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