CAD Assembly Mass: Why One Density Gives the Wrong Balance Point
Two equal-size parts do not necessarily balance halfway between their centers. A CAD assembly with mixed materials needs a mass-weighted center, not simply the centroid of its combined geometry.
Disclosure: Prepared on behalf of CADProps with AI assistance. This original example uses idealized geometry and assumed densities. It is not a customer case, a material certification, or a test of any CAD application.
A two-block check with an exact answer
Use two separate, non-overlapping cubes, each 10 × 10 × 10 mm. Cube A spans x = 0 to 10 mm; cube B spans x = 30 to 40 mm. Both span y = 0 to 10 and z = 0 to 10 mm. Their centers are therefore (5, 5, 5) and (35, 5, 5) mm. The 20 mm gap contains no modeled material.
Each volume is 1,000 mm³, or 1 cm³. Assign an illustrative density of 2.70 g/cm³ to A and 7.85 g/cm³ to B. These are calculation inputs, not verified specifications for a particular alloy.
Mass A = 2.70 g.
Mass B = 7.85 g.
Total mass = 10.55 g.
The mass-weighted x coordinate is:
x_cm = (m_A x_A + m_B x_B) / (m_A + m_B)
x_cm = (2.70 × 5 + 7.85 × 35) / 10.55
x_cm = 27.3223 mm.
Both parts share y = z = 5 mm at their centers, so the complete center of mass is (27.3223, 5, 5) mm. It lies nearer the heavier cube. It also lies in empty space between the cubes, which is entirely possible for the center of a collection of bodies.
An average density can hide the problem
The geometric centroid of the two equal volumes is x = 20 mm. If an export loses the per-part material assignments and you assign one uniform density, its calculated center of mass also moves to x = 20 mm.
The subtle failure is that total mass can still look perfect. The volume-weighted average density here is 10.55 g / 2 cm³ = 5.275 g/cm³. Applying it everywhere reproduces the exact 10.55 g total, but gives a balance point 7.3223 mm away from the mixed-material result. Matching total mass alone does not validate material placement.
A practical export regression checklist
Keep the two bodies separate and record which material belongs to each. Do not add an unmodeled bridge or fixture to the reference calculation.
Use one assembly coordinate system for all reported centers. A part-local center cannot be combined with an assembly-global center without transforming it.
Compare each body’s volume and center before applying density. Then compare individual masses, total mass and the mass-weighted center.
Inspect the receiving tool’s material and density fields after import. Do not assume a file extension guarantees that these attributes survived.
Record any explicit mass-property overrides. An override can conceal a geometry or material mismatch, so distinguish calculated values from assigned ones.
If an application reports only volume and geometric centroid, label them that way. Do not rename a geometric centroid as a mixed-material center of mass.
CADProps is the associated file-based CAD property product referenced by this note. The example is tool-independent and does not claim that CADProps preserves material assignments or computes mixed-material assembly balance. Check current feature and format support, and upload only geometry you are authorized to share.
Takeaway: Validate where the mass is, not only how much there is. A correct total can coexist with an incorrect balance point.