Reverse engineering

Under Reverse (engl .: surface reconstruction is meant) in the field of computer-aided design process in which a polygon surface in Non-Uniform Rational B-spline ( NURBS ) surfaces is converted. Special software is used for this. From a mathematical point of view, the conversion means a reduction in the descriptive parameters. The NURBS surface is normally a least squares approximation , so not all of the original points are in the surface. Reverse engineering is used for freeform surfaces.

Overall process

Reverse engineering is part of what is known as the reverse engineering process. This includes the digitization, the filtering of the measured points, the conversion of the point clouds into polygon areas and finally the actual reverse engineering.

The need for reverse engineering is based on two requirements. On the one hand, the object surfaces must be free-form surfaces; on the other hand, the returned surfaces must be the starting point for at least one further process step. This can be an FEM or CFD calculation or the further construction of a component.

Areas of application

Typical industries for the use of reverse engineering are the automotive industry, the aviation industry, tool making and shipbuilding. Used gas turbine blades are a typical example. The returned blades are used as the basis for calculations in order to determine the influence of wear and the resulting changes in geometry on performance.

Reverse engineering as a service

Numerous companies offer reverse engineering together with digitization as a service.

Accuracies

The accuracy of the reverse engineering can be determined from the distance between the original triangle points and the generated NURBS surface. In the case of complex surfaces, 1/100 mm are more likely to be seen as the lower limit, 3-5 / 100 mm are precision levels typical in practice.

software

Reverse engineering with conventional CAD software is theoretically possible, but due to the lack of mechanisms, it is very complex and therefore only useful in exceptional cases. There are therefore numerous programs for reverse engineering , such as Rapidform , Geomagic , PolyWorks , Pointmaster , Rhino Reverse , VRMesh and Imageware .

The most complex sub-process in reverse engineering is the patch generation described below. Some software products have integrated automation functionality for this.

Details

The starting point for reverse engineering is a polygonal area made up of triangles. These areas are saved in STL format, which can be binary or ASCII.

The main effort in reverse engineering is dividing the polygon surface into patches. A few hundred thousand triangles for the polygon mesh of a component are nothing special. The classification is recorded in a network of curves. Only very simple surfaces can be modeled with a single patch. Since the original surface has a flat structure, the network of curves must be generated in software by hand or by an automatic algorithm. An important criterion is the local curvature distribution of the surface.

The actual generation of the NURBS surfaces (a NURBS surface is generated from a patch) then happens automatically, but for larger models it can take a long time (up to a few hours). The required continuities are taken into account immediately.

The parameters of the NURBS surfaces can be saved in a STEP or IGES file or exported to a proprietary data format.

Remarks

continuity

The patches generated are, since they are separated by simple curves, a priori - continuous , i.e. H. the patches have no gaps or overlaps. In addition, continuity (no kinks) and continuity (no jumps in the curvature distribution) can be required. Compliance with continuities are additional boundary conditions and can lead to an increase in the deviation from the STL network. ${\displaystyle C^{0}}$ ${\displaystyle C^{1}}$ ${\displaystyle C^{2}}$

edge

The digitization cannot capture edges with a radius below the point distance. Therefore the edges have to be generated synthetically during the reverse engineering. Some software products offer functions for this.

literature

• Tamal Krishna Dey : Curve and surface reconstruction: algorithms with mathematical analysis , Cambridge University Press, 2007, (Cambridge monographs on applied and computational mathematics 23), ISBN 978-0-521-86370-4

Individual evidence

1. ↑ Manfred Weck, Christian Brecher : Machine tools 4 - Automation of machines and systems , 5th edition, Springer, 2006, ISBN 978-3-540-22507-2 , page 269