With the increase of the diameter of the silicon carbide (SiC) mirror, the amount of material removed increases dramatically. The SiC material has the characteristics of high hardness and chemical stability, so the higher requirement of the convergence efficiency is put forward. The current SiC mirror processing is carried out through a series of different processes or parameters. The combined processing converge the surface residual error. But this method relies on the experience of personnel experience to determine the transfer conditions of process sequencing. This paper research on improve the convergence efficiency at the same time ensuring the accuracy of processing. The quantified evaluation function is put forward to help estimating the transfer conditions of process sequencing. On this basis, the multi-process combination calculation is carried out. The dwell time of multiple remove functions is solved at the same time in the optimization process. The combination of large and small removal function is optimized. Combined with the simulation calculation, the optimized process of large diameter silicon carbide mirror is given.
As the basis of fewer-axis grinding of complex surface, the grinding mathematical model is of great importance. A mathematical model of the grinding wheel was established, and then coordinate and normal vector of the wheel profile could be calculated. Through normal vector matching at the cutter contact point and the coordinate system transformation, the grinding mathematical model was established to work out the coordinate of the cutter location point. Based on the model, interference analysis was simulated to find out the right position and posture of workpiece for grinding. Then positioning errors of the workpiece including the translation positioning error and the rotation positioning error were analyzed respectively, and the main locating datum was obtained. According to the analysis results, the grinding tool path was planned and generated to grind the complex surface, and good form accuracy was obtained. The grinding mathematical model is simple, feasible and can be widely applied.
In order to achieve aspheric high efficiency and low damage fabrication，and find the balance between damage and efficiency during the lapping, a SSD model based on fracture mechanics is given. Based on three body wear theory， a material removal model has been given. Based on the above two models, effective removal rate of damage model has been presented. The relationship between effective removal rate of damage and various factors has been analyzed by simulation. The effective removal rate of damage of diamond powder is smaller than the silicon carbide, so it is easy to achieve high efficiency and low damage fabrication. The presentation of effective removal rate of damage provide an evaluation criterion for aspheric high efficiency and low damage fabrication.
Computer Controlled Optical Surfacing (CCOS) is an important technology for manufacturing optical aspheric mirrors. Edge effect of small tool manufacturing restricts the machining precision and efficiency of CCOS technology. Edge effect is mainly caused by the polish tool cannot move to the very edge of workpiece and the change of pressure distribution when the tool move to the edge of workpiece. This article corrects the rolled edge effect of CCOS by different dimensions of polishing tool combination process and incorporated with the locality residual error trace contour path planning. Provide feasibility for the rolled edge by different dimensions of polishing tool combination process.
An ion beam figuring system (KDIBF2000) for final figuring of large size optics has been designed and built by National University of Defense Technology in China. It can figure optics up to the maximum dimensions of 2.0m×2.0m×0.4m with five axes of servo-motion used to control ion source movement. For operational facility, there are two vacuum chambers with main work chamber and a small supplementary chamber isolated by a flapper valve. The main chamber has two work zones, which can meantime hold a large optics with Φ1.5m and a small optics with 0.4m. The small optics can be transferred through supplementary chamber with a moving vehicle. By this way, it is very convenient and economical to gain the material removal function and check the system’s process performance. Now, this system has been gone into running to figure large SiC off-axis aspheric optics. Next step, a 1.2m SiC aspheric primary mirror will be figure by this system.
Specifications made on optical components are becoming more and more stringent with the performance improvement of modern optical systems. These strict requirements not only involve low spatial frequency surface accuracy, mid-and-high spatial frequency surface errors, but also surface smoothness and so on. This presentation mainly focuses on the fabrication process for square aspheric window which combines accurate grinding, magnetorheological finishing (MRF) and smoothing polishing (SP). In order to remove the low spatial frequency surface errors and subsurface defects after accurate grinding, the deterministic polishing method MRF with high convergence and stable material removal rate is applied. Then the SP technology with pseudo-random path is adopted to eliminate the mid-and-high spatial frequency surface ripples and high slope errors which is the defect for MRF. Additionally, the coordinate measurement method and interferometry are combined in different phase. Acid-etched method and ion beam figuring (IBF) are also investigated on observing and reducing the subsurface defects. Actual fabrication result indicates that the combined fabrication technique can lead to high machining efficiency on manufaturing the high-precision and high-quality optical aspheric windows.
