Fusion Foibles: Editing Mass In Fusion 360

A few days ago I tried to update the mass of an off the shelf component in Fusion 360, something I do often in SolidWorks and Inventor, and found that I couldn’t. One of my most often used features in CAD packages (especially when the weight of a design is important) didn’t exist.

Now, for many projects, this wouldn’t be much of an issue.  Typically, the overall mass of a final assembly isn’t important (as long as it isn’t something ridiculous) but sometimes, like when designing competitive robots (sumo, battlebots, etc.) the weight really does matter.  So, if you really do need the mass of your CAD model to be correct (because you don’t want to keep track of the mass of all your components in Excel) what do you do?

Well, right now you have 2 options:

  1. Be annoyed that this feature doesn’t exist and post about it on Fusion 360’s IdeaStation.  And then be even more annoyed that the feature is closed, and revert to using excel.
  2. Fake it.

So, how do we fake it?  Well, we have an easy 4 step solution:

  1. Assign a material to the part.
  2. Get the volume and mass of the part.
  3. Calculate required volume to achieve the proper mass.
  4. Cut fake holes in your off-the-shelf part to make the mass correct.

Now, this solution isn’t actually easy and has a few trade offs associated with it.  First, you may want to calculate mass/volume and then try to find a material whose density would give your part approximately the appropriate mass.  This way you don’t have to add/remove much material to/from your part.  Probably the biggest issue with any of these methods its that it is very difficult to get the center of mass for your part correct.  You have to be very careful about material selection and placement of any cutouts you make.

Unfortunately, I do not have a great solution at this time to this problem.  I may write a simple script that takes the volume, density, and center of mass into account and helps you select cutout size, material selection, etc.  Hopefully, Autodesk fixes this issue soon so I don’t have to come up with crazy workarounds.

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