To smoothen supply of a cooling fluid to an excavating position to keep satisfactory excavation efficiency.
A direct motor 2 is formed of a cylindrical rotor 17 having a cylindrical rotating shaft 11 extending through, a cylindrical stator 18 provided on the circumference of the rotor 17, a rotary joint 21 rotatably connected to the rear end of the rotating shaft 11 and having a passage 22 for guiding the compressed air supplied from a compressed air supplying device through a tube 24 to a nozzle 33 provided within the through-hole 11a of the rotating shaft 11. A cylindrical core bit 13 having a bit 15 at the tip is mounted on the rotating shaft 11, and the compressed air is expanded and blown out from the tip of the nozzle 33 into the through-hole 11a of the rotating shaft 11 while rotating the cylindrical shaft 11 at a high speed, and sent to the tip direction of the rotating shaft 11 so as to cool the excavating position by the bit 15 and also discharge chips to the outside.
IMAOKA TOSHIO
SATO KUSUO
NIPPON DAIYAMONDO KK