If you look at the way the machine changes tools, the extra drive key would hit the inside of the tool drum. The arrow machine does not have the power at the spindle to require two keys. It is only a 40 taper, a 50 or 45 taper spindle needs two keys because of more H.P. I agree that the spindle would seem to be better balanced with two keys, especially at higher speeds.

Mike cncmachineservices (dot) com