Now showing 1 - 10 of 184
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Manufacturing of graphite electrodes with high geometrical requirements

2019 , Uhlmann, E. , Kuche, Y. , Polte, J. , Polte, M.

Graphite is widely used for the die-sinking electrical discharge machining (EDM) process, especially for the roughing process. For the manufacturing of graphite electrodes the milling process is mainly used. The process enables fast processing times tP and high geometrical flexibility. In consequence of the cutting behaviour of graphite micro-components with geometrical features can be manufactured. In this contribution the manufacturing of micro-pins and bridges with aspect ratios of A = 1:50 were machined with diamond coated milling tools. By variation of the depth of cut ap and the width of cut ae it can be shown that the influence of the depth of cut ap is quite bigger than the influence of the width of cut ae. This results in consequence of the higher stability of the geometrical features by improved force distribution.

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Application of laser surface nanotexturing for the reduction of peri-implantitis on biomedical grade 5 Ti-6Al-4V dental abutments

2019 , Uhlmann, E. , Schweitzer, L. , Cunha, A. , Polte, J. , Huth-Herms, K. , Kieburg, H. , Hesse, B.

The annual revenue of dental implants is estimated on 33 billion USD in 2019 and the efforts to keep the teeth functionality and aesthetics is continuously growing over the years. However, loosening of dental implants induced by infection is still a critical and common problem worldwide. In this scenario, the development of new implant manufacturing strategies is of utmost importance. Every surface exposed in the oral cavity, both the tooth and the implant surface, are covered by a layer of salivary proteins, the so-called pellicle. The initial formation of a pellicle is followed by the attachment of bacterial cells onto it. Well-developed biofilms on dental implant surfaces become the main source of pathogenic microbes causing Peri-Implantitis, which is one of the main causes of dental implant failure. The surface topography and chemical composition of an implant are key factors in controlling surface wettability, which directly affects the formation of the biological films. In this sense, ultrafast laser surface nanotexturing rises as a promising advanced technology for controlling implant surface biological properties. Laser-processing parameters such as laser wavelength l, fluence F and number of pulses N are essential for surface texturing. Thus, this paper presents promising results on the influence of different laser induced periodic surface structures (LIPSS) on the composition of the pellicle and the biofilm formation on biomedical grade 5 Ti-6Al-4V dental abutments. Moreover, a biofilm reactor was built and adapted to assess the effect of the LIPSS on the biofilm formation.

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Investigation on the effect of novel cutting fluids with modified ingredients regarding the long-term resistance of polycarbonate used as machine guards in cutting operations (KSS-PC)

2019 , Uhlmann, E. , Haberbosch, K. , Thom, S. , Drieux, S. , Schwarze, A. , Polte, M.

Machine guard enclosures prevent the accessibility of hazard zones and safeguard against ejection of parts during operation, such as chips, tool and workpiece fragments as well as cutting fluids (DGUV 2008, DIN EN 12417). Vision panels as part of machine tool enclosures allow the observation of the machining process. Due to its high ductility, impact resistance Y and its optical properties, polycarbonate is the primarily used material for machine guard windows (Uhlmann and Duchstein 2008, Uhlmann and Duchstein 2010). By influence of cutting fluids, chip impact, ultraviolet radiation and heat, polycarbonate underlies a reduction of long-term resistance, which is called aging. In order to improve the protection of human health and environment, the European Union adopted the regulation ""Registration, Evaluation, Authorization and Restriction of Chemicals"" (REACH) in 2007. The implementation of REACH leads to modified cutting fluids, which need to be analyzed regarding their effect on long-term resistance of polycarbonate. This is important due to a not yet verified suspicion that cutting fluids with increased alkalinity, use of solvents, such as phenoxyethanol, or application of amines, such as dicylohexylamine, might increase aging of polycarbonate. Two approaches are presented in this paper. First, a qualitative approach is proposed to compare cutting fluids by determination of resistance of environmental stress cracking. Secondly, the effect of three cutting fluids on the long-term resistance in a quantitative way is presented. Aging of the polycarbonate is carried out by a time-controlled exposition with cutting fluid and under defined conditions over ten weeks. Impact resistance Y is determined by impact tests. Material properties are measured by tensile tests.

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Modelling of brushing processes

2019 , Sommerfeld, C. , Uhlmann, E. , Hoyer, A.

