said grinding wheel means having a generally frustoconical configuration and an outer periphery defined by {TAIL ~ first and second axially distinct helical grinding ribs} having {HEAD ~ different contours}; and 
said grinding wheel means having a generally frustoconical configuration and an outer periphery defined by { TAIL ~ first and second axially distinct helical grinding ribs } having { HEAD ~ different contours } ; and
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH