Machine tool

A machine tool comprising:

• a workpiece spindle unit including a workpiece spindle and a workpiece spindle motor, the workpiece spindle being rotatably held and configured to hold a workpiece, the workpiece spindle motor being configured to rotate the workpiece spindle;
• a tool spindle unit including a tool spindle and a tool spindle motor, the tool spindle being rotatably held and configured to hold a cup grindstone having a concave surface, the tool spindle motor being configured to rotate the tool spindle;
• a first-axis movement mechanism configured to move the tool spindle unit relative to the workpiece spindle unit in a direction of a first axis that extends along an axis of the workpiece spindle;
• a second-axis movement mechanism configured to move the tool spindle unit relative to the workpiece spindle unit in a direction of a second axis that is perpendicular to the first axis, wherein the tool spindle unit is arranged with an axis of the tool spindle positioned in a plane that is parallel to a plane formed by the first axis and the second axis and that includes the axis of the workpiece spindle;
• a detector configured to detect an outer diameter of the workpiece reaching a predetermined dimension during grinding of the workpiece;
• and
• a controller configured to control the workpiece spindle motor, the tool spindle motor, the first-axis movement mechanism, and the second-axis movement mechanism,
• the first-axis movement mechanism including a first-axis drive motor,
• the second-axis movement mechanism including a second-axis drive motor,
• the tool spindle unit being arranged such that the axis of the tool spindle intersects the axis of the workpiece spindle,
• the controller being configured to drive the first-axis movement mechanism and the second-axis movement mechanism to relatively move the workpiece spindle unit and the tool spindle unit along the axis of the tool spindle through combined operation of the first-axis movement mechanism and second-axis movement mechanism, thereby grinding the workpiece held by the workpiece spindle with the cup grindstone held by the tool spindle;
• the controller being configured to finish the grinding when the detector detects the outer diameter of the workpiece reaching the predetermined dimension and configured to store as a machining completion position a relative positional relation between the cup grindstone and the workpiece at time of detection;
• and
• the controller being configured to, in machining a plurality of workpieces with the cup grindstone, compare a machining completion position obtained in machining of a current workpiece with a machining completion position obtained in machining of an initial workpiece, and when an amount of variation between the machining completion positions exceeds a predetermined limit amount, judge that the cup grindstone has reached an end of its life and make a notification to outside.