{HEAD ~ means} for selectively raising and lowering {TAIL ~ said work surface} to a predetermined distance from said reflector when said laser beam is broken 
for selectively raising
{ HEAD ~ means } for selectively raising and lowering { TAIL ~ said work surface } to a predetermined distance from said reflector when said laser beam is broken
HEAD HEAD HEAD HEAD HEAD REL REL REL OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH