I noticed something about the order I use to join solids in Fusion 360. Until recently, I didn’t consider the effects of Combine Order; It does matter. This is what happened and what I learned.

Combine Order

I had initially exported two solid bodies and four fluid bodies from Fusion 360 to a STEP file. This was imported into Autodesk CFD without a hitch. I realized that changes needed to be made, and returned to Fusion 360. I altered the design, exported again, and started a new study in CFD. This time things were not so good. Instead of two solid components, I had approximately 550. There was no way I wanted to troubleshoot the mesh with that kind of overlapping complexity. I tried a few Fusion 360 alterations, but nothing mattered until I adjusted the combine order of operations.

What doesn’t work

You can create one singe solid body from many, by using the Combine tool. This works well as you would expect. One option within the tool, create a new component, will create a new component comprised of the newly joined/combined bodies, as a single body. All seems well in Fusion 360. However when exported to a STEP file, the model remains as numerous separate pieces (as if you imported a block into AutoCAD, and then exploded it). Not what I wanted.

What does work

Combine Tool in Fusion 360    Combine Tool in Fusion 360

I  recommend combining the bodies first, then separately create the new component. In the image above notice that the “New Component” option is not selected.

Create New Component in Fusion 360    Create New Component in Fusion 360

Afterward, Adding the joined, single-body into the component will produce a single body in the STEP file.

NOTE: if you add an additional body into the component after one has been added, either by dragging or using the combine tool, you may get multiple bodies in the exported STEP file.

Fusion 360 Combine before Component    Fusion 360 Component After Combine 

Notice the history bar at the bottom of each image, and the components at the bottom of the Browser Tree at the left side. In the first operation, the components are combined into the single hub body (highlighted). In the second operation, that single-body is added to a new component on-the-fly, called HPC Rotor. Inside that component is only the single hub body. This will come out in the STEP file properly.

The key is to join all bodies in one step as a single body in the history list. Then add that single-body to the component later. If you realize later that you need to add more bodies, back-up the history list and include the bodies in the single combine operation first. While these symptoms might sound suspiciously like saving your tool bodies, or having multiple bodies evident in your components, this is not the case in this situation. Only one body was evident in each component, viewed in Fusion 360.