| 摘要 |
To provide a drill which has a small diameter but deteriorates neither in rigidity like the type having two cut blades and two chip discharging grooves nor in cutting chip discharging property like the type having one cut blade and one cutting chip discharge groove, and which is extremely excellent in practical use while performing the enhancement in hole position precision by a simple improvement, there is provided a drill in which a chisel edge 6 is provided at a tip end portion, a spiral first chip discharging groove 1 and a spiral second chip discharging groove 2 are provided, a land portion 3, 4 is provided between the first chip discharging groove 1 and the second chip discharging groove 2, and a cut blade 5 is provided at an edge of a rake surface 7 of the first chip discharging groove 1, characterized in that the first chip discharging groove 1 and the second chip discharging groove 2 are provided asymmetrically with respect to a drill rotational center O and a land portion 3 behind the first chip discharging groove 1 is larger than a land portion 4 behind the second chip discharging groove 2. |
