Machine tool having workpiece reference position setting function by contact detection

A machine tool controlled by a numerical controller and having a function of setting a reference position of a workpiece, in which the workpiece has a machining surface symmetrical with respect to two orthogonal lines and is mounted on the machine tool such that the two orthogonal lines are respectively parallel to axes of a machine coordinate system, said machine tool comprising:

• movable axes supported by fluid bearings;
• position detectors for respectively detecting positions of said movable axes;
• position deviation determining means that determines a position deviation between a command position for each of the movable axes and a position of each of the movable axes detected by said position detectors;
• a measuring device arranged on the machine tool to be moved by said movable axes and including a probe having a stylus with a minute ball at a distal end thereof;
• drive control means that drives said movable axes to move the probe toward the workpiece from opposite sides of straight lines which are respectively parallel to the two orthogonal lines so that the distal end of the stylus is brought into contact with the workpiece;
• contact detecting means that detects the contact between the distal end of the stylus and the workpiece based on increase of the position deviation detected by said position deviation determining means;
• stopping means that stops said movable axes when the contact is detected by said contact detecting means;
• coordinate value obtaining means that obtains coordinate values of said movable axes when the contact is detected by said contact detecting means;
• and
• setting means that determines a central position of the workpiece based on the coordinate values obtained by said coordinate value obtaining means and sets the obtained central position as the reference position of the workpiece.