| 摘要 |
A bit (12) has a triangular drill portion (14), a frustoconical grinder portion (24), and a cylindrical reamer portion (28). The drill portion (14) has three longitudinal cutting edges (16) that define the angles of its triangular cross section. The grinder portion (24) extends axially inwardly from the drill portion (14) and has a minimum diameter less than and a maximum diameter greater than the maximum diameter of the drill portion (14). The reamer portion (28) is an axial continuation of the grinder portion (24). The outer surfaces of the grinder and reamer portions (24, 28) are plated with diamond grit (30) to enlarge and finish a hole produced by the drill portion (14). The relative diameters allow the grinder portion (24) to gradually engage the sidewall of a hole. The bit (12) is supported and centered in the hole by three point contact between the cutting edges (16) and the hole sidewall.
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