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My math teacher hated me. She would beat me unmerciful and use sounds of warrior robot apes fighting over canned meat in equations in a feeble attempt to stifle my creativity… It didn’t work. And neither did her or anyone’s warnings of manipulating the estimated mass of objects.

In SolidWorks you can apply materials to your solid geometry to get a weight. You can also add your own densities to approximate a weight, so that, based on the volume of, lets say, a robotic ape face, you’ll get a mass that works perfectly to strike fear into the minds of others. In doing so, you may find a need to manipulate mass here or there. I’m here to lead you astray and show you a few tricks that will aid in your endeavor.

Why change the densities?
In SolidWorks, densities, and therefore weight, are determined by the material you apply to a part, right? However, in a manufacturing environment, the weight of the overall assembly, and therefore the density, are determined by the amount of epoxy, welds or gum it takes to stick everything together or fix a production error.

In SolidWorks you can adjust the weight of a part by modifying it’s density, however, you can’t (and generally don’t want to) do it with assemblies. Why would you want to give an assembly a single density? Here are four reasons. One, I actually used, the other three, I simply made up for this post.

  • To adjust estimated weight to actual weight
  • To fine tune the weight of an assembly
  • To manipulate weight based on volume
  • To determine density of material not modeled

The Super Quick Way to Manipulate Weight of an Entire Assembly

Just a warning, this, in all regards, is not the most practical way of modeling or using models to create usable geometry, but it does have it’s place… somewhere, and, it’s really simple. There are two steps…

  1. Save the assembly as a part
    Ya can probably see where I’m going with this. Save the assembly as a part and make sure to check the “All components” option at the bottom of the save screen.
  2. Adjust the density in Mass Properties
    Open the part and go to the Mass Properties (Tools, Mass Properties…) You’ll want to uncheck “Assign Mass Properties” and then click “Options…”, Select “Use custom settings” and adjust the density. This, in turn, adjust the weight of the entire ‘assembly.’

Where I’ve used this

I’ve used this to estimate weight for assemblies, against other know densities, based on volume of a space. That so nerd, and makes my head ache, but it’s handy if you’re limited to a certain volume, limited to a certain weight or if you need to control both. Typically, I wouldn’t do this with an assembly I’ve already modeled. However, if I had a project where I needed to shave weight off an existing design, this would work great up front to see where I could trim material to reduce overall assembly weight.

Three more methods to manipulate weight

Add a Custom Property
This is really going way to far, but I thought I’d mention it. You can add a custom property that allows you to ‘apply’ a new weight to the assembly. It’s not actually changing the weight though. It’s a property that can be pulled into BOM’s or Notes, to replace the actual weight. Here’s an example design table, you can use to try it out. Download assembly-mass.xlsx

Add a design table to manipulate weight with custom densities. overkill? oh yes.

Add a design table to manipulate weight with custom densities. overkill? oh yes.

Add a dummy part
If you need to fudge an assembly weight a little you can add a blank dummy part and apply a certain weight to it. The only problem is, you can’t add negative weight. This is pretty common for things like adhesive, paint or boiling hatred that add weight to an assembly, but are not typically modeled. If you model glue and paint, tell me, so I can get a good shock.

Just change it in the Mass Properties
Of course, the extremely lazy way, to adjust mass it to simply change it in the Mass Properties of the assembly (Tools, Mass Properties.) This is really ideal for imported parasolids that you’ll be using as reference in your assembly model, things like purchase parts, referenced structure and

Have you actually saved assemblies as parts to adjust the density? Ever?

Filed under: CAD

  • Matthew

    Moments of inertia came back with assigned masses in the 2013 release. Finally…

  • Matthew

    It won’t work if you have a material assigned. Remove the material and the density option is changeable.