The life of a table is not easy. Legs get kicked; the table gets pushed and pulled across uneven floors, leaned against, and sometimes even sat upon.
To make matters worse, the very nature of wood adds to the stress. As the tabletop shrinks and swells with seasonal changes in humidity, the movement works against the integrity of the table’s structure. All that stress is felt in the leg-to-apron joint (above), which holds a table together and gives it rigidity. When the leg-to-apron joint fails, the table falls apart.
Leg-to-apron joints must withstand three different kinds of stresses: shear, bending, and twisting. Shear stress is caused by a vertical load directly above a joint, such as when someone sits on the corner of a table. Leaning heavily on the top of a table midpoint above the apron causes the joints to undergo a bending stress that tries to lever them apart. Shoving the table sideways or bumping against a leg gives the joints a mixture of twisting forces. Also, as a tabletop that is fastened too tightly to the apron expands or shrinks, it can try to twist the joints.
The best defense against these stresses is a well-designed, tight-fitting mortise-and-tenon joint that locks apron to leg. The mortise and tenon is not only a good joint for tables, but it’s also widely used in the construction of all types of furniture, including cabinet doors and chairs.
– When deciding on the sizes of joinery components, the key is to attain a workable balance. Too large a mortise, and you risk weakening the leg; too skimpy a tenon, and you lose glue and mechanical strength. Ideally, you want the tenon to be as big as possible, with the joint located to maximize the mechanical connection. That means the shoulders on both sides of the tenon (which butt against the leg) must be substantial enough to resist bending and twisting forces.
When laying out the size and placement of tenons, a full-scale, top-view drawing will help you understand the orientation and relationship of all the parts and will help as you cut the joints.
– You want the tenon to be as thick as possible. A good rule of thumb is to size the tenon thickness a little more than one-third the thickness of the apron. While the one-third rule is a good general guide to follow, sometimes it’s better to make exceptions. If I’m building a table out of a soft-wood, such as butternut, with aprons only 3⁄4 in. thick, I make the tenons at least 5⁄16 in., maybe even 3⁄8 in. thick. Any smaller, and a sharp bump to the leg might snap the tenon right off. Because you rarely see the thickness of an apron, one good design strategy is to make it thicker – 7⁄8 in. or 1 in. will provide larger, stronger shoulders.
– When it comes to tenon length, you want to create as much strong long-grain glue surface as you can. Naturally, a longer tenon has more glue surface along its cheeks (the wide faces of the tenon) and provides more mechanical strength to the joint. As a general rule, the longer the tenon, the better, assuming the leg it mates with can accommodate it without risk of damage. A tenon that’s three to four times longer than its thickness is quite adequate.
– When designing a mortise-and-tenon joint, look to create shoulders that are as large as possible and to have a shoulder on each side of the tenon (rather than one side only). Such a design is better suited to resisting bending stresses from either direction. The shoulders have the added benefit of covering any bruised edges on the mortise that result from cutting the joint.
– One engineering principle states that the stress on any part is least along the centerline or neutral axis. A centered mortise and tenon is strong because it leaves plenty of wood on both walls (sides) of the mortise and creates substantial shoulders on the tenons. But a centered mortise and tenon does not always work visually.
Deciding on the exact placement is a judgment call that varies with each project. The farther to the outside of the leg you position a mortise, the longer the respective tenon will be. Too far out, though, and the mortise wall will be more vulnerable to splitting under stress. To maximize the tenon length in such situations, I often design the joint so that the tenons meet inside the leg. I have butted tenons together, but doing so makes one tenon shorter than the other. I prefer to miter the tenons within the joint (see drawing, left). This is easy to do, and it can add 15% to 20% more glue surface and length to the tenons.
– A tenon the full height of the apron affords lots of glue surface and strength against bending and twisting forces. But there’s a trade-off: A full-height mortise weakens the leg, especially if there are two mortises at the corner of the leg, and serious stress on the apron can more easily split the top of the leg.
A simple solution is to shorten the tenon at the top by 3⁄4 in. to 1 in. or so and cut an angled haunch near the top shoulder.
This design leaves material at the top of the mortise, which makes for a stronger joint.
I cut the haunch with a dovetail saw and then clean it up with a chisel (see photos, facing page). For speed and accuracy, I mark out the haunch on the tenon using a template. After cutting the mortise for the tenon, I use another angled template to guide the chisel as I cut the haunched area.
Because I cut many of my mortises with a router bit, I keep the top of the mortise below the haunch round for a small measure of added strength. Also, a small 1⁄ 8-in. shoulder at the bottom of the apron tenon will hide any small inaccuracies in cutting the mortise, and it allows for vertical alignment when the table is assembled.
– The best design and the strongest glue won’t overcome a joint with carelessly fit shoulders or a sloppy fit between tenon and mortise. Even when I cut these joints with accurate machine setups, I still often find it necessary to improve the fit with a few passes of a shoulder plane or a chisel. I want the shoulders to fit tightly over their entire surface and the tenon to slide into place with a minimum of force for a good glue bond.
Part of the long-term strength of the joint is the snugness of the fit, or what I call its mechanical strength. Glue adds strength, but how long does a glue bond last? By its very nature, a mortise-and-tenon joint has wood fibers running cross-grain to one another, which weakens the bond. Flexible modern glues can accommodate some of this movement.
Before gluing, I always dry-fit and clamp the parts together to discover any problems that may arise while there’s still time to solve them. To ease assembly, I chamfer the ends of each tenon. Glue-ups can be stressful, but it is worth taking care to place the glue so as to avoid drips and oozing joints that would be a headache to clean up later. With a thin stick about half the width of an ice-cream stick, I apply a light amount of glue into the mortise and on both tenon cheeks. The flat edge of the stick is perfect to squeeze out the glue in a thin, even layer. Another trick that works well is to cut a light chamfer around the mortise to contain any squeeze-out. Ideally, the joint should slip together under light clamping pressure.
For large tables and for peace of mind, I often pin the leg-to-apron joints. I use a hard, straight-grained wood such as rose-wood, ebony, or maple for the pins. A contrasting wood can add a pleasing visual detail, and two small pins are stronger than one large one. Most often, I drill holes for the pins after gluing and drive them in either from the outside or inside of the leg, depending on whether or not I want them to show.