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