Markus Schinhaerl

Temporal stability and performance of MR polishing fluid

The lifetime of standard magnetorheological (MR) polishing fluids, used for example in polishing machines for optical applications, is limited. Scanning electron microscope examinations as well as chemical analyses of the fluid had been undertaken in order to investigate reasons for limited lifetime. We found out that the removal rate decreases during the course of time. However, the usable fluid life is most limited by the point of time when the critical minimum amount of fluid, necessary to ensure circulation, is reached. The results in association with a new fluid conditioner show, that a standard MR polishing fluid may be used for longer periods than common periods of about 2 weeks.

Relationship between influence function accuracy and polishing quality in magnetorheological finishing

Magnetorheological finishing is a typical commercial application of a computer-controlled polishing process in the manufacturing of precision optical surfaces. Precise knowledge of the material removal characteristic of the polishing tool (influence function) is essential for controlling the material removal on the workpiece surface by the dwell time method. Results from the testing series with magnetorheological finishing have shown that a deviation of only 5% between the actual material removal characteristic of the polishing tool and that represented by the influence function caused a considerable reduction in the polishing quality. The paper discusses reasons for inaccuracies in the influence function and the effects on the polishing quality. The generic results of this research serve for the development of improved polishing strategies, and may be used in alternative applications of computer-controlled polishing processes that quantify the material removal characteristic by influence functions.

Correlation between influence-function quality and predictability of a computer-controlled polishing process

A mathematical method has been developed to analyze influence functions that are used in a computer-controlled polishing process. The influence function itself is usually generated by some kind of calibration where the exact procedure is dependent on the process used. The method is able to determine asymmetries in an influence function. Application of this method yields a value that may be used to judge the quality of an influence function. That quality is also an indicator of the variance of the evolving surface error profile, since a close relationship between it and the polishing process exists. On the basis of an ideal, theoretical process, a model to handle and quantify the result of a real polishing process is described. Practical application of this model demonstrates the effect of influence-function quality on the polishing result. Based on this model, the predictability of the polishing result is evaluated. This initiative to judge influence functions by their quality is an important contribution to the development of computer-controlled polishing. Due to improved process reliability, the reject rate will decrease, and the result will be more economic manufacture.

Physical marker based stitching process of circular and non-circular interferograms

The usage of stitching technologies in the interferometrical precision optics measurement technique becomes more and more popular. There exist already a few metrology stages providing the stitching principle, such as, for example, the well known Sub-Aperture Stitching Interferometer for Aspheres (SSI-A1) [1] [2] [3] from QED technologies. For measurements with the SSI-A the greatest measurable diameter of the test object is approximately 280 mm [1]. As a consequence the University of Applied Sciences Deggendorf develops an own measuring system in order to test large telescope mirrors with a diameter of more than one meter which should be ready for application in 2012. The expected positioning accuracy of the measuring patches is significantly lower in comparison with the high-accurate SSI-A. Therefore a cross-correlation based translation detection tool is implemented in our current software solution. Since the metrology system is currently being established the SSI-A and the μPhase2 interferometer from TRIOPTICS are used as input data sources for the software development. Further this paper discusses the robustness of the translation detection tool and presents a stabilisation method of the stitching result with the aid of physical markers.

Material removal study at silicon nitride molds for the precision glass molding using MRF process

High-technology applications which are using high precision optic components in high and medium quantities have increased during recent years. One possibility to mass-produce e.g. such lenses is the precision glass molding (PGM) process. Especially for aspheric and free-form elements the PGM process has certain advantages. Premise is to manufacture accurate press molds, which have to feature smaller figure errors as the required lenses and may be made of materials, which are difficult to machine, like silicon nitride ceramics. These work pieces have to be machined in economical and steady process chains. However, due to the complex shapes and the corresponding accuracy an error dependent polishing is required. The Magnetorheological Finishing (MRF) as a high precision computer controlled polishing (CCP) technique is used and will further be presented in this work. To achieve the postulated demands a previous study of the material removal at selected machining parameters is needed. Changing machining parameters modify the removal, which is presented through values like the peak and volume removal rate. The value changes during the controlled variation of process parameters are described and discussed. Magnetorheological Finishing (MRF) provides one of the best methods to finish PGM molds that are relatively inaccurate to high precision in an economical, steady and efficient way. This work indicates the MRF removal selection and removal interference for the correction and finishing of precise silicon nitride molds for the precision glass molding.

Design and development of a novel computer controlled power device for electrical-assisted optical grinding

The main objective of this article is to introduce a novel power device for electrical-assisted micro-grinding, which could reduce the ambiguities reported and experienced during grinding. For example, the devices software is equipped with a knowledge database that automatically sets suitable electrical parameters for the instructed fine grinding parameters. The parameters are controlled throughout the process in order to achieve the stringent specifications required for further advanced polishing processes or establishing mirror surface finish on optical components.

Analysis of thermal sources in a magnetorheological finishing (MRF) process

Magnetorheological finishing (MRF) is a computer controlled polishing (CCP) technique for high quality surfaces. The process uses a magnetorheological fluid which stiffens in a magnetic field and thus acts as the polishing tool. At the University of Applied Sciences Deggendorf thermal sources in a MRF polishing unit have been analysed using an infrared camera. The result of the research is a warming of the fluid in the fluid conditioner caused by the mixer motor. The existing cooling is therefore essential, in order to ensure a constant polishing tool characteristic during polishing runs. A new fluid conditioner, which was developed at the University of Applied Sciences Deggendorf, with the aim of an extended fluid lifetime may be used without cooling, because an increase of the fluid temperature in the conditioner could not been detected. Furthermore, a warming of the workpiece during the polishing process was not ascertainable.

A new approach to predict computer controlled polishing results

A novel approach to handle and quantify a computer controlled polishing process will be introduced. This approach will be compared to real data. This comparison indicates the correctness of this approach. Based on it a formula has been developed to predict the results of a computer controlled polishing process. The formula will be used to predict real polishing processes and the results will be compared to the real results. The limits when using this formula will be shown along with suggestions when the formula would be useful. This rough prediction of the computer controlled polishing results may be used to enhance the automation of a computer controlled polishing process. Also a way to improve the formula itself will be introduced. It is the opinion of the author that by further stabilizing of the whole computer controlled polishing process the whole system becomes more robust, the prediction more accurate and the whole system improves in reliability and the results become better.

Prediction of MRF polishing by classification of the initial error with Zernike polynomials

The magnetorheological finishing (MRF) process makes use of a magnetically stiffened magnetorheological abrasive fluid to polish the surface of a workpiece in a precise fashion. The process may be used to finish the surface of high quality optical lenses. Investigations have been undertaken to quantify the operation of MRF and to identify those parameters key to an optimal operation of this lens production process. A correlation has been developed to relate the parameters important to the removal characteristics and to the precision of the polishing result and to the duration of polishing. A relationship to indicate the most appropriate MRF processing parameters for a lens is presented. In the examples discussed Fringe-Zernike polynomials are used to quantify the error on a lens.

Roughness improvement in Active Fluid Jet Polishing (A-FJP) by optimization of the polishing pin

This paper reports on an improvement of the surface roughness in A-FJP by utilisation of a perforated polishing pin. The University of Applied Science Deggendorf is currently working on the A-FJP process to investigate the effects on the resulting surface roughness on optical lenses.