Thoughts over coffee - Limitations of the system?

When i was building the large scale traction engines, everything was made with a tolerance of only 0.1mm or better - easily obtainable on a hand controlled Bridgeport with a three digit DRO. It really was not needed to go tighter on these engines and many say that if too tight they never worked properly due to expansion when running etc.

Anyway, the third digit on the DRO was pointless as you could never set that precision by hand and merely looking at the mill out of the corner of your eye made it wobble more than that!

So, now (when done) I will have ball-screws and servo's with Mach3 acting as my DRO display - this is all running on my bench very nicely now.

But - the encoder/motor is connected to the screw via a toothed belt - surely this must introduce an error possibility, especially with a 100kg sitting on the bed??

My main point here is - If i want to use the mill manually, say to drill some holes without cam-ing the job, I can put Mach into reset, this drops out the servo motors and i can spin the handles like before and Mach tracks the position on screen - I am hoping here that this is a workable situation with decent accuracy when positioned manually??