The mortise and tenon joint is another one of the strongest and most appealing woodwork joints able to be made because of its flush fitting design.
Like the dovetail joint this woodwork joint can be difficult to properly construct but it is incredibly strong and aesthetically pleasing if constructed well.
This woodwork joint consist of a tongue that is secured into a slot and it is used in areas such as table legs. For structural areas like these the joint must be tight fitting to ensure maximum strength but also to achieve a neat flush look.
There are slightly different versions of this woodwork joint which include double tenons, twin tenons and haunched mortises and tenons.
The haunched version of this joint consist of an extra piece of wood that is half the depth of the mortise and it is mostly used at the end of the timber to prevent twisting.
The double and twin tenons are exactly as they sound being two tenons next to each other which create an even stronger joint. The double tenon is made of two separate tenons while the twin tenon is two tenons which are joined in the middle.
Areas where the mortise and tenon joint or some form of it are most effective and best suited are areas which are required to support a large amount of weight or are structurally important such as the rails and legs of a table or chair.
The tenons should be made to the correct length and thickness depending on how much weight will need to be supported.
For example if the joint is for a large dining table you will need a larger and thicker tenon or even a double or twin tenons to support the extra loads and weights the table will likely endure but also ensure the mortises are tight fitting to create the strongest and most aesthetically pleasing joint possible.
On long-legged projects, any twisting force applied to the tabletop amplifies stress on the connections between the stretchers and legs. Although the simple notch-and-screw joints used for the table provide ample strength, a double mortise-and-tenon would be an even stronger choice. It provides more physical resistance to twisting, and twice the glue surface of a single mortise-and-tenon. To cut it, you need only a spiral upcut bit for your plunge router, a tablesaw saddle jig, a couple of hand chisels—and these easy-to-follow instructions.
Sizing the joint
When you choose to use a double mortise-and-tenon, keep in mind the minimum dimensions shownhere. These dimensions create tenons at least [fraction "1" "4"]" thick with [fraction "1" "4"]" between them, at least [fraction "1" "16"]" tenon shoulders, and allow for a [fraction "1" "4"]"-thick outside wall on each mortise. Make [fraction "3" "4"]" and deeper mortises at least [fraction "5" "16"]" wide, as a [fraction "1" "4"]" router bit may break when routing that deep.
Cut the mortises first
Prepare your workpieces, along with one extra for testing the mortise setup and another for testing the tenon setup. Mount an edge guide on your router and install a spiral upcut bit to match the mortise width.
Lay out the mortises; then clamp two legs together to provide a broad surface for the router to ride on. Set the edge guide, as shown in photo, and double-faced-tape stopblocks to define both ends of the mortise. For mortises near the end of a workpiece, there may not be room for a second stopblock; in this case, simply rout to the layout line.
Rout the inside mortise in each piece, reposition the workpieces so the edges that were inside are now outside, reclamp them, and rout the second mortise in each piece. Repeat these steps to rout mortises in all of the workpieces and the test piece.
Reset the edge guide to remove the waste between the mortises in the test piece only, making one large mortise. You'll use this when dialing in the setup for the tenons in the next step. With a chisel, square up the ends of all the mortises, including the test mortise.
Tenons times two
Set the tablesaw blade height to match the depth of the tenon cheek. Measure from the far face of the blade and position the rip fence to match the length of the tenon. Make shoulder cuts on all four faces of the workpieces and test piece.
Build the saddle jig shown here.
Set the jig over the rip fence and position the fence to cut the wide cheek on the outside face of the test workpiece [Photo C].
Cut the two wide cheeks, flipping the test piece face-for-face between cuts, and check the fit of the tenon into the wide test mortise cut earlier [Photo D]. Make any adjustments needed to the fence position or blade height. When you achieve a satisfactory fit, cut the wide cheeks on the project parts.
Reset the rip fence and follow the same procedure to cut the narrow cheeks on the test tenon. Then cut them on the workpieces.
Transfer the locations of the inside cheeks from a mortised workpiece onto the test tenon [Photo E].
Set the rip fence to cut between these marks, and make cuts with each face against the saddle jig [Photo F], leaving the tenons a little too thick. Test the fit in a double mortise. Adjust the rip fence as needed to achieve a snug fit; then cut the inside cheeks on the project parts.
Use a chisel to clean up the small ridge left between the tenons [Photo G].