More progress. Yesterday I marked the top mortises and chiseled out an eighth of an inch or so. Then I temporarily screwed each top beam miter joint together and drilled them out using the started pocket as a guide. I used my Rube Goldbergian setup of lowering the drill press table and putting the drill press over the workbench, and then plumbing and leveling everything.
Today, I chiseled mortises and sized tenons and got 3 top beams up to finally see what it's really going to look like..
So this shows some of the constant thinking and adjusting that I do on this project.
The sill beams are "shingled" in layout. That is, each beam's joints overlap the one on the left, and is overlapped by the one on the right. This was because I cut the laps on the radial arm saw, and it wouldn't cut left end laps on a 6x6 due to the design of the machine. This isn't a big deal because the base is assembled first and then the joints are pinned by the post tenons.
OTOH, the top beams are cut with either 2 top laps or 2 bottom laps. Originally, I was going to shingle them like the base, but realized that they would have to go on skewed because of the shingling (which, if the joints were tight, wouldn't work), or I would have to assemble the octagon on the ground and try to lift it into place in one piece.
Luckily, I managed to configure the radial arm saw to do left hand laps with a quarter inch to spare.
And, when assembling the top, I have to install one bottom lap beam, skip a bay, install another bottom, and then join them with a top.