Fraunhofer-Institut für Produktionsanlagen und Konstruktionstechnik IPK

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  • Publication
    Accuracy in force estimation applied on a piezoelectric fine positioning system for machine tools
    ( 2021)
    Uhlmann, Eckart
    ;
    Polte, Mitchel
    ;
    Triebel, Florian
    ;
    Overbeck, Rasmus
    ;
    Thom, Simon
    In order to improve the accuracy of machine tools, the use of additional active modules meeting the requirements of the ""Plug & Produce"" approach is focused. In this context one approach is the installation of a high precision positioning table for online compensation of machine tool deflections. For the model-based determination of the deflection, the knowledge of the effecting process force is crucial. This article examines the use of displacement sensors for force estimation in a piezoelectric system. The method is implemented on a high precision positioning table applicable in milling machine tools. In order to compensate nonlinear effects of piezoelectric actuators, a hysteresis operator is implemented. Experimental investigations are carried out to quantify the influence of preload stiffness, preload force and workpiece weight. Finally, a resolution d < 78 N could be achieved and further improvements to meet the requirements for online compensation of machine tool deflection are discussed.
  • Publication
    Modeling of the wet immersed tumbling process with the Discrete Element Method (DEM)
    ( 2021)
    Uhlmann, Eckart
    ;
    Fürstenau, J.-P.
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    Kuche, Yves
    ;
    Yabroudi, Sami
    ;
    Polte, Julian
    ;
    Polte, Mitchel
    Immersed tumbling is an industrially established process for finishing of components made of metal, ceramic or plastic. In this process, the components are completely surrounded by a wet, abrasive medium, which allows burrs to be removed and surfaces to be polished. In order to gain specific insights into the influence and flow properties of the abrasive media used in this process, numerical approaches using the Discrete Element Method (DEM) with the Rocky DEM software are presented within these investigations. A complete process simulation could be realised by means of a digital machine tool. The immersed tumbling process with cone-shaped polymer abrasive media is implemented by use of a liquid bridge model. The results were validated by experiments with an industrially used immersed tumbling machine tool and for the first time allow sound statements about the contact conditions and interactions of the abrasive media with the workpiece.
  • Publication
    Hyperparameter Optimization of Artificial Neural Networks to Improve the Positional Accuracy of Industrial Robots
    ( 2021)
    Uhlmann, Eckart
    ;
    Polte, Mitchel
    ;
    Blumberg, Julian
    ;
    Li, Zhoulong
    ;
    Kraft, Adrian
    Due to the rising demand for individualized product specifications and short innovation cycles, industrial robots gain increasing attention for machining operations as milling and forming. Limitations in their absolute positional accuracy are addressed by enhanced modelling and calibration techniques. However, the resulting absolute positional accuracy stays in a range still not feasible for general purpose milling and forming tolerances. Improvements of the model accuracy demand complex, often not accessible system knowledge on the expense of realtime capability. This article presents a new approach using artificial neural networks to enhance positional accuracy of industrial robots. A hyperparameter optimization is applied, to overcome the downside of choosing an appropriate artificial neural network structure and training strategy in a trial and error procedure. The effectiveness of the method is validated with a heavy-duty industrial robot. It is demonstrated that artificial neural networks with suitable hyperparameters outperform a kinematic model with calibrated geometric parameters.
