The y carriages came out of the 3d printer and they fit to the rails almost perfectly. I'm very lazy, so I made the carriages so they fit tightly around the ends of the rails, thereby covering the ugly hand-sawn ends. They are firmly held in place by M3 screws. The ball bearings roll over the y rails smoothly enough, though I have discovered that even a small piece of dirt makes the wheels slide instead of spin. I guess I'll have to keep the rails clean.

The gantry is driven by one NEMA23 motor at each end. The beauty of this design is that the gantry will align itself automatically to be square to the base as long as the endstops are mounted accurately (and the motors don't lose steps, which would be bad anyway). I only have to add one bearing to each side to keep the gantry from sliding left to right. One of these bearings will be mounted firmly to the carriage, while the other will be pushed against the rail to eliminate clearance.

The next step will be to attach the GT2 belts to the y rails and make the gantry move on its own.