US Patent 7278222 Method for measuring a program-controlled machine tool

A method for measuring a program-controlled machine tool using a measuring sphere. According to this method the measuring sphere having known dimensions is positioned on a machine part. A sensing element installed in the machine is moved to a selected initial position in which it is located approximately 10 mm above the vertex section of the sphere. Then the sensing element is repeatedly moved until it contacts various points on the surface of the sphere, and the coordinate values of those points of contact are input into the controller. Thereafter the spatial position of the center of the sphere is calculated and compared with target values stored in the controller.