Mate & Assembly Failures: Why Parts Float, Spin, or Won't Move

The single biggest source of broken FRC assemblies is sloppy mating. Understand the model first: a mate connector is a local coordinate system attached to a part; a mate joins two connectors and defines how many degrees of freedom remain between them.

Symptom: parts jump or rotate to the wrong place when you add a mate. Root cause is almost always conflicting connector orientation — the two connectors' Z or X axes point opposite ways. Fix: click the connector and use the reorient/flip controls in the mate dialog until the triads agree before confirming.

Symptom: 'over-constrained' / red mate errors. You stacked redundant mates (e.g., a Fastened mate plus extra Coincident mates on the same two parts). Onshape only needs enough mates to remove the intended degrees of freedom; one Fastened mate fully locks two parts. Fix: delete the redundant ones. Over-constraining causes solver instability and phantom failures.

Symptom: mates turn red and parts scatter after you edit a Part Studio. You mated to implicit connectors (auto-generated on faces/edges). When the underlying face regenerates or splits, the implicit connector disappears and the mate orphans. The community consensus is blunt: avoid relying on implicit and especially 'Center' mate connectors, which sit at a face/part centroid and 'are dangerous/useless' because the centroid moves unpredictably when holes or geometry change. Fix: create your own explicit mate connectors at hole centers and named features, then re-point the broken mate to them.

Debugging workflow: (1) Open the Assembly's feature list and look for red mates. (2) Edit the first red mate; Onshape highlights the missing reference. (3) If the reference is an implicit/face connector, add an explicit connector at that location in the Part Studio. (4) Re-select it in the mate. (5) Confirm the cascade is clear. Work top-down — fixing the first broken mate often clears the rest.

Prevention: standardize on Origin Cube as the first feature in every Part Studio so all parts share an origin, ground one part to the Assembly origin, and build everything else off explicit connectors. A swerve base mated this way survives resizing the #track variable without a single orphaned mate.