A drilling apparatus adapted to be fed into a hole to be drilled and extendable in a longitudinal direction includes a casing and a drilling unit. A drilling head of the drilling unit includes a first drilling unit for drilling a center hole and a second drilling unit for reaming the center hole for...http://www.google.co.uk/patents/US5590726?utm_source=gb-gplus-sharePatent US5590726 - Drilling apparatus
