To prevent the rattling of a draw bar during rotating a spindle and the vibration of the spindle to thereby improve machining precision.
A spindle device for a machine tool includes a hollow spindle 1 at the end of which a tool is mounted, and a draw bar 6 coaxially arranged in the spindle 1 so as not to be rotatable with respect to the spindle 1 but to be movable to the axial direction for mounting/demounting the tool at/from the end of the spindle 1 with its axial movement. A groove 12 is formed in part of the outer periphery of the draw bar 6 in its peripheral direction to thereby shift the gravity center of the draw bar 6 from the rotational center axis of the spindle 1. A nut 13 is threadly fitted to the spindle 1. A plurality of screw holes 14 are formed at peripherally equal spaces in the nut 13. A male screw 15 is threadedly engaged with the screw hole 14 located opposite to the direction of shifting the gravity center of the draw bar 6 with the rotational center axis of the spindle 1 interposed therebetween to thereby locate the gravity center of a rotating system including the spindle 1 and the draw bar 6 on the rotational center axis of the spindle 1.
KATSUMATA SHIHOU
SATO KOJI
