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In-process monitoring of inhomogeneous material characteristics based on machine learning for future application in additive manufacturing

Jaquemod Andre, Palalic Marijana, Güzel Kamil, Möhring Hans-Christian

Journal of Machine Engineering

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2024

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Vol. 24, No. 2

83--93

EN

Additively manufactured components often show insufficient component quality due to the formation of different defects. Defects such as porosity result in material inhomogeneity and structural integrity issues. The integration of in-process monitoring in machining processes facilitates the identification of inhomogeneity characteristics in manufacturing, which is crucial for process adaptation. The incorporation of artificial defects in components has the potential to mimic and study the behaviour of real-world defects in a more controlled way. This study highlights the potential benefits of cutting force and vibration monitoring during machining operations with the goal of providing insights into the machining behaviours and the effects of the artificially introduced defects on the process. Detection of anomalies relies on identifying changes in force profiles or vibration patterns that might indicate the interaction between the tool and the defect. Machine learning algorithms were used to process and interpret the collected data. The algorithms are trained to recognize patterns, anomalies, or deviations from expected behaviours, which can aid in evaluating the effect of detected defects on the machining process and the resultant component quality. The main objective of this study is to contribute to enhancing quality control of machining processes for inhomogeneous materials.

2

Detection and identification of nonlinear contact dynamics at workpiece clamping positions

Feng Qi, Maier Walther, Braun Steffen, Möhring Hans-Christian

Journal of Machine Engineering

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2023

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Vol. 23, No. 1

114--122

EN

All mechanical systems behave nonlinearly to a certain extent since there are always reasons for nonlinearities, such as friction and slip effects, in the actual structures. It is important to detect and identify the nonlinearity due to friction and contact in order to investigate their effect on the global behavior of the workpiece-fixture system. That is a prerequisite for modeling the dynamic contact behavior at the interface between the workpiece and clamping elements. In this research, the workpiece-fixture system was excited with a shaker using the swept sine signal. The nonlinearities could be detected by comparing and analyzing the frequency responses of the structures in Bode plots. However, the nonlinearities behaved differently at various frequencies within the observation range. Different mechanisms such as nonlinear stiffness and damping, micro-slip friction, are responsible for that. Then the nonlinear contact behavior at the clamping positions was successfully identified by means of the Hilbert transform. In addition, the clamping force directly influenced the nonlinear stiffness of the workpiece-fixture system.

3

A study of low-frequency vibration-assisted bandsawing of metallic parts

Tandler Tobias, Stehle Thomas, Möhring Hans-Christian

Journal of Machine Engineering

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2023

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Vol. 23, No. 2

110-122

EN

Sawing is often the first work step in the metal cutting production chain. Especially for larger workpieces, bandsawing is used for this purpose. Nevertheless, studies on sawing have led a niche existence in the research landscape for a long time. However, as a result of the optimization of manufacturing processes in terms of economic efficiency, bandsawing is increasingly becoming the focus of research, since there are still saving potentials here. The aim of this paper is to investigate the extent to which the bandsawing process can be influenced by active, low-frequency vibration superimposition in the feed direction. First, analogue tests were carried out and parameter combinations were determined which have a positive influence on the process. Subsequently, these parameter combinations were investigated on a real sawing machine with an excitation unit, analysing the extent to which the results from the analogue tests could be transferred to the real process.

4

Influence of LPBF parameters and strategies on fine machining of pre-built bores

Teich Heiko, Maucher Clemens, Möhring Hans-Christian

Journal of Machine Engineering

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2021

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Vol. 21, No. 2

91--101

EN

Additive manufacturing changes the classical possibilities of production. However, post-processing is usually unavoidable for these components to achieve functional performance. To obtain an optimum product, knowledge of the characteristics of the additive manufactured part and the machining mechanisms depending on these characteristics is required. In this paper, the influence and the interaction of the laser powder bed fusion process parameters on the subtractive post-processing are shown. The effects of the parameters on the geometry of bores are examined and subsequently the precision machinability is analysed using reaming. In addition, a process simulation is carried out to correlate the simulated deformation to the required machining allowance for subsequent reaming. The aim of this investigation is to examine the capabilities of the laser powder bed fusion process to produce bores at angles of 90° (vertical), 60° and 45° that can be machined directly with a reaming tool without the need for drilling.

