Construction started today by figuring out exactly where the 4x4 posts for the jig were going to go. Using existing 2x10 joists exposed in the ceiling, and using a temporarily screwed-in 2x4x10, I found a good, albeit arbitrary spot. I traced the outline of the post on the ground with pencil, then superimposed the metal post stand and traced its outline.
Then, using a 1/2" masonry bit, I drilled down into the concrete floor. The "slide wedge" masonry bolt took a little hammering (of course hammering on the nut, not the bolt end) to go completely down into the hole, but it fit like a glove.
Here's the post stand, bolted down and tight.
The next step was to find that sweet spot where the post, standing on the base, was perfectly level. I found the spot and clamped the post in place.
I had to run out to Home Depot to pick up an extra long 1/4" drill bit so I could drill holes for the carriage bolts (instead of using lag bolts, I used carriage bolts with nuts for this). I drilled the holes, ran the bolts through and tightened them down. Rechecked all four sides to make sure they were level, made some tiny tweaks (by adjusting the post in the base), and slapped some 10d nails into the post through the base's pilot holes. That finished the first post.
I was lucky enough to use an existing ceiling joist for the first post, but wasn't so fortunate in terms of the placement of the second post. The plans called for an approximately 114" spread between posts. That placed the second post smack dab in the middle of two joists. Had to fashion up a mock joist, reinforced by doubling 2x4s. In the background, you can see the temporary 2x4 that I used to line up the first post. I had already pushed it out of the way, and it came down shortly after this step.
The second post went in exactly like the first one. Got all four sides to be perfectly level. Next step was to cut the 4x4 cross beam to length. That was uneventful. Next I decided to provide a little support for the 4x4 cross beam. I arbitrarily picked 36" as the height for the top of the beam. I clamped some 2x4 scraps to the posts, eyeballing the height at first. Laid the beam across and fine-tuned the heights so the beam was perfectly level. Screwed the scrap supports into the posts.
Next I clamped the cross beam and drilled holes for 6" lag bolts. They go through the posts and into the cross beam. Here's the "completed" jig -- well, at least it's ready for a plumb bob wire. I still have yet to decide whether or not I'm going to stick a 2x4 under the center of the cross beam. If it starts to sag I will definitely do that, but it seems pretty straight and level to me so far...
Coming soon...running an air hose over the jig, setting up the plumb bob wire, tracing the reference plane, etc.