Jeff, it is the possibility of mechanical damage to the machine by hitting a wrong button in Mach which frightens me. I want my machine to be fool proof. Also, I still keep getting the "SmoothStepper ran out of data" message every now and then, and I am not sure about this, but I think it would also cause that sudden stop. Again, not sure about this one as I've just fitted the large chuck and with small one it really never made a difference how it stopped so don't recall.
Russ, it is a 2kW Delta servo. Don't have an external resistor. It has a 6:1 reduction transmission to the spindle, so the motor doesn't experience the full inertia of the load. Still easily tears an HTD-5M 15mm wide belt not even tripping an error.