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Wireframe and Surface Analysis Techniques #2
Joe Costa, MSC Software Corporation

This is the second in a series of articles discussing the tools available within CATIA to analyze the quality of surfaces and curves. The previous article discussed just wireframe analysis techniques. This article will focus on analyzing the continuity and the quality of surfaces.

The procedure for analyzing surfaces is the same as analyzing wireframe. First check the continuity of the surfaces and then check the quality. Once we have verified that we have an acceptable surface, then we can analyze the surface to check such things as minimum draft angles and surface manufacturability. However, these topics will be discussed in the next article.

The first tool to look at is the surface connection checker. It is used to check the continuity of a single surface or the interface between multiple surfaces. Simply select the type of continuity to check for, set a tolerance range and select the surface or surfaces to analyze. CATIA will then determine the trouble spots, if any. In the example below (Fig. 1), we are looking for curvature discontinuities that have a deviation greater than 5%. CATIA then highlights the problem areas (blue for curvature, yellow for tangency and red for point continuity) and displays the maximum and minimum Curvature values. Curvature deviation varies from 0% to 200%. Ideally, curvature continuity should be as close as possible to 0%.

Figure1: Quick Surface Analysis Checker

The only problem with this quick analysis is it that while it correctly identifies the problem areas, along with the maximum and minimum extreme locations, it does not quantify the problem areas along the entire interface. This is where the full analysis is beneficial. It uses a customizable color palette. All edges of the surfaces are color-coded based on the magnitude of the deviation. In fig. 2, the blue areas identify tangent continuities; the purple areas represent a maximum deviation of greater than 2 degrees. Notice the areas of yellow and green which identify varying degrees of tangency discontinuity, more than 0 degrees, but less than 0 degrees.

Now that we have checked the continuity, the next step is to check the surface quality. The quickest and easiest check is a quick visual search. It isn't as accurate as some of the other techniques, but it will identify any obvious defects. For best results, change the color of the surface to a brighter color and adjust the light sources.

Figure 2: Visual Surface Quality Check

A visual analysis will not identify all problems on a surface, but CATIA has several tools to check surface quality. Before we look at the tools, we need to define surface curvature.

At any given point on a surface, curvature is measured in two directions. Gaussian curvature = where R1 and R2 are the radius of curvature in each direction.

Gaussian Curvature can be positive or negative. If the curvature direction is the same for both measurements, the curvature is positive. If the curvature directions are opposite, the curvature is negative. This represents a saddle point on the surface

Figure 4: Gaussian Curvature Direction

This tool is used to identify surface quality problems. It color codes the surface based on Gaussian Curvature at every point. The magnitude of the curvature is not too important. Rather, what is more important is how the surface transitions from areas of low curvature to areas of high curvature. Try to avoid sudden large curvature changes. The colors should change gradually over a larger area. When the transitions occur over a small area, the result will always be a bumpy surface. If the transition occurs over a larger area, the result will be a smoother surface.

Figure 5: Gaussian Curvature

A sudden change in the color indicates a curvature discontinuity.

In summary, CATIA has excellent tools to analyze surface quality. For anyone designing class-A surfaces, these tools are a must. In this article, we have looked at two of those tools. The surfaces connect checker checks surface continuity and the Gaussian Curvature tool checks surface quality. In the next article, we will look at the tools available to check for part manufacturability.

To receive a PDF version of the complete presentation covering this topic, send an email to: info@mscsoftware.com include your contact information and ask for the "WireFrame and Surface Analysis Techniques" presentation by Joe Costa.

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