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.