5

Discrete wavelet transformation as a tool for analysing the borehole quality when drilling Carbon Fibre Reinforced Plastic aluminium stack material

Duntschew Jonas, Eschelbacher Sarah, Schluchter Ivo, Möhring Hans-Christian

Journal of Machine Engineering

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2021

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Vol. 21, No. 1

78--88

EN

To meet the higher demands of the lightweight industry, composite materials are used for weight-bearing structures. Due to their superior properties, CFRP (Carbon Fibre Reinforced Plastic) – aluminium stacks are especially attractive for many applications in the aerospace industry. Using rivets to connect these materials requires precise boreholes in large numbers, which are usually drilled. Because of the anisotropic material properties, high demands on the tool performance and process stability are set. Damages such as delamination, fibre pull-outs and inadequate surface properties are frequently observed at the drilled boreholes. To improve the monitoring status and failure detection, acoustic emission during drilling was analysed using the discrete wavelet transform method. By converting time signals into multiple independent time-frequency signals, temporary events during the monitoring of the cutting processes can be found. The studies show that certain combinations of wavelet families are particularly useful for describing correlations between acoustic emission and the delamination of the CFRP surface.

6

Development of an adaptronic spindle for a faultless machining of homogeneous and inhomogeneous materials

Dobrinski Alexander, Möhring Hans-Christian, Stehle Thomas

Journal of Machine Engineering

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2021

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Vol. 21, No. 2

5--23

EN

One of the latest topics in construction concerns the difficulty of producing faultless drill holes in parts made of composite and sandwich materials. At the Institute for Machine Tools of the University of Stuttgart, a prototype of an adaptronic drilling spindle for a machining of components made of homogeneous and inhomogeneous materials was developed within the framework of a research project, funded by the Federal Ministry for Economic Affairs and Energy. The spindle made it possible to limit the axial force and the torque acting on the tool, thus protecting the tools and the workpieces from any possible damage. The tests carried out with the spindle prototype proved that the axial feed force acting from the spindle can be reliably controlled by means of the developed spindle. It might be possible to restrict critical machining situations arising due to the removal of drill chips or the reduction of process temperatures.

7

Multiaxial force platform with disturbance compensation for machine tools

Palalic Marijana, Möhring Hans-Christian, Maier Walther, de Mendoza Adrian Gonzalez, Riemeier Friedrich

Journal of Machine Engineering

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2020

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Vol. 20, No. 3

5--16

EN

As part of a cooperation project between the Institute for Machine Tools, Stuttgart, Germany, and EMO Systems/Nuton GmbH, Berlin, Germany, a prototype of a multiaxial force platform with disturbance compensation for the measurement of the cutting forces in machine tools was developed. Commercially available products based on piezoelectric technology are subject to a degree of measurement uncertainty and therefore characterized by various disturbances. Interpreting measurement data of the commercially available products is complex, time-consuming and prone to errors. This paper describes the mechanical design and the simulation of the multiaxial force measuring system with the finite element method (FEM) as well as the conceptual development of a reduced model for a multibody simulation with the purpose of implementing a compensation algorithm. The measurement uncertainty was reduced by using appropriate hardware and software for the compensation of the various disturbances so that the application of the force platform would also be possible in the industrial application for the process diagnosis, the control and the regulation in machine tools. Systems based on strain gauge technology have some advantages in the field of zero-point stability and also provide a less expensive solution. For the disturbance compensation, an additional force and torque sensor system with eight channels was used for the detection of the platform displacement and inclination. With the help of appropriate algorithms for the disturbance compensation and their integration in the evaluation software, the disturbances could be reduced to a minimum.

8

Integrated strain gauge printing in a CFRP structure

Misch Sandra, Welzel Florian, Möhring Hans-Christian

Journal of Machine Engineering

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2019

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Vol. 19, No. 3

74--81

EN

Our approach is to integrate printed strain gauges into a structure of laminated carbon fibre reinforced plastics (CFRP). This can provide minimizing disturbances caused by an additional sensor weight. Another point is to reduce the occurrence of pre-damage, as a printed structure is integrated directly into the CFRP. Due to the printing, no additional masses are applied to the CFRP by cables. To this end, the boundary conditions for the print are first explained. Subsequently, the strain gages were printed. For this purpose, studies were carried out regarding the orientation of the strain gage printing direction, the influence of repeated printing, the overlapping during printing and the subsequent lamination in CFRP plates. The sensors are to be used in the structure of the CFRP plate in a machine tool.