Polishing tools for high mechanical loads

To ensure that polishing tools can withstand high loads and temperatures, a unique manufacturing technique has been developed that bonds the PU polishing layer to the base on a molecular level.

When the first modern progressive lens came onto the market in series production in 1959, a breakthrough in spectacle optics was achieved. Since then, the production technology for aspherical surfaces has developed rapidly: since the early 1990s, the methods of classic optics have no longer been practicable on an industrial scale. Thanks to the use of modern CNC machines, the polishing time for aspherical optics has been reduced from 40 to around four minutes. The problem with this was that the required high precision of 1 µm - which is even tighter in the field of fine optics - could only be achieved with elastic polishing runners by means of a lengthy approach. Spot tools, on the other hand, can reduce polishing errors from the outset and correct them better later. However, due to the small contact surface, significantly more force is exerted on the optics. Polishing wheels and membranes must therefore have particularly high mechanical and thermal properties. OAT-Technologie GmbH achieves this through a unique manufacturing technique that bonds the PU polishing coating to the base on a molecular level. The tools can withstand the high temperatures of more than 200 °C in the working gap and can process around 2,000 surfaces per polishing wheel with 100% reproducibility.

One of the most spectacular and at the same time most widespread applications for aspheres in the last 20 years has been individually optimized progressive lenses. Although these are everyday objects, they pose major challenges for manufacturing technology. During the refraction process, the ophthalmologist or optician not only determines the need to correct a vision defect such as myopia, hyperopia or astigmatism. Individual physical characteristics such as interpupillary distance, corneal vertex distance, eye pivot point and the optimum frame tilt are also taken into account. Usage habits such as office work or outdoor sports can also be taken into account. A combination of all parameters results in a theoretical number of variants of around 160 billion lens types. This number of variables makes stocking progressive lenses simply impossible: they can only be produced individually on demand.

"Shaping aspherical lenses is technically feasible and efficient with CNC machines," says Lutz Küpper, founder and Managing Director of OAT-Technologie. "However, the final work step, polishing, is a different story." For varifocal lenses, optical deviations must not exceed 0.05 dpt. For production technology, these requirements mean geometric accuracies of around 1 µm - with a total polishing removal of around 20 µm. With flexible polishing shells, as used in the early days of CNC production in this field, it is not so easy to maintain these tolerances, as local pressure differences and non-uniform properties of the polishing runner cause fluctuations in the removal. The production of an optic is therefore subject to several iteration cycles, which makes the process complex, time-consuming and expensive - and unsuitable for biometric progressive lenses, which nowadays have to be produced within a very short time.

"In order to reduce the uneven polishing removal of free-form optics, the contact surface to the workpiece was reduced to a maximum of 5 to 8 mm using so-called spot tools," explains Küpper. "Thanks to the precise calculation and control of a CNC machine, any aspherical surfaces can be polished true to geometry in this way." However, this poses a new challenge for production technology: in order to operate the more complex CNC systems profitably, the previous polishing time of 40 minutes had to be reduced to around four minutes. In combination with the reduction in tool surface area, this means that many times more energy is applied to the glass in the effective gap. In addition to the high mechanical pressure, process temperatures of more than 200 °C are generated there. Conventional adhesives, which are usually used to apply the polishing layer to the base body of the polishing wheels or membranes, become brittle under these conditions. This not only necessitates frequent machine downtimes and tool changes, but also reduces the dimensional stability of the spot tools themselves. As a result, the polishing results are again subject to major deviations that go beyond the tolerable range.

For Lutz Küpper, a mechanical engineer at leading German eyewear manufacturer Rodenstock, who played a key role in the development of the first polishing wheels in the 1990s, it quickly became clear that in order to meet the high demands on the mechanical and thermal properties of the polishing tools, it was necessary to start with the production technology. Under the umbrella of OAT-Technologie, he has therefore been implementing a unique manufacturing principle for polishing wheels since 2008 and for polishing membranes since 2015. "The polyurethane (PU) polishing coating is injected into the material of the base body," explains Küpper. "The result is a bond at molecular level that makes the use of sensitive adhesives superfluous." Because the tools are produced in a mold, they have a very high degree of dimensional accuracy, which is ultimately the prerequisite for the required reproducibility of the polishing results.

Today, OAT's spot tools are used worldwide in varifocal spectacle optics and precision optics. Due to their robustness and longer service life, polishing wheels are particularly suitable for processing aspherical lenses: they represent an economical solution for established processes with known products. Polishing membranes, on the other hand, are more flexible and allow the spot size and polishing pressure to be decoupled from each other via separate pressure control. However, due to the lower coating thickness, they have a shorter service life and are therefore particularly suitable for small series and new start-ups. "Thanks to the mechanical and molecular cross-linking of the PU coating with the base, the polishing wheels and membranes achieve a significantly longer service life than conventional PU-covered spot tools," adds Nils Koch, Managing Director of Optic & Electronic Koch GmbH, which supports OAT users as a sales partner. "Our customers report 2,000 processed surfaces per polishing wheel. And OAT polishing membranes have already been used continuously for more than five days to polish telescope mirrors." (OM-2/25)