| 摘要 |
PURPOSE:To eliminate the chip packing and to improve the machining efficiency by enlarging the helix angle at the cutting edge section of a drill while simultaneously making the drill groove deeper than a predetermined value and forming a web so as to have a parallel thickness and no taper. CONSTITUTION:A complex material, e.g. a printed wiring board, is bored such that the helix angle theta at the cutting edge section of a drill having the diameter shorter than 0.5mm and employed for boring a hole deeper than five times of the drill diameter is larger than 30 deg., the depth (h) of the drill groove is in the range of 50-70% of the drill diameter D/2 and the thickness (t) of a web 2 is uniform. Consequently, the chip packing is eliminated, the breakdown of drill is prevented and the machining efficiency is improved.
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