How NOT to Make Joints
Woodworking could be described as the science – or art- of making joints in lumber. Granted, it’s an oversimplification, but as oversimplifications go, it’s a pretty useful one. When you’re talking about a joint in woodworking, you’re referring to any connection between two pieces of wood. That’s it.
Of course, there are lots of different types of joints, but the basic idea is the same for all of them. Everything a woodworker does is a prelude to connecting those chunks of lumber. That doesn’t mean it’s always easy, of course. Making a joint is simple; making a good joint is a different thing altogether.
A Classic Bad Joint
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The worst joint in woodworking is the nailed butt joint in a right angle. Even a glued right angle butt joint is stronger than a nailed one, due the nature of wood. Find a piece of lumber and study it for a moment. The wood grain is a lot like very small, very compact tubes running through the wood. When you drive a nail into these ‘tubes’ there’s very little friction to hold the nail in place. Thus, when you make a butt joint with nails, the nails slip out very easily, resulting in a weak, wobbly joint.
A glued right angle butt joint is a little stronger and stiffer, since the glue covers the entire surface of the joint, rather than the single points that nails cover, but it’s still very weak. This is, again, due to the nature of wood. Even glue can’t get a good grip in end grain.
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A mitered right angle butt joint is stronger than a normal butt joint, as long as it is glued. The extra surface area of the angled butt allows for more glue, providing more strength.
Unfortunately, if you don’t glue it, the mitered joint will be weaker, due to the reduced amount of wood gripping each screw or nail. Also, the mitered angle will change the way the stress acts on the screw or nail, making it more likely to split the wood.
The Problem With Nails
Nails are the fastest and easiest way to make a joint. However, they are also the weakest and least reliable. This doesn’t mean that nails don’t have a place in the woodshop; it means that you should be aware of their limitations. You wouldn’t use white glue in a project you expect will get wet, would you? Similarly, you don’t want to use nails in certain situations.
Nails have three limitations as far as woodworkers are concerned: they rely on friction, they create single-point connections, and they are prone to rust.
Since they rely on friction, they can be pulled out. In some cases, this might be a good thing. Unfortunately, they can be pulled out by a lot of things, the most common being simple stress on the joint. Like in the right angle butt joint, any stress on the joint will pull the nails out slightly, eventually leading to a loose, wobbly joint. Worse, loose joints that have been nailed are difficult to permanently repair, since the nails have to either be re-driven or removed and either method is likely to damage the wood.
Since nails are thin pins of steel, they are ‘single-point connections’. That is, the only place where there is actually a joint is where the nail itself is. For example, if you use one nail to hold two boards together, they will be able spin around the nail. This is easily fixed by adding a second nail, but that simply adds a second single-point connection. Obviously, the more connections you have, the better, but you can only use so many nails for each joint.
The last problem, rust, is usually not too bad. However, you do need to keep it in mind if your project might get wet or if you live in an exceptionally humid climate. Even water-based paints might cause nails to corrode, if the nails aren’t stainless steel. Rusted nails are very bad for lumber. Best case scenario, they leave an unsightly red stain in your wood. Worst case, they act as a passageway for moisture into the wood, creating a decayed spot.
Screws are better than nails, because their threads prevent them from slipping and they make a tighter initial joint. Under enough stress, a screwed joint will become wobbly and loose, but it takes more than a nailed joint. Screws are usually made of higher quality metal, as well, so rust isn’t as much a problem.
The single-point connection is still a problem with screws, though they are somewhat better than nails. Since the screw can be tightened far more than a nail, the wood on wood friction is increased. This ability to be tightened also means a loose joint can be easily fixed by giving the screws a few turns.
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One type of joint you’ll often see made with screws is a pocket joint.
Do your best to avoid this type of joint. The drilled ‘pocket’ hides the screw, but it drastically weakens the board, making it prone to splitting. It’s a poor joint, in any case, since it relies on the weak end grain for its strength.
Pins and Pegs
Pegs and wooden pins are a vast improvement on nails or screws, while still being fairly easy to use. Since they’re made of wood, they can be glued to wood, making a very strong bond. Even a butt joint can be made quite strong with pegs, providing plenty of good quality glue is used. Pegs really shine when you’re joining boards along their length.
This sort of joinery is somewhat more difficult, since it requires very precise positioning of the pegs. However, once the joint is made, with the butt itself glued as well as the pegs, it is very sturdy and durable. The glued butt gives a great deal of shear strength, while the glued pegs provide strength across the grain and help resist bending stress.
Basics of a Good Joint
The rule of thumb for a joint is ‘more surface area is better’. If you’re using screws or nails, this doesn’t apply, but goes for any glued joint. The more surface area you have, the more glue you’ll have on the surface, allowing for more strength.
The best quality of glue is its ‘shear strength’. This means its hold is strongest when holding two pieces from sliding across each other. This is why dovetail joints, box joints, mortise joints and other similar joints are the strongest types for gluing. They are constructed so that almost any force applied will be a shear stress, allowing the glue to work at its best.
Even if you’re planning to use screws or nails, backing them up with glue is always a good idea. However, regardless of gluing, high surface area joints tend to be better than low surface area joints. A nailed dovetail joint will be stronger than a nailed butt joint. In fact, a good friction-fit dovetail is usually stronger than a nailed butt joint.
A good joint uses the strengths of the wood to neutralize or even utilize the wood’s weaknesses, making tough, long-lasting workpiece.