Brushes consist of a body with fixed highly flexible filaments and can be used for deburring and surface finishing operations. During the brushing process, axial and tangential deflections of the highly flexible filaments lead to an adaptation to the shape of the workpiece and interaction between the filaments. The described complex contact behavior has been insufficiently investigated so far. For a better understanding of the contact between a brush and the workpiece surface, this paper presents a model based on physical principles. The model describes the dynamic behavior of a brush in contact with different workpiece geometries and consists of separate physical descriptions for the filaments of the brush, the workpiece surface and the occurring contacts. A description of a single filament is given by a multi-body system of rigid links. The rigid links are connected by joints which approximate the material behavior of the filaments. To approximate different geometries, the workpiece surface is specified by a polynomial. Contact can occur between the filaments and the workpiece surface as well as between the filaments. For the description of the occurring contacts, Hertz's theory of elastic contact and Coulomb's law of friction are used. The aforementioned physical descriptions are included in the Lagrange's equations to obtain a system of equations of motion that calculates the deflection of the filaments of the brush and the acting forces during the contact with the workpiece surface. A numerical solution to the system of equations of motion was calculated by using experimentally determined material and contact properties of the filament. A comparison of the calculated forces with experimentally determined values shows good correlations for different workpiece surfaces and process parameters. In this context, the developed model calculates the progression and the maximum value of the acting contact forces. The results show a shorter contact length of the filament lc for a circular surface compared to a plane surface, and a rise of the maximum normal force Fn with the depth of cut ae. Furthermore, consideration of the filament interactions leads to a more accurate approximation of the brush-workpiece contact. Based on the findings, the developed model can be used to calculate predictions for different brushing processes which reduce the number of time-consuming preliminary tests for the process design.

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Safety of slim tool extensions for milling operations

2019 , Uhlmann, E. , Thom, S. , Barth, E. , Pache, T. , Prasol, L.

The development of 5-axis machine tools (MT) allows complete machining of complex workpiece geometries. In order to counteract lacking operation space and to improve the accessibility of the workpiece, slim tool extensions (STE) are applied. Operating errors, e.g. by crash, can cause plastic deformation of STE during machining operation and therefore lead to an increased moment of inertia, and thus to rotational energy due to the spindle speed control of machine tools. Currently applied machine tool enclosures are not designed for such failures with corresponding kinetic energies EKIN. The described failure implies a risk potential for operators and a high damage potential for machine tools. In this paper, the failure scenario is identified and modeled. This includes the calculation of elastoplastic deformations of STE based on finite element analysis and analytical calculation of kinetic energies EKIN considering deformed STE. Based on the described model a parameter study is carried out considering the geometry of the STE as well as the spindle speed nS. The safety of existing machine tool enclosures is evaluated according to DIN EN 12417 in order to identify safe operating conditions. Finally, the authors suggest possible solutions addressing both, the STE and machine tool enclosure. The research presented in this paper is funded by the German Machine Tool Builders Association (VDW).

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Cutting force prediction in micro-milling considering the cutting edge micro-geometry

2019 , Uhlmann, E. , Polte, J. , Wiesner, H.M. , Kuche, Y. , Polte, M.

The micro-milling process is used for a wide range of materials and enables the manufacturing of complex geometries with micro-features. One important factor for the tool life is the cutting force Fc, which depends on the applied technology, process parameters and cutting edge micro-geometry. High cutting forces Fc can lead to tool breakage in the transition between the shank and the cutting part of cemented carbide end mills. The prediction of cutting forces Fc in micro-milling processes through cutting force models could potentially decrease the hazard of tool breakage. By including the cutting edge radius rv into the prediction model, additional correction factors can be avoided. Therefore, further knowledge about the applicability of those models for the micro-milling process with chip thickness h < 0.01 mm is needed. In this investigation, the cutting force model of KOTSCHENREUTHER [1], which takes the cutting edge radius rv into account is used for the cutting force prediction in micro-milling. In order to validate this model, an innovative lead free copper alloy CuZn21Si3P is machined. Cemented carbide micro-milling tools with tool diameter D = 1 mm were used. The manufacturing of different cutting edge radii rv was realised with the immersed tumbling process. During milling experiments with a five-axis high precision machine tool the cutting forces Fc were measured. Cutting forces in a range of 6 N < Fc < 26 N were detected. The results show good correlations between the predicted and experimental determined cutting forces Fc. Furthermore, the measured cutting edge radii rv show a high influence on the deviation of the measured and predicted cutting forces Fc.

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Ultra-precision-milling of silicon by means of single crystal diamond tools

2019 , Uhlmann, E. , Polte, M. , Rolon, D.A. , Kühne, S.

Silicon is an important material often employed on most of micro-electro mechanical systems (MEMS), integrated circuits, micro-chips, and micro-fluidic devices. Therefore, strategies and process parameters to machine those planar 2.5-D geometries of silicon are essential. Moreover, silicon belongs to the group of hard-brittle materials, which means that it is very likely to originate cracks during the milling operations as a result of the intermittent interaction of the cutting edge and the silicon surface. Besides, the machining of silicon results on severe tool wear. The ductile-brittle transition and tool wear reduction of the silicon-milling are aspects still not completely investigated. Consequently, this paper aims at finding the proper parameter range for ductile ultra-precision milling (UP-milling) of 2.5-D silicon geometries employing single crystal diamond cutting tools. Furthermore,the evaluation the tool wear after the process is a crucial part of the investigations. In order to fulfil such knowledge gap, single groove experiments are proposed. The milling process to generate those grooves is monitored by means of force measurements. Also, surface aspects of the machined grooves are measured through white light interferometry (WLI). For evaluating tool wear, dry UP-milling investigations are conducted and images of the cutting edges are taken by means of a scanning electron microscope (SEM). The experiments show that the machining of silicon is feasible and the ductile material removal is possible. Moreover, the process forces Fpr generated by the UP-milling process of single crystal silicon are able to be employed for monitoring and avoid the transition from ductile to brittle material removal.

