| 摘要 |
The problem to be solved for a small-diameter drill as an object to be improved is how to restrain reduction in the strength of the outer circumferential portion of the cutting edge without providing a return portion while assuring sharpness of the outer circumferential portion of the cutting edge without an insufficiency, as well as how to achieve favorable machining accuracy and favorable chip processing performance. A small-diameter drill having a diameter of Õ3mm or less and the end that has undergone cross thinning is provided such that in a front view, a flute portion cutting edge 4a is curved in concave with a degree of concavity d with respect to an imaginary straight line in a range of 0.5 to 2% of a drill diameter D, the imaginary straight line connecting the radially outer end P of the flute portion cutting edge and the radially outer end Q of thinning portion cutting edge, the ratio of length B of the thinning portion cutting edge to length A of the flute portion cutting edge is set to 0.6 to 0.9 : 1, and in an axially perpendicular cross-sectional view, curvature radius R1 of a heel-side flute surface of the flute is set to be smaller than curvature radius R2 of a flute surface along the flute portion cutting edge. |
