Mass and inertia

Select one object, or several. The selection panel then shows mass properties for that selection. With several objects the panel title is Group (n objects).

Density (g/cm³) and Mass offset (kg) are on the panel itself. Results are in the Analysis folder, and moments of inertia are in Moment of inertia inside it. The numbers are computed when Analysis is opened, and while Show Center of Gravity is on. Keep open on a folder leaves that folder open for the next selection.

Density and mass offset

Density (g/cm³) – Mass from the volume of this object. 0 adds nothing from volume. The value is stored on the object and is kept in a saved GLB.
Mass offset (kg) – Kilograms added or subtracted. Use it as a correction, or as the whole mass when density is 0. 0 adds nothing. Stored on the object and kept in a saved GLB.

On a group, the number you type is stored on every selected object.

Set Model unit in the View panel to the unit of the coordinates in the file: mm (default), cm, m, or inch. Area, volume, and mass are read in that unit. The model is left where it is.

How mass is combined

The selection is a set of roots. Each root includes everything under it. For one object the root is that object. For a group, each selected object is a root, and the results are one combined total.

The mass of an object is:

its density times the volume of every mesh under it, children included,
plus the mass of each child,
plus its mass offset.

A density on a parent and a density on a child both count. A negative mass on a child reduces the parent. Selecting a parent and that same child together counts the child a second time, so select the assembly or the parts.

A mass offset is a point mass. It is placed at this object's center of volume when a density is set. Otherwise it is placed at the center of gravity of its children, then at the center of volume of the meshes under it, then at the object's position.

Hiding an object leaves it in the mass. Section cuts, measurements, annotations, and center-of-gravity markers are left out.

With no density and no mass offset on the selection or anything under it, mass, the center of gravity, and the moments stay –. Show Center of Gravity and Save CoG then tell you that none is specified.

Analysis

Surface area – Area of the meshes, in the model unit squared.
Volume – Enclosed volume, in the model unit cubed.
Mass – From the combination above. The unit follows the size: mg, g, kg, or t.
Center of gravity (X, Y, Z) – World coordinates, in the model unit.

A value ending in (open?) means the shell is open or its faces point inconsistently. Treat the volume, mass, center of gravity, and moments as unreliable.

While you drag or rotate the selection, the center-of-gravity marker follows the part. The numbers refresh when the move ends. A scale change updates area, volume, mass, and the moments.

Center of gravity in the view

Show Center of Gravity – Draws a cross at the center of gravity. Its axes are the principal axes: red is axis 1, green is axis 2, blue is axis 3. The switch applies to the current selection.
Save CoG (Center of Gravity) – Asks for a name and stores a marker in the scene, in the same place and with the same orientation. The marker is a row in the scene tree.

Distance, angle, radius, and dimension tools snap to the shown center of gravity and to a saved marker.

Moment of inertia

Values on screen use kg·m², g·cm², or g·mm², whichever fits the magnitude. Export analysis (TXT) always writes moments in kg·m².

Ixx, Iyy, Izz (at origin) and Ixy, Ixz, Iyz (at origin) – Tensor about the world origin (0, 0, 0), along the world axes. Moving or rotating the part changes these.
Radius of gyration (Rx, Ry, Rz at origin) – Lengths, in the model unit, for those three moments.
Ixx, Iyy, Izz (at CoG) and Ixy, Ixz, Iyz (at CoG) – The same tensor about the center of gravity, still along the world axes. A translation leaves them as they are. A rotation changes them, because the axes stay aligned with the world.
Radius of gyration (Rx, Ry, Rz at CoG) – Lengths, in the model unit, for the moments about the center of gravity.
Principal moments (I1, I2, I3) – The three moments in the part's own principal axes. In those axes the products of inertia are zero, so only these three numbers are shown. Moving or rotating the part leaves them as they are. Scale changes them.
Principal axis 1 (red), Principal axis 2 (green), Principal axis 3 (blue) – Direction of each principal axis in world coordinates. The same colors are on the center-of-gravity cross.
Radius of gyration (r1, r2, r3) – Lengths, in the model unit, along the principal axes.

Ixy, Ixz, and Iyz are the products of inertia: how mass is coupled between a pair of axes.

Export

Export analysis (TXT) downloads a text file. The name starts with Analysis_ and the object name, or Group_ and the number of objects. The file records the name, date and time, user name when one is set, frame (world), model unit, density, mass offset, surface area, volume, mass in kg, center of gravity, both tensors, the radii of gyration, and the principal moments and axes. An open shell adds a note that the shell may be open or inconsistently oriented.