The dynamics of the structure of a lathe at the cutting point are directly identified under normal machining operation using an improved version of a method originally proposed by Opitz and Weck. The improved method is based on the interrupted cutting of a specially designed, surface-modulated workpiece that provides a strong, broadband excitation. This interrupted cutting has been theoretically shown to permit the identification of the structure’s dynamics from input-output measurements, despite the intrinsic coupling of the structure with the cutting process during machining. All three components of the cutting force and the tool’s acceleration are measured simultaneously. It was found that the input force components were correlated, so that measurements from two distinct cutting configurations had to be used to obtain the elements of the structure’s transfer matrix. The modal parameters measured during the interrupted cutting tests were compared to the results of commonly used impact tests, which can only be applied when there is no cutting. This study showed that the damping of the machine tool’s structure increases during cutting by 20 to 40 percent of that measured by impact testing.

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