Mates and degrees of freedom
Pick the mate that describes the real joint, so the model moves the way the robot will.
By the end you can
- Choose the right mate type for a given physical joint
- Reason about degrees of freedom instead of guessing at mates until it looks right
- Place mate connectors deliberately rather than accepting the default
On this page · 4 sections
A mate is a statement about what a joint can do. Get it right and the assembly behaves like the robot. Get it wrong and you get a model that looks assembled but tells you nothing.
Start from degrees of freedom
A free part in space has six degrees of freedom: three translations and three rotations. Every mate removes some. The question is not "which mate looks right" but "which of the six should survive?"
- Fastened — 0 remaining
- Nothing moves. For anything bolted solidly: a motor mount to a plate, a bearing block to a rail.
- Revolute — 1 rotation
- Spins about one axis only. Wheels, arm pivots, any shaft in a bearing.
- Slider — 1 translation
- Slides along one axis only. Linear slides, a cascading lift stage.
- Cylindrical — 1 rotation + 1 translation
- Spins and slides on the same axis. A shaft that is not yet retained — often a sign the design is missing a collar.
- Ball — 3 rotations
- Rotates freely about a point. Rare in FTC outside of linkages.
- Planar — 2 translations + 1 rotation
- Stays flat on a surface but is free within it. Useful for a part that rests rather than mounts.
Mate connectors are the real skill
A mate joins two mate connectors — small coordinate systems on each part. Onshape will infer them from what you click, and the inferred one is often nearly right and subtly wrong.
Nearly right is wrong
A revolute mate on a connector that is 0.4 mm off the true bore axis produces an assembly that appears to work and a wheel that wobbles in a way no real bearing would. Worse, the number is small enough that nobody questions it.
Place connectors on the geometry that defines the joint
For a shaft in a bearing, put the connector on the bore axis, not on a face near it. The bore is what physically locates the shaft, so it is what should locate the mate.
Under- and over-constraining
Under-constrained parts still move when dragged. Sometimes that is correct — a lift stage should slide. Sometimes it means you forgot a mate.
Over-constrained assemblies have contradictory mates. Onshape will tell you, and the fix is almost always to remove a redundant mate rather than fight it.
A useful check
Drag every part in the assembly. Each one should move exactly the ways the real part can and no others. This takes a minute and catches more mistakes than reading the mate list does.
Check yourself
You are mating a dead-axle wheel: the axle is bolted solidly to the plate, and the wheel spins on bearings around it. Which mates describe this correctly?
Next
Structure and motion — keeping a robot-sized assembly workable.
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