| 摘要 |
A small drill includes one chip-discharging flute which is helically formed in the exterior surface of a cutting portion of the small drill. The small drill also includes a flute-shaped portion which is formed in the front region of the chip-discharging flute in the rotating direction, which is connected to the chip-discharging flute, and which is open at the exterior surface of the cutting portion. The core diameter ratio d/D, in percent, of the small drill is set to 60% or more, and a margin angle theta thereof is set to 120° or more.
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