Tenon shoulder-cutting jig
The mortise-and-tenon joint offers two major advantages: strength and invisibility, making it ideal for furnituremaking. Shaping the mating parts requires multiple setups and various cuts. Tenons alone require two basic cuts: shoulder cuts and cheek cuts. Shoulder cuts establish the length of the tenon; cheek cuts, the tenon's width and thickness. (See Drawing 1.) This project handles shoulder-cutting tasks..
Thanks in part to an adjustable stopblock, this jig gives crisp 90º shoulders quickly and accurately. Note that the jig rides in the miter slot located on the right side of the saw blade.
Referring to Drawing 2, cut all of the parts, except the stopblock, to the dimensions shown. Drill the 1/4" machine screw hole in the fence.
Attach the fence to the base with glue and countersunk screws, flushing the fence along the base's back edge. Screw this assembly to the miter gauge, ensuring it protrudes 1" or more to the right of the miter gauge. Next, set the miter gauge and assembly into the miter-gauge slot, raise the saw blade 1/4" above the jig base and cut through both the base and fence. Use the kerf as a guide to center and install the blade cover with screws and glue.
From 3/4" stock, cut a 6" blank ripped to 1 1/4" wide. With a dado blade, cut the 1/4x1 1/2" notch on the bottom edge. Now, cut the stopblock to finished length. To form the 1 1/4"-long slot used to adjust the jig for tenons of various lengths, drill 1/4" start holes, where shown, then scrollsaw between the holes. Drill a centered pilot hole in the notched end and screw a panhead adjustment screw into the hole. The notch in the stop and the adjustable screw prevent sawdust build-up from altering the location of the shoulder cut.
Now, insert a 1/4" machine screw through a washer, the fence, and the stopblock, secured with a small pull knob. Finally, install a 1/4" plate of plywood over the base, but only on the right side of the saw kerf. The raised surface prevents sawdust from getting beneath the end of your board, which would create an unwanted angle on the next shoulder cut. Further, this provides adequate space for small falloffs that potentially could bind the blade and result in kickback.
Putting the jig to work
Determine the length of your tenons. Then, slide the stop over to the desired location and tighten it in place. Raise the blade to the depth of the intended shoulders. Slide the workpiece against the stop edge and run it through the blade. Now you just rotate the workpiece to cut the remaining edge and faces, as shown in the photo. It's best to design your tenons with equal shoulders on all four sides. That way you will only have to adjust the blade height once.
Keep in mind that when cutting the shoulders, you don't need to push the jig all the way through the saw blade. Once the top of the blade reaches the fence, the shoulder cut is complete.
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