Fraunhofer-Institut für Produktionsanlagen und Konstruktionstechnik IPK

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Modeling of the wet immersed tumbling process with the Discrete Element Method (DEM)

2021 , Uhlmann, E. , Fürstenau, J.-P. , Kuche, Y. , Yabroudi, S. , Polte, J. , Polte, M.

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.

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Accuracy in force estimation applied on a piezoelectric fine positioning system for machine tools

2021 , Uhlmann, E. , Polte, M. , Triebel, F. , Overbeck, R. , Thom, S.

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.

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Tool wear and surface roughness in micro-milling of aluminium and high-alloyed aluminium materials using cutting tools made of binderless carbide

2021 , Uhlmann, E. , Polte, M. , Hein, C. , Polte, J. , Wiesner, H.M.

Micro-milling can be applied for manufacturing in a wide range of materials and complex geometries. This process is especially important for the aerospace industry. High-alloyed aluminium is a common material for aerospace applications with complex micro- and macro-geometry due to its high wear resistance. The costs-effectiveness of producing these parts can be increased by using tools with improved wear behaviour and higher life times. However, wear-resistant tools are often associated with higher tool costs, which reduces the cost-effectivness of the whole production. An innovative solution is offered by the use of a cutting tool made of binderless tungsten carbide. The micro-milling of conventional and high-alloy aluminium with a new cutting material based on a binderless tungsten carbide is analysed in this investigation. The absence of a binding phase leads to an increased hardness and improves the wear behaviour of these tools. Therefore, tools with a tool diamete r of D = 10 mm were manufactured and there machinability was successfully proven. The feasibility of these innovative tools is demonstrated in a series of experiments. The experimental investigations were carried out on the five-axis high precision machine tool PFM 4024-5D PRIMACON GMBH, PeiÃenberg, Germany, with a workpiece made of TiAl 48-2-2. A surface roughness of Ra = 0.202 µm was detected after a path length due to primary motion lc = 70 m without any noticeable wear marks on the cutting tool. These results show the economic potential for milling tools based on binderless carbide for achieving high precision surfaces while reaching high lifetimes.

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Precision finishing of additively manufactured components using the immersed tumbling process

2021 , Polte, J. , Polte, M. , Hocke, T. , Lahoda, C. , Uhlmann, E.

Additive manufacturing enables the production of highly complex metallic components with highest geometrical flexibility in dedicated lightweight construction. For titanium-aluminium alloys, which are used in particular in the aviation industry, powder bed based processes such as the laser powder bed fusion are established. Nevertheless, laser powder bed fusion is limited with regard to the producible surface roughness in a range of 5 µm ⤠Ra ⤠15 µm. According to the state of the art, the increase of the geometrical accuracy and the reduction of the surface roughness values of the additive manufactured components are realised by different cutting and non-conventional processes. In this investigation, a new approach for the reduction of the surface roughness values by immersed tumbling was realised. Therefore, additively manufactured square bars made of the titanium alloy Ti-5Al-5Mo-5V-3Cr were used as sample geometries. An immersed tumbling machine tool with plan etary kinematics for post-processing was applied and the lapping media QZ, HSC 1/500 and M5/400 were evaluated. In addition, the influence of the rotor speed and the holder as well as the depth of immersion were considered as influencing factors. As target values the surface roughness values as well as the rounded edge radius were examined. Within this investigations the surface roughness values could be reduced by more than 90 %. In addition, a targeted rounding of the edges could be obtained, which removed the excess edge height at the part resulting from the laser powder bed fusion process. As a result the immersed tumbling process shows a great suitability as a finishing process for additively manufactured components and is particularly suitable for automated and serial finishing processes.

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Application of tool electrodes oxidised with humid and dry air during the electro-discharge drilling of MAR-M247 alloy

2021 , Uhlmann, E. , Polte, M. , Streckenbach, J. , Camin, B. , Chocholaty, O. , Dinh, N.C.

The requirements and challenges of machining advanced materials in the field of aerospace, automotive and tool industry are increasing constantly. Due to their mechanical properties, cutting of high-strength materials such as superalloys is severely limited. Electro-discharge drilling can be used for the manufacturing of holes in hard to machine materials. Although electrical discharge machining (EDM) is successfully applied to the machining of holes in turbine blades, a lack of performance and challenges concerning the geometrical accuracy still remain. By applying inner flushing, the resulting electrically conductive debris is flushed through the lateral working gap, increasing the probability of arcs and short circuits. The resulting increased tool wear, conicity of the hole, limited hole depth and process instabilities are still challenging in electro-discharge drilling. In order to decrease the effects of the arcs and short circuits, a surface modification is appli ed to increase the electrical resistance of the lateral surface of the tool electrode. As a result, the mentioned impairments occur less frequently due to decreased occurrence of arcs and short circuits in the lateral working gap. For the present investigation copper tool electrodes were thermally oxidised in dry and humid air with different durations and used afterwards for electro-discharge drilling of MAR-M247. The tests were carried out on the machine tool AGIETRON Compact 1 from the company GF AgieCharmilles, Losone, Switzerland. Holes with a depth of t = 11 mm were drilled using various surface-modified tool electrodes with a diameter of d = 2 mm. Oxidation with dry air and a extended oxidation time resulted in a 18 % lower erosion duration, accompanied by an increase of the linear wear of the tool electrode Île below 10 %.

