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J. Eur. Opt. Society-Rapid Publ. 21, 12( 2025)
a certain load and chip thickness, which spread and lead to the formation of small material particles [ 5 – 7 ]. This is the grinding regime of brittle fracture, which always leads to the formation of a certain roughness structure and thus to a matt surface on glasses, which exhibits both surface cracks and crack depth damage under the component surface, the so-called subsurface damage( SSD). These must be completely removed in subsequent ultra-fine processing steps to produce an optical surface quality [ 8, 9 ]. Material damage can be successively reduced through multi-stage grinding processes with reduction of the abrasive grain size( pre-grinding, medium grinding and fine grinding), whereby the removal rates also decrease with each finer sub-process [ 8 ]. A second grinding regime, ductile grinding, is of increasing interest in the field of ultra-fine machining. In addition to the introduction and propagation of microcracks, plastic deformation processes described as ductile behavior can also be detected when looking at the chip formation zone on a microscopic basis [ 5, 6 ]. The ductile grinding regime was first described by Bifano et al., and then substantiated mathematically and experimentally in a paper published in 1991. The critical cutting depth or chip thickness, below which predominantly ductile removal takes place instead of brittle fracture, is decisive for achieving this regime. This is based on the energy consideration of the material removal, whereby it has been shown that at low removal depths the plastic flow of the material is energetically advantageous compared to fracture formation [ 10 ]. For glass, this results in low mathematical values for the critical cutting depth of well below 100 nm for silicate glasses in some cases [ 8, 10 ]. In addition to achieving such low cutting depths, which is generally only possible with ultra-precision machines, fine grain sizes, high concentrations, high cutting speeds and low feed rates can also be used to obtain or support the ductile regime [ 6, 11 – 13 ]. Ductile grinding can thus prevent the formation of microcracks in the interaction zone between the component and the tool, enabling the production of workpieces with high surface qualities and damage-free surface zones [ 5, 6, 9, 12, 13 ].
In this paper, an approach is described using special resin bond tools with fine diamond grain sizes, where the properties of the resin matrix can be described as highly porous and slightly elastic. This elasticity reduces the penetration depth of grains into the glass material, while the pores improve the supply of cooling lubricant and chip removal. The process is further called ultra-fine grinding, since it offers the potential to create partially reflective and transparent surfaces with roughness Rq < 10 nm and waviness Wt < 1 lm observed for planar fused silica samples before [ 14 – 17 ].
Given the considerable variability of local curvatures, polishing of freeform optics requires the use of very small sub aperture tools to ensure the retention of the shape after grinding. In this context, the application of mechanicalabrasive sub-aperture polishing is limited due to permanently changing tool engagement conditions. However, efforts have been made to overcome this challenge through the integration of process simulation [ 18, 19 ] and the development of adaptive tools [ 20 ]. Alternatively, beambased tools may offer a viable solution. These non-contact techniques are not susceptible to tool wear during the machining process. A considerable number of research groups around the globe have been engaged in the development of laser-based polishing techniques. The CO 2 laser has been established as a method for laser polishing of glass surfaces [ 21 – 23 ]. A fast-scanning laser beam heats the surface of the sample up to the material-specific softening temperature, which leads to smoothing due to elevated viscosity and minimized surface tension [ 24 ]. However, thermal stress often results in structures in the mid spatial frequency( MSF) range not being smoothed or new MSF errors occurring due to the fast scanning process. As a result, an additional fine correction step is required for the optical surface [ 25 ]. As a beam-based tool, the atmospheric plasma jet( PJ) is also capable of polishing optical freeform surfaces [ 26 ]. The preservation of surface figure and the ability to recover from surface damage have already been proven [ 27, 28 ]. Plasma jet polishing( PJP) is a thermal process that results in localized heating of the surface near the softening point. Convective heating by the PJ causes the surface of the glass to melt, resulting in a reduction in viscosity and a thermally induced redistribution of material on the surface. Furthermore, the minimization of surface tension leads to a reduction in roughness. While the micro- and nanoscopic plasma-surface interaction is not yet fully understood, the reduction in surface roughness can be compared to the principle of polishing with a laser beam [ 21, 25 ]. In PJP, the plasma-based polishing tool, with its highly localized effect and small contact area, operates at a scanning velocity of approximately 1 mm / s along a meandering tool path. This setup enables a uniform and gradual heating and cooling of the material through tangentially flowing plasma gas, preventing steep temperature gradients. Thus, the generation of MSF can be significantly reduced.
The polishing effect in PJP can be mathematically described by a two-dimensional Gaussian function [ 29 ]. Once, the polishing function is known, the polishing result for a given initial surface can be estimated.
In the present investigations ultra-fine grinding is combined with PJP in order to achieve a time- and cost-efficient production of freeform optics made of glass. The aim of the investigations is the application of a new process chain, schematically given in Figure 1, which combines freeform generation by coarse grinding, fine grinding and ultra-fine grinding with a subsequent PJP step to obtain a fully functional optical surface. Due to the combination of the 3-stage grinding process and the thermal induced polishing the size of the workpiece is mainly limited by the dimensions of the machines and the CNC axis strokes. In addition, there are minor curvature limitations, which has to be considered in the machining set-up. Obviously, the smaller the grinding tool, the finer the curvature radii that can be produced. Furthermore, for PJP, the PJ and the attached temperature measurement system must be able to tilt parallel to the local surface normal. The ability to manufacture various freeform surface designs by preserving the ground geometry is one of the main advantages of PJP, together with the absence of tool wear.
Aspects of the individual grinding steps as well as the PJP process are discussed with regard to process convergence. By selecting the optimal parameters, a successful manufacturing of an Alvarez-type freeform optic will be demonstrated.