Patent ID: 6841911

Claim:
A machine tool providing desired machining for a work by relatively moving a tool and said work, comprising: a main shaft for installing a tool; an electric motor provided at the shaft head, for rotating said main shaft; an inverter circuit for driving said electric motor; a controller for controlling said inverter circuit, and a tool data setting means for pre-setting the magnitude of the power output of said electric motor which is suitable for the working condition of each said tool used, the desired working being done to a work piece by moving relatively said tool and said work piece, wherein said electric motor comprises: a stator having a primary winding, and a stator field magnet and a rotor having a field magnet to be placed in opposition to said stator field magnet and a shaft, said field magnet comprising a first field magnet having different polarity magnetic poles sequentially arranged in a rotating direction and a second field magnet rotatable relative to said first field magnet and having different polarity magnetic poles sequentially arranged in a rotating direction, and a shifting mechanism for shifting said second field magnet relative to said first field magnet in axial and rotating directions depending upon a balance between a magnetic action force between said first field magnet and said second field magnet and a torque generated in said rotor; said rotor rotating in forward direction when said electric motor is driven at low speed for rotatingly driving said main shaft, and said shifting mechanism placing magnetic pole centers of said first field magnet and said second field magnet in phase by balance of said magnetic action force between said first field magnet and said second field magnet and said torque generated in said rotor when said rotor is rotating in forward direction, and placing magnetic pole centers of said first field magnet and said second field magnet out of phase for reversal of direction of the torque generated in said rotor when said rotor is rotating in reverse direction.