The mold core fabrication of a freeform surface on die steel by ultra-precision machining can make the optical elements of freeform be mass-produced by plastic injection and glass mold pressing. However, because steel is a typical difficult-to-cut material, the technical limitations of existing machining methods hardly meet the current requirements of design. In this paper, a novel machining method, one-dimension ultrasonic-assisted slow tool servo (UASTS) turning, is proposed to manufacture the freeform surface with high-precision and large-steepness on the die steel. Aiming at the characteristics of UASTS turning, the tool trajectory is generated by analyzing the compensation of tool radius and confirming the position of ultrasonic displacement. The 2D surface model contours of residual tool marks are established for predicting the 3D surface topography based on considering the effects of kinematics, material elastic recovery and plastic side flow. In the experiments, the large-amplitude bidirectional sinusoidal wave grid (BSWG) surface is successfully fabricated by UASTS technology on the material, Polmax steel, for which the value Rt of surface roughness is less than 70nm and the value Ra of surface roughness can achieve to 3.298nm. The results show that the freeform surface with high-precision and large-steepness can be machined by UASTS turning technology on mold steel.

Precision glass molding (PGM) is an important processing technology for aspheric lenses that has the advantages of low complexity, high precision, and short processing time. The key problem in the PGM process is to accurately predict the residual stress of aspheric lenses. In this paper, we examine the residual stress relaxation model for aspheric lenses, including a creep experiment of D-K9 glass, calculating shear relaxation function, and predicting residual stress of aspheric lenses with the finite element method. Validations of the proposed model are conducted for three different process parameters, including molding temperature, molding pressure, and molding rate. The experimental and simulation results show that the errors of the residual stresses of the three process parameters are within 0.358 Mpa, which proves the validity of the model. The model can be used to predict the residual stress of the optical glass lens fabricated by PGM and analyze the processing parameters.

Precision glass molding (PGM) has become a viable processing method for large-volume aspheric optical elements. The optimization of a PGM process is an obstacle to the realization of mass production. The current work is focused on optimizing the process parameters to gain satisfactory surface shape. But the machining cycle time is not optimized. When setting the process route in the machine interface, going to the next step after reaching the target temperature rather than reaching the target time is usually set for the heating and cooling phase. Thus, the time to complete the heating and cooling stages of the production cycle is known only in the actual production. As for chalcogenide glass, its physical and chemical properties are greatly dependent on temperature. So, it is necessary to effectively simulate these stages to obtain the cost in time for actual production. Due to the excellent availability of numerical simulation, the rapid development of computing technology, and the increase of task scale in data and information processing, the finite element method can be applied to simulate the whole molding process. In this paper, a heat transfer model is established with the partial differential equation toolbox in MATLAB software. MSC.Marc software is used to simulate the heating stage at the same time. The numerical results are consistent, indicating that the heat transfer model established in MATLAB is, at least to a certain extent, valid. The heat transfer model needs further improvement by considering temperature-dependent properties such as viscoelasticity to make it a more effective tool for process analysis and optimization.

Ultrasonic assisted diamond turning (UADT) is a significant machining technology for the fabrication of a crack-free surface on single-crystal silicon. However, due to insufficient understanding of intermittent cutting characteristic, most researches have been only focused on the mechanism of ductile-regime machining rather than the improvement of surface quality and machining efficiency. Therefore, the novel machining model in UADT, ductile-brittle coupled cutting, is proposed to reveal how to realize the high-precision optical surface with larger processing parameters. Two quantitative performance indices, crack length projection and maximum tolerance length, are employed to evaluate whether a smooth surface can be achieved. And the variation of microscopic crack is analyzed and discussed in a single vibration cycle with different machining and tool parameters. In the experiments, the odd cosine surface is fabricated and the surface roughness Ra can reach 1.739 nm after measuring. The results show that better surface quality and higher machining efficiency can be achieved on single-crystal silicon by ductile-brittle coupled cutting in UADT.