Entering a zero or NULL value for speed limit would mean that teh tool has not been set AND it will not start teh spindle for safety reasons. That is one way of doing it.
Another way is IF the speed is set to zero then it can calculate teh max speed based on tool diam from teh tool table.
Like I said there are at least a dozen ways of doing it.
One simple way is to just monitor teh Spindle speed and compare it to Max tool speed. IF it exceeds that speed then stop machine and warn operator.
(;-) TP