Patent ID: 7331741

Claim:
A numerically controlled machine tool having a spindle on which a tool holder having an outer surface having key grooves is mounted, whereby a rotational tool is attached to the tool holder and machining is performed by rotating the rotational tool, the numerically controlled machine tool comprising: predetermined-amount rotating means for stopping the tool holder at a predetermined rotational position and then rotating the tool holder by a given amount; and a distance sensor which is installed apart from the tool holder and measures a distance to the outer surface of the tool holder; wherein a numerical controller of the numerically controlled machine tool is configured to stop the tool holder by said predetermined-amount rotating means at a rotational position at which the key grooves do not face the distance sensor and to rotate the tool holder by the given amount, and to measure the distance to the tool holder by the distance sensor while the tool holder is being rotated by the given amount, and the measured distances are compared with a reference value to determine whether the rotational run-out of the tool holder is normal or abnormal such that measurements of the distance to the tool holder at the key grooves do not affect the determination.