Temperature is an important process factor in grinding. Cryogenic cooling has proven to be very successful in metal machining, for example, as oxidation is prevented by lowering the temperature level. This enables significantly higher cutting speeds and longer tool life. In our research, we aim to determine whether similar effects occur when grinding brittle-hard materials, which is why the temperature in the contact zone is explicitly examined here. This article presents a very simple and extremely cost-effective approach using a germanium window and an infrared temperature sensor. It explains how the system is constructed, what results can be achieved with it and how the system compares to other commercially available approaches.
Cryogenic machining is a cooling strategy that has recently been frequently found in research in machining processes for materials such as inconel or titanium. In particular it is reported that these processes are more efficient in terms of tool life and material removal rate compared to those with water-based cooling lubrication. An increase in efficiency and productivity has significant potential from a cost perspective, especially when grinding Ceramic Matrix Composite materials and similar. These processes are used, for example, in the lightweighting of telescope mirrors made of e.g. fiber-reinforced silicon carbide, or Zerodur. An increase in productivity would be clearly noticeable in the manufacturing costs due to normally long machining times. In the "KryoSonic" project, we investigated whether and to what extent the use of cryogenic machining affects the rough machining of Zerodur with and without ultrasonic support.
In this paper we present a feasible variant of a device for in-process roughness measurement during an optical polishing process. The system, already presented as Tirm respectively I-Tirm, has been technically varied and can now be integrated into almost any lever polishing process with little effort. This enables new possibilities regarding real-time optical manufacturing process monitoring and optimization.
The present paper shows a way for a cost-effective, integrated measurement of the density p of polishing suspensions. It is based on the principle of measuring the turbidity of a solution on the basis of the light transmitted through it, and of calculating back to the density on the basis of the turbidity. The tool is suitable for monitoring polishing agents and for detecting when the density of the polishing slurry leaves the permissible parameter space and can perspectively enable the automated monitoring and adjustment of polishing suspensions. The tool is named after an Octopus. These animals have amazing abilities, for example they can copy the behaviour of other sea animals or act with a plan. Their tentacles each have their own brain and can perform biochemical analyses.
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 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.
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.
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.
The contents of this work are based on [1], [2] and [3]. Using the three wagons approach, critical parameters were identified and the process window of ductile machining was considerably enlarged. This made it possible to increase the critical depth of cut, which is ten times greater than predicted by the Bifano formula. A new formula to describe the machining process was developed and verified experimentally. In addition, the level of surface roughness (Sq) generated in ductile mode was analyzed and a formula was generated that allows roughness prediction depending on the critical process parameters. Finally, both formulas were used to create optimized sets of process parameters that produce a "first light" in ductile machining for a) single point diamond turning (SPDT) on ultra-precision machines (UPM) of binder-free carbide form and b) non-UPM, standard CNC ductile grinding of WC and glass.
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.
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.
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 [1] Consequently, this paper reports on two experimental feasibility studies to extend the process window of ductile mode grinding of brittle materials. Applying ductile process parameter sets featuring values of the critical depth of cut larger than 1 micron depth two processes were experimentally analyzed that are up to now not applicable in industrial production environments: a) single point diamond turning (SPDT) of BK7 glass applying UPM machineries and b) ductile grinding of tungsten carbide molds applying standard CNC grinding machines featuring lower tool positioning accuracies than UPM. Experimental data of both tests 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.
