Milling thin-walled workpieces can suffer from severe and complicated vibrations. Self-excited vibrations can create subharmonics, making it difficult to determine the vibration type from measurement data. This paper introduces a phase analysis using signal processing of measured accelerations during machining. A piezoelectric impact excitation test is introduced to explain the nonlinear subharmonics caused by the engagement of the tool and the workpiece. Acceleration during machining is analysed by classifying the phase pattern with the aid of the data from the excitation test.