For maximum strength, try the double mortise-and-tenon joint

Long legs mean leverage that can challenge your best joints. Twin tenons trip-up twist.

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 shown here. These dimensions create tenons at least 14 " thick with 14 " between them, at least 116 " tenon shoulders, and allow for a 14 "-thick outside wall on each mortise. Make 34 " and deeper mortises at least 516 " wide, as a 14 " router bit may break when routing that deep.

Drawing of tenon and joint

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. 

Router /stopblock/two legs…
Position the bit within the layout lines of a mortise, set the edge guide, and secure stopblock(s) to fix the mortise's length.

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.

Illustration of saddle jig

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].

Tall jig thru saw
Raise the blade to cut just into the shoulder cut; then position the rip fence to cut a cheek on the outside face of the test piece.

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.

Inserting jig in test mortise
Check the tablesaw setup for the wide cheeks by testing the fit of the test tenon into the test mortise cut earlier. Work for a snug, not tight, fit.

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].

Using pencil on jig
Carefully align the outside tenon cheeks with the mortise walls; then mark the inside cheek locations on the test tenon.

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.

Jig over fence, going thru saw
Make the first cuts for the inside cheeks inside the marks; then adjust the fence to gradually widen the gap until the tenons fit in a double mortise.

Use a chisel to clean up the small ridge left between the tenons [Photo G].

“chisel away”
Because the blade doesn't cut the full height of the cheek, chisel away the ridges left between and around the tenons.

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