Clamping Pressure - Squeezing in a little understanding

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Blog entry by RobynHoodridge posted 11-26-2012 02:08 PM 2817 reads 1 time favorited 13 comments Add to Favorites Watch

First off, this is my understanding [opinion] of some aspects of clamping pressure. Emphasis on “my” and emphasis on “opinion”. Which should be obvious, this is a blog after all. But a large part of why i’m writing this is so that you can tell me if i’m wrong. There seems to be a lot of misunderstanding surrounding clamping pressure. And many times an understanding of what’s going on, rather than a set of absolute instructions, is helpful in letting folks decide and plan for themselves. So that’s kind of the idea here. I once had a botany professor who taught us the answer to everything. This secret I will now share with you. – “It depends.”
So “how much clamping pressure should i apply”, well, “it depends”. It depends what you’re clamping. It depends what adhesive you’re using. Etc. Etc. ... It depends.
And instead of knowing specific methods and numbers for every situation there could be, it’s better to consider a few things about what’s going on in a glue-up.

Glues are plastic
Whether you take this to mean what they are chemically, or that they have the physical properties of plastics (such as plasticity), it’s helpful to understand what you’re dealing with. For one thing, plastics aren’t indestructible. With enough force, you could bend almost any piece of wood to close a gap which in truth shouldn’t be there in the first place. But when you take those clamps off and take that force away, the wood wood hasn’t changed. It still wants to be in the shape it was before the glue-up, with a gap at the glue line. And whatever force it took to close that gap will be pulling at the glue line. I’ve heard many people say they’ve been heavy handed in clamping and that their glue-up failed, and that this must be due to the mythical thing called a glue-starved joint. Let’s be clear on something: There are a number of ways that a joint can wind up with not enough glue in it, but excessive clamping pressure is not one of them. Expecting a glue line to withstand immense tension when excessive clamping pressure is released however, may be expecting too much. And THAT can be a cause of delamination. This isn’t an article on preparation for glueing, so i’m not going to describe what to do. But needless to say, if the wood doesn’t need to be bent to meet the other glueing surface, and if there aren’t any high or low spots that glue needs to be squeezed to or from, then the glue-up gets a lot easier. And probably has a far greater success rate. Now since we’re talking about the strength of the plastic glue itself let’s consider the strength of a thicker glue line, since clamping pressure will often dictate this thickness. Plastic is about as strong as wood. Now there’s a statement. Obviously ‘it depends’ what plastic, and what wood, etc.. But it’s probably fair to say that you wouldn’t break a solid block of plastic (glue) much before you’d break a similar sized block of wood. So a lot of solid glue wouldn’t be a structural problem (though it would be an aesthetic one). Unfortunately it’s not as simple as that though. Some glues, most glues, occupy less space when they’ve dried or set, and having half of the filler between the two wood surfaces disappear … well that can’t be good. Now we’re no longer comparing a solid glue layer to solid wood, increasing the likelihood that the glue could fail. Just why this matters more when there’s a lot of glue than when there’s a little is too in-depth for a blog entry. But imagine you’re in a boat and half of the dam below you suddenly disappeared. That’s going to give you a lot more problems than if you were afloat in a puddle and the same thing happened. Same effect, just more of it in one case. The more clamping force the more of the ‘excess’ glue will squeeze out (squeezing the dam out to a puddle). So the more clamping force the better when all else is perfect. (But we’ve seen above that all else isn’t always perfect.) The contrasting case is glue that does not shrink (gap filling) like epoxy.

Since you’re not likely to physically break the material of modern glue, a LOT of how a glue joint performs has to do with the interface of the glue and the wood. It’s always going to be a matter of adhesion as opposed to cohesion. Cohesion is what the tree did when it built the wood, and adhesion is what we do when we try to get something sticky to bond to something else. (hence “adhesives”) Now the adhesion is greatly helped if we can get the glue intermingled with fibres of a complex, fuzzy surface, or penetrating and locking into the pores of the wood. And forcing this with increased pressure is likely an advantage of stronger clamping.

