MAIN SPINDLE DEVICE OF MACHINE TOOL

PROBLEM TO BE SOLVED: To provide a main spindle device capable of eliminating the vibration of a first sleeve even at a high speed and preventing a heat generated by a built-in motor from transferring to a main spindle. SOLUTION: This main spindle device 20 of the machine tool comprises a nose 5 rotatably supporting the main spindle 4, having a tool mounting tapered hole 3 at the tip part thereof, on the inner diameter part thereof, a built-in motor 6 adjacent to the rear end of the nose 5 and rotating the main spindle, a cylinder 11 fitted to the inner diameter part of the spindle 4 through sleeves 8 and 22 and positioned adjacently to the rear end of the built-in motor 6 axially moving forward and backward a draw bar 10 controllably opening and closing a collect 9 for holding and releasing a tool 2 on and from the spindle 4, and a cooling device feeding cooling medium from a cooling medium source located on the outside of the device to the inside of the spindle through a rotary joint 12 fitted to the cylinder 11. Insulating material 21 is installed on the nose 5 side of the built-in motor 6, and an axial cooling passage is provided in the first sleeve 22.