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