I hear you RCaffin. Most of the parts we manufacture are in the 0.01mm to <5mm range. On the 350FG in the lab we have held tolerances of ±0.1um and underthetire is correct, in this regime temperature, not to mention tooling, are extremely critical. Who cares if your machine can hold 0.3um form if your tools aren't even round to a micron? That said, both the temperature in the machine enclosure and the measuring area must be known and controlled very precisely. Although it is nice being able to pull your part back in tolerance simply by breathing on it for a few seconds
JK. All of our measurement systems are non-contact and most diameters are measured on laser micrometers from either Z-Mike or Keyence. As for the original post, I don't know what your budget is but the highest resolution linear scales of which I am aware are the Laser holographic linear scales used by Moore Nanotech. I am not sure but I believe they are made by Sony. *edit* also, we are disposing of two older precitech lathes in the near future. I am not sure what will happen to them. The laser scales on them are still good. I am not sure if they might be of use to you but I am willing to ask the uppers what the plan is for them. One machine is company owned the other is the property of the DOE. I am sure the DOE machine is off the table but I can probably ask about the other.