Simple Machine (help and advice needed)

[EddyCurrent]

. . .but I would say if you haven't already bought the linear rails then don't buy round rails.
Profiled linear rails have really come down in price in the last year or so and it's not worth buying the round type IMO. They simply don't compare in performance and accuracy.

I've just used some 20mm supported rail for a non cnc job, it was the first time I'd used them and it will be the last, as Jazz says, use Hiwin type rails every time, use 20mm all round.