Proceedings of the
9th International Conference of Asian Society for Precision Engineering and Nanotechnology (ASPEN2022)
15 – 18 November 2022, Singapore
doi:10.3850/978-981-18-6021-8_OR-14-0167
Dynamics and Surface Quality Analysis in Robotic Milling
1Singapore Institute of Manufacturing Technology, Agency for Science, Technology and Research (A*STAR), 73 Nanyang Drive, 637662, Singapore
2School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, 639798, Singapore
3School of Mechanical Engineering and Automation, Harbin Institute of Technology, Shenzhen 518055, PR China
ABSTRACT
Industrial robots excel Computerized Numerical Control (CNC) machines in pose dexterity and working range, making them suitable for machining operations of large-format complex workpieces. Stability characteristics of robotic milling need to be studied for milling performances enhancement. In this research, modal analysis is applied to a robot-spindle-tool assembly in order to obtain modal frequency and damping ratio which are two essential parameters for milling stability prediction. The milling stability prediction at three different robot configurations based on regenerative chatter theory is generated using Zeroth Order Approach (ZOA). Validation milling experiments are conducted on 6061 Aluminium alloy at each robot configuration. Vibration signals during milling processes are collected using a three-axial accelerometer and analyzed in the frequency spectrum. Results verify the accuracy of stability prediction, similar to many other existing works. However, most prior work lacks systematic investigation on the milling surface quality. The main contribution of this work lies in the detailed analysis of the milling marks left on workpiece surfaces. A criterion based on the continuity of milling marks is proposed to differentiate chattered surfaces from chatter-free surfaces. If the milling marks are smooth and continuous, the process is chatter-free and the surface quality is acceptable, whereas if the milling marks are rough and broken, the process is chattered and the surface quality is not acceptable. Based on surface roughness measurement, it is observed that chattered surfaces have significantly higher roughness value than chatter-free surfaces which further validates the proposed criteria. Furthermore, this paper establishes a qualitative relationship between the vibration signals during milling process and the after-milling surface quality. It is found that occurrence of chattered milling surfaces is closely related to occurrence of high vibration frequencies above 3500 Hz. The average lengths of broken marks on chattered surfaces are measured in order to calculate the chattering frequencies which are found to be close to vibration frequencies collected by the accelerometer. In other words, quantitative evaluation of frequencies of chattered milling marks further validates the correlation.
Keywords: Robotic milling; stability prediction; robot dynamics; high-frequency chattering; milling surface quality.
1Singapore Institute of Manufacturing Technology, Agency for Science, Technology and Research (A*STAR), 73 Nanyang Drive, 637662, Singapore
2School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, 639798, Singapore
3School of Mechanical Engineering and Automation, Harbin Institute of Technology, Shenzhen 518055, PR China
ABSTRACT
Industrial robots excel Computerized Numerical Control (CNC) machines in pose dexterity and working range, making them suitable for machining operations of large-format complex workpieces. Stability characteristics of robotic milling need to be studied for milling performances enhancement. In this research, modal analysis is applied to a robot-spindle-tool assembly in order to obtain modal frequency and damping ratio which are two essential parameters for milling stability prediction. The milling stability prediction at three different robot configurations based on regenerative chatter theory is generated using Zeroth Order Approach (ZOA). Validation milling experiments are conducted on 6061 Aluminium alloy at each robot configuration. Vibration signals during milling processes are collected using a three-axial accelerometer and analyzed in the frequency spectrum. Results verify the accuracy of stability prediction, similar to many other existing works. However, most prior work lacks systematic investigation on the milling surface quality. The main contribution of this work lies in the detailed analysis of the milling marks left on workpiece surfaces. A criterion based on the continuity of milling marks is proposed to differentiate chattered surfaces from chatter-free surfaces. If the milling marks are smooth and continuous, the process is chatter-free and the surface quality is acceptable, whereas if the milling marks are rough and broken, the process is chattered and the surface quality is not acceptable. Based on surface roughness measurement, it is observed that chattered surfaces have significantly higher roughness value than chatter-free surfaces which further validates the proposed criteria. Furthermore, this paper establishes a qualitative relationship between the vibration signals during milling process and the after-milling surface quality. It is found that occurrence of chattered milling surfaces is closely related to occurrence of high vibration frequencies above 3500 Hz. The average lengths of broken marks on chattered surfaces are measured in order to calculate the chattering frequencies which are found to be close to vibration frequencies collected by the accelerometer. In other words, quantitative evaluation of frequencies of chattered milling marks further validates the correlation.
Keywords: Robotic milling; stability prediction; robot dynamics; high-frequency chattering; milling surface quality.