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Verbundprojekt SmartStream: Intelligente Bearbeitung durch die Verwendung schaltbarer Fluide

2019 , Schmiedel, C. , Bierwisch, C. , Uhlmann, E. , Menzel, P. , Mohseni-Mofidi, S. , Breinlinger, T. , Nutto, C.

Strömungsschleifen und Hydroerosiv (HE)-Verrunden sind einzigartige Verfahren, die sich dadurch auszeichnen, dass sie funktionelle Oberflächen im Inneren eines Bauteils bearbeiten können, die sonst mechanisch nicht zugänglich sind. Jedoch unterliegen die Verfahren Begrenzungen aufgrund der Gesetzmäßigkeiten der Strömungsmechanik. Daher können die Verfahren nicht bei allen Anwendungen für eine technisch sowie wirtschaftlich sinnvolle Bearbeitung genutzt werden. Im Verbundprojekt SmartStream werden Möglichkeiten zur Überwindung bisher geltender Verfahrensgrenzen untersucht. Zur lokalen Beeinflussung der Zerspanungsleistung der auf die Oberflächen wirkenden Abrasivmedien werden diese durch ein externes magnetisches Feld schaltbar gemacht. Mit Hilfe des angelegten Magnetfeldes lassen sich zum einen strömungsmechanisch ungünstig gelegene Bereiche des Werkstücks bearbeiten und zum anderen die Zeitspanvolumina lokal gezielt steuern. Im vorliegenden Beitrag werden erreichte Entwicklungsziele am Beispiel des Strömungsschleifens vorgestellt.

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Die-sinking EDM of a SiC-boride-composite

2019 , Uhlmann, E. , Polte, J. , Jahnke, C. , Wolf, C.-S. , Degenhardt, U.

Silicon carbide-based composites are highly demanded for industrial applications, like heat exchangers in corrosive environments. In consequence of the mechanical properties like high hardness H and brittleness, cutting processes are still challenging. An opportunity for processing difficult-to-cut materials with sufficient electrical conductivity s is electrical discharge machining (EDM). In order to develop suitable machining technologies, known parameters for common material combinations can serve as a starting point. Hence, standard technologies from the database of a commercial die-sinking EDM machine tool were applied for machining a silicon carbide-boride-composite. The material removal rate QW and the arithmetical mean deviation Ra for finishing and roughing operations are observed. The formation of cracks and the extend of the typical deposition layer formed at the surface of the workpiece is analysed through scanning electron microscopy. While the material removal rate QW is increasing with higher discharge energies W in this investigation, the arithmetical mean deviation Ra is not dependent on this property. Furthermore, spalling is identified as main material removal mechanism occurring in this process. Overall, the machinability of silicon carbide-boride composites with EDM, applying commercial available parameter technologies, is successfully demonstrated.

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Micro-cutting of a MMC-composite for enhanced injection moulds

2019 , Uhlmann, E. , Polte, M. , Hein, C. , Polte, J. , Jahnke, C.

Tools for micro-injection moulding are currently made of hardened steel. These tools are exposed to high local loads, which significantly reduce the injection moulding tool life time tT. Furthermore, the occurring wear of the milling tool during machining of hardened steel leads to reduced surface roughness Ra and geometrical accuracy GF. Copper and aluminium alloys as mould materials provide an alternative to hardened steel with advantages regarding material removal rate QW and wear of the milling tool, but with a significantly reduced life time of the injection moulding tool tT. Until now, the combination of a good machinability and high wear resistance cannot be achieved. The approach, presented in this paper consists of an easy to machine material and the development of a wear resistant metal-matrix-composite (MMC) material layer with a hardness of up to 3,000 HV. Therefore, the pre-machined test specimens made of aluminium-bronze are coated by laser dispersing with wolfram-carbide-particles W2C-WC. Furthermore, for the finishing machining of the coated moulds, a cutting technology for the machining of W2C-WC-particles was developed. The verification of the developed technology was performed with an injection moulding process based on carbon-fibre reinforced thermoplastic material. By means of documented machining efforts, the quality indicators geometrical accuracy GF and surface roughness Ra as well as occurring tool wear, the feasibility of the developed technology was demonstrated.