Design of ultra-precision CNC grinding machine and its application in machining large aspheric mirrors

Large aspheric mirrors are needed for the remote sensing and ground based telescope optical systems, these mirrors are made of hard and brittle materials which require ultra-precision grinding process to guarantee the high profile accuracy and machining efficiency. The ultra-precision aspheric CNC grinding machine (UAG900) is presented by this paper, as well as its grinding capability. The hydrostatic bearings of high accuracy and stiffness are adopted by the linear and rotary motions to guarantee the mirror accuracy, material removal rate and subsurface damage. Disk type grinding wheel with arc edge is used. The material removal rate can be up to 360 mm3/min to guarantee the machining efficiency during rough grinding using D180 diamond grinding wheel while the fine grinding is performed using D15 grinding wheel. It indicates that the grinding wheel radius measuring error is proportional to the profile error induced by the grinding path. The grinding step size is better to be 0.01 mm for the reduction of the grinding movement accelerations and program length. The grinding path is planned and expressed based on the grinding mode according to the mirror shape. One 540 mm×450 mm×100 mm zerodur mirror is ground and re-ground using the measuring data acquired by the Leitz CMM. The final surface accuracy of P-V value is less than 5 μm after compensation grinding. ©, 2015, Harbin Institute of Technology. All right reserved.