光学反射镜CCOS研磨阶段边缘效应控制技术

Edge effect control technology of CCOS in the grinding stage for optical mirrors

计算机控制光学表面成形技术(CCOS)广泛应用于反射镜的研抛过程,其去除函数通常选用去除效率高、去除稳定的近高斯形函数,但在加工过程中容易出现边缘翘边现象,严重影响反射镜的加工效率和面形收敛率。针对CCOS研磨过程中出现的边缘效应问题,提出一种应用于多自由度机械手的无偏心加工技术,对无偏心工具头的去除函数进行了分析,采用控制变量法研究加工压力、磨头转速和驻留时间等工艺参数对去除效率的影响,并对无偏心盘修边效果进行了试验验证。结果表明,通过对加工压力、磨头转速、磨盘悬挂比以及加工区域等加工参数进行合理调控,无偏心盘可以对翘边进行有效去除,且加工后面形平坦,可以很好地解决边缘效应问题。最后,针对研磨阶段提出了新的工艺流程:采用有偏心修形和无偏心修边相结合的工艺加工方式,可以快速提高反射镜面形加工的收敛效率,实现高效、高精度加工。

Objective Computer controlled optical surface technology(CCOS)is widely used in the grinding and polishing process of mirror,it uses a small grinding head controlled by a computer to quantitatively grind and polish the surface of the workpiece.The removal function is a key parameter in CCOS,which is mainly related to process parameters such as the dwelling time of the grinding head on the workpiece,the rotation speed of the grinding head,and the processing pressure.The current CCOS removal function usually uses a near-Gaussian removal function,which has high removal efficiency and stable removal.However,it is easy to cause edge warping during processing.Generally,manual repair is used to remove the warping,which not only requires a large amount of labor costs,but also seriously affects the rapid improvement of processing accuracy.The larger the aperture of the mirror is,the larger the edge area is needed to be processed.If the edge warping is not properly controlled,it will seriously affect the processing efficiency and the convergence rate of the mirror.Aiming at the problem of edge effect in the CCOS grinding process,a non-eccentric processing technology was proposed and the removal function of the non-eccentric tool was analyzed.Methods The theoretical analysis of the removal function of non-eccentric tool was conducted.The removal function of non-eccentric tool was extracted by single factor fixed point pit test(Fig.1-2).Using the control variable method,the influence of process parameters such as processing force,grinding head rotation speed,and dwelling time on the removal efficiency was studied(Tab.1).In order to verify whether the non-eccentric tool could effectively control the edge effect,an experimental mirror with an aperture of 407 mm was selected;Firstly,the whole surface of mirror was processed with an eccentric tool whose removal function was a near-Gaussian type.When the edge was warped,the non-eccentric tool was used to process the edge(Fig.6).Results and Discussions Compared to the Gaussian shaped removal function of the eccentric tool,the noneccentric tool removal function was an inverted V shape,with the minimum removal amount at the center of the tool and the maximum removal amount at the edge.From the impact of process parameters on the removal efficiency of non-eccentric tool,it could be seen that when the processing force was within 21 N,the removal efficiency increased linearly with the increase of the processing force;When the force was greater than 21 N,the removal efficiency tended to decrease(Fig.3).This was because the force was too large,which prevented the grinding fluid from entering smoothly.With the increase of rotation speed,the removal efficiency increased significantly(Fig.4).The effect of dwelling time on the removal efficiency was linear(Fig.5).From the surface processing result of theФ407 mm experimental mirror,it could be seen that using the eccentric tool to process the whole surface and the non-eccentric tool to process the warped edge,the PV value of the mirror surface could be quickly converged from 66μm to 10μm by reasonably adjusting the processing parameters(Fig.6-7).Compared with previous mirror of the same type that only used eccentric tool to process,the surface convergence efficiency could be improved by more than 20%.It could be seen that by using the non-eccentric processing technology to process the edge part of the mirror,the edge effect problem had been well solved,which not only saved labor costs,but also greatly improved the processing accuracy and efficiency.Conclusions Aiming at the edge effect problem in the CCOS grinding process,a non-eccentric processing technology was proposed and the removal function of the non-eccentric tool was analyzed.The influence of process parameters such as processing pressure,grinding tool rotation speed and dwelling time on removal efficiency was studied by using the control variable method,and the edge processing effect using non-eccentric tool was experimentally verified.The results show that the non-eccentric tool could effectively remove the warping edge by properly adjusting the processing parameters such as processing force,rotation speed,grinding tool suspension ratio and processing area,the mirror surface after processing is flat,the problem of edge effect is solved.Finally,a new process flow was proposed for the grinding stage which was combining eccentric tool and non-eccentric tool,the eccentric tool was used to process the whole mirror surface and the non-eccentric tool was used to process the warping edge of mirror.This method could quickly improve the convergence efficiency of mirror surface processing,so as to achieve high efficiency and high precision processing.