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Additive manufacturing of precision cemented carbide parts

2021 , Polte, J. , Polte, M. , Lahoda, C. , Hocke, T. , Uhlmann, E.

Cemented carbide parts are commonly used as wear resistance components in a broad range of industry, e.g. for forming, mould making and matrices. At state of the art the machining of precision cemented carbide components by milling is strongly limited due to excessive tool wear and long machining times. Promising approaches for precision machining of cemented carbide components are dedicated cutting tool coatings, new cutting materials like binderless polycrystalline diamond and ultrasonic-assisted machining. Nevertheless, for all these approaches the components need to be machined of monolithic materials. The new approach addresses an innovative manufacturing process chain composed of near net shape Additive Manufacturing followed by a precision finishing process. Within this investigations for the manufacturing of precision cemented carbide parts, cemented carbide with a cobalt content of 17 % and a grain size in a range of 23 µm ⤠gs ⤠40 µm were used. As Addit ive Manufacturing technology laser powder bed fusion was used. Diamond slide burnishing and immersed tumbling were investigated as finishing technologies. Based on the investigations, a dedicated process chain for the manufacturing of precision cemented carbide parts could be realised. The findings show that the developed process chain composed of near net shape Additive Manufacturing and the finishing process diamond slide burnishing enables the manufacturing of precision cemented carbide parts with a geometrical accuracy of ag ⤠10 µm. Due to the finishing process the initial surface roughness after Additive Manufacturing could reduce by Ra = 89 %.

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Micro-milling of a sprue structure in tungsten carbide-based metal matrix composite

2021 , Uhlmann, E. , Polte, J. , Polte, M. , Hein, C. , Hocke, T. , Jahnke, C.

Many industries rely on plastic components manufactured by micro-injection moulding. There is a high potential to further increase the cost-effectiveness by machining the moulds needed for this process from non-ferrous metals and reinforcing the parts of the mould, which experience high loads during the micro-injection moulding. Inserting tungsten carbide particles locally into the surface of these non-ferrous metals is one possibility of reinforcement. The resulting metal-matrix-composites (MMC) exhibit the needed wear resistance, while the ground material can be machined very effectively through micro-milling. In contrast, the Micro-milling of these MMC-materials is challenging and so far not state of the art. Thus, this investigation is concerned with the development and qualification of micro-milling parameters for tungsten carbide-based MMC-materials. Binderless polycrystalline diamond as innovative cutting material was applied for this purpose. The goal of the mil ling parameter development was to optimize the surface roughness and the form accuracy for machining an aluminium bronze workpiece reinforced with tungsten carbide particles through laser injection. Based on an analysis of a wide range of process parameters, an optimised milling strategy was applied to machine a sprue structure from the described MMC-material. Different parameter sets are evaluated by analysing the form accuracy and measuring the surface roughness of machined structures. A surface roughness of Ra = 80 nm and form accuracy of a = 3 µm could be achieved with optimized micro-milling parameters and qualified the developed parameters for industrial applications.

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Cutting edge preparation of monolithic ceramic milling tools

2021 , Uhlmann, E. , Polte, M. , Polte, J. , Hocke, T.

Due to international competition, continuous increases in productivity, product quality and reduction of production costs are required. Especially, the development of milling tools made of innovative cutting materials and application-specific tool geometries for the machining of brittle materials are in focus to overcome these challenges. One approach to improve the performance and the tool behaviour concerning milling of graphite is the use of monolithic ceramic milling tools. Unfortunately, the high brittleness of the ceramic leads to breakouts on the cutting edge during the grinding process. This results in an increased maximum chipping of the cutting edge, which has a significant influence on the milling process. To improve the breakout behaviour, a cutting edge preparation with the immersed tumbling process was applied. To enable a process reliable cutting edge preparation, a suitable lapping medium, the influence of the processing time as well as the depth of imme rsion were investigated. Besides the maximum chipping of the cutting edge, the rounded cutting edge radius was also analysed. The results show that a process reliable cutting edge preparation of monolithic ceramic milling tools with a maximum chipping of the cutting edge RS,max ⤠3 µm and a rounded cutting edge radius of rβ ⤠7 µm could be realised. In future investigations, the experimental applicability of monolithic ceramic milling tools will be proved.

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Reduction of erosion duration for electrical discharge drilling using a nature analogue algorithm with nested strategy types

2021 , Uhlmann, E. , Polte, M. , Streckenbach, J. , Osmanovic, M.

The required high economic efficiency, combined with the corresponding high quality demands, in the aerospace industry as well as in mould and tool making, motivate the necessity of finding suitable parameter combinations for the process of electrical discharge machining (EDM), e.g. when introducing new materials. To counteract this, various methods are being investigated in research for the optimisation of EDM. One new method is the stochastic optimisation procedure evolution strategy (ES). Due to its metaheuristic approach, this optimisation method is excellently suited for very complex processes in which the interrelationship of the individual influencing variables is not known. This publication presents the results of the investigation of the suitability of the ES optimisation method using the example of electrical discharge drilling. For this purpose, two nested ES-types were investigated. The electrode materials used were brass for the tool and stainless steel X5C rNi18-1 for the workpiece. As a result, the erosion duration could be reduced by 30 %. This investigation forms the basis for the use of nested ES types in electrical discharge drilling.

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Hyperparameter Optimization of Artificial Neural Networks to Improve the Positional Accuracy of Industrial Robots

2021 , Uhlmann, E. , Polte, M. , Blumberg, J. , Li, Z. , Kraft, A.

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.

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