Understanding the high-point formation mechanism on large optical surface during ultra precision fly cutting process

It was found that there always exists high points during cutting in and cutting out in the fly cutting of potassium dihydrogen phosphate (KDP) crystals. This results in poor surface flatness of KDP devices and degrades their optical performance. It is believed that the high-point formation are related to the cutting heat; however, the manner in which the cutting heat produces the high points and its control are still unclear. A thermomechanical model of the tool system in the fly cutting process is established to reveal the high-point formation mechanism on a large optical surface during the ultra-precision fly cutting process. The temperature field distribution of the tool system and axial displacement variation due to thermal deformation are solved and analyzed. The results indicate that the slow cutting heat accumulation caused by the discontinuous fly cutting process leads to moving of the cutting point in the axial direction, which is the main reason for the height difference during the process. Further analysis demonstrates that moving of the cutting point is primarily caused by the thermal deformation of the tool holder, not the tool or tool handle. Therefore, reducing the tool holder thermal deformation is the key to further improving the surface flatness of the workpiece. This study is of great significance for the improvement of the surface quality of large aperture optical devices in fly cutting.