| 摘要 |
A blind hole (Fig 3, 7) is formed around a rotational axis O of a spindle 1 such that the hole is open at an end portion on a side opposite to a drill holding side. A center guide 30, having a plurality of air outlets 33 of a journal air bearing at its outer periphery, is fitted in the blind hole so as to be supported by a case 10. Compressed air is blown off from the air outlet so as to support the spindle in a diameter direction from a center side. Mass of such structure of the spindle can be made smaller, so that no unstable phenomenon occurs even if the spindle is rotated at high speed with high natural frequency. The centre guide may be fixed to the case. The push rod 20 is moved for attaching or detaching a tool. A further gas blowing off hole 13 may be formed at an inner peripheral face of said case. |
