I rechecked the diagonals and sure enough, now that the gantry is on, it’s pretty well dead on. (Or at least not measurably greater than a sixteenth of an inch ot of square). I used an old book, which was just about the right height, to set the height at the corners. I might cut a spacer on the table saw to be a little less kludgey in a day or so. … I guess I could measure, too. One moment.
It’s entirely possible the paper and excess pressure are to blame for the wiggle waggles. We’ll see how it does once I strap the 3D printing head on, or the DeWalt. It could also be the table I built. It was made out of what I had laying around and the ply that makes up the surface wasn’t the flattest I’d ever seen. I screwed it down to the frame, but even so, I wouldn’t be surprised if it’s led to some minor twisting. I KNOW it isn’t quite planar to the gantry, because some of my tests end up with the pen drifting off the paper as it moves toward center. I suspect a new table for this thing is in my future. At the very least I’ll be rigging up a levelable print surface for the 3D print tests.
Thanks very much for answering my posts, by the way.