If you removed the glue from the joints, most woodworking projects would be reduced to a jumbled pile of parts. Even the best-planned and well-executed joinery wouldn’t be worth a nickel without the contribution of the glue.
However, when gluing up a joint, it’s pretty easy to take a good end result for granted. The goal is simple — to get the best return on the time spent laying out and cutting a joint. But there’s more to producing a strong glue joint than you might think. And to ensure that the glue does its job, it’s helpful to have a basic understanding of a few simple gluing fundamentals.
HOW GLUE WORKS. Every woodworker knows from experience that glue makes one piece of wood adhere tightly to another. But not many could tell you how. The truth is, the way glue works isn’t completely understood. But there’s a broad consensus that two different mechanisms are responsible.
Although most woodworking is done with yellow polyvinyl acetate (PVA) glue, most glues work in essentially the same way. The glue creates both a mechanical and a chemical bond, as illustrated in the drawing above.
A mechanical lock is formed when the glue finds its way into the pores and around the fibers of the wood and then dries hard. Like a nail or screw, it forms a physical bridge between the two pieces of wood, holding them in contact.
But this mechanical connection is actually the weaker force. A stronger bond is formed by a chemical or molecular attraction between the glue and wood. The glue between the two pieces of wood essentially acts like a very powerful magnet to bind them tightly together.
THIN GLUE LINE. For this chemical attraction to be most effective, the two pieces of wood need to be in very close contact and the glue line should be very thin. You won’t achieve a strong bond across a wide gap in a joint even if there’s glue filling it. The excess glue will simply act as a weak link to diminish the long-term strength .
THE THREE ESSENTIALS. So the next s question is, what do I need to do to make sure the glue gives me the strongest possible bond? Basically, there are three aspects to successfully gluing up a joint. And they’re pretty easy to grasp.
A GOOD FIT. First, the joint needs to fit properly. The mating surfaces should be smooth, flat and make good contact. As I described, an extra dose of glue won’t make up for sloppy joinery. And on the other hand, a joint that fits too tight may not have space to establish a good glue line. “Hand pressure snug” is a good rule of thumb.
WHERE AND HOW MUCH? Second, you need to add the right amount of glue to the joint and put it where it will do the most good. Use too little glue and you’ll risk a “starved” joint — too much, and the joint may not close up tightly. The goal is to end up with a continuous layer of glue between the mating pieces. So the glue should be spread in a way that ensures even coverage across the joint.
CLAMP IT RIGHT. The final factor is proper clamping pressure. Clamping a joint serves more than one purpose. Obviously, the clamps close the joint and hold the surfaces in contact until the glue dries.
But just as important, the pressure applied by the clamps creates a thin glue line and also helps the glue penetrate and “wet” the wood. So you need to think about both the number of clamps you use and where you place them. As a general rule, when gluing a joint, the more clamps, the better.
SOME PRACTICAL EXAMPLES. So how does all this relate to gluing up a particular joint? Well, each type of woodworking joint is a little different. But a look at just a few common examples will give you a good general understanding of how to tackle a particular joint.
MORTISE AND TENON. When assembling a mortise and tenon, the fit of the joint and how you apply the glue are foremost. A clamp will close the joint for the best appearance, but doesn’t really apply force across the gluing surfaces.
The drawing and photo above illustrate this point. To end up with a thin, strong glue line, the tenon has to fit snugly, but not too tightly, in the mortise. The glue won’t form a strong bond across gaps between the mortise and tenon.
And you’ll notice in the photo that glue is spread on both the end of the tenon and at the top of the mortise. As the tenon is inserted into the mortise, the glue in the mortise is forced down while the glue on the tenon is forced back, resulting in full, even coverage.
AN EDGE JOINT. The strength of an edge joint is entirely reliant on the action of the glue. So the fit of the joint, how you apply the glue, and clamping pressure all play a pretty important role.
The edges to be glued should be smooth and straight so that you have good wood-to-wood contact all along the joint. A generous bead of glue should be applied to one edge and then spread evenly over the surface (main photo on the opposite page).
Finally, use closely spaced clamps to ensure overlapping “cones” of pressure along the joint, as in the lower left drawing and photo. This last step is a big key to creating the thin, “invisible” glue line that yields a strong joint.
A LAP JOINT. Thanks to an abundance of gluing surface, a simple lap joint can be very strong. But here again, gluing the joint has slightly different considerations.
In order to make the most of a lap joint’s gluing strength, you need to start with smooth, flat surfaces. Ridges, grooves, and unevenness will prevent good contact and greatly weaken the joint.
To assemble the joint, you’ll simply lay one half over the other. So unlike a mortise and tenon, you can’t rely on a “sliding” action to produce a continuous layer of glue. Instead, this is done by spreading glue liberally over one face of the joint, as in the photo below.
As with a mortise and tenon, applying clamps along the length of the pieces will pull the shoulders tight, but this won’t provide pressure across the gluing surfaces. To do this, a clamp should be tightened across the two halves of the joint, as shown in the drawing below.
What all this comes down to is keeping the basic goals in mind and then tailoring each particular glueup to achieve them. If you do this, you really will be able to take strong glue joints for granted.
