Unrealistic Positional Accuracy Specs?

[LazyMan]

Hi, I have looked at a shopbot's specs for positional accuracy and they list +- .002 inches. Now from my understanding of positional accuracy this spec would mean that if you tell the machine to move 96 inches it will move 96 +-.002 inches. Now I cannot see that as possible on a shop bot unless they are referring to positional accuracy over a shorter distance such as 12 inches. non ground racks are not individually mapped and and you would need laser measuring systems to map out such a large machine. I am planning out a large machine build and am just investigating commercial machines and their specs as a starting point. Am I missing something or what?

[Hirudin]

That would be phenomenal accuracy over 8'! Actually, I think that would be pretty good for just 1'. It looks like ±0.0018" over 12" is "C7" grade.
I bet they're talking about over a single foot.

[ger21]

They actually say ±.003 with no load, for their closed loop Alpha versions. ±.005 for their standard version.

http://www.shopbottools.com/PRSalphaspecs.htm
http://www.shopbottools.com/PRSstandardspecs.htm

[LazyMan]

I guess they got a discrepancy on there site because on this link http://www.shopbottools.com/prSalpha.htm in their feature list says ±.002. Either way, ±.003 would be great. CNT Motion specify ±.002 inches for their routers over the whole length of the table. Well I guess it can be done with precision ground racks, but there are so many other things that effect positional accuracy beyond just the accuracy of the lead screw or rack so I am just impressed.