Mates — hold parts together

Relate two bodies so one follows the other, and scrub a hinge or slider through its travel.

A mate is a relationship between two placed bodies — *coincident* faces, a shared *coaxial* axis, *parallel*, a fixed *distance*, a *hinge (revolute)*, or a *slider*. Add one and OwlCAD moves the parts to satisfy it, so a bolt drops into its hole instead of being nudged there by eye.

A mate has a direction, and picking the two bodies the wrong way round is the usual mistake. Body A is the fixed reference and body B is what moves. If the wrong part jumped, you do not have to delete the mate and start again — press Swap which body stays fixed. The parts stay the same distance apart; only which one is held still changes.

Pick faces, not bodies, when it matters. Choosing bodies from the dropdowns mates them by their centres. Clicking a face in the viewport anchors the mate to *that* face or axis, which is what you want for a bolt into a hole or a lid onto a rim. If a face you picked is later edited the mate reads Stale — it still solves against the frame it captured, but re-pick it to be sure it still means what you want.

Scrub before you print. A hinge or slider has one driven degree of freedom. Drag it through its range with the travel limits set and watch whether the parts foul each other — that is the check that saves a print, and it costs nothing. If they are too tight or too loose to move, the clearance is the thing to change; the print-in-place joint generators and the fit test coupon exist for exactly that number.

Your positions are never overwritten. Mates are solved at the display layer, so the transform you authored stays in the document and the solved position is derived each time. Delete a mate and the parts return to where you put them.

1. Open the Mates panel with at least two bodies in the scene.
2. Choose a mate type, then pick the fixed body (A) and the moved body (B) — or press Pick faces in viewport and click a face on each.
3. Add mate. Body B moves to satisfy it; body A does not.
4. For a Hinge or Slider, drag the angle or offset to scrub the motion, and set Min/Max to stop it where the real part would stop.
5. Check the parts clear each other, then export — mates are a relationship, not a change to the shape.