A ratchet allows {HEAD ~ the mill} to move toward {TAIL ~ the object} relative to the mandrel and prevents the mill from moving away from the object relative to the mandrel 
to move toward
A ratchet allows { HEAD ~ the mill } to move toward { TAIL ~ the object } relative to the mandrel and prevents the mill from moving away from the object relative to the mandrel
OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH