Objectives: Amid the booming development of large astronomical telescopes, space remote sensing cameras and other technologies, the surface quality and the manufacturing efficiency of ultra-large-diameter optics also put forward high requirements different from traditional optical components, bonnet polishing technology with its adjustable pressure, good surface conformability, high polishing efficiency, and other technical advantages make its application in the processing of ultra-large-diameter optics also has potential, according to the processing needs of ultra-large-diameter optics, and to localize the large-size bonnet, design the large size bonnet with a spherical crown radius of 320mm to adapt 1m-2m optical workpiece high-precision polishing.
Methods : A finite element analysis (FEA) model of the bonnet under load is established using a finite element software package. Then, the reliability of the simulation model is validated by static loading experiments with a small-sized bonnet with a spherical crown radius of 80 mm. In this study, a bonnet design method based on the principle of consistency of the average pressure is proposed. Based on this principle, the bonnet is optimized in combination with the simulation model so that the structure of a large-sized bonnet with a spherical crown radius of 320 mm is obtained.
Results : The FEA model of the bonnet under load is validated by taking the bonnet with a spherical crown radius of 80 mm as an example. When the compression ranges from 0.6 mm to 0.8 mm, the polishing-force error between the simulation and the measurement lies within 14% with the consistency of the contact pressure distribution with a near-Gauss shape. When the compression equals 0.9 mm, the error is 27.8%, while after the compression exceeds 0.9 mm, the error between the simulated and measurement polishing forces gradually exceeds 27.8% due to the gradual inward concavity of the actual bonnet. We can deduce that the simulation model established is feasible when the compression adapts to the spherical crown radius of the bonnet. Therefore, based on the principle of average pressure consistency and the FEA model of the bonnet, a large-size bonnet with a spherical crown radius of 320 mm is designed. The optimal design yields results in which thicknesses of three layers equal, respectively, inner rubber = 3 mm, middle metal = 1.2 mm, and outer rubber layer = 6 mm. The contact pressure distribution resembles a Gaussian shape when the applicable compression ranges from 2.4mm to 3.2 mm. The simulation analysis shows that not only the average pressure at the compression equaling 2.5 mm can be adjusted to suit the average pressure at the compression equaling 0.63 mm for the bonnet with a spherical crown radius of 80 mm, but also the area of the contact zone is greatly increased compared to the small size bonnet, with the radius of the polished contact zone reaching 40 mm, which about 2.5 times as much as the contact zone at the compression equaling 0.63 mm for the bonnet with a spherical crown radius of 80 mm, and the maximum contact pressure is 0.7889 MPa. Thus, the removal efficiency can be increased to accommodate the machining of ultra-large-diameter optics of 1 to 2 m with the same speed of the bonnet. Theoretically, the rationality and the suitability of the large-size bonnet with a spherical crown radius of 320 mm are validated.
Conclusions: Due to the widespread use of bonnet polishing technology in the field of ultra-precision machining, the processing of workpieces of different sizes and specifications has offered corresponding requirements on the suitability of the bonnet for processing. Therefore, in this paper, a simulation model of a bonnet with a three-layer structure of the spherical crown under load is established, and the commonly used bonnet with a spherical crown radius of 80 mm is used as the model to verify the polishing normal force. Also, contact pressure distributions obtained from the simulation match those in-lab measurements when the applicable compression ranges within 0.6-0.8 mm which adapts to the spherical crown radius of the bonnet. The design of the bonnet is based on the principle of average pressure consistency proposed herein, and the optimized design using the simulation model can lead to the expected bonnet structure and can help guide bonnet design in the future.