I finished the chair yesterday. Somewhere along the way I drew up this template showing the sightlines I use to bore the leg mortises. I tape it right to the battens, stick a block of scrap wood under it so the adjustable bevel will sit on it and set the bevel & bore away.
It works pretty well. The auger bit chews up the oak battens some because it’s canted over pretty far – 25 degrees. Having the back’s uprights in place helps keep the legs from hitting those through tenons coming down from above. The back edge of the template is 4 3/4″ from the seat’s back edge. It worked perfectly, the legs miss the tenons by about 1/4″ or more.
Usually on the old chairs, the leg tenons come through both the battens and the seat. This cross-grain construction – the battens run perpendicular to the seat – almost guarantees that the seat will crack. Except sometimes it doesn’t. In all of my previous versions of this sort of chair I did the joinery so the leg tenons only penetrated the battens. This time I made the leg tenons long enough to come all the way through. So I threw the switch in my head that told me not to do it – and bored the mortises through the battens and seat. And glued and wedged the legs in place.
I didn’t think I’d like the tenons poking through the seat board, but I do. It’ll show up better when I put a finish on the chair. Then decades from now it will be harder to see again. Ash legs, butternut seat and back.
I’m going to tinker some more with these 3-piece backs, but I do have some wide walnut waiting to be brettstuhls. And one more ash bolt to rive. Better get to it.