Force and Pressure
There’s a 10 pound hammer balancing on your hand, and a 10 pound knife balancing on it’s edge on your other hand. Which are you more worried about? That’s the difference between force and pressure. The objects weigh the same (have the same force on your hand), but the knife exerts that force on a much smaller surface area, at which the pressure is higher than the pressure under the hammer. Knowing as we do, that we want to get the most ‘excess’ glue out of a joint without being silly about it, we see that we want to apply enough of a force to do this. Okay, fine, but how much force IS that actually? Well it’s pointless measuring it or putting a number on it since conditions vary. For instance a thicker (more viscous) glue will take more force to move it around. And since it’s the effect we want, we can keep adding force until we get that effect. This is why people say “keep tightening down until squeeze-out stops flowing”. Now we’re very lucky that wood is a mostly rigid material. If we were applying enough force to squeeze glue from under a shopping packet we would have to apply an even force over the whole surface of the packet. For instance by laying something flat and heavy upon the packet, or by spreading sand upon the packet. In fact that’s a perfectly reasonable approach for forcing wood together. But we’re freed from the constraints of an even application of force since the wood itself will spread the point-force from a clamp out over a much wider area. And this is one reason that you’ll see absurdly high clamping pressures recommended by glue manufacturers. They assume a clamp only so often (clamps spaced apart). And whatever force that clamp provides is spread out over many time the clamp’s contact area so that the actual pressure between wood surfaces, at the glue line, is far less than the clamping pressure, but still enough for all the good effects like squeeze-out. Similarly, you don’t need to apply as much force to every square inch that you’re applying it to when you are applying it to more square inches of the pieces being pushed together. Maybe read that last sentence twice. This is why veneers or thin laminations such as in skis or skateboards can be pushed together by a large force that’s spread out over the entire surface. For instance, the pressure at any one point when vacuum bagging is being used to press parts together can reach a maximum of 14.7 psi (vs clamping pressures of between 200 and 2000 psi, depending on the clamps you use), but can work just as well.
To give you some idea of the forces we’re talking about, 14 psi would be achieved if a layer of water 33 feet (9.84 meters) thick could be laid on top of the pieces being glued. And the layer of water weighing down upon the patch the size of the clamp heads, to simulate clamps being used, would be 461 feet (140 meters) high to achieve 200 psi clamping pressure.
So the simple value of clamping pressure applied does not answer all questions about a glue-up. It’s also about how, and how long, etc..
And “how tight should my clamps be?”, well “it depends”. For instance, it depends how many clamps you use.

Wood is plastic
Okay, now i’m stretching the concept. But what I really mean is that wood is springy, elastic, and not perfectly rigid. So we use multiple clamps and distribute the necessary force between them. Fewer clamps may necessitate greater clamping pressure. This could damage the surface of the wood (white oak for example has a crushing pressure somewhere around 850 psi). Some will read into this that more clamps of lower quality are better than fewer high end ones for the same total cost. I mention that only because it highlights the idea that almost any clamp can provide clamping pressure in the region of what you need to create a good bond. And that you therefore have no excuse not to get to glueing and testing what works for you. Because the general clamping pressure we’re aiming for is usually not at any extreme. And because ‘it depends’ anyway.

Oh, hey, my glue must be set by now. Gotta go.

-- Never is longer than forever.

13 comments so far

View jap's profile


1251 posts in 1836 days

#1 posted 11-26-2012 02:46 PM


-- Joel

View casual1carpenter's profile


354 posts in 2258 days

#2 posted 11-26-2012 03:23 PM

Robyn, I too find this an interesting approach to understanding the logic path of glue joints. Most of what you stated becomes obvious with my metal trades background and exposure to materials properties/engineering, yet a light bulb flashed on with the logic of treating the wood and glue with their plastic deformation properties. I guess that I come away from reading your post with the thought that properly applied clamping pressure will not make up for inadequate joint preparation to a great extent without introducing the possibility of joint failure once the plastic deformation limits are reached.

There is a difference between hearing, reading, knowing, and understanding. Often we have heard, read, and know a thought or principle without truly understanding the thought logic path, so our subsequent application is often flawed because when the ‘it depends’ factor comes into play it is our thought process (understanding) not book knowledge which is applied when setting the clamping pressures.

View oldretiredjim's profile


206 posts in 2168 days

#3 posted 11-26-2012 03:30 PM


View Raymond Thomas's profile

Raymond Thomas

189 posts in 2001 days

#4 posted 11-26-2012 03:33 PM

Robyn, an interesting thought provoking post. I agree that we, as normal human beings, sometimes over-think a situation without fully understanding the sciences of what is actually happening when we force our will on inanimate objects. I also have to agree with casual1carpenter’s statement on “inadequate joint preparation…”; and now I have to get back to “clamping the heck” out of my current project.

-- Raymond, Charlotte, NC -------- Demonstrate the difference!

View Monte Pittman's profile

Monte Pittman

26313 posts in 2120 days

#5 posted 11-26-2012 03:54 PM

I think most use too much pressure. Probably most should use more clamps to spread the pressure. I am guilty of both.

-- Mother Nature created it, I just assemble it.

View bonobo's profile


297 posts in 1839 days

#6 posted 11-26-2012 04:20 PM

Can you squeeze all the glue out of a joint?

