Quote Originally Posted by Jonathan View Post
SBR20-400mm:0.03mm height variation.
Hiwin 15mm (760mm long): 0.06mm variation.
NSK 20mm (520mm long): 0.05mm variation.
When comparing the parts yep but a more real world example would be that whilst installing on a machine you can further level the profile rails providing you've got the measurement tools. So those figures you've hinted at aren't set. They could even be made worse if you make a mess of the fixings which is less likely with the round rail as its only fixed both ends and not at regular intervals. Its as Jazz said, they're designed for precision and will give more of that over round rail but you have to exercise another level of precision when installing them to reap that.