基于砂轮原位自动修整的大型SiC离轴非球面精密铣磨研究

SiC off-axis aspherical mirrors are the key components in a space camera. Due to the severe wheel wear in grinding of SiC, the machined surface accuracy is low and the surface quality is poor. The grinding efficiency is also low due to the frequent truing and dressing of grinding wheel. The ELID technologies mainly solve the dressing problem but it is inefficient in truing of grinding wheels having complex shapes. To solve the above problems, a wheel-swinging method based on the mechanism of dwell time control is proposed to realize the in-process self-truing of wheel. The control principle of dwell time and key technologies will be mainly studied. Furthermore, a two-dimensional rotary ultrasonic grinding is proposed to strengthen the self-dressing of grinding wheel, and also reduce the wear amount on the grinding wheel surface. Thus, the in-process self-truing/dressing will be achieved. Moreover, the material removal mechanism of SiC under the wheel-swinging, the wheel-rotation and the ultrasonic vibration of wheel, will be studied. At last, the grinding mechanism and key technologies for the 2-meter-scale SiC off-axis aspheric mirrors will be built. This project will provide new ideas and new technologies with independent intellectual property rights for the precision machining of large-scale hard and brittle materials with complex surfaces.

SiC离轴非球面反射镜是空间相机的核心元件。铣磨SiC反射镜时，砂轮磨损剧烈，导致工件面形误差大、表面质量差；砂轮频繁停机修整，导致加工效率低下。ELID技术可实现砂轮的在线修锐，但很难完成复杂型面砂轮的在线整形。本申请提出加工过程中的砂轮原位自动整形新思想：基于磨削点驻留时间控制算法，创建砂轮摆动铣磨法，使砂轮表面逐点依次参与磨削，将加工时间内的砂轮磨损总量均化到整个砂轮表面，从而实现砂轮表面的原位自动整形。重点研究磨削点驻留时间的控制原理及关键技术。同时建立大功率二维旋转超声铣磨新方式，研究对砂轮表面的自动修锐能力及磨损速率的影响。并研究在砂轮的摆动、旋转及超声振动等多个周期激励共同作用下，SiC材料的微观去除机理。最终形成面向2 m量级SiC离轴非球面反射镜的精密铣磨基础理论及关键技术。本项目将为大型超硬脆性材料复杂曲面元件的高效精密加工提供新思路和具有自主知识产权的新技术。

SiC陶瓷是建造空间环境下服役的高精度复杂曲面反射镜的理想材料，但SiC陶瓷材料硬度高、脆性大，导致其机械加工性差。在磨削SiC陶瓷时，最重要的现象是树脂基结合剂金刚石砂轮随着磨削的进行发生了剧烈磨损，不断产生磨损让刀，进而引起被磨削表面的面形误差。本项目以2 m直径反应烧结SiC反射镜的精密铣磨为研究目标，紧紧围绕高精度复杂曲面模型的建立与快速成形技术、铣磨面形误差在线精确补偿加工技术、复合周期激励下SiC材料的微观去除机理、金刚石砂轮磨损机理与原位在线修整等若干问题开展细致研究工作。最终形成面向大口径SiC复杂曲面反射镜的精密铣磨基础理论及关键技术。实际加工SiC反射镜面形精度PV值达到18 μm，完成了任务书设定的小于20 μm的技术指标。磨削表面的亚表面损伤分析表明，在磨削过程中，结合剂Si相发生了延性去除，因而获得了光滑表面；SiC颗粒内部发生了多处的微观脆性断裂坑，亚表面损伤层的估算深度小于5μm。金刚石砂轮发生了严重磨损，但形貌观测显示金刚石颗粒的突起程度和新砂轮没有明显区别，表明即使砂轮表面产生剧烈磨损，但仍然保持锋利状态，充分说明了砂轮原位在线修整的效果。本项目为硬脆材料复杂曲面的高精加工提供了可行的技术方案。