Mass Properties
Mass Properties opens the Mass Properties window: the mass, volume and surface area of the model, where its center of mass is, and its moments of inertia. A small marker shows the center of mass in the view while the window is open. It does not change the model.
When to Use It
- You need the center of mass to place a lifting point, a counterweight or a support.
- You need the moments of inertia for a motion or vibration check, or to send to someone who does.
- You want to know how much of a model's weight sits on one side. Use Measure Mass when you only need the weight.
Step by Step
- Open a 3D model, click the Measure tab, then Mass Properties. The Mass Properties window opens.
- Read the numbers. The center of mass is marked in the view by a small red ball with three short ticks along the axes. The ball is drawn on top of the model, so you can see it even when it sits inside the part.
- If the model has more than one body, a list at the top of the window picks Whole part or one body. The numbers and the marker follow your pick.
- Click Copy to copy the numbers as plain text.
- Close the window with the X or Esc, or click another measuring tool. The marker goes away with it.
What the Window Shows
- Mass, Volume and Surface area. Surface area adds up every face, the walls of holes included.
- Center of mass: X, Y and Z measured from the part origin (the assembly origin for an assembly), in the length unit you chose.
- Principal axes of inertia and principal moments of inertia, taken at the center of mass. Ix, Iy and Iz are the three axes as direction vectors of length 1, and Px, Py and Pz are the moments about them, smallest first.
- Moments of inertia taken at the center of mass and aligned with the output coordinate system: the 3 by 3 inertia tensor Lxx to Lzz, with the axes parallel to the part axes.
- Moments of inertia taken at the output coordinate system: the same tensor Ixx to Izz, but about the part origin.
The output coordinate system is the part origin, with the part's own X, Y and Z axes. Other coordinate systems are not offered.
In the two tensors, the diagonal terms are the moments of inertia about each axis. The other terms are the products of inertia with their own sign: Lxy is the sum of x times y times mass over the whole body, and the same for xz and yz. This is the positive tensor notation, and the window says so under each tensor. The principal moments above are worked out from the full tensor, so they do not depend on this choice. A term that is only rounding noise next to the largest moment is shown as 0. Numbers show up to 8 significant figures, whatever the Decimals setting.
Units
The units follow the length unit in Settings:
- Inches and feet give the mass in lb and the inertia in lb * in² (or lb * ft²).
- Millimeters, centimeters and meters give the mass in kg and the inertia in kg * mm² (or kg * cm², kg * m²).
- Volume is in the length unit cubed and surface area in the length unit squared.
Where the Mass Comes From
Each body's volume is multiplied by the density of its material, exactly as for Measure Mass. Right-click a body in the Model Tree to give it a material; see Materials and Weight. Bodies of different materials are added up with their own densities, so the center of mass lands closer to the heavier ones.
Tips and Common Problems
- Only what is showing is counted. A body hidden with its eye is left out of every number and out of the center of mass, just as it is for Volume and Mass. Show it again and the window updates by itself.
- It follows your changes. Change a material, hide a body or move a component and the window updates.
- Mass says unknown. No body showing has a density (a material named in the file with no density, and none picked). The center shown is then the center of the volume, and the window says so. Pick a material for the body to get the mass and the moments. A body without a density is left out of the mass and moments when other bodies have one, and the window names it.
- Assemblies. Every component showing counts, where it sits in the assembly, and the numbers are about the assembly origin.
- Curved parts. The numbers are worked out from the same triangles that are drawn, like Volume. Flat-sided parts come out exact. A round part comes out a hair off the true value, because its curved walls are drawn as many small flat triangles.
- The button is greyed out on a flat DXF drawing and while you are sketching. Exit the sketch first.
- Not saved as a mark. Props shows a window, not a pill, so nothing is added to Kept Measurements. Use Copy to keep the numbers.