Cut precise tenons with a shop-made jig and a standard saw blade.
A dado blade is probably the fastest means of cutting tenons, but it's an expensive investment. As with most woodworking techniques, there's more than one way to skin that proverbial cat, and tenons are no exception. Here's how to save yourself some money and still make first-rate tenons with just a standard saw blade and a tenoning jig you can build from scrap.
Step 1: Assemble your tenoning jig. It's simply a saddle that fits over your saw's rip fence, with a tall fence attached to it to support long workpieces vertically for cutting. Make the saddle portion of the jig about 8 to 10 inches wide. It should fit snugly around the rip fence with just enough clearance to slide smoothly along the fence without binding. Build the saddle from plywood or MDF instead of wood so it won't be subject to wood movement from season to season. Fasten a taller 10- to 12-in.-tall fence to the saddle with screws. Next, screw a vertical brace flush with the back edge of the jig fence to provide backup support for your workpieces. The brace should be at least as thick as your workpieces to help prevent blade tearout. Finally, fasten a locking clamp to the brace about midway up the jig fence. The clamp will hold workpieces securely as well as give you a convenient "handle" for pushing the jig through the cut (see Photo 1). If you don't want to buy a locking clamp, you can substitute a C-clamp or short bar clamp instead.
Step 2: Measure the depth of your mortise with a combination square, subtract 1/16 in. and mark off this distance on the faces and edges of your tenon workpiece (see Photo 2). This will establish the final position of the tenon's shoulders as well as its length. (Note: The narrow areas that surround the base of a tenon are called the shoulders.) By making the tenon slightly shorter than the mortise depth, it will seat fully and leave a little clearspace at the bottom for excess glue.
Step 3: Mark the tenon's thickness on both long edges of your workpiece using the mortise as a reference (see Photo 3). Generally, for single-tenon joints the tenon is centered on the thickness of the workpiece, and the widths of the opposite shoulders match. Check to be sure with a combination square. Extend these marks up to the shoulder lines to lay out the tenon's cheeks.
Step 4: Fasten a long scrap of wood to your miter gauge fence to serve a backup board for the workpiece during cutting. Now you're ready to begin cutting the tenon. Raise your saw blade until the tips of the teeth touch the bottom cheek layout line you drew in Step 3. Slide the rip fence over so the outside face of the blade intersects the shoulder layout line you drew in Step 2. Lock the fence. With the workpiece's end pressed against the rip fence, make the first shoulder cut (see Photo 4). Flip the part to the opposite face, and cut the second broad shoulder.
Step 5: Now that the broad shoulders are cut, your tenon probably needs a short shoulder and narrow cheek on each end. Use the mortised workpiece again as a reference for laying out the position of these cheeks. Adjust the height of the blade as necessary to meet the new cheek lines, but leave the rip fence locked in place. Tip the workpiece on edge, and make both narrow shoulder cuts (see Photo 5).
Step 6: Stand the workpiece on end, and raise the blade until it meets the shoulder kerf lines (see Photo 6). You'll want to cut away all of the broad cheek waste in one pass in the next step.
Step 7: It's finally time to put that new tenoning jig to good use! Clamp the workpiece to the jig with the tenon end resting on the saw table. Fit the jig over the rip fence, and slide the entire assembly over until the outside face of the blade meets the broad outer cheek line (see Photo 7). Lock the fence.
Step 8: Back the jig up, start the saw and feed the jig and workpiece through the blade to saw the first broad cheek. Then, shut off the saw, flip the workpiece to the opposite face, re-clamp and saw the second cheek (see Photo 8).
Step 9: Switch back to your miter gauge again to nibble away the waste and create the narrow "end" cheeks (see Photo 9). When setting up these cuts, lower the blade until its height matches the short shoulder kerf cuts you made in Step 5.
Step 10: Test the fit of the tenon in its mortise. If it takes force to push things together, the tenon is still too thick or wide. Use a rabbeting or shoulder plane, or a file, to pare the cheeks down until the tenon slides into the mortise with just a bit of effort (see Photo 10). You shouldn't need a mallet or clamp to close a properly made mortise and tenon joint.
Now that you've got a good tenoning jig, be sure to keep it handy for cutting lap joints, tongue-and-grooves and even rabbet joints. It'll help you get every bit of value out of your standard saw blade!