<p>The ball end mill is used as a tool for cutting the metal mold and some complicated shape lake a screw. The demand for the precise machining of the ball end milling is increasing. The ball end milling has the unavoidable problem of machining error by the elastic deformation of tool because of its low rigidity. So this report is set up the new method of the high precision machining used by the tool orientation control. The machining area at the point of the surface generation is changed depend on the tool orientation of the ball end mill. The geometric mechanism is analyzed and the relationship between the machining area and the tool orientation is clarified. And the cutting force and the machining error are measured by the machining test that the tool orientation has been varied. As the result, it is verified that the machining error becomes large at the point, which the machining area is large. And the machining error estimation index has been proposed and calculated about three dimensional surface.</p>