  • Publication
    Electrical discharge dressing of honing stones
    ( 2020)
    Uhlmann, Eckart
    ;
    Yabroudi, Sami
    ;
    Zimmermann, Sascha
    ;
    Gerlitzky, Georg
    ;
    Polte, Mitchel
    ;
    Klink, Ulrich
    ;
    Sihling, Bernd
    Honing processes require a systematic preparation and a regeneration of the honing tool to guarantee a sufficient cutting behaviour and prevent unintended wear of the tool. These dressing processes are particularly important for the regeneration of the macro- and microscopic topography of the honing stones. The established method of manual shaping subsequent to a cylindrical grinding process demands a high skill level, a great amount of non-productive time and is not reproducible. In this paper the preparation of honing stones by use of electrical discharge machining (EDM) is presented. The aim is to ensure an automatic, systematic and reproducible process of resetting the bond of honing stones. Different tool electrode materials were analysed systematically for two different bond specifications of the honing stones. EDM process parameters were studied within a design of experiments (DoE) strategy and a subsequent effect analysis to assure the identification of most appropriate EDM process parameters. After this procedure various cycles of long-stroke honing processes were carried out. As a result, the new approach makes it possible to prepare honing stones with reproducible sharpness and macroscopic topography within an automated process. The dressing time td was reduced by 88 % compared to the established method.
  • Publication
    Dry-ED milling of micro-scale contours with high-speed rotating tungsten tube electrodes
    ( 2020)
    Uhlmann, Eckart
    ;
    Perfilov, I.
    ;
    Yabroudi, Sami
    ;
    Mevert, Ricardo
    ;
    Polte, Mitchel
    This paper presents machining results for dry-ED milling of micro-scale contours by use of a new machine tool, including a relaxation generator, designed for dry-EDM. To implement the dry-ED milling process tungsten tube electrodes were used. The gas is injected through the tubular tool electrode under high pressure. Additionally, a high-speed EDM spindle was used to overcome process instabilities and increase the material removal rate due to better flushing conditions. Further, fluid simulations of the flushing conditions are presented in this paper. It is found that the use of gaseous dielectrics enables much better machining results in terms of shape accuracy and tool wear. The simulations provide new insights in the flushing conditions when using gaseous dielectrics. The calculated fluid flow pattern shows a great agreement with the observed depositions of reattached molten material on the tool and workpiece surface. The results presented as well as the hardware introduced enable the industrial application of dry-EDM for the first time.
  • Publication
    Development of Tool Paths for Multi-axis Single Stage Incremental Hole-flanging
    ( 2020)
    Besong, Lemopi Isidore
    ;
    Buhl, Johannes
    ;
    Ünsal, Ismail
    ;
    Bambach, Markus
    ;
    Polte, Mitchel
    ;
    Blumberg, Julian
    ;
    Uhlmann, Eckart
    In this paper, multi-axis single stage incremental hole-flanging is explored in place of three axis multi-stage tool paths in order to prevent flange conicity in hole-flanging by single point incremental forming (SPIF). The angle of inclination of the forming tool, the starting radius and step size of the tool paths are varied to investigate the feasibility of new tool paths. Using the proposed tool paths, an optimization is performed by Finite element (FE) analyses. The geometrical accuracy of the flange is maintained within tolerances relevant to industrial applications while minimizing the resultant forces acting on the tool and maintaining high formability (hole expansion ratio, HER). The FE analyses of the process reveal low formability and high forming forces for flanges formed at high angles of inclination of the forming tool. The tool paths with small diameters at the start point of the tool path result in high bulge heights and high resultant forces. Hence, the existing three axis tool paths are better than the proposed strategies. To prevent flange conicity, over bending of the flange edge to compensate for springback after forming is implemented on the three axis tool path strategy. This is achieved by adding a second tool path with an over bend angle at the end of the three axis tool path.
