In the high-end optical instrument application, aspherical lenses have replaced spherical lenses and became a key component owing to the aberration correction characteristic it benefits. Though aspherical lenses truly provide multiple advantages, as long as the uncertainty and time-taking issue remain unsolved in CNC polishing process, the term “mass production” will still be far from realization. In this paper, we have developed a method based on Preston’s equation and the Hertz-Contact theorem (HCT) to construct the tool influence function (TIF), hoping to increase the convergence of the process result. We will also discuss how different tool offsets affect the polishing force against the workpiece. We firstly obtained velocity distribution between bonnet and workpiece from dynamics in polar coordinates, then applied the equivalent contacting-Young’s modulus in Hertz-Contact theorem to calculate the pressure distribution model. Subsequently, we conducted a series of experiments under IRP1000 by Zeeko Ltd. and avoided unstable outcomes caused by both machine vibration and deficient tool offset. We modified the parameters into five different feed rates while remaining the equivalent dwelling time, to create more observable features of material removed and further proved the linearity relationship between the dwell time and the removal depth. We applied LP66 as the polishing pad and fused silica as the workpiece to acquire the experiment result.
Spherical lenses lead to forming spherical aberration and reduced optical performance. Consequently, in practice optical system shall apply a combination of spherical lenses for aberration correction. Thus, the volume of the optical system increased. In modern optical systems, aspherical lenses have been widely used because of their high optical performance with less optical components. However, aspherical surfaces cannot be fabricated by traditional full aperture polishing process due to their varying curvature. Sub-aperture computer numerical control (CNC) polishing is adopted for aspherical surface fabrication in recent years. By using CNC polishing process, mid-spatial frequency (MSF) error is normally accompanied during this process. And the MSF surface texture of optics decreases the optical performance for high precision optical system, especially for short-wavelength applications. Based on a bonnet polishing CNC machine, this study focuses on the relationship between MSF surface texture and CNC polishing parameters, which include feed rate, head speed, track spacing and path direction. The power spectral density (PSD) analysis is used to judge the MSF level caused by those polishing parameters. The test results show that controlling the removal depth of single polishing path, through the feed rate, and without same direction polishing path for higher total removal depth can efficiently reduce the MSF error. To verify the optical polishing parameters, we divided a correction polishing process to several polishing runs with different direction polishing paths. Compare to one shot polishing run, multi-direction path polishing plan could produce better surface quality on the optics.