Computer-controlled polishing has introduced determinism into the finishing of high-quality surfaces, for example those used as optical interfaces. Computer-controlled polishing may overcome many of the disadvantages of traditional polishing techniques. The polishing procedure is computed in terms of the surface error-profile and the material removal characteristic of the polishing tool, the influence function. Determinism and predictability not only enable more economical manufacture but also facilitate considerably increased processing accuracy. However, there are several disadvantages that serve to limit the capabilities of computer-controlled polishing, many of these are considered to be issues associated with determination of the influence function. Magnetorheological finishing has been investigated and various new techniques and approaches that dramatically enhance the potential as well as the economics of computer-controlled polishing have been developed and verified experimentally. Recent developments and advancements in computer-controlled polishing are discussed. The generic results of this research may be used in a wide variety of alternative applications in which controlled material removal is employed to achieve a desired surface specification, ranging from surface treatment processes in technical disciplines, to manipulation of biological surface textures in medical technologies.
High quality optical lenses are usually finished by magnetorheological finishing (MRF). In this process an abrasive
fluid, with the ability to stiffen in a magnetic field, is used as the polishing tool in a computer-controlled machine
tool. Although the machine is automated it is necessary for a skilled operator to set the machine and make
judgments with regard to its operation.
An investigation has been under way to examine the detailed operation of the MRF process, and the information
that is necessary to establish best practice. This has resulted in the incorporation of a knowledge based
system (KBS) into the machine control regime, and a methodology for the creation of artificial polishing tool
characteristics, the machine influence function. The incorporation of the these elements has been instrumental in
the operation of an enhanced MRF machine. This has been subject to extensive test procedures, and it has been
demonstrated that the production process may be enhanced significantly and consistently. Batch production
time may be significantly reduced, a figure in excess of a 50% reduction was met consistently during prolonged
operation. Furthermore the incorporation of the KBS is instrumental in increasing the automation of the MRF
process, reducing the levels of manual input necessary to manage machine operation.
Magnetorheological finishing (MRF) is a commonly used computer-controlled polishing (CCP) technique for
high precision optical surfaces. The process is based on a magnetorheological abrasive fluid, which stiffens in a
magnetic field and may be employed as a sub-aperture polishing tool. Dependent upon the surface error-profile
of the workpiece and the polishing tool characteristic (influence function) an individual polishing procedure is
calculated prior to processing. However, determination of the influence function remains a time consuming and
laborious task. A user friendly and easy to use software tool has been developed, which enables rapid computation
of MRF influence functions dependent on the MRF specific parameters, such as, magnetic field strength or fluid
viscosity. The software supersedes the current cumbersome and time consuming determination procedure and
thus results in considerably improved and more economical manufacture. In comparison with the conventional
time period of typically 20 minutes to ascertain an influence function, it may now be calculated in a few seconds.
An average quality improvement of 57% relating to the peak-valley (PV) value, and approximately 66% relating
to the root-mean-square (RMS) of the surface error-profiles was observed during employment of the artificial
computed influence functions for polishing.
KEYWORDS: Control systems, Process control, Surface finishing, Polishing, Optics manufacturing, Databases, Power supplies, Lens grinding equipment, Surface roughness, Software development
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 device's 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.
Magnetorheological finishing (MRF) is a computer controlled polishing (CCP) technique for high precision
surfaces. The process uses a magnetorheological fluid which stiffens in a magnetic field and thus acts as the
polishing tool. A standard MR fluid consists of magnetic carbonyl iron (CI) particles, nonmagnetic polishing
abrasives and liquid. To delaying oxidation of the iron particles and avoiding agglomeration the liquid consists
of water completed with stabilizers. For the material removal and smoothing of the surface mostly cerium
oxide or diamond is used. The materials to be polished may tend toward to different sedimentations of
the MR fluid on the machined surface. These sedimentations result from the machining and may develop a
polishing layer with MR fluid components. At the University of Applied Sciences Deggendorf analysis of the
machined surface are made by the scanning electronic microscope (SEM) to determine the sedimentations
of the finishing. The results of the research display the influence for the surface properties due to developing
polishing layer by magnetorheological finishing.
Magnetorheological finishing (MRF) is a computer controlled polishing process (CCP), which is commonly used in the field of high quality optical lens production. The process uses the material removal characteristic of the polishing tool (influence function) and the surface error-profile to calculate individual, surface error-profile dependent polishing sequences. At the University of Applied Sciences Deggendorf a testing series with a magnetorheological finishing machine has been performed, and effects of the influence function size and its removal capacity on the polishing quality and the process time have been investigated. The result of the research shows that the influence function size has a major effect on the process time, whereas the polishing quality is nearly independent of the influence function size. During the testing series the process time was significantly reduced using an appropriate influence function size. The process time decreased about 9% relating to the original influence function.
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.
Magnetorheological Finishing (MRF) is commonly used to finish high quality optical surfaces. The process is based on a magnetorheological fluid, which stiffens in a magnetic field and thus may be used as a polishing tool. The fluid removal characteristic depends on several parameters, for example the magnetic field strength or the relative velocity between workpiece and polishing tool. Another parameter is the fluid itself. Different compositions of polishing abrasives result in different removal characteristics. At the University of Applied Sciences Deggendorf, five different magnetorheological polishing fluids have been analysed. The results of the research are scanning electron microscope analyses as well as spectra analyses. The removal characteristic for each fluid has been determined for different glass materials. Finally, the fluid conditions during polishing have been analysed. For this purpose, the fluid flow rate, the fluid pressure and the fluid viscosity have been investigated.
In Magnetorheological Finishing (MRF) a magnetic field is applied to a stream of abrasive magnetorheological fluid, in order that the fluid behaves as the polishing tool. The process may be used to finish the surface of high quality optical lenses. The fluid viscosity is one important parameter the polishing tool characteristic depends on. At the University of Applied Sciences Deggendorf a new viscosity measurement, which uses the inductance of the fluid had been tested. The result of the research is a close relationship between viscosity and inductance. The new viscosity measurement is not an absolute, but a comparative system, based on inductance of the flowing fluid and the fluid age.
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.
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.
The demand on quality of optical surfaces is increasing from year to year.
Computer controlled polishing is one way to fulfill these demands.
The process depends on the error-profile of the optical surface.
In this paper the usage of the TII measurement machine is discussed to manufacture optical surfaces.
Since end of 2003 the TII-3D - the new contact topography measuring device for measuring aspherical and spherical surfaces - is available on market. Due to its novel technology, the system is specified to measure a large range with λ/10 accuracy, therefore being a very flexible tool for pre- and post-measurements in high quality zonal polishing processes like MRF. At the University of Applied Sciences Deggendorf a testing series has been carries out to compare the results of the TII-3D with CGH-interferometric measurements on aspherical surfaces. An analysis of the measurement errors is shown and ranking of the different metrology systems for production processes of high quality aspherical lenses is given.
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.
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.
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