Study on traceability and suppression method of medium-frequency error for ultra-precision machining optical crystals

The medium-frequency error on the surface of ultraprecision flycutting has an important effect on the performance of the optical crystal. In this paper, firstly, the characteristic phenomenon of "knife-like grain" in the medium frequency surface of the square and circular optical crystal machined by ultraprecision fly-cutting is revealed. Besides, the error traceability is realized and the results show that the periodic low-frequency fluctuation of 0.3 Hz between the tool tip and the workpiece is the cause of the medium frequency error of "knife-like grain". Secondly, through the frequency domain waterfall diagram of vibration signal and the analysis of spindle speed signal, it is proved that the surface shape characteristic is caused by the fluctuation of spindle speed during the cutting process. Then, the variation rule of the cutting amount caused by the fluctuation of spindle speed is explored theoretically and experimentally, and the formation mechanism of medium frequency error in flycutting is revealed. Finally, in order to reduce the medium frequency error of "knife-like grain", the control parameters of the aerostatic spindle are reasonably optimized based on the analysis of the mechanical and electrical coupling control performance of the spindle, so that the RMS values in the medium frequency band of the workpiece are greatly improved after machining.