The bit assembly has at least one rotary cutting {HEAD ~ cone} for drilling into {TAIL ~ a relatively hard material}, such as rock 
for drilling into
The bit assembly has at least one rotary cutting { HEAD ~ cone } for drilling into { TAIL ~ a relatively hard material } , such as rock
OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH