Christian Vogt

Forces acting between polishing tool and workpiece surface in magnetorheological finishing

Magnetorheological finishing is a computer-controlled polishing technique that is used mainly in the field of high-quality optical lens production. The process is based on the use of a magnetorheological polishing fluid that is able, in a reversible manner, to change its viscosity from a liquid state to a solid state under the control of a magnetic field. This outstanding characteristic facilitates rapid control (in milliseconds) of the yield stress, and thus the pressure applied to the workpiece surface to be polished. A three-axis dynamometer was used to measure the forces acting between the magnetorheological fluid and the workpiece surface during determination of the material removal characteristic of the polishing tool (influence function). The results of a testing series using a QED Q22-X MRF polishing machine with a 50 mm wheel assembly show that the normal forces range from about 2 to 20 N. Knowledge of the forces is essential, especially when thin workpieces are to be polished and distortion becomes significant. This paper discusses, and gives examples of, the variation in the parameters experienced during a programme of experiments, and provides examples of the value of this work.

ELID supported grinding of thin sapphire wafers

Sapphire material is, due to its crystal structure, difficult to machine in an economic way. There is a request for thin, i.e. below 0.2 mm thickness, sub surface damage free wafers to produce sensor elements. ELID -- electrolytic in process dressing -- is an innovative high end grinding technology, using small grain sizes, which enable to manufacture surfaces in a quality that is close to polished. ELID grinding requires exactly aligned machining parameters of the grinding process. To grind sapphire the materials behavior is additionally to be considered. Studies on the necessary oxide layer on the grinding wheel and influences on its build-up process will be presented. The presentation shows the results of comparing grinding experiments on different -- c-plane and r-plane -- sapphire materials. Different tool specifications are used. Infeed and grinding velocity are varied and the results on wear, removal rate and surface quality are shown. The process parameters the stiffness of the machine, the grinding forces and pressure are evaluated. The ELID grinding is compared in its results to conventional grinding steps. The material removal rate on sapphire is relatively small due to the extreme hardness of sapphire. The achieved excellent surface roughness will be discussed.

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.

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.

From turning to grinding: ductile machining with gPVA

Grinding processes are usually set up experimentally, product-specifically and iteratively. The selected parameters are often based on empirical values, whereby the user can set very different parameters and achieve similar results. In 2017, the so called “grinding process validation approach” (gPVA) was introduced to determine suitable parameter windows. The method allows the definition of parameter windows for grinding tools and materials. Parameter adjustments for optimum results are possible due to the experimentally determined dependence on specific chip volume and tool pressure. The system was originally developed to describe brittle grinding processes on standard grinding machines. Tests on an ultra-precision lathe provide process parameter data on ductile mode machining of tungsten carbide using UPM machines. This paper reports on gPVA being applied to transfer the ductile machining process from UPM machineries to a standard CNC grinding machine.

Ductile mode single point diamond turning (SPDT) of binderless tungsten carbide molds

Ductile mode grinding is a finishing process usually being applied to generate molds in brittle materials (e.g. tungsten carbide) to be used for precision glass molding (PGM). To that aim, ultra-precision machineries (UPM) are applied controlling depth of cut not to exceed a critical value, hcu,crit (e.g. 160 nm for tungsten carbide). Recent process analyses of the ductile mode grinding process of brittle materials have demonstrated that the critical indentation depth hcu,crit, that determines the transition from brittle mode to ductile mode removal, can significantly be shifted to higher values by adjusting process parameters such as the type of coolant and its pH value: e.g. for tungsten carbide up to 1600 nm and for BK7 glass up to 350 nm depth. This paper reports on a feasibility study to extend the process window of ductile mode material removal. Applying optimized ductile process parameter sets, enabling values of the critical depth of cut larger than 1 micron, single point diamond turning (SPDT) of binderless tungsten carbide molds has been successfully tested applying UPM machineries. Experimental data will be presented demonstrating that by controlling and adjusting ductile process parameters only, it is possible to extend its process window into regimes that are today not yet machinable: binderless tungsten carbide molds for precision glass molding have been processed in a ductile removal mode by SPDT generating surface roughness levels of less than 2 nm rms. An analysis of the adjustment of the critical process parameters will be presented together with a detailed description of the First Light experiments towards SPDT of binderless tungsten carbide molds.

Ductile grinding of tungsten carbide molds applying standard CNC machines

Ductile grinding offers several advantages over conventional approaches with grinding and polishing. Ductile grinding is usually carried out on Ultra-Precision-Machines (UPM), e.g. for the production of press molds. However, the machining volumes for ductile processes are limited and UPM are usually not suitable for high volume removal by brittle machining. Therefore, the pre-machining often takes place on conventional CNC grinding machines. In this paper, we publish first results on ductile machining of tungsten carbide dies on a standard CNC grinding machine. This approach enables the pre-machining and finishing of tungsten carbide dies in one machine, in one clamping with the same tool.

gPVA: a system for the classification of grinding tools

The Grinding Process Validation Approach (gPVA) presented in 2017 enables the determination of suitable parameter windows for grinding tools. The abrasion properties of grinding tools are determined experimentally. The collected data can be used to derive optimum parameters for defined grinding tasks so that service life, process stability and productivity can be maximized. In this publication, the gPVA method is used to compare different grinding tools. Differences in stock removal performance with identical specified tools from different manufacturers are investigated. In addition to that, recommended tools for fine grinding of fused silica are examined also.

Filled-Up-Microscopy (FUM): a non-destructive method for approximating the depth of sub-surface damage on ground surfaces

Subsurface Damages (SSDs) can cause a wide variety of defects to optical lenses and other components. In addition to the adhesion and quality of coatings, the mechanical stability, the transmission quality and the laser-induced damage threshold (LIDT) of the products, is also affected. It is, therefore, attempted to get components as SSD-free as possible at the end of the production chain. Already during the individual production steps, it is important to know the depth of the SSDs in order to remove them in the following manufacturing steps. To design the manufacturing processes efficiently and avoid damage, it is important to be able to measure the depth and characteristics of SSDs as precisely as possible. There are a several approaches and methods to determine SSDs known in literature. However, many of them inevitably lead to the destruction of the workpiece. Although others are non-destructive, but very complex in design and/or associated with large investments. Likewise, only a few are suitable for determining SSDs on ground rough surfaces. Filled-Up Miicroscopy (FUM) is an alternative approach to approximating the depth of SSDs, even on rough surfaces without destroying them. At a first glance at the method, the procedure is described in detail and all necessary steps of preparing the samples are shown. A first comparison with the known Ball Dimpling Method confirms the functionality of the concept.

Closed-loop next generation laser polishing

A novel fabrication parameter controlling method for laser polishing processes called CLasso (Control of LASer Surface Optimization) is presented, monitoring within the footprint the smoothening process as well as the removal of ssd in situ. Therefore, it is possible to determine and control the optimum dwell time a footprint needs to stay at a certain point before moving further enabling a more stable and cost optimized polishing.