On-machine measurement and Zernike-based compensation cutting for large-aperture additive metal mirrors with ultra-high lightweight ratio

In recent years, large-aperture additively-manufactured metal mirrors have been widely used in optics, astronomy, and aerospace. However, achieving precision diamond cutting of these mirrors remains challenging due to their intrinsic non-uniform lightweight structures. On-machine measurement of surface errors and compensation cutting offer promising solutions; however, current techniques for large-aperture metal mirrors remain insufficiently mature. Furthermore, compensation cutting is hindered by the coupling of multiple error factors, including inherent equipment errors, measurement uncertainties, and surface fitting deviations. To address these challenges, this study developed an on-machine measurement system combined with a compensation cutting method based on Zernike polynomials. Unlike compensating for the entire surface error, the first 36 Zernike terms of the entire surface error were extracted for compensation. In the experiment, an additively-manufactured AlSi10Mg mirror with a diameter of 510 mm and a lightweight ratio of 86% was fabricated. To the best of our knowledge, an additively-manufactured mirror of such a diameter with such a high lightweight ratio has never been reported in the literature. Using the proposed on-machine measurement system, a repeatability of 50 nm root-mean-square (RMS) was achieved. Furthermore, comparative validation with coordinate measuring machine results confirmed the reliability of the on-machine measurement system. Subsequently, Zernike polynomials were employed to reconstruct the surface errors. After a single compensation cutting process, the surface error decreased from 0.636 μm to 0.30 μm (RMS), achieving a convergence rate of 52%. The experimental results demonstrate that the proposed on-machine measurement methodology, combined with an error compensation strategy, significantly improves both precision and efficiency of ultra-precision machining.