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