If you are going to make a flying gantry model have a look at this method of making a bearing:

www.thingiverse.com/thing:233402

When I made my CNC machine, I used this system of skateboard bearings on bright steel bar. Saves big bucks and all you need to do is spray the bright bar with WD40 to keep the rust away. You can see that the roller bearings run on the top half of the bar, so you can quite easily make some support block to put under th bar to stop it flexing out of alignment.

Old Man Emu