Milling machine arbors were inserted with satellite spindles having a maximum speed of 1500 rpm, and broke out between the groove and the flange. The appearance of the fracture surface was the same on both arbors. The pronounced scan lines characterized the fractures as fatigue fractures. The appearance of the fracture in the arbors indicated ductile fatigue fracture which had its origin in the radii between groove and flange. These radii of 0.15 and 0.2 mm were too small for the load on the milling machine. In addition there were grooves at the base of the radii which had an unfavorable effect on the life of the component by acting as notches with their resulting stress concentration. Considering the great hardness of the case, the small radii would have been critical even without grooves. Measures were taken so that the critical radius of the milling machine was increased and the surface roughness measured more precisely.