Abstract: The cutter tower mandrel of a machining center fractured during use. The causes of mandrel fracture were analyzed by means of macroscopic observation, chemical composition analysis, metallurgical quality analysis, metallographic examination, fracture analysis, hardness test, mandrel size and surface processing quality analysis. The results show that the hardness of the cutter tower mandrel was not up to the technical requirements and the content of A-type inclusions was high, which reduced the fatigue resistance of the cutter tower mandrel. At the same time, the transition fillet of the undercut groove was damaged during grinding, and the stress concentration was intensified when the grinding wheel groove was not designed, and the cutter tower mandrel had fatigue fracture under the action of alternating load.