A control device 100 of a machine tool contains a rectifier 11, of a bidirectional conversion between an ac voltage to a ac power supply side 4 and a dc voltage to a direct-current intermediate circuit, a feed shaft inverters 12, of a bidirectional conversion between the dc voltage at the direct-current intermediate circuit and an ac voltage on the side of a feed shaft 2, a main spindle motor-inverter 13 in the form of a bidirectional conversion between the dc voltage at the direct-current intermediate circuit and an ac voltage on the side of a main spindle motor 3, a blackout-detection unit 14, which on a power failure the ac power supply side 4 of the rectifier 11 can be seen a voltage-detection unit 15, the said dc voltage of the dc-intermediate circuit is detected, and a control unit 16, which in the case of a power failure on the ac power supply side 4 a feed shaft delay command at the outputs feed shaft inverters 12, and to the outputs a main spindle motor-acceleration command main spindle motor-invertor 13, if the of the voltage-detection unit 15 detected direct voltage is greater than a predetermined upper limit value, and a main spindle motor-delay command, if the of the voltage-detection unit 15 detected direct voltage is smaller than a predetermined lower limit value.
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