| 摘要 |
A spiral or twist drill having a hard metal cutting edge and particularly useful for drilling concrete or artificial stone includes a cutting edge having a centering projection of a diameter which is smaller than the outside diameter of the drill. The cutting edge is formed by a hard metal plate which is secured such as by soldering within a cross groove of the drilling head. The centering tip is formed either by a separate hardened metal element which projects outwardly from the center of the cutting plate or is defined by notches which are formed on each side of a central centering tip portion of the hard plate.
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