| 摘要 |
PURPOSE:To achieve complete elimination of burr at the lower surface of a workpiece by continuing rotation of a feed motor for a predetermined period of time to lower a drill after load current flowing through the drill motor is decreased and thereby termination of drilling is determined. CONSTITUTION:A drill motor 2 is rotated at a speed corresponding to the drill diameter in use, and at the same time, no-load current other than a starting current is stored. Then, a feed motor 4 is rotated so as to lower an electric drill 3 at a speed corresponding to the drill diameter in use and when a workpiece is drilled, the current flowing through the drill motor is increased to load current. When drilling is finished, the current through the drill motor is reduced to the above stored no-load current, and thereby drilling state is discriminated and termination of drilling is detected to continue rotation of the feed motor 4 for a predetermined period of time. Burr remaining at the lower surface of the workpiece is removed during the no-load current operation. This allows complete drilling, providing a bore having a fine finished inner surface. |
