Table of contents:
- Picture gallery
- Layer by layer to the desired density
- The LOM machine concept
- FE modeling of the machine
- Comparison of the simulation results
Video: Fast Route To The FEM Model
Due to the increasing spread of additive manufacturing processes, workpieces with geometries can now be manufactured that were not considered to be producible a few years ago. In the generative process, the workpiece is created directly from the CAD data. For this purpose, software is used to create cuts from the 3D model, which can be produced using various processes and assembled to form the final component.
Generative manufacturing technologies can also bring advantages in topology optimization. Until now, penalization (e.g. the SIMP method) has usually forced the convergence of the optimized topology to a 0-1 design. This only contains areas with full material (ρ = 1) and areas without material (ρ = 0). It can therefore usually be manufactured using conventional manufacturing technologies. Penalization, however, results in a less optimal solution. With the help of generative manufacturing processes, it is possible to manufacture components with an almost continuous density distribution. Penalization can thus be dispensed with and the optimal topology can be used.
Picture gallery with 5 pictures
Layer by layer to the desired density
One approach to manufacturing two-dimensional components with continuously distributed density is the top-gene process. For this purpose, several structured layers are created from the optimal solution. Due to the different structuring of the layers, the desired density value is averaged at every point of the component. The manufacturing process used should therefore work in layers and be able to create cavities. In order to achieve the strength required in technical applications, steel should also be processed.
Laminated Object Manufacturing (LOM) is currently the only widespread generative manufacturing process that meets these requirements. The individual layers are first cut out with the help of a laser beam and then bonded to each other with adhesive. The process with sheet metal has already been successfully used to produce large forming tools. However, a compact processing machine that automatically produces metal LOM components is not yet commercially available. In the context of this article, a concept for such a machine tool for use in additive manufacturing processes is presented and checked using an FEM simulation with two different software programs - Ansys Workbench and Meshparts.
The LOM machine concept
The necessary functions of the machine include laser cutting, the application of glue, the pressing together of the sheets (spreading and hardening of the
glue) and the handling of the sheets. The
individual components, such as the laser cutting optics, the adhesive system and the feed axes, can be obtained as finished components from different manufacturers. So the challenge is to use these components to design a compact and functional machine. For this purpose, various machine concepts were developed and
compared with each other. The most suitable design is now equipped with three linear axes and contains all of them
Machining functions, i.e. the sheet gripper, the laser cutting optics and the adhesive nozzle, on a common machining head (Fig. 1).
To investigate the dynamic and static behavior of the machine, an FEM simulation is carried out in two different programs, Ansys Workbench and Meshparts. Meshparts is software that was specially developed to simulate machine tools. For this purpose, like many CAD systems, it has the option of assembling the entire machine from parameterizable modules. Meshparts also has a component library with many pre-networked components. These can be inserted and adjusted with parameters.
FE modeling of the machine
For the simulation in Ansys Workbench, a parametric, simplified 3D model is first built in Solidworks and coupled with Ansys Workbench. To simplify matters, the extruded profiles are replaced by rectangular tubes, for example. Their dimensions are determined so that they have the same tensile, compressive and flexural rigidity. The elements of the feed axes are also mapped with the help of replacement stiffness. In Ansys Workbench, the replacement stiffness is inserted as a socket element. This allows the definition of a stiffness matrix. The rolling contact in the ball screw drive is also represented by an alternative stiffness. In addition, the pitch of the thread is taken into account by a coupling equation.
In mesh parts, the dimensions of the components, replacement stiffness, spindle pitch and other properties are taken into account as parameters in prefabricated components. The stiffness and the spindle pitch are then automatically taken into account by mesh parts. Only a few specific parts (e.g. the end caps of the linear axes) are imported as Parasolid or as a networked CDB file (press). Otherwise, the simulation model is rebuilt in mesh parts. By using parameterizable modules, the y-axis could be derived directly from the identical but longer x-axis in mesh parts. The extruded profiles are taken from the component library and, unlike the workbench model, do not have a simplified cross-section.
The results of a modal analysis of the entire machine are given as examples. The machine is stored fixed on the floor and the engine brakes are activated. The following results then result in Table 1. As can be seen from the table and the representation of the fourth eigenmode (Fig. 2), the results are largely the same. The deviations can be attributed to the different degree of simplification of the models.
Overall, the natural frequencies are very low. This is due to the light construction of the frame with aluminum profiles and the use of feed axes of small size. Since there is no vibration excitation during gluing and laser cutting, the natural frequencies only limit the dynamics of the processing. In the course of further investigations, it can be clarified whether this is acceptable or whether the machine construction should be revised.
Comparison of the simulation results
Both programs made it possible to create a FEM model of a machine tool in a relatively short time using a comfortable graphical user interface and to carry out the simulation on a machine with 24 cores. A great advantage of Ansys Workbench is that positions of the subcomponents are imported directly from the CAD system and do not have to be modeled from scratch. In the case of mesh parts, on the other hand, the assembly definition only takes place in the mesh parts software at the FE level, which makes it easier to examine variants in the case of major model changes. Furthermore, the search for replacement stiffnesses can be omitted for the components that are stored in the program's library. (mz)
The authors thank the Deutsche Forschungsgesellschaft (DFG) for financial support of the project.
* Andreas Pfaff, Mahdi Mottahedi M.Sc, Dr.-Ing. Armin Lechler, Prof. Dr.-Ing. Alexander Verl, Institute for Control Engineering of Machine Tools and Manufacturing Facilities (ISW), University of Stuttgart
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