Jason;
With regards to having 2 machinists (one a CNC robot);
I've found that I can't concentrate on doing complex stuff when my CNC is running. Cleaning the shop, doing the washing up in the kitchen; that kind of stuff I can do.
I've always got an ear open listening to the CNC for "strange sounds" like material coming loose, cutter dulling, etc.
I don't know why; the little KX1 (running LinuxCNC) runs for hours and hours without issues. I'd presume that Mach3 would have the same reliability, but no experience with that SW so can't comment.
Maybe it's just me being paranoid. I'd (honestly) like to hear on how others do with leaving the CNC mill running, while going and doing something else complex.
Note that I did have ONE problem with the KX1. Actually, my software and hardware; I had set up the software (hand-tuned by me – yeah right! bad idea!) so that pulses to the stepper controllers were right at the limits according to the spec sheets, and one of the stepper controllers missed a pulse (about 1 in a million) when stepping one direction. Backing off the minimal numbers solved the problem. Lesson – never trust the minimum numbers on spec sheets!