It is expected that the vibration characteristics of NC machine tools are affected by the contact between the tool and the workpiece during the cutting operations. However, the influences of the contact have not been clarified up to now. In this study, a method to evaluate the contacting effect and evaluated results are described. Frequency response of a vertical type milling machine during cutting operation is evaluated in this study. The evaluation tests of the contacting effect are carried out with and without cutting operations. In order to clearly evaluate the influence of the contact between a tool edge and a workpiece, boring operations of 50 mm diameter are carried out. The frequency responses are measured by using feed motor torque. Impulse signal is applied to the motor torque command during the cutting operations to oscillate the machine tool, and the axial acceleration of the table is measured to obtain the frequency responses. The impulse signal can be applied by refereeing the spindle rotational angle to control the relationships between the cutting edge and workpiece surface. As the results of the evaluations, it is clarified that the proposed method can evaluate the influence of the contact adequately. The natural frequency slightly increases and the vibration amplitude decreases when the tool contacts with the workpiece, regardless of whether non-cutting or cutting. It has also been confirmed that the vibration amplitude of the frequency characteristics is changed due to the contact length and the relative direction of the cutting edge.

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