Your quiet right though I think you meant micrometer not mili.
Machine tools do need a nice really really flat surface to work on when you working to micrometer or single thou's of tolerance even the smallest tilt in the x or y plane is unacceptable - you probably also want a very stable temp environment.
ps I never thought id have the chance to use the old unit "thou's" in a hn comment
Your comment implies that micrometers and thous aren't an order of magnitude different. Regardless of which one you are talking about small tilts are not going to make a big difference. As long as everything is lined up properly within the machine then you aren't going to have a problem. Hell, even if it is a horrible, cheap, backlash ridden machine it is quite possible to machine something down to 1 thou tolerance provided you are careful and know a few tricks. I'm not even trained as a machinist and I have done that without too much trouble.
Machine tools do need a nice really really flat surface to work on when you working to micrometer or single thou's of tolerance even the smallest tilt in the x or y plane is unacceptable - you probably also want a very stable temp environment.
ps I never thought id have the chance to use the old unit "thou's" in a hn comment