Complex boundary conditions can make NURBS surface design challenging. Engineers using general purpose CAD software are not always able to solve easily such special purpose tasks, as shown in the figure above. This is a single scull...

iCapp develop CAD applications - libraries, with or without GUI - for special purposes in tool design, manufacturing or mechanical engineering in general. We have extensive experience in the calculating of curvature- and process optimized spline-faces, depending on arbitrary criteria and boundary conditions.

Examples include the calculation of faces for turbine blades, boat hulls or car body construction. The following will show typical iCapp solutions for free form surface design.

Holzbau Amann, one of the leaders for timber constructions requested a smooth single NURBS surface for the roof construction of the "Centre Pompidou Metz" building. The dimensions are approximately 100m x 100m.

The task was to transform given 3000 3d points from the architectural concept while having regard to lots of different boundary conditions coming from aesthetic, civil engineering and production criteria.

The project was realized in close collaboration with designtoproduction.

Technical details

The use of only one NURBS surface (left), enables global optimization of smoothness. The right figure shows a zebra-plot of the resulting face, allowing the designer to assess curvature. In the form.

The timber construction comprised 6 layers of wooden beams. Each layer surface was calculated from the first NURBS surface, using a normal offset distance (around 0.4 m).

The length and shape of each beam varies due to the torsion in the layer surface. This causes the ends of the beam to vary (see left figure above). To be successful all beams have to be manufactured automatically by an NC milling machine. The digital data need to achieve this is easily taken from each of the 6 layers.

Martin Pfundt (Holzbau Amann) and Fabian Scheurer (designtoproduction) report in the German journal for architecture ARCH+ on "Zur Kommunikation zwischen Zimmermeistern und CNC-Fräsen".

Stryker develops medical technologies for individual patient requirements. To work on virtual data they digitized thousands of bones of the lower extremities by CT. For each bone STL data of three different levels has derived from the scans: outer marrow, outer spongiosa and outer bone surface. To get sufficient input data for engineering tasks like solid modelling a transformation of the discrete STL data to continuous NURBS faces had to be done.

iCapp developed a fully automated software solution to work in batch mode: Reading each STL-file, calculating the faces and writing a STEP file. This enables automatic translation of hundreds of STL meshes, by applying a suitable script.

Technical details

STL data of the surface level is shown - above left. The resulting NURBS-face model is shown - above right. The program creates smooth faces, trimmed with 3 to 15 boundary edges. Most reverse engineering tools need a maximum of 4 curves to define one face. Breaking this restriction was one precondition in general to solve the problem. We were able to adjust the seams within 0.01mm tolerance - to get a waterproof model for each layer. These features allow production of high quality watertight forms.

The picture above shows the three layers of the bone mentioned above. It is common to produce very much larger files when transforming meshes to NURBS surfaces. Our program will significantly control resulting file sizes. In this medical application, 200 faces were typically needed to represent one femur bone layer described by approximately 20 000 curvature based triangles derived from the original scan. The file size of the face model is here nearly the same as the STL mesh file.

For calculating the faces no special hardware was required. Approximately 6min calculation time by a 2GHz processor was used to transform one layer.

For each tissue layer, NURBS models have been calculated for every given bone. The images above and below show the complexity of the created surface models for the upper and the lower part of a femur.

The advantage of the algorithm used in this application is, that an approximation to the mesh data is done for every single face. Each point is calculated with a distance-to-mesh tolerance of 0.1mm. This ensures that all bone details are represented in a smooth face model.

Since 1896 Stämpfli has designed rowing boats. Today, it is one of the worlds leading manufacturers of high performance sculls. A few years ago they started in collaboration with hs composite to design and produce sculls with carbon fiber. It is critically important to reduce water turbulence and friction forces in symmetrical boat designs. CAD solutions able to produce high surface quality needed to be used. The surface data from the final model was to be used in manufacturing the product.


Technical details

Design development became a blend of traditional, trial and error methods together with technical advances in CAD and CNC control. This employs the skills of experienced craftsmen to create a preliminary shape. Professional rowers optimized the 8 m long body of the prototype by several trials. The final shape then was digitized by an optical scanner (ATOS from company GOM). The result was a closed triangle mesh (see figure above).

When trying to secure any long symmetrical part for scanning, inevitably distortions occur in the shapes because of the objects own weight. The first step was to correct the scanned point data to eliminate these torsion and bending effects.

Some special approximation techniques had to be developed to transform the corrected scan data to continuous NURBS faces. The aim was to control the smoothness of the surfaces - reducing water resistance and to improve performance. Curvature changes had to be avoided while the surfaces had to fit as well as possible to scanned data.

iCapp designed a symmetrical set of NURBS-faces representing a solid part. The figure above shows the resulting model.

Ultimately, faces had to form a symmetrical solid part. The quality of the resulting body over all surfaces can be shown by zebra plots. The figure above shows the light lines on the most critical face, the front side of the scull.