Quote Originally Posted by Jonathan View Post
so it's arguably OK to use but you will need to have realistic expectations of the accuracy.
Bingo he's Hit the nail on the head.! . . . . . . . People are getting silly on this forum regards the level accuracy they are trying achieve or THINK they are achieving with what is essentially Cave man tools in engineering terms.

Be realistic and remember it's a router based machine that will probably spend most of it's time cutting materials that have high variations of in-stabilty that far excede the accuracy machine can cut too.!

If you take your time to align the rails with sensible means IE: Clive S suggestion of taught wire along with feeling of bearings etc you'll be very close if not spot on for the use of the machine.
Yes rail alignment error will affect performance and live span of the rails but with care and patience the level of error will be so small to make little difference at DIY levels. Even in small/medium scale production enviroment it will take good few years to have any affect on wear if care is taken on setting up and alignment.
Just throw them on and you'll pay the price but with careful setup and sensible attention to detail you won't go wrong.!! . . . . Stop over thinking it and take sensible precations with checking straightness/twist along with feeling your way and you'll be fine.!! . . CRACK ON