-- “The easy confidence with which I know another man's religion is folly teaches me to suspect that my own is also.” ― Mark Twain

View RobynHoodridge's profile


126 posts in 2112 days

#7 posted 11-26-2012 05:45 PM

Ooh, thanks for the link to that vid bonobo. And thanks go to Mathias for making it.
I wonder if the same results apply to longer (larger) joints, and different woods, etc.
His gap filling joint wasn’t under tension, but the results seem to contradict my thoughts on ‘too much’ glue.

casual1carpenter, we seem to think alike. :)

-- Never is longer than forever.

View kjel00142003's profile


11 posts in 2473 days

#8 posted 11-26-2012 08:49 PM

very thought provoking-thanks!

View shipwright's profile


7717 posts in 2580 days

#9 posted 11-28-2012 12:27 AM

To clarify, are you speaking of PVA glues only?

You can squeeze epoxy too hard and starve the joint causing failure. Common practice is to roughen the surfaces or include a bit of thin glass cloth in tight laminations to avoid it.

Hot hide glue requires virtually no clamping pressure and will render a joint as strong as “modern” (pva) glues.

Urea formaldehyde glues do require high clamping pressures.

Urea formaldehyde and hot hide glues are not “plastic” but hard crystalline drying types so maybe they are outside of the area of your discussion.

I do agree with most of what you are saying if you are referring to pva glues.

-- Paul M ..............If God wanted us to have fiberglass boats he would have given us fibreglass trees.

View RobynHoodridge's profile


126 posts in 2112 days

#10 posted 11-28-2012 04:45 AM

Hi Shipwright
Am i speaking of PVA glues only? Well, yes and no. It depends .. how technical one wants to get. I was hoping to generalize. First because getting too technical gets boring and long winded for a reader. And because (for the purposes i was talking about) all glues, even the epoxies and hide glues, are plastic (“any of a wide range of synthetic or semi-synthetic organic solids that are moldable” – wikipedia). So that generalizing is okay i feel.
For instance, one of the inspirations that got me thinking about my blog topic was the video ( which i got to through a link in your series on Hide Glue for Beginners ( Thanks for that by the way. And in the video it’s mentioned that hide glue is a polymer (not crystaline) and that it’s specifically useful when movement is going to occur after bonding, because it retains its plasticity (
However, the take-away messages from my writeup are probably more beneficial when it comes to using PVA glues.

What would you guess (or perhaps you know) is the reason that heavily squeezed epoxy glue lines fail? My guess is that, while the epoxy is a very strong and very rigid plastic, it doesn’t ‘wet’ as well as other glues (hence, as you say, roughing the surface is more important to give the epoxy something to cling to as it sets around the loosened fibres). In which case it’s probably the tension on the weaker interface from bending the wood a lot during clamping that causes the failure in a starved joint. Which would make epoxy especially relevant to my thoughts on starved joints being more about overclamping than a shortage of glue.

-- Never is longer than forever.

View shipwright's profile


7717 posts in 2580 days

#11 posted 11-28-2012 05:25 AM

Not to argue but to clarify my comments.

I’ve used epoxies in many situations for many years and have always kept the clamping pressure down on the advice of my supplier who was actually the chemist who designed it. I don’t know the exact science behind it but I do know that you are better off if there is ample glue left in the joint.

As for my remarks about hide glue, the main point I was making was that in many instances no clamps are needed at all.
The common woodworking strengths of hide glue (185 lb and up, usually 192 lb) set hard and are not “plastic” in the “mold-able, flexible” definition of the word. The animal glue that was traditionally used to minimize movement problems was fish glue which becomes plastic at warmer temperatures where marquetry elements like brass expand more than wood. Fish glue is not considered strong enough to be a viable wood glue.

I don’t think we are far apart on these things and I didn’t mean to sound critical. I mostly just wanted to put a word in for hide glue in it’s capacity as a perfect clampless glue for woodworking joints.

-- Paul M ..............If God wanted us to have fiberglass boats he would have given us fibreglass trees.

View Nicholas Hall's profile

Nicholas Hall

352 posts in 1889 days

#12 posted 12-07-2012 01:08 AM

Fine woodworking did an article on this by an engineering professor that does glue research for industry. He’s also an avid woodworker. He had an interesting perspective based on thousands of tests:

-- Outside of a dog, a book is a man's best friend. Inside of a dog, it's too dark to read. -Groucho Marx

View RobynHoodridge's profile


126 posts in 2112 days

#13 posted 12-07-2012 08:28 AM

Thanks Nicholas. Sounds very interesting.
Unfortunately, when i follow that link – “The information you requested is premium content only available to Online Members”.

-- Never is longer than forever.

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