A rolling cutter shell is mounted upon the rotary head or {HEAD ~ the bit} for {TAIL ~ rotary motion} to roll along the formations being cut 
for
A rolling cutter shell is mounted upon the rotary head or { HEAD ~ the bit } for { TAIL ~ rotary motion } to roll along the formations being cut
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH