I have this exact same problem with my Zenbot. It will happen randomly in a job - unpredictable. Although restarting Mach3 will does reset everything - this is NO SOLUTION! Because you have to re-zero the job / bit! I too have a spindle which is just manually controlled. Is there a setting I need to change to prevent this?

Is there just a simple way to disable spindle control in Mach3 or the machine's XML file Would that prevent this from happening even if the gcode (I generate from Cut2D) calls out the speed defined by the tool (in Cut2D)?

So what happens, the code stops running or Mach freezes or?
Hood

Thanks for your help. The machine does not respond to Mach3 at all. I have to restart Mach3 program. There are no error /pins red just the "pwm bumped to minimum setting" message. Another thing I have not solved: I have finally figured out how to re-zero the machine though with saving the fixure and re-homing the machine after restarting Mach3. However, I am always restarting the job from the start instead of "run from here" because Mach3 DESTROYS the work because it always moves the Z-axis first in the "preliminary move" which is just so retarded. Why Mach3 does not move x and y first and then Z last *ALWAYS* is baffling.

Mach does do that if you have your safe Z set up correctly.

Not sure why PWM bumped to min would cause issues as it should just be a error message but if you attach your xml I will see if I can find an issue with the spindle.
Hood