  • Publication
    Fundamental research of applying tungsten carbide-cobalt as tool electrode material for sinking EDM
    ( 2020)
    Uhlmann, Eckart
    ;
    Polte, Mitchel
    ;
    Bolz, Robert
    ;
    Börnstein, Julian
    The manufacturing process milling is limited in machining specific geometries like inner contours with sharp edges, high aspect ratios or defined radii. Sinking EDM (S-EDM) is an appropriate process to machine complex features if the negative geometry is machinable. The tool electrode wear is a critical factor, because it affects machining accuracy and process efficiency. The tool electrode wear depends on S-EDM processing parameters, the geometry and material of the tool electrode. The material tungsten carbide-cobalt owns suitable thermophysical properties for the application as tool electrode material in EDM. Most applications of tungsten carbide-cobalt in EDM are limited to EDM drilling processes, where tungsten carbide tube electrodes are widely used. The advancements in milling of hard materials enable an economic manufacturing of tungsten carbide-cobalt form electrodes for sinking EDM applications. However, it is not investigated which composition of the material tungsten carbide-cobalt is the most appropriate for the application as tool electrode material in S-EDM. Additionally, there are no suitable EDM process parameters in order to benefit from the material characteristics of the respective tungsten carbide-cobalt. Focus of this work is the analysis of different tungsten carbide grades regarding the average grain size and the cobalt content in order to identify correlations between the material characteristics and the EDM process results. Therefore, eight different tungsten carbide grades with four different grain sizes and five different cobalt contents were applied in S-EDM experimental analysis. The experimental studies showed a general suitability of tungsten carbide-cobalt tool electrodes for EDM-processing, where the material removal rate is comparable to commonly used tool electrode materials. Nevertheless, the relative tool wear shows inferior results. It could be observed that higher cobalt content and coarser grain size of the applied tungsten carbide tool electrodes are advantageous for S-EDM.
  • Publication
    Modelling of a thermoelectric self-cooling system based on thermal resistance networks for linear direct drives in machine tools
    ( 2020)
    Uhlmann, Eckart
    ;
    Salein, Sebastian
    ;
    Polte, Mitchel
    ;
    Triebel, Florian
    The use of direct drives in linear and rotary axes as well as increased power density of main drives offer the potential to raise feet rate, acceleration and thus allow higher productivity of machine tools. The induced heat flow rates of these drives could lead to thermo-elastic deformations of precision related machine tool components. In order to reduce thermally caused displacements of the tool-center-point and to prevent a negative impact on the achievable accuracy, the induced heat flow rates of main drives must be dissipated by effective cooling systems. These systems account for a major share of the machine tool's total energy consumption. With the intention to overcome the area of conflict regarding productivity and energy efficiency, a so called thermoelectric self cooling system has been developed. To convert a proportion of thermal losses into electrical energy, thermoelectric generators are placed in the heat flow between the primary part of a linear direct drive and the cooling system. The harvested energy is directly supplied to a pump of the water cooling circuit, which operates a decentralised cooling system with reasonable coolant flow rates. For predicting the thermoelectric system behaviour and to enable a model-based design of thermoelectric self cooling systems, a thermal resistance network as a system simulation in MATLAB/Simulink is presented. The model is applied to a feed unit with a linear direct drive and allows the calculation of harvested energy as well as the simulation of steady and transient states of the cooling system. The comparison of simulative and experimental determined data indicates a predominantly high model prediction accuracy with short simulation times. At an early stage of development the model turns out to be a powerful tool for design and analysis of water flow thermoelectric self cooling systems.
  • Publication
    Influence of cutting edge micro-geometry in micro-milling of copper alloys with reduced lead content
    ( 2018)
    Uhlmann, Eckart
    ;
    Kuche, Yves
    ;
    Polte, Julian
    ;
    Polte, Mitchel
    Especially copper-zinc alloys (CuZn) with good machining properties are used for electrical components and fittings. By using copper alloys with lead content of 1 % < Pb < 3 % an improved chip breakage can be achieved. Legal regulations require the reduction of lead and demand further knowledge about the effect of the material properties in interaction with the used micro-milling tools. In this contribution the cutting conditions of copper as well as four copper alloys were examined. The results show considerable differences in the resultant surface roughness and burr formation. Furthermore, the influence of two different tool geometries and variied cutting edge micro-geometries were investigated while machining CuZn21Si3P. Thereby, tools with increased cutting edge radii rv showed increased active forces Fa, burr height h0 and decreased surface roughness.