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