The 7 Deadly Sins of Fusion 360 3D Modeling Pt. 4: Loose Tolerances 

Paul Munford Paul Munford August 29, 2023

2 min read

Let’s uncover why loose tolerances are a common cause of modeling failures in Fusion 360 and how to avoid them.

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 about the 3D Modeling deadly sins of ‘Singularities’ and ‘Non-manifold Topology’, and we learned how to avoid them! 

In this post, we’ll learn why loose topology isn’t helpful and bring the series to a conclusion. 

Loose Tolerances 

Fusion 360 is a highly accurate 3D CAD modeling tool. When we import files from other CAD systems that work to different tolerances, Fusion 360 handles any non-manifold gaps between surfaces by increasing the tolerance around some of the edges and vertices. 

For example, when we use the ‘Stitch’ command to join surfaces together, Fusion 360 allows a tolerance. In fact, Fusion 360 doesn’t change the underlying geometry—it uses a tolerance zone to join the two adjacent surfaces to a single common edge 

You could image this tolerance zone as a tube for edges, and a sphere for vertices. 


Loose tolerances could be considered geometry that shares a tolerance zone bigger than 1 Nanometer or 40 nanoinches). This is not a problem in itself, but it could cause problems when adding to the model downstream. 

For example, thickening a face by a small distance could place a vertex outside the tolerance zone of an Edge, but inside the tolerance zone of a Vertex. This is likely to cause a modeling failure. 

Loose tolerances are nearly always brought into Fusion 360 from other CAD systems. If you need to allow a large tolerance to stitch surfaces together, this is an indication that you’ll have problems later. You may also see weird graphical anomalies around areas that have been interpolated. 

You can use the feedback from the stitch tool to identify edges that failed to stitch, and which may need your attention. 

The best solution is to return to the original CAD system, tighten up the tolerances, and re-export the model. If this isn’t possible, you may have to rebuild the problem surfaces in Fusion 360. 

The 7 Deadly Sins of Fusion 360 3D Modeling

Now you know the 7 deadly sins of 3D part modeling and how to avoid them. Read the rest of the series below:

Part 1: High Curvature & Near Tangency
Part 2: Near Coincidence & Sliver Faces
Part 3: Singularities and NonManifold Topology

And remember, We can avoid these conditions by following this best practice:

Avoid teeny tiny ambiguous features. Maintain manifold Topology. 

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.

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