| 摘要 |
PURPOSE:To enable accurate perforation by forming a sub-groove in such a way that a secondary cutting edge is positioned on the thick-wall part side of a drill from a straight line connecting the sub-groove side end of a main cutting edge and the end of outer periphery on the sub-groove side of a chisel edge. CONSTITUTION:A sub-groove 4 is formed in such a way that a ridge line which is the edge of a secondary cutting edge 1b is positioned on the thick-wall part 6 side of a drill 10 from a straight line connecting the points on each cutting edge line of the sub-groove side end of a main cutting edge 1a and the opposite sub-groove side end of a chisel edge 3. Thereupon, the drill 10 having thus formed end parts will have a fairly shorter chisel edge 3 than the chisel edge of a drill having no sub-groove 4, improving the biting property into a workpiece while making it hard to cause a walking phenomenon. Accordingly, the rotation center of the drill 10 can be effectively prevented from moving, reducing cutting resistance while improving a cutting property to carry out accurate perforation.
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