Mass properties (mass, CG, inertia)

TeamCraft computes the mass, the centre of gravity (CG) and the inertia tensor of an assembly – a vehicle, a rocket, a robot – from the 3D files of its parts, the densities of their materials and the masses you measured. Nobody has to load the whole assembly in the CAD system to get the numbers: open the assembly revision and choose Mass properties (on the revision page next to the BOM, or on the BOM page).

The method and its limits are described in the research note Mass properties from exported meshes.

The workflow

1. Export every part in the assembly frame. In the CAD system, export each part as STL (or 3MF, OBJ, STEP …) in the coordinate system of the whole assembly or vehicle, not in the part’s own frame. TeamCraft then only adds the parts up – no placements, no CAD assembly file needed. A part used several times (four fins) is exported once with all its instances in one file; set Units shown in the file to 4.
2. Attach the files to the part revisions as usual. Attach the files of one part to a part revision, in as many formats as you like. The geometry file is then picked automatically (STL, 3MF, OBJ, PLY, OFF, AMF, glTF, 3DS, then STEP, then the lightweight data in NX, SOLIDWORKS, JT and Rhino files; the native format of your preferred CAD program first among the native ones) and the page says so: Geometry: box.stl (chosen automatically; 2 other formats of the same model). Files belong to one model when their names agree apart from case, _, -, spaces, the extension and a trailing rev-X (box.prt, Box_rev-B.SLDPRT, box.stp). If the revision holds files of several different parts, TeamCraft does not guess – see Geometry ambiguous.
3. Name the material. The material attribute of the revision (e.g. Aluminium 7075-T6) is looked up in the material library, ignoring case. The Material field of New item and Edit revision (and of the desktop app) suggests the library while you type, with the density next to each name; picking a suggestion enters exactly the library name. Free text stays allowed – for a bought part with a measured mass, say – but a name that is not in the library shows a hint, and the mass properties report a missing density. A density override in the mass settings wins over the library – for 3D-printed parts with partial infill, for example.
4. Enter measured values for bought parts and weighed sub-assemblies: mass, optionally the CG and the inertia tensor about that CG.
5. Check the result on the mass properties page and export it as CSV or JSON for the flight simulation, the structural analysis or the balancing.

Where each number comes from

For every revision the first source that applies is used:

Lines below a measured assembly are shown as covered by measured parent. An assembly’s own 3D file and entered mass are not added to the sum of its lines (they would count the parts twice); enter a measured mass to replace the sum.

A BOM line with quantity n adds n units. In the common frame these units sit at the same place unless the file shows them all: a line whose quantity differs from the units shown in the file is flagged copies stacked at the same place – the mass is right, the CG and the inertia usually are not.

Geometry ambiguous

A plausible but wrong number is worse than none, so a revision whose 3D files belong to several different parts (typically an assembly attached as one item: the parts, the assembly and the neutral JT or STEP file) does not get the mass of whichever file sorts first. Its source is geometry ambiguous, it adds nothing to mass, CG and inertia, and the totals are flagged incomplete: N files of M different parts – choose the geometry file in Mass settings, or split the parts into BOM lines. The same applies to a part revision inside an assembly. An entered mass still counts (without position).

• TeamCraft suggests the likely whole assembly: the file that references other files of the revision and is referenced by none of them, else the file named like the item (number or name). The Mass settings card offers a one-click Use NIST mtc crada assembly.jt (the revision must be checked out); the choice is stored like a manual one. It is a suggestion only – nothing is used without the click.
• A JT 10 file of the assembly is often the best choice: it is the one file with every part placed in the assembly frame (the NX assembly .prt has no geometry of its own). For the NIST box assembly its volume matches the sum of the 36 placed part files to within 1 %.
• The file selector lists the candidates grouped by model, the likely assembly first, and marks the file in use as automatic or chosen; Automatic restores the automatic pick.
• The better fix is usually to model the assembly as an assembly: one item per part with the part’s files, and BOM lines with quantities (see The workflow).
• REST, the JSON export and the MCP tool carry geometry per line and root_geometry: choice (automatic, chosen, ambiguous), the file, the number of other formats, the candidates by model and the likely file. The CSV has the columns geometry_file and geometry_choice.

Warnings

• open mesh – edges with one triangle only (holes): the volume is not well defined. Fix the export (closed solids only).
• inconsistent orientation – some triangles face inwards.
• non-manifold edges – bodies touching along an edge or duplicated faces; usually still correct.
• inside out – every triangle faced inwards; TeamCraft flipped the mesh.
• no density – no material, or the material is not in the library.
• no position – a mass without CG (entered mass, measured without CG): counted in the total mass, left out of the CG and inertia.
• restricted lines – items of project groups you are not a member of are left out, like in the BOM roll-up.
• The entered mass attribute differs by more than 5 % from the computed mass.

Units and conventions

• The length unit is set per file: millimetres by default (glTF: metres), metres or inches. Results are always SI: kg, m, kg·m².
• The CG and the tensors are in the frame of the geometry files.
• The inertia tensor is given about the CG and about a reference point you enter (x,y,z in metres, e.g. the nose tip), in tensor form: the off-diagonal entries are the negative products of inertia, Ixy = −∫ xy dm. The principal moments are listed as well.
• Accuracy: a mesh approximates curved surfaces from inside. With an export chord tolerance of 0.01–0.05 mm the error is far below that of handbook densities; an icosphere of 1 280 triangles is 0.9 % light, one of 20 480 triangles 0.05 %.

Mass settings

The Mass settings card on the page holds, per revision: the geometry file with its length unit and number of units shown, the density override, and the measured mass, CG, inertia tensor and a note (scale, data sheet, date). Changing them needs the revision checked out, like every other attribute; Revise carries them over to the next revision.

Material library

Admin → Materials (/materials) lists the densities. Every signed-in user can read it; administrators add, rename and remove materials. Names are unique ignoring case. Removing a material does not change any revision; their masses then report a missing density.

Exports and interfaces

• CSV and JSON from the Export menu of the page (the totals, both tensors and every line with its source and warnings). Like the BOM downloads they follow the project group’s download switch.
• REST: GET /api/v1/items/{number}/revisions/{code}/mass-properties?rule=&ref=x,y,z and GET /api/v1/materials (REST API).
• MCP: the read-only tool get_mass_properties (MCP server).

Limits

• Placements (transforms per BOM line) are not stored: parts exported in their own frame cannot be combined yet.
• Overlapping bodies in one file are counted twice.
• Point clouds and simulation results have no volume.
• Measured values can only be changed in an in-work revision.