Grinding processes offer a huge range of parameters affecting quality and quantity of the optical elements being produced. Systematic optimization can reveal unexploited potential even in already "stable and efficient" processes. In this paper we describe a method for optimizing CNC grinding processes under industrial conditions by applying a Preston-based approach. As already reported by Mr. Preston [1], for load controlled grinding processes, the workpiece material removal rate (MRR) is proportional to the applied tool pressure (P) multiple the applied relative speed between tool and workpiece (vr): MRR = c*P*vr. Consequently, while maintaining the relative speed (vr = const), there is a linear dependency of MRR on the applied pressure P existing within the process window. The method reported in this paper uses the inverse conclusion that outside of the process window, the linear dependency of MRR on P is not valid any more. This effect is being used to determine the process window of feed controlled CNC grinding processes, since spindles and slides of modern feed controlled CNC grinding machines have finite stiffness values. Therefore, we can apply a load controlled approach to determine the process window of feed controlled grinding processes. To that aim, a dynamometer was mounted onto a fixed abrasive, feed controlled, standard optics CNC grinding machine, monitoring tool pressure in situ and in process for different values of CNC machining parameter sets such as tool rotation, workpiece rotation and infeed speed etc. Experimental data will be presented determining the process window of various CNC grinding processes demonstrating that applying grinding process parameter sets within the process window boundaries guarantee a stable and predictable production with high yield values.
While setting up grinding processes for optics fabrication, the optimum tool and process parameters have to be chosen. Unfortunately, datasheets of commercial grinding tools contain only information about grit type and size, bond material, tool shape etc., leaving out any information about process parameters to be applied or tool lifetime or performance. For this reason, tools and machining parameters are commonly adjusted during iterative experimental test runs using actual workpieces from production batches which is time consuming, expensive and without guarantee that the optimum set of machining parameters are achieved. In this paper we present a method to determine grinding tools process window for a given workpiece material. That way, ideal machining parameters can be determined offline for maximum reliability and productivity. In addition, the gained data provides the basis to identify the most suitable grinding tool for the desired application.
Optical surfaces are usually machined by grinding and polishing. To achieve short polishing times it is necessary to grind with best possible form accuracy and with low sub surface damages. This is possible by using very fine grained grinding tools for the finishing process. These however often show time dependent properties regarding cutting ability in conjunction with tool wear. For a successful grinding process the tools must show a constant chipping performance. A constant, at least predictable wear and cutting behavior is crucial for a deterministic machining. The paper describes the results of a testing series on a test bench for grinding tools. The test bench was presented for the first time on a SPIEConference in San Diego in August 2015 [1]. It is used to investigate the machining behavior of single grinding pellets on different materials under certain conditions, for example load and cutting speed. The bench is equipped with a linear measuring system and allows in-process-analyzing of material removal, tool wear and self-dressing effects. The gained data is used to determine suitable parameters for grinding processes, such as form grinding with ball-shaped tools or cup wheels and also gives information about tool wear and lifetime. This work describes a method to determine the characteristics of pellet grinding tools by tests conducted with a single pellet. The determination of the effective material removal rate and the derivation of the G-ratio are investigated. Especially the change from the newly dressed tool status via the quasi-stationary to the worn status of the tool is described. From the results of these tests the usability of a pellet grinding tool for a specific grinding task can be determined without testing a comparably expensive serial tool. The collected data can be stored and used in an appropriate data base for tool characteristics and be combined with useful applications.
Optical surfaces are usually machined by grinding and polishing. To achieve short polishing times it is necessary to grind with best possible form accuracy and with low sub surface damages. This is possible by using very fine grained grinding tools for the finishing process. These however often show time dependent properties regarding cutting ability in conjunction with tool wear. Fine grinding tools in the optics are often pellet-tools. For a successful grinding process the tools must show a constant self-sharpening performance. A constant, at least predictable wear and cutting behavior is crucial for a deterministic machining. This work describes a method to determine the characteristics of pellet grinding tools by tests conducted with a single pellet. We investigate the determination of the effective material removal rate and the derivation of the G-ratio. Especially the change from the newly dressed via the quasi-stationary to the worn status of the tool is described. By recording the achieved roughness with the single pellet it is possible to derive the roughness expect from a series pellet tool made of pellets with the same specification. From the results of these tests the usability of a pellet grinding tool for a specific grinding task can be determined without testing a comparably expensive serial tool. The results are verified by a production test with a serial tool under series conditions. The collected data can be stored and used in an appropriate data base for tool characteristics and be combined with useful applications.
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.
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 material's 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.
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.
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