October 13, 2001 at Woodworker’s Supply

Mortise and tenon is one of the oldest methods for joining wood at right angles. It continues to be a superior choice, because the large surface area of the joint spreads the loads and stresses across a maximum amount of wood fiber, and provides a large side grain to side grain gluing surface.

A mortise and tenon joint resists racking and twisting forces effectively, as long as the tenon isn't pulled out of the mortise. Windsor chairs solve this problem by placing the joints in compression. In most other furniture, the mortise and tenon is held together by glue, wedges, tusks or pins.

The key liability of the mortise and tenon design, is that it always involves cross-grain construction. Wide tenons, like those in the side boards of bed frames, will often fail due to wood movement. This expansion and contraction may partially break the glueline, as usually happens with round tenons. On wide rectangular tenons, the glue usually holds, and the tenon's wood fibers are compressed as the wood expands. Later, when the wood shrinks, the wood will fail in tension, and crack along the grain. Tenons wider than 4" need to address wood movement by being split into multiple smaller tenons and/or allowing part of the tenon to slide within the mortise.

The mortise and tenon joint comprises many forms and variations. Tenons may be round, rectangular or square. Mortises may be through or blind. Tenons can be wedged, tusked, pinned, haunched, split or loose. They may have shoulders cut on zero, one, two, three or on all four sides. A mortise may have only three sides, forming a bridal joint. The chosen mortise and tenon form must harmonize with the design, both structurally and visually.

The standard rule for mortise and tenon layout is to make the tenon and each side of the mortise 1/3 of the total joint thickness. However, old, broken furniture reveals many broken tenons, and few broken mortises. Increasing the relative width of the tenon may make the joint stronger, up to the point that the sides of the tenon begin to fail. My experiments suggest that the strongest joint will have a tenon between 1/3 and 1/2 of the total joint thickness. In most designs, the mortise and tenon joint will be strong enough, that this is not a concern. Where joint members are small, and forces great, as in many chair designs, it is worth experimenting with wider tenon proportions.

Books tell us to cut mortises and tenons with the same face of the member always against the fence, so that minor variations in stock thickness will not affect tenon or mortise width. This can work, but it leads to multiple setups and possible errors in setting and resetting fence positions. I recommend milling the stock precisely, so that a tenon can be milled registering on all four faces of a tenon member. At the workshop, I cut a tenon in just a few seconds, using a simple, shop-made jig, and a single fence position. I use this method on both the router table and the tablesaw, when the tenon has the same width shoulders on all four sides.

Your can cut either the mortise or the tenon first. The other half of the joint is cut to fit the first, and requires a micro-adjustable cutting method. I usually cut my tenons first on the table saw, and then cut precise mortises with a hand-held router, and a shop-made jig. Routers make smooth, precise cuts, but they aren't a great tool for hogging out a lot of wood, nor for extracting chips from deep mortises. I recommend removing most of the wood from a mortise using a drillpress, before routing. Particularly for deeper mortises or harder woods, pre-drilling is faster and puts less wear on your tools. Several members suggested that a sharp chisel can quickly and accurately fine-tune the fit of a mortise and tenon joint.