Stool: Cutting an Angled Mortise and Tenon Joint

In my last post, I sawed the angle at the bottom of one of the legs of the stool. Now, I felt it was time to try one of the angled mortise-and-tenon joints with which half of the stretchers will be attached to the legs.

I again went back to my scale drawing and marked out where the mortise would go:

Unlike most of the joints I’ve made in the past, I couldn’t use one mortise gauge to mark both the mortise and the tenon. This is because a 1″ wide stretcher will meet a 1.5″ leg, so there has to be about a quarter-inch of space on either side of the stretcher so that it’s centered.

So I had to use a little bit of actual calculated measurement for this. Fans of the metric system may want to skip to the next paragraph. My mortise chisel is 3/8″, and half of that is 3/16″. The leg is 1.5″, or 24/16″, and half of that is 12/16″. That means that to center the mortise, I needed to align the near edge at 9/16″ (12/16″ – 3/16″). (Gee, I guess there was a good reason for not going with the 7/16″ chisel for this project after all.)

I used a marking gauge set at 9/16″ to scribe just one line for the mortise. From that point, it’s just like chopping a regular mortise, except at the ends. This mortise is not rectangular, but rather, a parallelogram. I used my sliding T-bevel to get the approximate angle on both ends while chopping.

The preceding image is actually a bit of a fib, because when I was doing this, my hands were switched (left on the chisel, right on the bevel), because I use the mallet with my right hand. I think you get the idea, though.

Next up was to saw the tenon. For this, I set up the mortise marking gauge. The first thing I did was to chop a mortise dead center in a 1″ width into a piece of scrap. Then I stuck the knives into the mortise and set the gauge:

Notice that this piece of scrap is roughly sawn on the face that you see here and it’s actually a little more than 1″ wide. This doesn’t matter, because the reference face of the scrap is flat, and I scribed a line at 1″ on the top, but I admit that it makes for a confusing picture.

Proceeding to the actual tenon, I went back to my scale drawing and marked out the limits of the tenon shoulder. When I had a mark in place, I used the sliding T-bevel again to mark the shoulder:

After using the mortise gauge set earlier, it looked just like almost any other tenon, except that the shoulder was at an angle. Sawing at angles turned out to be tricker than I thought, but then I remembered another tip from that book I mentioned in my last post; you can put a piece of scrap with one side relieved between the work and whatever you’re holding it against to bring it roughly square. This worked surprisingly well:

And soon, I test-fit my first angled mortise-and-tenon joint:

There’s a little gap in the joint in this photo; the test-fit revealed that I still had a little more trimming to do on the shoulder.

Three more of these plus four non-angled mortise-and-tenon joints to make, and the frame will be together, ready for its joinery to the top.

4 thoughts on “Stool: Cutting an Angled Mortise and Tenon Joint

  1. Have you thought of leaving the back shoulder of the tenon a little (a mm or two) deeper cut than the front? That may help the front shoulder pull up tight and appear seamless when you peg (are you pegging these joints?) the joint together.

    • Hi CB, that’s a pretty common trick and I usually end up doing something like it (although I wouldn’t go as far as a millimeter, you don’t need nearly that much. Most of the time, you can see if a tenon will draw tight just by using a try square on the shoulder.

      The joints aren’t pegged. I usually don’t peg. Mortise-and-tenon joints are really very strong due to all of the glue surface available.

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