A table saw is all you need to make this sturdy variation of a mortise and tenon joint.
Building Open Mortise and Tenon Joints
There are all sorts of mortise and tenon joints ... and just about as many ways to build them. But here's the only style that won't have you switching to a router, drill press or mortising machine to cut the mortise. As the name suggests, the mortise is open on two ends so you can cut it with a table saw. The interlocking parts make it a super choice for frame construction, and it offers twice the glue surface area of a traditional lap joint. Plus, the interplay of edge and end grain looks great, no matter where you use it. You'll need a tenoning jig to cut the mortise - a shop-made version such as the one shown in Photo 3 works fine - and a dado blade. Here's how to build this rugged and showy joint.
Step 1: The best approach is to cut the mortise first, then size the tenon to match it. In terms of proportions, you'll get the strongest joint if you make the width of the mortise about equal to the combined thickness of the walls on either side of it. Lay out the location and width of the mortise on the end of one workpiece. Install a dado blade in your table saw, stacked to cut a slot a little narrower than your mortise. Clamp the mortise workpiece in the tenoning jig, and adjust the jig until the dado blade aligns with one of the two mortise layout marks (see Photo 1).
Step 2: Raise your dado blade so it will cut the full depth of the mortise. The depth of the mortise needs to match the width of the tenon, so use the tenon workpiece as a gauge for setting blade height accurately (see Photo 2).
Step 3: Start the saw and make the first pass to open the mortise. This cut will create one of the two mortise walls (see Photo 3). After the first pass, flip the workpiece to the other face and make a second cut to form the other mortise wall and center the slot perfectly in your workpiece. If the blade leaves burn marks in the wood, which could compromise the glue bond, shift the jig slightly closer to the blade and make two more light passes to clean up the walls. (Here's another good reason to start with the mortise instead of the tenon!)
Step 4: With the mortise cut to size and shape, it's time to make the tenon. Install a few more chipper blades in your dado setup to create a wider cut. Raise the blade until the teeth intersect the inside wall of the mortise (see Photo 4). This establishes the depth of the tenon cheeks.
Step 5: You'll cut the tenon to shape against the miter gauge. Fasten a long auxiliary fence to your miter gauge to provide plenty of support for your tenon workpiece and a place to clamp a stopblock. A quick way to dial in the length of the tenon is by standing the mortise workpiece on end and against the miter gauge. Line up the outside edge of the workpiece with the outer edge of the blade teeth, and clamp a stop block against the other edge of the workpiece (see Photo 5). Now the tenon length will match the width of the mortised part.
Step 6: Switch to the tenon workpiece, and set it face-down against the stop block. Cut across one face, flip the workpiece, and make a second cut to create the tenon shoulders (see Photo 6). In addition to establishing the tenon length, the stop block also ensures that both of the shoulder cuts are perfectly even with one another.
Step 7: Make a series of side-by-side passes to remove the rest of the waste from both faces of the workpiece and create the tenon cheeks (see Photo 7).
Step 8: Slide the tenon into the mortise and check how the parts fit together. You should be able to seat the tenon in the mortise without resorting to a mallet - a light-friction fit is what you're after. If the parts are too loose, the strength of the glue joint will be compromised, so you'll need to cut a thicker tenon. If the parts are overly tight, you could distort the walls of the mortise or even break the joint. Relax the fit by taking a few shavings off the tenon cheeks with a shoulder or rabbeting plane (see Photo 8).
Variation: By moving the tenon in from the end of the workpiece, you can transform this joint into a bridle joint instead. It's a great option for installing intermediate rails in a frame. Cut the open mortise first, then create the bridle side of the joint using the tenon-cutting procedure explained here (see Photo 9). Notice that you'll need to cut two pairs of shoulders to set the length of the bridle cutout. Shift and re-clamp the stop block to make the shoulders, then hog away the waste in between.
There you have it: mortise and tenon joints made with one machine!