| 摘要 |
The device comprises a solid drill bit (28) and an annular drill bit (40). Axial impacts are exerted on the solid drill bit (28) which are transmitted onto the annular drill bit (40) through impact transmission surfaces. The annular drill bit (40) is freely axially displaceable within limits. It has rearwardly projecting tongues (43) that engage windows (15) of an outer pipe end piece (12). Thus, the movability of the annular drill bit (40) is ensured without any keyings or the like being necessary, which entail the risk of a clogging with bore material. All movable parts are included in the scavenging so that jamming is avoided.
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