{HEAD ~ A second drive unit} moves {TAIL ~ the tool} along one of the axes in a very finely controlled manner 
{ HEAD ~ A second drive unit } moves { TAIL ~ the tool } along one of the axes in a very finely controlled manner
HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH