Tool making research Mobile milling machines always cut precisely thanks to the use of FEM
At the IFW, researchers are investigating how displacement compensation for mobile machine tools, which are used to repair heavy press tools, for instance, can be made feasible with the help of simulations and sensors.
Large, complex pressing tools weighing up to 60 tonnes are used for the production of car body panels. Each of these tools must be adapted, optimised or repaired due to wear several times during its life cycle. The machine tools required for this are usually not located where the pressing tools are used. This results in a long and expensive transport of the moulds to the machine. Against this background, Picum MT GmbH has developed a mobile machine that eliminates the need to transport the pressing tool. The machine comes to the pressing tool instead.
Due to the forces acting in the milling process, the milling cutter suffers a displacement from the ideal path, depending on the rigidity of the machine. The use of simulation software and the concept of the "feeling machine" counteracts this situation. This concept is based on the measurement of process forces with the help of integrated sensors and the subsequent control compensation of the displacement. In collaboration with the Institute of Production Engineering and Machine Tools (IFW) at Leibniz Universität Hannover, such a concept for increasing accuracy and quality is being implemented as part of the research project “Mobile technology platform for hybrid process chains”.
The vision of the feeling machine
The use of strain gauges offers the possibility of retrofitting a machine tool with sensory capabilities. First, suitable application points are identified with the help of finite element simulation (FEM). The strain gages applied at these points form the sensor array and detect the process forces that lead to the displacement of the cutter. The measuring amplifier processes these signals and transmits them to the machine control. Here, the displacement of the tool centre point (TCP) is calculated by a compensation algorithm using a calibration matrix and included as an offset value in the calculation of the position to be approached in this time step. The compensation of the displacement is carried out via the subsequent controller of the machine axes.
How to find the right sensor positions
For the FEM model of the machine to be used to identify application points, the properties and behaviour of the FEM model and the machine must first be aligned. This ensures that the theoretically determined variables (elongation, displacement) match those of the real machine. For this purpose, the CAD model was validated by measurement in the first step. Subsequently, the course of the stresses and strains within the structure under the effect of force was investigated by means of FEM simulation. For this purpose, simulations of the entire model were carried out with the maximum expected process forces at the TCP. Subsequently, the resulting maximum strain on the individual components was considered and an evaluation was carried out with regard to the applicability of strain gages. The investigation showed that the Z-axis is the most suitable for the application of strain gages. Here, the highest strains occur in the main direction Z. The main direction is relevant for the arrangement of the measuring grids. The measuring grids of the strain gages should always be aligned in the main direction in order to achieve the highest sensitivity. In addition, there is sufficient surface area here to mount the strain gages and the associated protective housings.
Application of force sensors needs to be deliberate
Full strain gauge bridges (N2A-13, ME measuring systems) were applied to both the front and the back of the component. In order to ensure an approximately equal positioning and thus an equivalent signal of the strain gauges, an assembly aid was used, which was created by rapid prototyping.
Evaluation of the concept for measurement
Subsequently, measurements were carried out again with the prepared machine to displace the TCP under the influence of defined, external forces in different poses. For the poses, the centre position of the machine as well as the positive and negative end positions of the X and Y axes, thus a total of five positions spanning the entire working space, were chosen. In addition to this, the height of the Z-axis was varied in three steps at each position (start, middle & end position). Furthermore, different rotation angles of the A-axis (angle around the X-axis) were investigated. The level of the forces was based on the maximum expected process forces. The measurement of the displacement was carried out with a Renishaw XM-60 6D laser interferometer. Based on the measured values, the sensor system was calibrated and an algorithm was developed that determines the TCP displacement online during the process so that it can be compensated.
Due to the direction-dependent stiffness of the Z-axis, the strain gauge values show the expected harmonic course. The directional dependence results from the guide rails located on the side of the Z-axis. These guide rails cause a higher resistance cross-section for forces in the transverse direction and thus ensure a higher stiffness.
In order to determine the displacement of the cutter based on the strain gauge values in the process, correlations between the displacement of the TCP and the measured strain were identified in the following step. The displacement quotients determined in this way are integrated into a calibration matrix, with which the displacement can finally be compensated by the control. These results will be used in the future to further increase the manufacturing accuracy of milling processes.
The project partners would like to thank the NBank and the European Regional Development Fund for funding the research project “Mobile technology platform for hybrid process chains”, funding number ZW3-85040350.