摘要 |
PURPOSE: To easily drill a hole having a substantially square cross section through formation of a machining member by providing a drilling blade at each top of an equilateral triangle and its inside and by rotating and revolving this machining member in a prescribed relation. CONSTITUTION: A drilling blade 11 is provided at each top of a substantially equilateral triangle and its inside, and the downside of a drilling action surface is formed as a machining member 10. This machining member 10 is mounted to a machining-member rotating support device 14 rotatably about its center G, this machining-member rotating support device 14 is mounted to a machining- member revolving support device 13. The center G is rotated by the machining- member rotating support device 14 at a radius (1/3<1/2> -1/2) times one side length of the triangle of the machining member 10. The machining member 10 is made to revolve three times in the direction to opposite the rotating direction. |