The shape, size and material of grinding tool not only affect the machining accuracy, but also the machining efficiency in
the process of Computer Controlled Grinding. The hardness of the SiC aspheric mirror, the misfit of grinding tool and
the work-piece also emphasize the importance of grinding tool optimization. By means of analyzing the misfit property
of grinding tool and aspheric optic theoretically, as well as the wear character and the process of the grinding tool
experimentally, this manuscript establishes the rule of grinding tool optimization satisfying different machining
objective. Based on this, the adopted grinding tool was optimized in the grinding process of SiC off-axis aspheric
(634mm×560mm). The simulation provide reasonable grinding tool for the off-axis aspheric grinding, and good results
(large amount material removal and edge error figuring) are obtained when the optimized grinding tool are applied. Both
of the simulations and experiments demonstrate the feasibility and correctness of the grinding tool optimization method.
Due to the different curvature everywhere, the aspheric surface is hard to achieve high-precision accuracy by the traditional polishing process. Controlling of the mid-spatial frequency errors (MSFR), in particular, is almost unapproachable. In this paper, the combined fabrication process based on the smoothing polishing (SP) and magnetorheological finishing (MRF) is proposed. The pressure distribution of the rigid polishing lap and semi-flexible polishing lap is calculated. The shape preserving capacity and smoothing effect are compared. The feasibility of smoothing aspheric surface with the semi-flexible polishing lap is verified, and the key technologies in the SP process are discussed. Then, A K4 parabolic surface with the diameter of 500mm is fabricated based on the combined fabrication process. A Φ150 mm semi-flexible lap is used in the SP process to control the MSFR, and the deterministic MRF process is applied to figure the surface error. The root mean square (RMS) error of the aspheric surface converges from 0.083λ (λ=632.8 nm) to 0.008λ. The power spectral density (PSD) result shows that the MSFR are well restrained while the surface error has a great convergence.
The high-precision aspheric surface is hard to be achieved due to the mid-spatial frequency error in the finishing step.
The influence of mid-spatial frequency error is studied through the simulations and experiments. In this paper, a new
polishing process based on magnetorheological finishing (MRF), smooth polishing (SP) and ion beam figuring (IBF) is
proposed. A 400mm aperture parabolic surface is polished with this new process. The smooth polishing (SP) is applied
after rough machining to control the MSF error. In the middle finishing step, most of low-spatial frequency error is
removed by MRF rapidly, then the mid-spatial frequency error is restricted by SP, finally ion beam figuring is used to
finish the surface. The surface accuracy is improved from the initial 37.691nm (rms, 95% aperture) to the final 4.195nm.
The results show that the new polishing process is effective to manufacture large-aperture and high-precision aspheric
Modern optical systems require high quality and compaction at the same time. Steep aspheric mirrors are more and more
widely used due to its excellent optical property and high compact structure, but they also put new challenge to optical
polishing machine and polishing technology. A novel offset tool axis method is presented to improve the old machine’s
polishing ability of steep aspheric mirrors and make good use of the old machine. The simulation shows the offset tool
axis method can alleviate non-uniformity distribution of pressure between polishing tool and workpiece, while it also
shows the method can enhance the stability and controllability of the polishing process. The pseudo ρ-θ scanning method
which combines merits of raster scanning and spiral scanning is presented for offset tool axis method. A 475mm
diameter RB-SiC mirror with a relative aperture of 1:1 is polished and surface error reduces to 0.175λ (PV)/0.009λ
(RMS) from 0.527λ (PV)/0.079λ (RMS). The result proves the feasibility of the offset tool axis method and pseudo ρ-θ
Combination technology of MRF and sub-aperture smoothing in off-axis asphere manufacture was researched. The
asphere polishing with computer control polishing is not very deterministic , as removal rate of computer control
polishing is variation over time in asphere polishing due to the tool misfit, pad wear, or slurry variation.
Magnetorheological Finishing is deterministic, subaperture finishing technology in asphere manufacture, but it is limited
to smooth mid-spatial-frequencies and high-spatial-frequencies. An example was given in a 290mm circle aperture offaxis
aspherical optic polishing process with combination technology of MRF and sub-aperture smoothing. The final
figure error was λ/50 rms from the initial 0.8λ rms. The result shows that the combination technology is practical and
have high convergence efficiency.
Rigid tools can confer advantages at certain stages of manufacturing off-axis mirror segments, but the misfit due to surface asphericity and asymmetry poses constraints on their application. Types of misfit are classified and, using least squares, the best-fit tool forms with different distances from the pole of the parent asphere are calculated. The outer mirror segment for the European extremely large telescope is taken as a case-study, assuming a rigid tool size of 150 mm. A simple independent approximation validates the calculation. A close parallel is wavefront misfit in subaperture interferometry, which is also considered.