Experimental Studies On Grindability Of Soft-brittle KDP Crystals

KDP (Potassium Dihydrogen Phosphate, its chemical formula is KH2PO4) crystal is a non-linear optical material used as laser frequency conversions and photoelectric modulators for its high nonlinear and high laser damage threshold value. KDP has several weak points such as soft, brittle and deliquescent from the machining point of view, so it is the most difficult machining crystal in the modern optical engineering. So the most important problem is to find a kind of method, which is high efficient, to get high accuracy of finished surface. In order to explore a new process of KDP crystals using grinding and abrasive-free polishing, the grindability of KDP crystals is studied in this paper,which is supported by National Natural Science Foundation of China.Precision surface-grinding machine MM7132 is used as test platform, The effects of the technical parameters (grinding depth, workpiece's feeding speed etc.) and wheel granularity on the grinding force and surface perfectness are analyzed under the horizontal peripheral grinding. It is shown that under the certain condition there is a critical speed, below which the normal grinding force increases with the increment of the workpiece' s speed and above which the normal grinding force goes down gradually. However, there is still a gap between the roughness of surface ground and that of the required surface by optimizing the technical parameters, which is due to the poor rigidity of grinding machine, so the ultra-precision machining of KDP crystals is very difficult realized on the MM7132.In order to explore the grindabilty of KDP crystals under the vertical rotation grinding, a kinematics geometry model of the rotation grinding is established in the paper. The grinding track of grits on the KDP crystals under different speed ratio are predicted and simulated using MATLAB software, and the principles of the track in the center of crystal are analyzed at the same time. Based on the above theory, the results of computer simulation and theoretical analysis are verified by different experiments, which were conducted on the ultra-precision grinding machine. The relationships between the grinding forces, spindle currents and technological parameters are obtained. The technological parameters to get better surface quality are predicted, and the theoretical basis to improve the surface roughness is provided.In addition, based on the analysis of the influencing factors of fracture toughness of KDP crystals, such as the micro-hardness and the Young's modulus, the best machining orientation of KDP crystals is presented. In order to reveal the material removal mechanism of KDP crystals, the single diamond grain grinding is conducted on precision surface-grinding machine MM7132. The volume of groove in scratching is presented by considering the geometrical relation of the single grain grinding.