamping surface will also be over half the surface area of the glue face surface area. A clamp with platens exactly the same size as my glued surface area would be too heavy, not to mention really only applicable to that one size…I will have four clamps, each with an area of 90 square inches. My math is unquestionable, which i know may seem hard to believe. I didn't think so, but i cant explain it any better than i have : )


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## art3427 (Feb 6, 2008)

I regularly glue edge bands on with nothing more than TB2 and blue masking tape. I spread the glue evenly over the entire surfacer of one edge then hold it in place with a strip tape. I pull the tape just enough to create a squeeze out. I have also used duct tape for this. 'narry a joint has failed me in about 40 years.

art


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