| 摘要 |
A counterbore for enlarging an existing hole in the presence of a wire having an end extending through the hole. The counterbore includes a twist drill bit of a size of the enlarged hole to be drilled, a shank at the rear end of the bit for engagement with a drill chuck, and a pilot shank extending from the cutting end of said bit. A swivel is mounted at the leading end of the pilot for engagement with the wire during counterbore drilling. The swivel and pilot guides the wire into the hole without twisting during drilling of the counterbore and extracts the wire from the counterbore as the twist drill bit and pilot shank is withdrawn.
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