| 摘要 |
PURPOSE: To provide the accuracy of a finished hole and the accuracy of the inner peripheral finished surface, suppress the thickness of a machining affected layer, prevent the enlargement of the inner peripheral diameter of a liquid shim part, and moreover prolong a durable life and prevent the fluffing of carbon fiber. CONSTITUTION: A helical flute 34 twisted in the opposite direction to the rotating direction A of a core drill 10 is formed at a cutting edge 32, and a leading edge 40 is provided between the helical flute 34 and the outer peripheral surface. A spiral point flute 50 with the flute bottom approaching an axis toward the tip part is formed on the tip side of the helical flute 34. The axial length L of the spiral point flute 50 is to have the ratio of 0.4-1.5 to a core drill diameter D, and the inclinationβof the spiral point flute 50 to the centerline C is to be 25-60 deg.. The cutting quality of the main cutting edge 52 of a bite part 44 mainly involved in cutting is thereby heightened, and cutting resistance is suitably reduced. Chips generated from the main cutting edge 52 are discharged forward, that is, toward the tip of the core drill 10.
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