It was finally decided by the powers that be, that we (meaning the Design Team, aka me) need to have actually produced something. Since 2 of the 6 benches are for customers, they have been jumped up to the top of the priority list, and I have been working on them solidly for the past week.
All of the tenons were through tenons, where the tenon penetrates through the legs and seat. The seat tenons will be flushed up, and have a wedge of ebony, while the stretcher tenons are left proud and are keyed with more ebony.
The mortises in this case were better served to be square, made with a hollow chisel mortiser. The hollow chisel, a sharp chisel blade on 4 sides, with a drill bit in the center, echoes the construction technique of yore, where they would drill out the waste wood first, and then come back and square everything up with a chisel. The hollow chisel mortiser does take some getting used to; making sure that everything is lined up just so, cutting just over halfway through, and then flipping, and cutting from the opposite side, leaving you with a clean surface.
To complicate all of this, the legs are splayed in both directions, necessitating the tenons going into the seat to be cut in with compound angles, which seemed simpler than cutting the mortise other than straight. The leg mortises were cut with only one angle, so the tenons could be straight with only an angled shoulder.
Then the tenons were cut by hand, with a tablesaw and dado blade, then mortised in the tenon to allow for the key. The seat was cut with our CNC router, with required a bit of tricky programming of what turned out to be 1/4 of a sine wave. Fortunately, I laid this problem to one of our showroom guys, who happens to have a background in engineering and a love of calculus, and he set me up with the equation I needed.
It was only after everything was cut and finished (for 6 benches) that I could clean one up enough to dry fit, and hope I was accurate enough for it to go together.
Fortunately, the dry fit went fine. Unfortunately, it is not easy making sure each tenon fits into each mortise. There is a lot of sanding of tenon faces, and paring of mortise cheeks until the proper fit is found.
Too bad I didn't think to make one for myself, I do really like them. I just don't want to ever have to make them again for any reason.