I have been a CATIA instructor for a long time and there is a situation I often encounter. Many CATIA users, including those with surface modeling experience, have trouble creating multi-section surfaces and lack knowledge about couplings.

If used correctly, the multi-section surface command can be a very powerful tool. It is important to understand its full potential to make the most out of it.

Here is a quick guide on using the coupling option for Multi-section Surfaces. For a demonstration video, please refer to the end of the article.

Couplings determine how sections are paired with each other. With using couplings options, in some cases no need to use guide curves, getting surfaces with fewer patches and more quality surface transitions obtained.

There are four types of couplings: ratio, tangency, tangency then curvature, and vertices.

• Ratio: the curves are coupled according to the curvilinear abscissa ratio. That is, they are coupled in proportion to their lengths. Point on ratio 1/L1 on Section1 is coupled with point on ratio 1/L2 on Section2 where L1 and L2 are curve lengths of sections 1 and 2 respectively. (Figure.1)
• Tangency: the curves are coupled according to their tangency discontinuity points. (Figure.2 and Figure.3)
• Tangency then curvature: the curves are coupled according to their tangency continuity first then curvature discontinuity points. (Figure.4)
• Vertices: the curves are coupled according to their vertices. (Figure.5)

It should be noted that curves with different numbers of vertices are not coupled by tangency, tangency then curvature, or vertices types.

Figure 1. Ratio type coupling

Figure 2. Tangency type coupling

Figure 3. Tangency type coupling for tangent continuity sections

Figure 4. Tangency then curvature type coupling

Figure 5. Vertices type coupling