Sorry. Here it is.

Ok your Z Axis  has a velocity of 6, that would mean with a thread pitch of 0.031 inch the max RPM you could command would be 192. If you commanded a greater RPM then the Z axis could not physically move fast enough and thus Mach will give you that error message.
 Ideally you would not want to try at 192rpm as there is a chance the spindle may be rotating slightly faster so for a safe value you would have to use about 170 or so RPM.
The best thing would be if you could increase the Velocity of the Z axis in motor tuning, that would allow you to have a greater RPM. Whether that is possible however will depend on your stepper and whether it can be set to a greater velocity without stalling.
Hood

For those following this thread......

That's why one should always click the button to calc the number of passes just to make sure that
your velocity is ok for the intended thread at some rpm.

RICH