Woodworkers use clamps for many tasks, but most importantly for clamping work together while the glue sets. Some clamps, like spring clamps are fairly weak and apply about 35 lbs of force. At the other end of the scale, heavy duty bar clamps can exert up to 6800 lbs, and C-clamps can exert well over 7500 lbs.
So how much pressure should you put on a glue-up? The answer: it depends. Obviously, you don’t want to put so much pressure on that you crush the wood fibers. And scrap pieces of wood, called cauls, are very useful in distributing pressure and avoiding fiber-crushing. But how much pressure really depends on what type of glue you are using.
The most commonly used glue in woodworking is PVA, or polyvinyl acetate. Examples of PVA glues are white glue or yellow (carpenter’s) glue. These glues are entirely synthetic and bond wood very well. If glued and clamped well, the glue line will in fact be stronger than the wood itself.
(Hide glue has been the traditional glue used in woodworking and it has many benefits that can be credited for its continued use today. Other glues used in woodworking include epoxy and cyanoacrylate (Super Glue).)
To test how strong your glue joint is, try gluing to pieces of wood together, edge to edge. Once the glue has dried, break the two boards apart. If your joint is good, the wood adjacent to the glue line will fail, often along the grain, so part of one board will remain bonded to the other. If your joint was weak, the two pieces will separate along the glue line.
So how do you create a strong glue joint? Well, it starts with proper glue application. With PVAs, you really can’t use too much glue. Well, you can over-apply PVA, but you won’t create a weaker joint – you will only make more clean-up for yourself later. While it is better to have too much than too little, it isn’t hard to get the right amount of glue in the joint.
We will use, for example, gluing two boards edge-to-edge. The long grain (edge or face) of a board creates a much stronger bond than end grain. Any joint involving the end grain should be reinforced if it needs to endure any stresses. The first step in a successful glue up, as with anything, is proper preparation.
The two edges should fit well, with no light visible between them. Even though clamps are able to close some small gaps, I don’t like to rely on them for that too much. But some woodworkers do, and they in fact take advantage of this and create what is known as a sprung joint. A sprung joint is made with two slightyly concave edges, so that the ends touch, but there is a slight gap in the middle. The great benefit of this is that fewer clamps are needed to hold the joint together (as few as one!).
Once you’ve prepared the mating edges, it is wise to do a dry run. That is, to put the joint together, in clamps but without the glue. Just to make sure that everything works as it should. It also gets your clamps set to the right opening, making for a more efficient glue up. Glue ups are one of the most stressful parts of woodworking, right up there with finishing. It’s the pressure of time, knowing you only have so long before it starts to set. The more prepared you are, the smoother and the glue-up will go and the less stressed you will feel.
Run a bead of PVA down the center of the board from end to end. For 3/4″ stock, I find that a 1/8″ wide bead of glue is about right. I also start and stop the bead about 2″ from either end. For thicker stock, I run the bead in a zig-zag pattern down the edge. Then it’s time to spread the glue. For narrow surfaces, I just use my finger, running it up and down the edge looking for even glue coverage over the entire surface, from end to end. For wider surfaces, I use a the long edge of a 4″x3″ piece of 1/8″ plywood. Then do the same for the mating edge.
Put the two boards together and slide them back and forth against each other a few times. You will notice that in doing so, a sort of hydraulic lock – there will be more resistance to sliding. Put the boards in the clamps and tighten the clamps either from the center outwards or from one end to the other. It’s not good practice to clamp from the ends towards the center.
I find that PVA glues work best with a thin glue line, so I apply as much pressure as I can comfortably. Use lots of clamps. As you apply clamping pressure, watch that the boards stay properly aligned with each other. You may use either dowels or biscuits to aid in alignment, but I have gotten into the habit of gluing up before the final surfacing, so minor misalignments aren’t a big deal.
Once the clamps are applied, you should ideally see little beads of glue on the surface all along the glue line. This means that you used the right amount of glue.
Some woodworkers like to clean up the squeeze-out right away with a damp cloth, but I feel the best way is to wait for the glue to semi-harden to a rubbery consistency. Then it is easy to remove without making a mess using a chisel. Sometimes, I also wait for the glue to dry completely, then scrape it off. This works well on hard woods, but on softer wood, it tends to take pieces of wood with the glue resulting in what looks like tearout.
If you are using hide glue, your procedure will be different. Sprung joints don’t work very well because you don’t use clamps! Instead, go for perfect fit between the two mating edges. Then apply a thin layer of hide glue to both edges, just enough to cover the surface. Then let it dry. Now apply the glue to one surface as we did with the PVA. Don’t apply any more glue to the other edge. Put the two edges together and rub them together, back and forth to distribute the glue evenly along the entire joint and to create that hydraulic lock. Then set the glue up aside. No clamps, no nothing. That’s it.