The dynamic behavior of spindles is known to determine the accuracy and efficiency of most machine tools owing to their significant influence on the chatter vibration. To this end, experimental-based precise determination of the dynamic stiffness of machine tools based on the chatter vibration is necessary. It is worth noting that rotating and resting spindles exhibit different dynamic behaviors due to the change in the lubrication status of the spindle bearings. This temporal change often results in a complex dynamic stiffness behavior, which makes it difficult to accurately determine the dynamic stiffness of spindle during rotation.
At present, hammering test is the most used technique to measure dynamic stiffness. However, it is unsuitable for measuring moving objects at high speed due to the lack of direct contact between the harmer and the object. Several contactless dynamic spindle test (CDST) methods have been developed to address this challenge. Specifically, CDST based on electromagnetic mechanisms can clarify the relationship between different spindle dynamic parameters over a wide frequency range. However, the constraints of the measurement device hinder its widespread application. It also requires significant effort and time to test many samples and establish the individual differences in the spindles. Therefore, developing more effective and convenient CDST techniques is highly desirable.
To address the above problems, Professor Keigo Takasugi,Ms. Hitomi Sakai and Professor Naoki Asakawa from Kanazawa University, together with Mr. Masahide Ooshima from Suwa University of Science and Mr. Daisuke Noda from Komatsu NTC Ltd. proposed a novel CDST testing approach based on eddy current brake. Generally, eddy current breaking on an electrical conductor surface occurs when there exists a relative velocity between the conductor and the magnets surrounding it. Their work is currently published in the research journal, Precision Engineering.
In their approach, a dummy tool was used as an electrical conductor and magnets were arranged around it. The spindle rotation generated relative velocity further generated eddy current braking, which also served as the excitation force in this CDST. The working principle of the novel CDST was illustrated. FEM analysis was carried out to determine the excitation force induced by the eddy current braking. Finally, the applicability of this approach was verified by comparing it with the conventional hammering test.
The researchers demonstrated the simplicity and the ease of generalizing and miniaturizing the proposed CDST. Whereas the generation of the excitation frequency was significantly dependent on the rotation speed, this method offered several advantages as it did not require external control or power supply. Moreover, the system was relatively simple and could be realized by using only the electrical conductor and permanent magnets. Unlike other CDST methods, it was difficult to directly measure the excitation force in the proposed CDST. Thus, a magnetic field analysis was adopted to determine the excitation force.
Furthermore, the compliance at the tip of the dummy tool was determined based on the measured excitation force and generated vibration displacement. A comparison of the compliances of the proposed approach and hammering test showed that the compliance peak and profile positions matched. Despite the partial differences that were attributed to the dynamic behavior, the effectiveness and reliability of this method were verified.
In summary, the authors reported a relatively a new simple and easy-to-develop CDST method for precise determination of the dynamic stiffness of machine tools under high speeds. This novel approach has several advantages over the conventional methods. In a statement to Advances in Engineering, Professor Keigo Takasugi, first author stated that the novel CDST approach is a promising tool for determining and optimizing the dynamic behavior of spindle and will play a vital role in improving the performance of machine tools.
Takasugi, K., Sakai, H., Ooshima, M., Noda, D., & Asakawa, N. (2022). Development of contactless dynamic spindle testing using an eddy current brake. Precision Engineering, 74, 396-402.