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