My first major problem with this build. After all the glue was dry on the gantry, I rotated it 90 degrees on my work surface (the machine base) to see what it would look like in its final orientation on the machine. One corner of the gantry sat above the surface by about 3/32”. I first thought it had warped after the glue-up, but back in the position where I constructed it the gantry laid perfectly flat against the base. After doing a lot of checks with a straight edge and feeler gauges I found that my base was not perfectly flat as I initially thought. It was flat along the edges in both length and width, but there was some variation throughout the center. So, between the 40 or 50 thousandths of twist introduced where I constructed the gantry, and another 30 to 40 thousandths in the final orientation, I ended up with the gantry having this 3/32” warp or twist in relation to the base.
The way I decided to fix this was to create a large planing jig for my router. Following some videos online, I mounted two long pieces of plywood (with the factory cut sides up) along the sides of my machine base and used a level and two wires running from corner to opposite corner to get the rails level and without any twist. Then I built a sled for the router that would slide along these two rails, allowing me to run the router over the whole surface with a 1 ¾” surfacing bit. I waxed the router sled and the rails, and used some clamps for stops. My wife helped out by holding the shop vac to collect the dust while I routed the entire surface of the machine base. I clamped one end of the base to my leveled workbench so that the corner with the twist would be up the air and would get milled down. If you want to see more details, this is one of the videos that provided inspiration: youtube.com/watch?v=qtkBZHLJyD0
After routing the top, I flipped the base over and repeated the operation on the other side. In theory, these two sides should now be parallel and without any twist. This will hopefully get me back to square one for having a flat work surface to build the machine on.
Rather than re-constructing the whole gantry on this newly flattened surface, I decided to use the same planing jig to get the top and bottom of the gantry to be flat and parallel. This seemed to work well and now the gantry sits perfectly flat against the base.