A Lap-MRF process is proposed for large aperture mirrors. In Lap-MRF, a lap is used to expand the polishing area, which improves the material removal rate. Moreover, the MR fluid can be renewed continuously to ensure the stability of the material removal rate, which improves the convergence efficiency of the surface profile error. In this paper, the figuring ability of the Lap-MRF removal function is analyzed. The Lap-MRF process, which is based on surface profile filtering, is presented. Finally, a series of figuring experiments on a Φ350 mm K9 mirror are carried out. For the Lap-MRF removal function, the volume removal rate is up to 0.48 mm<sup>3</sup> /min and the cut-off frequency is about 0.03 mm<sup>-1</sup> . After four times figuring using Lap-MRF, the surface profile error throughout the whole surface is improved to 4.84 λ (λ= 632.8 nm) PV (Peak-to-Valley), 0.69 λ RMS (Root Mean Square) from 25.24 λ PV, 4.31 λ RMS and its total convergence ratio of the RMS error is up to 6.32. These results verify the validity of the proposed method for large aperture mirrors.
In this paper, from two aspects of abrasion of grinding disk and stability of removal function, this paper compares the machining methods of planet movement and smooth running. On the basis of the Preston hypothesis, based on the kinematics theory, the grinding disk theory wear model of planet movement and smooth running is established, and the three-dimensional model of grinding disk wear under two kinds of motion is simulated by MATLAB. The correctness of the theoretical wear model and three-dimensional simulation wear distribution model of the grinding disk is verified by experiments. The experiment found that the wear of the grinding disc under smooth running tends to be uniform, and the wear of the grinding disc under the planet movement increases along the direction of the radius increasing. It is assumed that with the continuous abrasion of the grinding disk, the degree of fit between the grinding plate and the workpiece surface becomes worse, and the removal quantity of workpiece material is affected, which will affect the stability of the removal function. Through the removal function experiment of an hour, we find that the stability of the volume removal rate of the removal function fluctuates within 7% and the stability of the peak removal rate fluctuates within 6% under the smooth running, while the stability of the volume removal rate of the removal function fluctuates within 29% and the stability of the peak removal rate fluctuates within 12.3% under the planet movement. The results show that the wear of the smooth running is uniform and the removal function is stable. Therefore, the smooth running is more suitable for high-precision modification than planet movement.
A magnetorheological finishing (MRF) method, which is named lap-MRF, is proposed in this paper. A lap, instead of a large polishing wheel, is used to expand the polishing area, which significantly improves the material removal rate. Lap-MRF uses the flexible magnetorheological fluid (MR fluid) as a polishing pad to match the surface well. Moreover, the MR fluid in the polishing zone can be renewed continuously to ensure the stability of the material removal rate. The principle of Lap-MRF and the theoretical model of the material removal rate are presented. The effects of the spindle speed and the working gap distance on the polishing normal pressure and the material removal rate are analyzed. Finally, a series of polishing experiments and simulations are carried out. The volume removal rate for K9 is up to 0.76 mm3 / min and the comprehensive material removal stability is 6.7%. Using MATLAB simulation process method, the convergence efficiency for a ϕ1000 mm mirror is 98.5%. The results verify the validity of the proposed method.