Linear rail accuracy question

[pstockley]

I am looking to build a small very precise milling machine. My question relates to the accuracy grade of linear rail required. Looking at the spec's it appears the biggest difference between standard, precision and superprecision is the accuracy of the blocks with regard to height and width etc. It seems to me you can compensate for differences by using shims or scraping the surfaces they bolt to. The only other area is the parallelism of the tracks/rails. Now this is going to effect preload but how critical is it? I would be using medium preload carriages anyway and aligning the rails against a precise reference edge to ensure straightness.

[Bruggles]

Which linear rails are you looking at, there are hundreds of different types. A pacific type bearing is super precise, long lasting and economical. One problem with anything super precision is that you will need them to be super parallel etc. If I were trying to make an accurate small machine I would go with thompson shafts, and pacific bearings the combo should be virtually free of play. One of the most important things for precision is the screws themselves, but everything does matter.

[ViperTX]

It's not only the variation of height...but also the variation of the height over distance traveled....I would shoot for buy the most precise rails I could afford....also you need to qualify what you mean by building "a very precise milling machine."

[kdoney]

I was able to create a very small and precise 3 axis by using daedal linear slides, bought new from ebay. Crossed roller bearings and daedal precision quality made it more accurate then I could measure <.0005 in all axes but the travel was small 8" X 8" X 10". I mounted the 10", the base, on an optical breadboard. I use it to measure pieces from larger machines. Total cost was about $1200, not counting the electronics, probe and servo motors.