Let’s uncover two common causes of modeling failures in Fusion 360 and learn how to avoid them: near coincidence and sliver faces. 

Have you ever tried to model a shape with Autodesk Fusion 360 and failed? I have, many times! In the previous post in this series, we learned how to avoid the 3D Modeling deadly sins of ‘High Curvature’ and ‘Near Tangency’.

In this post, we’ll learn why tiny faces and teeny overlaps can cause havoc with your 3D Models. 

Near coincidence

Near-coincidence occurs when two geometries overlap by a tiny amount. They are nearly the same, but not identical. 

This condition causes problems with downstream Boolean operations (Join, Cut, Intersect). Boolean operations calculate the intersections between bodies. If the intersection between bodies is very small, finding the precise intersection is difficult. 

You can accidentally create near coincidence inside Fusion 360, but it’s more likely to be a problem with an imported body. 

If you’re having problems with Boolean operations, try creating the feature (Extrude, Revolve, Sweep, etc.) with the ‘New Body’ operation selected. If this works, the underlying problem is likely to be a near coincidence. 

One workaround is to use the unstitch command to turn your solid into surfaces, delete the overlapping faces, create new faces as needed, and then use the ‘Stitch’ command to connect the surface faces back up into the bodies you need. 

Sliver faces

Sliver faces are simply very small, unexpected faces that are not part of the design intent. Sliver faces are too small to be seen at the normal zoom. Sometimes they’re very long but very thin. 

Sliver faces cause problems because Fusion 360 is trying to do very precise math in a very small area. Sliver faces can be created accidentally, but they are more likely to be seen on imported models. 

Sliver faces are very difficult to identify because you can’t usually see them without zooming in. One method is to search for broken edges, by selecting edges in the problem area. If the highlighted edge doesn’t run all the way from one edge to the next, this could indicate a problem. Zoom in for a closer inspection. 

Sliver faces are usually quite easy to deal with once you track them down. If you can’t go back and fix the inputs to eliminate the Sliver faces, use the ‘Delete’ tool, which will remove the Sliver face and re-intersect the surrounding geometry. 

In extreme cases, you may need to use the Unstitch command to turn your Solid model into Surfaces, delete the Sliver faces and the faces surrounding them, build new surfaces using your preferred boundary geometry, and then Stich the surfaces back together to make a Solid model. 

The Seven Deadly Sins of Fusion 360 3D Modeling Part 03: Singularities & Non-manifold Topology

In the next post, we’ll learn about ‘singularities’ and ‘non-Manifold topology’ and how to avoid both! Read the rest of the series below:

Part 1: High Curvature & Near Tangency

This blog post is based on an Autodesk University class. You can read the handout, download a dataset, and try out some exercises that illustrate the Seven Deadly Sins from the Autodesk University website here: 7 Deadly Sins of 3D Part Modeling in Fusion 360.