profiled rail wich preload do you use ZO (light) ? ZA (Medium) for a diy router ?

[Jonathan]

For a CNC router, like you're building, I would go for 'ZA' (medium) preload. Preload results in higher stiffness for the bearing, at the expense of reducing the life and increasing the driving force required. These two factors are both negligible for the vast majority of home builds, so getting bearings with zero preload would make little sense.

There are stiffness values in this datasheet, if you want to compare - though they are for different bearings the general trend will be the same.