Boundary Surfaces in SolidWorks

Boundary surface is a feature in SolidWorks, with the help of this feature you can create models which have surfaces instead of solid.

What boundary surfaces do is it forms a surface in a closed boundary.

In SolidWorks this happens when trying to apply a boundary surface to a boundary that is not four sided. The result is that SolidWorks forces the curvature into the corner of a boundary, which creates a singularity at the corner. This may seem trivial, but when you go to thicken the surface or add fillets, you’ll find that more often than not the feature will fail or be produced incorrectly.

How to use Boundary surface tool and to avoid degeneration.

Step-by-Step use:

A common mistake in working with SolidWorks is using boundary surfaces to form the surfaces for the part which has 5 or more boundaries or 3 or less boundaries.

The boundary surface tool is specifically meant for a 4 boundary sketch.

For instance let the sketch be:

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To start boundary surface feature, go to surfaces, click on ‘boundary surface’.

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This is the pallet for boundary surface.

Select the sketches which form the boundary.

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This is the preview for boundary surface. After selecting the sketches you can analyze this preview.

The green lines projecting out of the sketch cause degeneration.

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You can opt for different vectors for sketches but it will not solve the problem for degeneration.

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Select curvature to face for both of the sketches.

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This model looks good even after you have used boundary surface to bound 2 boundary.

But the problem of degenerated surface still exists.

To check that problem, go to thicken surfaces and thicken the boundary surface.

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First let’s try to understand what boundary surface is actually doing to bound these two surface.

To do that edit the boundary surface feature and increase the mesh density which will increase the visibility of what’s happening here.

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When you use the boundary surface tool, it selects and two boundaries, and tries to bound them perpendicularly. In the above picture, red pointers represent one boundary and green pointers represent the second boundary.

The boundary represented by green pointers acts as the guided curves for the surface.

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To thicken the surface, go to surfaces, then click on thicken. After thickening the surface, you will see some irregularities, these irregularities are known as surface degeneration.

Because while using boundary surface, the far side is a point. So after thickening, SolidWorks does not know where to extrude that point so the result is gibberish.

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In this example this weird shape is formed at both far ends of the model.

To overcome this problem, we use surface filled tool to form surfaces with different no. of boundaries than 4.

To do this extrude two sketches, these will act as the reference for surface filled feature.

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Then go to filled surface, and select the sketches. Now at first this filled surface feature will do the exact same as boundary surface, to check this, make sure that surface optimization is not checked.

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Uncheck optimize surface.

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The surface mesh with filled surface is finer.

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Surface after using surface filled feature.

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After thickening, there is no problem of surface degeneration. That is why it is recommended to use surface filled instead of boundary fill except in the case when there are 4 boundaries.

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