Drilling crown has cutter element and centre drill which has on its surface a restraint element to hold back drilled out material section

摘要

the drilling crown has a cutter element (5) fixed on the crown body and a centre drill (3) which has on its surface a restraint element (6) to hold back the drilled out section. The restraint element extends from a point on the drill in front and outside of the cutter element up to in the front end of the drill. The restraint element has threaded pitches (6) which can protrude beyond the surface of the drill.