> that's .01mm or about 4thou

Pretty sure 0.01 mm is more like 0.4 thou FWIW.

The other thread where the OP describes the project is at

http://www.mycncuk.com/forums/mill-b...html#post47999

Says we're looking at 5' x 10' working area, Z travel 14".

As Irving says this sounds like a major project and which screws/rails/servos to choose is, I would think, secondary alongside making the structure sufficiently accurate and rigid to machine steel at an economically sensible speed and finish. And resolution is the least of the problems I would think - more important will be making sure everything is flat and orthogonal (even with e.g. 500 lb of steel plonked on it), that steel can be machined at an economic rate without excessive vibration and with a reasonable surface finish... etc.

Just had a quick nose around to see if there's commercial equivalents out there... best I could find:

Milltronics: BR60IL (100" x 60" x 28")

Obviously you don't need to emulate the same speed, horsepower or accuracy necessarily to have a useful home built machine but it's notable that it weighs SIX TONS (14000 lb). You could get away with much less of course but it will compromise the accuracy and finish attainable over those large travels. Rather than guessing on this, I'd probably want to get some calcs done on (as a minimum) how much your proposed structure will deflect under the machining forces you intend to put on it. This'll let you know whether it can achieve realistic tolerances for commercial work. Damping/resonance could also be an issue worth tackling at the design stage rather than later...

You'll need to be confident of its accuracy before taking on paid work, I would think - worth thinking now about how that would be proved - measuring things accurately over those sizes is not trivial...

Some good stuff here on design BTW in case you've not come across it before:

http://www.mycncuk.com/forums/faqs-p...mn-design.html

Apologies if you already know all of this - just some random thoughts after reading your plans so far. Good luck with what sounds like an ambitious build.