Feed axis monitoring device and monitoring method for machine tool

A feed axis monitoring device for a machine tool that determines an abnormality of a feed axis in the machine tool performing machining of a workpiece while rotating a tool mounted to a main spindle or the workpiece and driving a moving body by the feed axis, the feed axis monitoring device comprising:

• a position deviation calculating unit that calculates a position deviation of the feed axis;
• a machined surface perpendicular direction calculating unit that calculates a direction perpendicular to a machined surface at a time of a position deviation calculation;
• a position deviation component converting unit that converts the position deviation of the feed axis into a component in the perpendicular direction;
• a threshold value calculating unit that calculates a threshold value based on a preset relation between an operating state of the main spindle and the threshold value, the operating state of the main spindle being at least one of an absolute value or a variation amount per unit time of a load of the main spindle at the time of the position deviation calculation;
• and
• an abnormality determining unit that compares the position deviation of the feed axis converted into the component in the perpendicular direction by the position deviation component converting unit with the threshold value calculated by the threshold value calculating unit and determines that the feed axis has an abnormality when the position deviation exceeds the threshold value.