| 摘要 |
PURPOSE:To prevent a frame from over-drilling by decreasing rotating speed of a drill at the starting point of drilling with the thick drill of a double-stage drill. CONSTITUTION:As a double-stage drill 3 goes down and starts drilling, a slender drill 1 drills a face plate 5, and after the drilling is finished drilling is started on a frame 4. A thick drill 2, thereafter, drills the face plate 5, and forms an idle hole. After the idle hole is formed, and at the time the thick drill is about to touch the frame 4, an inverter 7 operates by a signal from a torque meter 6, the rotating speed of the double stage drill 3 decreases suddenly therefrom, and the drill turns to go up. Accordingly, the frame is prevented from over- drilling with the thick drill beyond the limit. |
