Main spindle device for machine tool

A spindle device for a machine tool including a hollow spindle which is rotatably supported in a housing, the spindle device comprising:

• an annular boss member which is affixed to the housing so as not to rotate with the spindle, which is formed so as to penetrate into the spindle, and which includes an opening formed in the axial direction of the spindle,
• a cooling passage formed in an outer circumferential surface of the boss member so as to face an inner circumferential surface of the spindle,
• a first bearing, arranged between a rear end of the spindle and the housing, for rotatably supporting the spindle,
• an annular bearing casing which includes an inner circumferential groove configured to receive the first bearing, and an outer circumferential groove,
• a first coolant passage extending through the boss member from an inlet port formed in the boss member to the cooling passage, and
• a second coolant passage extending through the boss member and the housing from the cooling passage to an outlet port formed in the housing, wherein
• the cooling passage is formed by a groove extending along the outer circumferential surface of the boss member, and a cover member in the form of a cylinder arranged so as to be in close contact with the outer circumferential surface of the boss member, and separated from the inner circumferential surface of the spindle by an air gap,
• the spindle is cooled by the air in the air gap which is cooled by contacting with the outer circumferential surface of the cover member which is cooled by flow of coolant through the cooling passage, and
• the outer circumferential groove of the bearing casing forms a part of the second coolant passage.