So, I printed out the CORRECT pineapple coupler, and adjusted the center so all the rollers were evenly tensioned and it seemed sqaure as far as I could measure. I put the gantry back on the frame, and it’s STILL off by the same 3mm on the left side. So…I guess there is something wonky with my frame; I doubt two different centers with new gantry rails would be out of square the same amount in the same direction. The good news then is I know what I’m looking for now; the bad news is I don’t know how to more accurately measure my frame, as I’ve checked those measurements so many times and they always seemed within 1/32″ or less.
Part of the problem is I’ve assembled my machine inside an enclosure, and it makes measuring and setting the corners very difficult. In retrospect, I would have made a seperate removable base (which I may have to do now anyway) to make adjustments easier.