Flsun cube linear rail X-Y gantry

I wanted to try linear rails on my Flsun cube, but I wanted to do it in a way that kept things largely as they were originally, and not require a bunch of extra parts, other than the rails themselves.

This is the result. The front and rear Y rods are still used, but the carriage travels on 3 MGN12H linear rails.

The main advantage the rails provide is a massive reduction in the middle-of-bed sag caused by the deflection of the original smooth rods due to the weight of the carriage. The sag mak