I realized that I’m getting ahead of myself with some of this beautiful Japanese joinery I’ve been practicing. I skipped over the wedging of the tenon for Joint No. 1 and really missed out on picking up some important details. Namely, paring a taper on the outside of the mortise such that the wedge truly locks the tenon in the mortise. Having planned to use this wedged tenon in my shoji garden gates I’m currently working on, I figured a practice try was in order.
This is a shoji stile, 1-1/8″x1-3/16″, with the back taper on the mortise 1/8″ wider at either end than the tenon. This puts the wedge taper at a 1:5 ratio, quite obtuse by most measures. I figured if I could get away with this heavy of a wedge, I could comfortably relax the taper ratio for my garden gates and not worry about splitting the tenons when driving the wedges. My reference for the proportions was Jay Van Arsdale’s book on shoji making. He describes how the back taper extends about half way down the depth of the mortise, and the cuts in the tenon for the wedge are made at a diagonal. This was news to me, even having read this book…
I thought tenon wedging was done with a wide kerf saw to make the cut for the wedge in the tenon, no back taper, and wedges with very little taper. It hurts to be wrong, but the time to learn is now before I produce a joint that lacks strength and longevity.
I’m used to tapering in the sides of my blind mortises so that the tenon is compressed as it is driven together with the mortise. Toshio Odate, in his book “Making Shoji”, describes the shape of tategu through mortise as an hour glass, pinching the tenon in the middle. He mentions nothing about flaring the outside of the mortise wider than the tenon, but perhaps there is a middle road. This picture isn’t representative of the taper I pared in the end grain. In general, I’m trying to taper the mortise in so that the middle is about 1mm narrower for softwoods. I figure compressing the tenon at the point that it most wants to split when the wedges are driven will go a long way to keeping the joint together. If I was to cut the end grain to a barrel shape the tenon would have room to split.
Can you see the saw kerf for the wedges in the tenon? They were cut with a very fine saw starting towards the outside edge of the tenon and leaning inward. I’ve always seen these cuts made straight down the tenon, but I imagine this diagonal cut lowers the chance of a split running out from the tenon, as well as allowing more flexibility at the tip of the tenon where it needs to move over the most. I should have made the wedges about a centimeter longer than needed and cut off a bit of the sharp point on the wedge, but this still worked out.
Look at how much room there is! This is the sweating moment, when you wonder if wood really is this flexible of a material (and I’m a bowyer).
Success! One of the wedges needed to be driven a little deeper than the other necessitating the use of a nail set to drive it down, but it doesn’t show much after planing. I feel like the back taper on the mortise was more than needed, but it comforts me to see just how much the wood will move over without splitting. The compression at the middle of the mortise was definitely a good idea.
And now in practice, using a small sliding bevel to gauge the back taper of the mortises for my garden gates. I used 1/8″ more width either side for these mortises as well, but the stiles are 2-1/2″ wide, making a taper ratio 1:10, half of the previous example. Screw the glue, wood is good.