[MacMarty]

(Thanks for the advice on steppers, everyone. Gonna get some bigger ones.)

AND, at long last I have some progress, with photos to share. I've built up both of the x-axis rails, and the gantry to boot. Here's what I -think- I've learned. Note that in all three photos, the table is lying on its back.

The gantry is 80-20 1530 series, with triangular gussets at all four corners. Strong like bull! The first time I put it together, I put the gussets on the inside of all four corners of the gantry. Then I realized that this would leave the bottom horizontal member pretty far away from the table, requiring a thick spacer block to get the drive screw into position. Came back to it next day and realized that things would work out a lot better if I flipped that bottom member over, so that the gussets are on the outside of the rectangle. ("Bottom gussets outside")

You'll note that I've constructed my gantry so that the vertical rails mount to the end of the horizontal rails. My intention here was to design a solid rectangle with 90 degree corners. With that as a given, the next challenge was how to arrange for adjusting the fit between the rails and gantry. Gotta be perfectly snug, right?

The current solution was to mount the front X rail with 1/4" holes and 1/4" lag screws, nice and parallel to the edge of the prefab table, and screwed down tight. The back X rail uses the same lag screws, but I've oversized the holes so that the rail can be moved in the horizontal plane. The third photo shows a slack adjuster, (there's one at each end of the table,) which allows me to carefully push the rail out, to take out all slack. There's no room for the gantry to "yaw" as it rolls to and fro. I'm planning to upgrade to Ahren's 8" bearing blocks, to make this relationship even more rigid in the yaw axis.

With those in place, now I'm moving on to the Acme rods and bearing blocks. For some reason, the bearings don't slide smoothly along the rod: the bearing starts out smoothly enough, but gets really tight on the X screw, and then binds up at one point. Something's out of spec somewhere, where'd I put that micrometer? I have 2-start 1/2" Acme screw, and the VXB 1/2" radial bearings Ahren recommends. Any there any curative measures known to everyone but me?