Convert a Complex STL into a Body in Fusion 360

by | Oct 2, 2017 | 3D Printing, Fusion 360 |

Almost daily technology blogs features stories about advances in the field of desktop 3D printing. New techniques and technologies, not to mention new 3D printers, are constantly being developed. But one of the most underrated advances in the consumer 3D printing market is the proliferation of 3D model repositories. Thingiverse, Youmagine, pinshape, GitHub, and Sketchfab are just a few of the dozens of different sites hosting 3D models for download.

These sites and many others are fantastic for downloading and printing models, as long as the models provided suit your needs. But what if you need to or want to edit the models? This seems like it should be easy; however, the major problem you will encounter is that the vast majority of 3D models online are provided in STL format. STL files can be downloaded and printed directly, but it can be difficult to edit these models in 3D CAD software like Fusion 360.

If you want to, for example, download a model from Thingiverse and edit it, you will need a way to convert the STL file into a form that the tools provided by your software can understand. This guide will show you how to convert complex STL files into solid bodies in Autodesk Fusion 360. A previous tutorial demonstrated how to convert relatively simple STL files into solid bodies.

What is a “complex” STL file?

An STL file consists of a bunch of triangular surfaces joined together to create an object. The reason most 3D CAD programs cannot edit STL files the same way they edit models created directly in the software is because STL files have no internal structure. An STL file is simply a complex surface consisting of two-dimensional triangles.

This T-Rex skull by MakerBot (thing:308335) is a complex STL made from a large number of triangular surfaces.

Therefore, in order to convert an STL into a solid body, Fusion 360 must build a solid object based on every triangle in the STL. This process becomes more and more difficult as the number of triangles in the STL increases. For simple STL files, Fusion 360 can convert the STL to a solid body directly, without requiring any modifications to be made to the STL mesh first. So, for the purposes of this tutorial, a complex STL is one that Fusion 360 fails to convert using the Mesh to Brep toolat least not without requiring modifications to the STL first.

For complex STL files, Fusion 360 will fail to convert the model using the Mesh to Brep tool without modifying the STL first.

Convert a Complex STL into a Body

The procedure below will allow you to convert a complex STL into a body that can be edited in Fusion 360 using your normal editing tools in the Model workspace.

Import your STL into Fusion 360

First of all, the STL file will need to be imported into Fusion 360. If you’ve done this before, skip to the next step. Otherwise, in the upper-left corner of the Fusion 360 window, click the Upload button. Then, click the Select files button and navigate to the STL you wish to import.

upload button location

Upload an STL file using the Upload button.

After selecting the STL file you want to convert, click the Upload button, this time the one in the lower-right corner of the file upload window. Then, the STL file you selected will be imported into Fusion 360. The process can take a few minutes.

Click the Upload button after selecting your STL file.

Reduce the Mesh Density

The problem we have with converting the complex STL to a solid body is the complexity of the STL mesh itself. Therefore, in order to allow the tools in Fusion 360 to convert the STL to a solid body, we need to reduce the complexity of the mesh. Fortunately, there is a newly released workspace in Fusion 360 that gives us access to tools for editing Meshes. In the upper-left corner of the screen, switch to the Mesh workspace.

Switch to the Mesh workspace.

The Mesh workspace allows a variety of actions to be performed on mesh bodies. The one we will use for this tutorial is the Reduce function. First select the entire STL mesh body, this is easiest to do by selecting the Mesh Body 1 in the Browser in the upper-left corner of the screen. Then, in the ribbon at the top of the screen, select Modify > Reduce.

Reduce the complexity of the Mesh using Insert > Reduce in the Mesh workspace.

The dialog box that appears will allow you to reduce the Mesh by a certain density, which is a decimal value. Reducing the mesh reduces the total number of triangular faces in the STL file. You will need to experiment a bit with the exact density to use for your STL file. You need to reduce the density of the mesh enough to allow Fusion 360 to convert the mesh into a body, but if you reduce the mesh too much, you will lose detail in the model. The more complex your original STL file is, the more you will need to reduce the density of the Mesh.

Experiment with the density value until you find one that works for your model.

Input the desired density decimal value into the Reduce dialog box and click the OK button. You will end up with a mesh body that should look almost the same as the original but with fewer faces in the mesh.

On the left is the original Mesh body and on the right is the reduced version.

Disable Design History

Normally Fusion 360 tracks all the edits you make to a model in a timeline at the bottom of the window. However, we actually cannot convert the STL file into a body while Fusion 360 is tracking history. Therefore, the history tracking functionality will need to be turned off.

First of all, switch back to the Model workspace using the dropdown in the upper-left corner of the screen. Then, at the top level of the Browser tree is the file name itself. If you right click on the file name, you will get a menu. To turn off history tracking, select Do not capture Design History.

Disable design history from the Browser.

When trying to turn off Device History, you may find that it is already disabled, in which case the option at the bottom of the menu will say “Capture Design History.” If this is the case, you will need to turn on history and then turn it off again.

Convert Mesh to BRep

Now that the density of the STL mesh has been reduced enough to allow conversion to a body to be a feasible operation, we can do the actual conversion. Looking in the feature tree again, if you expand the Bodies section, you will find an item called MeshBody1. The mesh body is the STL file. To convert the STL file, right click on the mesh body and select Mesh to BRep.

Convert the body using the Mesh to BRep tool.

If the tool still fails to convert the STL to a body because of the number of faces, you will need to repeat the previous step to reduce the mesh density a bit more. You may also get a warning that Fusion 360 can convert the Mesh to a solid object, but the conversion is not recommended. If you get this error message, you can still convert the Mesh into a solid body, but the performance you will experience in Fusion 360 while editing the converted body may be a bit sluggish.

With the STL converted into a solid body, you can perform whatever modifications you would like to the body just like you would with any other model in Fusion 360. Therefore, you can get STL files off online repositories, and then modify them to suit your needs.

With the STL converted to a solid body, you can edit it in any way you want. For example, I made a unicorn T-Rex.