| 摘要 |
A drill bit assembly comprising a plurality of cutter units, with each cutter unit comprising a circumferentially forward, relatively blunt positioning cutter, followed by a fixed cutter having a relatively sharp cutting edge, with the cutting edge of the following cutter being positioned a predetermined distance below an operating position of the positioning cutter. Thus, the cutting depth of the following cutter is controlled by the positioning cutter locating the drill bit assembly against the hole end wall from which material is being cut. Also, additional fixed cutters can be provided in circumferential alignment in said cutting units, and liquid jet cutters can be used in combination with said cutting units.
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