The manufacturing of optical freeform surfaces offers a high potential for optical approaches in the future, since they can make new optical systems lighter and more compact or even enable completely new functions, compared to conventional optics. However, the expanded possibilities go hand in hand with higher complexity in production of freeforms for precision optical applications. This leads to high prices and long delivery times. This paper shows an approach to improve manufacturing of freeforms in small batch sizes with a high degree of customization, by a process chain consisting of (ultrasonic-assisted) pre- and fine grinding combined with ultra-fine grinding using resin bond tools. The process chain is suited for efficient fabrication of optical surfaces. A main focus of the experiments is on reduction of low- and mid-spatial frequency surface deviations, as well as surface roughness. Several different influencing factors in a 5-axis CNC grinding process of fused silica freeforms are investigated and their effects on the resulting surface topography (from the low to the high frequency range of surface deviations) are observed using white light interferometry measurement principles. Various optimization approaches can be concluded.
The investigations carried out deal with real-time evaluation and recording of vibrations during a CNC grinding process, as well as the analysis and control of process influences on the surface quality of optical components. The experiments were carried out on a 5-axis CNC machine. At the beginning, in planar grinding experiments the present vibration is detected with state-of-the-art vibrometric and optical measurement technology from Polytec. It enables in-process recording of the process vibrations and rapid subsequent topography analysis with the aid of white light interferometry. The measurements aim is to reduce the surface deviations (roughness, mid spatials, waviness) influenced by process vibrations. By carrying out a detailed modal analysis of the CNC machine used, it is possible to detect vibrations and vibration transmissions on the machine and to derive the necessary sensor concept a nd setup. Initial in-process measurements with the aid of a fiber vibrometer showed that the vibrations detected during the grinding process correlate to a high degree with the recorded topography data. It is thus possible to find the characteristic vibrations dependent on the grinding parameters as surface structures on the components, which makes it possible to predict the surface quality produced in the process.
In many optical applications plane optical elements are needed for deflection or splitting of light. The functionality of these parts is usually reduced to one certain task and manufacturing of them needs a big amount of effort to achieve high precision. A new optical element is presented, which can fulfill different optical functions simultaneously, is simply built and robust. It is based on glass cuboids joined together under a certain angle while introducing the possibility of fine adjusting the angle in the manufacturing process. The basic components can be modified to offer a lot of different applications. A technological process development is presented as well for more efficient machining of these new parts and existing part geometries. Novel manufacturing processes like ultrasonic grinding, ultrafine grinding and diffusion joining are experimentally investigated, since they promise significant improvements compared to conventional methods. Finally, an appropriate process chain is developed.
The production of complex glass components with 2.5D or 3D-structures involves great effort and the need for advanced CNC-technology. Especially the final surface treatment, for generation of transparent surfaces, represents a timeconsuming and costly process. The ultrasonic-assisted grinding procedure is used to generate arbitrary shaped components and freeform-surfaces. The special kinematic principle, containing a high-frequency tool oscillation, enables efficient manufacturing processes. Surfaces produced in this way allow for application of novel smoothing methods, providing considerable advantages compared to classic polishing. It is shown, that manufacturing of transparent glass surfaces with low roughness down to Rq = 10 nm is possible, using an ultra-fine grinding process. By adding a CO2-laser polishing process, roughness can be reduced even further with a very short polishing time.
The fabrication of high-quality optical components involves great effort. Polishing often functions as the final step in a manufacturing chain. To reduce the conventionally time-consuming, complex polishing process with loose grain, an interesting approach with novel resin bond grinding tools is presented for surface smoothing. Various processing-experiments were carried out, regarding different silicate materials such as BK7® and fused silica. Among other results it is shown, that good surface qualities with low roughness down to Ra = 8 nm or Rq = 10 nm can be achieved so far, a quality that already allows speaking of “ultra-fine” grinding. This results in remarkable possibilities to reduce conventional fine-machining procedures with loose abrasives. The fine grinded components can directly be polished to finally smooth the surface and remove remaining defects. Total-processing-times can be strongly reduced, involving significant economic advantages.
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