The system I outlined above, is a safeguard which places a PHYSICAL limit on the depth of cut. I am not at all certain, that lost steps are the actual culprit which causes this problem to occur. Could be due to a temporary voltage drop, when the well pump cuts on (Since my shop is actually permitted as the pump house). But, whatever the cause, it ain't gonna move any deeper than the Limit Switch! My machine is using a Gecko G540 drive, 48 VDC power supply, 960 oz in motors on the XY and Z axis, and 1200 oz in on the A axis. And the speed is usually set to 10 inches per minute (for finishing cuts) driving 1/2" 10 TPI acme lead screws (direct drive Lovejoy couplings). And for over 2 years has operated flawlessly, for the most part.
Every time the problem has occurred, it seems to be at random (making it very difficult to diagnose the actual problem). All drives, and axes glide smooth as silk with motor couplings un-coupled, the lead screws can be turned between your fingers. (NO physical binding, or racking on any axis). I have retuned the motors on each axis in Mach3, and can usually get 60 inches per minute feed rates for hours on end without problems. (Although the fastest I usually cut is at 30 inches per minute, the slowest is at 10 inches per minute feed rates).
The addition of the (-) Z Limit Switch has solved the problem, since when triggered, (whether I am there watching it, or left it running and gone to bed) all that happens is a single plunge maybe 2 thousandths deeper than programmed is cut into the work piece. And the machine is there waiting to be reset, re-homed, and Start Cycle to pick up right where it left off at. I made the original request, in hopes of getting some constructive pointers in how to automate the task from some of the more experienced Mach users. I apologize if I have offended anyone here.