To improve rotation balance of a collet chuck for coupling a collet by machining an annular protrusion and to achieve high-speed machining.

A nose ring 50 is fitted between a fastening nut and the collet, and an eccentric flange 54 eccentrically fitting into a peripheral groove provided around the outer periphery of the collet is provided around the inner periphery of the nose ring 50. In addition, the annular protrusion 55 centered at the axial center is provided on an end surface of the nose ring 50. Figs. (a) to (c) show front and sectional views when the annular protrusion 55 of the ring 50 is adjusted by cutting depending on an amount of eccentricity. By adjusting the shape of the annular protrusion 55 of the nose ring 50, imbalance on a coupling between the ring 50 and the collet during rotation can be balanced.

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