Overview#
A normal Joint moves components so that two origins coincide. An As-built Joint does the opposite: the components are already where you want them, and you only declare the relationship and motion type between them. Nothing moves when you create it.
Use As-built joints when:
- You modeled the parts in place (top-down), so they already sit exactly where they belong.
- You imported a STEP or other assembly in which the parts are positioned correctly but have no joints.
- Moving the parts would disturb a carefully positioned arrangement, such as a mechanism linkage.
- You want to avoid the origin-matching step because the geometry has no convenient snap points.
Autodesk's help describes it as joining components that are already positioned rather than moving them. The Joint dialog and the As-built dialog differ mainly in the missing Joint Alignment section (Angle, Offsets, Flip: nothing to shift). Because nothing is repositioned, either selected component may be grounded, and you can select the two components in any order.
Where to find it#
- Design workspace > Solid tab > Assemble panel > As-Built Joint.
- Shortcut: Shift+J (Joint is J).
- Command Search (S): type "as-built".
Step by step#
- Ensure both parts are components and are in their final positions. If one is grounded, that is fine.
- Choose Assemble > As-built Joint.
- Select the two components to join.
- For any joint type except Rigid, choose the Mode to place the joint origin:
- Simple: click a Snap point.
- Between Two Faces: select Plane 1 and Plane 2 (the origin is centered between them), then a Snap point.
- Two Edge Intersection: select Edge 1 and a non-parallel Edge 2; the origin is at their extended intersection.
- In the Motion section choose the joint Type (Rigid, Revolute, Slider, Cylindrical, Pin-Slot, Planar, Ball) and its axes (Rotate, Slide, Axis, Normal, Pitch, Yaw as applicable).
- Optionally set Joint Motion Limits (Motion, Minimum, Maximum, Rest) and click Preview Motion.
- Click OK. The joint appears in the canvas and in the Joints folder in the Browser.
Verifying#
Drag the moving component or use Drive Joints. The part should move only through the freedom you declared. If it jumps, the joint origin was misplaced or the axis was wrong; edit the joint (right-click > Edit Joint).
Options reference#
| Option | What it does | Notes/default |
|---|---|---|
| Components | The two components being joined. | You select components first, then the origin geometry. |
| Mode: Simple | Origin on a face, edge or point at the Snap. | Not used for Rigid. |
| Mode: Between Two Faces | Origin centered between Plane 1 and Plane 2. | Useful for slots and pockets. |
| Mode: Two Edge Intersection | Origin at the extended intersection of Edge 1 and a non-parallel Edge 2. | |
| Joint Type (Motion section) | Rigid, Revolute, Slider, Cylindrical, Pin-Slot, Planar, Ball. | |
| Rotate / Slide / Axis / Normal / Pitch / Yaw | Axis of the motion (X, Y, Z or Custom Axis). Roll is automatic. | |
| Joint Motion Limits | Motion, Minimum, Maximum, Rest. | Optional. |
| Preview Motion / With All Limits | Animates Component 1. |
Note there is no Offset, Angle or Flip: those would move the part, which an as-built joint never does.
As-built vs regular Joint#
| Question | Joint | As-built Joint |
|---|---|---|
| Does it move the parts? | Yes | No |
| Good for bottom-up inserted parts? | Yes | Only if already positioned |
| Good for top-down parts? | Not needed | Ideal |
| Needs matching snap points on both parts? | Yes | No, one origin is placed |
| Closed kinematic loops | Can fight | Handles well since nothing is forced |
In an as-built joint you place a single joint origin. In a regular joint you place an origin on each component, and the two are made to coincide.
Converting between kinds#
To change an existing joint's motion type, right-click it and choose Edit Joint. Switching between a regular joint and an as-built one is not a documented toggle; delete one and create the other.
Note: not confirmed in Autodesk's documentation: that Edit Joint can change a joint between regular and as-built, or the API timeline-rollback rule for changing motion type.
Using as-built joints for a four-bar linkage#
Closed loops are where as-built joints shine:
- Model four link components, each already in the correct position.
- Pin the base link.
- Add an as-built Revolute at each of the four pivots, one pivot at a time, choosing Between Two Faces or Simple origins at the pin centers.
- Drag the crank. If every joint axis is parallel, the linkage moves; if an axis is off, the linkage locks. Check axes and joint origin Z directions.
