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