Academy Notes and Tips
the Shopsmith Woodworking Academy
Any experienced woodworker will tell you that top-quality, enduring furniture is not built piece-by-piece...but rather, joint-by-joint.
For over a hundred years, the Shakers at New Lebanon, New York manufactured chairs and benches of all descriptions. These were light, airy creations with stiles and rails no thicker than your thumb in many places. Yet, despite their delicate construction, hundreds - perhaps thousands - of these fine furniture pieces are still in service after decades of use.
The men and women who made this furniture understood the subtleties of wood and woodworking: The strength, durability and continued appeal of a design doesn't depend on the dimensions of the lumber so much as the cut and fit of the joints.
Apply the principles behind this joinery in your home workshop, and you can expect your own hand-built furniture to last for generations.
Just three rules
The truth is...there really aren't hundreds of different joints; only a dozen (illustrated here). Everything else is merely a variation or combination derived from these 12 joints. The rules that govern which of these basic joints you use are even fewer - just three. Whenever you have two pieces to join together, remember:
1: Allow for the movement of the wood. A seasoned board is fairly stable along the length of the grain, but it will expand and contract with changes in humidity and temperature by as much as 1/4" for every 12" of width across the grain. In a small joint, this movement may be insignificant. However, in a joint with larger pieces, swelling, shrinking or splitting could easily cause your joint to come apart. That's why, whenever possible, you should always try to align the grain of adjoining boards so they can swell and shrink in the same directions. When this isn't possible, leave room for movement or make a series of smaller joints.
There are some movements you'll want to restrict. Wood has a nasty tendency to cup and warp as it grows older.. The proper joinery will the pieces of a furniture project from distorting, while still allowing the wood to expand and contract.
2: A joint must support the anticipated load. A butt joint only has one adjoining face. As a result, it will only stand up under continual stress or a load coming from one direction. This is fine when you're joining a tabletop to a table frame, as an example...where the only anticipated load comes from whatever's on the table. However, the legs will be bumped and pushed from many different directions. In this case, a mortise-and-tenon joint with five or more adjoining faces between the legs and table frame members is much more suitable for absorbing this kind of multi-directional stress.
Remember, too, that a board is strongest along the direction of the grain. The load-bearing members of a furniture project should use the grain to evenly distribute the stress throughout the piece. Cut tenons, fingers and dovetail pins parallel to the grain of the wood; otherwise, they may snap off.
3: Wood should be glued long grain to long grain, whenever possible. Every side of every board shows either the long grain or the end grain (see Fig. 1). Boards glued end grain to end grain won't hold together under heavy loads; neither will boards glued end grain to long grain. Most woodworking joints are designed to increase the long grain to long grain gluing surface...and thus, the strength of the joint.
As with anything, there are, of course, exceptions and trade-offs - you can't expect every joint in every project to fulfill all three of these requirements. For example, a dado joint may provide no long grain to long grain contact, even though it shoulders a load from three directions and allows for wood movement better than many other joints. On the other hand, a mortise-and-tenon joint leaves no room for expansion and contraction, yet provides a healthy amount of long grain to long grain gluing surface.
Sometimes, a variation on a basic joint...or a combination of two or more joints...will help your joinery design meet an important requirement. But don't get hung-up on trying to decide among the myriad variations and permutations in the furniture design books -- at least, not at first. There's one other rule that you can call on if you need it: Simple works just as well as fancy.