This will be an interesting read. I will tell you this. We just purchased a Hermle, C42 5x machine. The utmost in positioning accuracy. When I check the kenematics with the ball over the entire 800mm range of travel, I get an error or around .003-.005 microns, very impressive to say the least. BUT, trying to do a simultanious cut and hold tight tolerances, that is another story. Machining accuracy consists of, programming software, toolpath tolerance, tool tolerance, holder accuracy, shop temperature, tool runout, the list goes on and on. Take a ball endmill with a .0004 tolerance, then add a .0002 toolpath tolerance, then add holder runout of .0001 or so, now the tool is hanging out a way and you have some deflection, the material is heat treated so add some more deflection, now warm up the spindle of the machine a while, now the shop heats up.. you are out .0005-.0006 before you even hit a button. IF I am very, very careful, I can hit a .0005 range, +-.00025. But that is taking everything into account. To just set up a tool, run it thru the laser and hit the button, maybe .0007 if your lucky. And this is with premium tolerance tooling.