Using the method of double-sided polishing with oscillating and rotating upper disk, we have investigated the material removal behavior of flat fused silica. It is suggested that the material removal characteristics of the optics depend on the positon and size of the upper polishing disk. Measurements together with the kinematic model results show that the effect of the swing position and size of upper polishing disk similar to sub-aperture polishing, leading to the non-uniformity distribution of the material removal on the surface of the optic component. the surface figure errors of 0.5 wavelengths is achieved after polishing process. The efficiency and quality of double-sided polishing can be improved through changing polishing disk and swing position according to this study.
Double-sided rapid polishing machine is a new type of polishing equipment which utilizes the principle of chemical mechanical polishing and the shape precision copying mechanism to remove materials from the upper and lower surfaces of optical elements. To analyze the principle of double-sided rapid polishing machine, a material removal model based on Preston equation was established, and the relationship between the parameters such as eccentricity, rotational speed ratio and swing stroke and the material removal uniformity was obtained, which provided a theoretical basis for the double-sided rapid polishing process. With 430mm×430mm×10mm large diameter optical element as the experimental object, the double-sided rapid polishing test was carried out, and the precision index of surface shape accuracy λ/2 was obtained, which realized the high efficiency and high precision machining of plate optical element.
Based on the motion mechanism of double-sided polishing, the motion trajectory model of polishing pad relative to medium-aperture optical components was established. This research simulated the trajectory of the abrasive particle relative to the optical component, and then the trajectory was obtained under different processing parameters. Based on the simulation results, different processing parameters were optimized which had been used for double-sided polishing experiments of optical components. The surface roughness and figure of both sides of the optical component after double-sided polishing were detected. The detection results showed that the surface roughness approached 0.5nmRMS, and the surface figure was 0.5λ(λ=632.8nm) for 150mm×150mm×7mm specification medium-aperture optical components.
The edge roll off seriously restricts the further improvement of the overall surface figure accuracy in CP of large planar optics. Firstly, the half width and depth of large planar optics’ edge region is proposed to quantify the roll off degree by analyzing its radial equivalent profile. Then, a two-dimensional symmetrical model of the large planar optics and the pitch lap is established. Next, based on the finite element model, the influence of extension block’s bonding state on the large planar optics edge’s stress concentration is analyzed. Finally, the effectiveness of this proposed method is verified by CP of large planar optics with extension blocks.
In planar optics machining, surface flatness is a critical requirement for making high-quality optical devices. Chemical mechanical polishing (CMP) is an important process of smoothing surfaces for hard and brittle planar optics. Thus, there are growing interests in developing high-precision flatness on-machine measurement system for tin plate CMP process. However, the traditional evaluation measurement cannot meet the measurement accuracy requirement of approximate 2 μm. In this study, we propose an effective on-machine measuring system for the large-caliber optical tin plate, which consists of linear guideway, rotary table, and measuring sensor. In the system, the straightness error of the guideway and the axial runout error of the rotary table are compensated using a spiral method and the Keyence LK-G5000 measuring sensor. First, the spiral measuring path is formed by the movement of the line guideway and rotary table, which can prevent from multiple calibration. Then, the 3D flatness error of the dressing process is measured by linear movement of the LK-G5000 and the rotation of the tin plate. After the primary measurement process, the obtained 3D flatness error is compensated for straightness and axial runout error in sequence. To verify the accuracy of the flatness measurement, the tin plate is dressed by the natural diamond tool according to the measured 3D flatness error. After the dressing process, the peak-valley value of the remeasured flatness error can achieve 2.12 μm through a single-step dressing. The experimental results provide the accuracy and reliability of the high-precision on-machine flatness measurement system for the large-caliber tin plate.
In order to improve the grinding quality and efficiency of SiC elements, the influence of grinding parameters on the grinding quality was studied by using 2MK1760 ultra-precision surface grinder. Based on the surface roughness and surface precision, the relationship between grinding volume and grinding force of metal bonded and resin bonded diamond grinding wheels was researched, and it was obtained that the metal bonded grinding wheels could maintain enough sharpness during a large amount of grinding removal. The grinding force was used to characterize the grinding wheel wear, and through experiments studied the relationship between grinding parameters (including grinding depth) and the SiC material removal volume after grinding wheel one-time dressing. The results showed that different grinding depth will not affect the grinding wheel life under the same other conditions. On the basis of technological experiments, aiming at optimizing grinding efficiency and quality, resin bonded plane grinding wheel was used to grind SiC elements with three diameters of 75mm, 150mm and 320mm, and very small surface precision (PV=3.758μm) and surface roughness (RMS=35.472nm) could be obtained.
Based on the ultra-precision grinder, an experimental platform for single abrasive indentation was built to investigate the effect of single abrasive particle on the fused silica optical element in the grinding process. In the paper, the process of crack formation and propagation in the element was analyzed. Two crack growth nuclei were formed on both sides of the indentation point just after indentation. With the increase of the indentation depth, the crack size increased and those two crack growth nuclei joined together, and the longitudinal cracks torn and extended to the interior of the material. Experiments were carried out at different indentation depths, and the variation of force with indentation depth was obtained. The crack morphology was measured by microscopy, and the variation of transverse crack radius and longitudinal crack depth with indentation depth was obtained. The results had important engineering significance for improving the grinding process.
Chemical mechanical polishing (CMP) is the most important process for global planarization. The micro material removal and planarization of the optical surface is a complicated process, and the surface shape of optics is effected by kinematics, pressure, and chemical conditions. Moreover, it is a remarkable fact that the distribution characterization of polishing particles also has an important effect on material removal uniformity, especially for leather pad and Tin polishing lap. Large optics were always polished to a convex shape for the low density of valid abrasives in optic center. The porosity and grooves distribution of pad plays a major role in slurry delivering. The novel model of contact and material removal is presented in which pad characterization, and polishing particles delivery and distribution effects are included. With the modified pad asperity and optimized grooves, the particles have been inclined towards well-distributed, and experiments validated that the optic figure is significantly promoted.
In this paper, the polishing properties of the zirconia slurry on the fused silica were studied, comparing with the effect of the ceria polishing. The polyurethane without dopant was applied as polishing pad. The experiment results show that it is easier to produce surface scratches when compared with ceria polishing owing to the high Mohs hardness of zirconia. The surface roughness(Rq) of the workpiece was less than 1nm after polishing with submicron size polishing slurry and suitable polishing pad. The R-on-1 test of 355 nm laser-induced damage threshold for fused silica show that zirconia polishing performance is 3.02% higher, however the exposed subsurface damages with HF etching were more than ceria polishing.The subsurface defect density is 0.10def/cm2 (@1μm)with ceria polishing and 1.19def/cm2 (@1μm)with zirconia polishing. Based on the fine polishing performance and abundant resources in nature, the zirconia slurry may be an alternative for fused silica polishing in prospect with the appropriate technological solution.
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