What size profile rails for Y axis?

[pippin88]

For a wood router 15mm profile rails are more than enough load wise.

Bigger rails are sometimes easier for a couple of reasons:
Bigger mounting screws - easier to tap the holes

Slightly more room around the mounting holes (to deal with misalignment). For example my Hiwin 15mm rails use M4 screws and have 4.4mm holes, not much room for error. Larger rails have bigger clearance).

Height compared with ball screws / ball nut. Ball nuts are quite bulky, and higher than rails+cars. Going a step up in rail size can be useful.



Preload is how tight on the rail. Zero or low preload moves with very little resistance but are not as stiff. For most CNC machines you want a medium preload.