said grinding wheel means comprising first and second separate grinding wheels mounted on {HEAD ~ a common shaft} in axially spaced relationship, and said first and second grinding ribs being formed on {TAIL ~ said first and second grinding wheels}, respectively 
said grinding wheel means comprising first and second separate grinding wheels mounted on { HEAD ~ a common shaft } in axially spaced relationship , and said first and second grinding ribs being formed on { TAIL ~ said first and second grinding wheels } , respectively
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD 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 OTH OTH