CAD-FEM import of a machine tools feed drive system model.

Parpala, Radu Constantin

1. INTRODUCTION

Today manufacturers can no longer afford to consume time and money building and testing real prototypes of the large models, instead they use virtual prototypes.

In the virtual prototype approach it is possible to simulate the kinematic, static and dynamic behavior of the machine tools including all aspects of real life exploitation. Using different software packets one is able to simulate even the cutting process. This complex design concept was enabled by the use of high performance computer technology (Altintas et al., 2005).

Unfortunate at the moment there are specialized software packages so engineers are forced to use more than a software suit during the design phase. Mainly there are two categories of softwares that engineers use: CAD/CAM softwares and FEA softwares, so as a result there are many problems that appear when importing models from these two different software applications.

In this paper it's presented as a case study the model of a machine tools feed drive system. (Weck 2001)

2. COMPUTER AIDED DESIGN

The 3D model of the feed drive must be designed in order to be accepted as input by various software suits that will be used for further analyses.

Because it's hard to find an integrated environment in which to perform all analyses engineers must take into consideration software's compatibilities (Jeong Hoon Ko et al., 2003).

The 3D model of the feed drive was designed by using the CATIA V5 CAD software mainly because of his good integration with the ANSYS software which was used for statically and dynamical FEM analyses. All 3D part where fully parameterized in order to optimize needed parameters in FEM analyzes.

In order to generate all the contacts between surfaces it's necessary to correctly design the 3D assembly, it's also very usefully to check all the clashes and clearances within the CAD environment. Using the information provided by the CAD software we can set the correct tolerances for the automatic contact generation.

3. PREPROCESING THE MODEL

Because the model was not design in ANSYS Workbench native CAD system we must first check if all the model's features are imported before we proceed to further analyses.

Getting the geometry Into Design Modeler, simulation or advanced meshing is now much easier than in almost any other FEM software. The first thing to know about formats is that there are to classes readers and plug-ins. Readers simply translate the CAD format into workbench's internal format. A plug-in actually uses software from the CAD vendor and open up the geometry in the native format and gives the workbench all the information it needs in native format. Often the geometry provided by readers is referred as dumb and plug-in geometry as smart because this kind of geometry can be transferred back to the CAD files. It's a good practice to import all the parts into the Design Modeler before proceeding to simulation

After a successful import of the model it's very easy to automatically generate all the contacts between components, setting up the correct parameters for contact creation.

For a proper meshed structure it is necessary to repair the geometry before any further operations.

The main purpose of the CAD repair tool is to detect and close gaps between neighboring surfaces. Typically this procedure involve two steps:

* Build topology--build curves and points which will help to diagnose the model for geometrical problems. If the curves are within a geometric tolerance, they are merged together as one. The curves are then displayed in a specific color to illustrate their connectivity in the surface data (Table 1), which can be used to determine any gaps or holes in the geometry (Fig. 2).

* Repair any gaps or hole in the topology For any two faces (surfaces) that meet at a common edge (curve), there is typically a finite distance between the two edges. By default, a curve is associated with all the edges of each face.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

This topology of the two surfaces would indicate a gap in the model. Typically, ANSYS meshers can handle this if the gap is smaller than the proposed element size on the surfaces or curves. Therefore, you would set a tolerance larger than the gap if you are using a large element size.

A tolerance smaller than the gap would create yellow curves which could be fixed. The recommended tolerance is approximately 1/10th the size of the average mesh size

The quality of the mesh needs to be checked before applying loads and constraints. It gives an idea of how close the mesh is to an ideal mesh. Mesh

Quality can be measured by the various criteria. For Hexa dominant meshes the quality is calculated as the determinant. The Determinant, more properly defined as the relative determinant, is the ratio of the smallest determinant of the Jacobian matrix divided by the largest determinant of the Jacobian matrix, where each determinant is computed at each node of the element. The Determinant can be found for all linear hexahedral, quadrahedral, and pyramidal elements.

[FIGURE 3 OMITTED]

A determinant value of 1 would indicate a perfectly regular mesh element, 0 would indicate an element degenerate in one or more edges, and negative values would indicate inverted elements.

To automatically improve the quality of the mesh elements different smoothing algorithms are available depending on which mesh type is loaded. Mesh can be smoothed with respect to a particular quality criterion and with a specified number of iterations to achieve a given quality level. A mesh containing tetras, pyramids, prisms and triangular and quad surface elements can be smoothed.

The mesh can be also improved manually by moving nodes. An example is shown in Figure 2 some nodes where moved as a result we have in the left side of the figure a smother mesh this can be checked by visualizing element colors. Those colors change as the form of the elements changes.

4. CONCLUSION

Today the main problem in checking structures consists in importing and preprocessing the CAD model. It is well known that the geometry of the model can dramatically change FEM results (Zaeh & Oertli 2004)

Initiated mainly by the automotive and aircraft industry, the development of modern software tools for integrated simulation of products has been enchanted lately.

Unfortunate at the moment there are no integrated software platform for the virtual design and analyze of the machines tools.

5. REFERENCES

Altintas, Y.; Brecher C.; Weck M. & Witt S. (2005). Virtual machine tool, Annals of the CIRP, 54/2: 651-669.

Gross, H. & Hamann, J. (2001) Electrical Feed Drives in Automation. Basics, Computation, Dimensioning. Siemens, Publicis MCD Corporate Publishing, Erlangen and Munich

Jeong Hoon Ko; Won Soo Yun; Dong-Woo Cho; Kyung Gee Ahn & Seung Hyun Yun (2003). Development of a virtual Machine tool--Part 2, International Journal of the KSPE, vol. 4, No.3

Zaeh, M. & Oertli, Th. (2004) Finite Element Modeling of Ball Screw Feed Drive Systems, Annals of the CIRP, 53/1: 289-292.

Weck, M., (2001) Werkzeugmaschinen. Mechatronishce Systeme, Vorschube, ProzeBdiagnose (Mechatronical system, feed motion, Process diagnosis ). Springer-Verlag, Berlin, Heidelberg, ISBN 3-540-67614

Tab 1 Build topology colors

Color Semnification

Yellow Single or free edge curves
Red Double edge curves
Blue Multiple edge curves
Green Unattached curves