Demystifying a classic connection
The poor misunderstood twin tenon joint. The Internet seems fraught with dubious ways of cutting it and impractical uses for it. So what is it good for, and how do you cut the tricky thing? Well, traditionally, the most common use of a twin tenon joint is to connect drawer rails “on the flat” to table legs or cabinet stiles—the form I’ll focus on here, and the one I used in the side table on page 32.
Typically, the glue-strength of any joint depends upon sufficient intimate contact between its adjoining long-grain surfaces. In the case of a connection like this, a single tenon would provide minimal contact. So the main purpose of a twin-tenon joint is to double the long-grain glue surface. But the joint can be tricky to cut well. Unlike a single tenon, which can be sawn a bit fat, and then trimmed to a perfect fit with a shoulder plane, the inner cheeks here can’t be planed, and are hard to chisel perfectly.
After trying many methods, I’ve found that using spacers consistently yields a perfect or near-perfect joint every time. I’ll show you how to make the most common form of a twin tenon joint to demonstrate the system. You can then apply it to other configurations using the appropriate spacers. In addition to a hollow chisel mortiser, all you need is a tenon jig and a good quality dado head for your table saw.
Spacers and mortises first
For our example joint here, mill a piece of 1-1/2 × 1-1/2" leg stock, and two or three pieces of 3/4 × 1-1/4" rail stock. Set your dado head for a 1/4"-wide cut, and saw a groove about 1/2" deep in a short piece of dressed scrap. Check to make sure this groove matches the cut from your 1/4" hollow chisel. Leave the dado head installed on your saw until you finish the process. Carefully prepare a 48"-long strip of approximately 1-1/2"-wide spacer material for a tight fit in the 1/4"-wide groove. Saw off any planer snipe at the ends, then crosscut the strip into four approximately equal lengths. Finally, lay out your mortises, and cut them with a hollow-chisel mortiser as shown.
Spaced-out mortising. After laying out your mortises on the sample leg, cut them with a hollow-chisel mortiser, sandwiching two spacers between the work and fence for the innermost mortise (above), Then remove the spacers to cut the second mortise.
A jig, spacers, and dado head does the tenon trick
Set a marking gauge to the length of your tenon, and scribe the tenon shoulders. Then mark out the first cheek cutline as shown. Using a tenon jig and your previous dado head configuration, set up for the first cheek cut. Make this first cut and check it as shown. After cutting all of the rails at this setting, remove two spacers and saw all the center notches. Then make the third cut with no spacers. Finish up at the saw by cutting the narrow tenon cheeks as shown. Now fit the joint, which may or may not require just a bit of finessing.
First cut. After raising your dado head to tenon-length height, mount your test piece on your tenon jig against all four spacers. Locate the set-up as shown to saw the innermost tenon cheek, then make the cut.
First cut check. Presss the test piece against the leg’s innermost face to check the cut’s position. If necessary, readjust the rip fence and recut until the inside rail edge is flush with the innermost mortise wall. Then cut all your rail ends at this setting.
Final cut. Reposition the jig to saw the narrow tenon cheeks, flipping the piece between cuts to create a 1⁄16"-wide shoulder across both wide faces of the rail.
A Routed Approach
Spacers can also be used with a router to cut the mortises for a loose-tenon version of this joint. For a detailed look at this method, see our Counter-Height Stool story in issue 95, June/July 2020.
