Hand-cut joinery is, to me, one of the most satisfying aspects of woodworking. The ability to shape two pieces of wood in such a way that they fit together in a structurally sound configuration is the heart and soul of what we do. While much attention is given to dovetails, an even more important joint is the mortise-and-tenon. This joint is part and parcel to everything that we do, and is one of the strongest joints there is. Another great thing about it is that it is, for the most part, invisible. While the pins and tails of the dovetail joint are in plain view for all to see, mortise-and-tenons are usually invisible. On a housed mortise-and-tenon, all but the line where tenon shoulder meets the mortised piece is hidden, and any slight errors there will be hidden as well. Only the line remains – and that is the source of our discussion today.
It is essential for a good-looking mortise-and-tenon that the tenon shoulders fit flush against the mortised piece on all exposed surfaces. For this to happen, the shoulder line must be parallel to the mortised piece, and the shoulder faces must be coplanar. While this is easy enough on a tablesaw, cutting the shoulders with a hand saw is a little harder. There are various tricks to make this easier, and all work well on smaller pieces. However, when the shoulders are 4” wide, as they are in my dummy stand, this becomes a little harder to pull off. The longer the line, the easier to mess it up. The following is my way of dealing with the problem.
I start off by marking the shoulders with a marking gauge. This gives a nice, clean line that matches all the way around. Then, I align a straight piece of wood with the scribe line. I used to do this by eye, but now place a wide chisel in the kerf left by the marking gauge. Kari Hultman takes this one step further and uses a plane iron. Either way, butt your piece of wood up against the blade, thus creating a vertical surface perfectly in line with the gauge kerf. Once this is done and the wood clamped down, I use it as a jig to keep the saw blade properly aligned as it starts the cut. It’s generally not necessary to keep the block in place for the whole cut, but I usually do anyway.
That’s all there is to it! While this won’t guarantee you a perfect mortise-and-tenon joint, it definitely gives you a head start with good shoulders. The rest is up to you.