I miss JohnS in these kind of discussions.
I always remember a discussion somewhere (possibly on a ye olde usenet group), where somebody wanted to machine a bit steel to exactly 12" long, to within a stupid tolerance of a couple tenths.
John simply asked at what temperature did he want the tolerance met?
The guy didn't have a clue, and had to have it explained to him that for every degree of temperature change, the bar would grow/shrink by over a thou.