If you used regular joints on a loop, each joint would try to move parts to meet the previous joint, and Fusion would probably fail or produce a distorted result.
Tips and pitfalls#
Tip: Model in position first, then add as-built joints. It is the natural pair of the top-down workflow described in Top-Down vs Bottom-Up Design.
Tip: After creating the joint, test it by dragging immediately. A joint with the wrong axis looks fine until you move it.
Warning: An as-built joint keeps the components where they are, so it never corrects misalignment. Edit the joint rather than moving parts afterward (general guidance; Autodesk's page does not describe a stored zero).
Warning: Parts that are almost aligned (off by a tenth of a millimeter) cannot be corrected by an as-built joint. It will preserve the error. If you need exact alignment, use a regular Joint with joint origins.
Tip: If Two Edge Intersection fails, switch to Simple and snap to a shared vertex, or create a joint origin first.
Warning: Do not use as-built joints on imported parts that are slightly interfering and expect motion; the motion will collide. Use Contact Sets sparingly or fix the clearance.
Tip: Rigid as-built joints are a fast way to "freeze" an imported sub-assembly: join every part to a base with Rigid as-built joints, or use a Rigid Group (see Rigid Groups and Grounding).
Worked example: a rocker arm imported from STEP#
Suppose you receive a STEP file of a rocker arm assembly: a 120 mm long arm, a 10 mm pin, and a bracket. The parts import as bodies or loose components with no joints. A regular Joint would try to move the arm so that its origin lands on the bracket's origin, which scatters the model. Instead:
- Make sure each part is its own component (right-click the Bodies folder in the Browser and choose Create Components from Bodies), then pin the bracket.
- Start As-Built Joint, pick the arm as Component 1 and the bracket as Component 2.
- Choose Mode Simple and snap to the center of the pin's circular edge, so the joint axis runs along the pin.
- Set Type to Revolute. Check the preview arrow: it must run along the pin axis. If the preview shows rotation about the wrong axis, change the Rotate axis (X, Y, Z or Custom Axis).
- Set Minimum -25 and Maximum +25 degrees and a Rest value of 0 in Joint Motion Limits.
- Drag the arm. The tip, 120 mm from the pin center, should swing on an arc of radius 120 mm. Over the full 50 degree range the straight-line distance between the two end positions is 2 x 120 x sin(25 degrees), about 101 mm. Measure it with Inspect > Measure to confirm that your model matches.
If the tip does not sweep a clean arc, the origin was placed on the wrong circle (for example the chamfer edge instead of the bore edge) or the pin hole is not concentric with the pin.
Choosing the origin mode in practice#
| Situation | Best mode | Why |
|---|---|---|
| Pin through a round hole | Simple, snap to a circular edge | The center of the edge is the pin axis. |
| Slot or tab fitting between two walls | Between Two Faces | The origin is midway, so the slider axis is centered in the slot. |
| Two straight edges crossing at a corner | Two Edge Intersection | The origin sits exactly at the corner, even with no vertex there. |
| Origin point has no geometry (center of a flat plate) | Create a joint origin first, then use it | Snap modes need geometry to grab. |
For a slider in a 20 mm wide slot with a 19.8 mm wide block, Between Two Faces on the two slot walls puts the origin on the slot centerline, and the block has 0.1 mm of play either side that the joint ignores. A Simple snap on one wall would place the origin 10 mm off center and the limits would then read from the wrong place.
Common failure modes and fixes#
| Symptom | Likely cause | Fix |
|---|---|---|
| The part jumps when you first drag | Joint axis not parallel to the real pin | Edit the joint and change the Rotate axis or Custom Axis. |
| Mechanism locks after the third joint | Over-constrained closed loop, usually with an axis tilted by a fraction of a degree | Change one joint of the loop to Cylindrical or Ball so it can absorb the error; check parallelism with Inspect > Measure. |
| Rigid as-built joint "does nothing" | It is working: it freezes the relative position | Test by trying to drag; parts should move together. |
| Cannot select a component | The target is a body, not a component | Use Create Components from Bodies first. |
| Joint misbehaves after editing an earlier feature | Parts shifted or the snapped geometry changed | Edit the joint and re-pick the origin, or roll the timeline to the joint and recreate it. |
Workflow advice#
- Create all joints for a mechanism in one session, in the order of the kinematic chain from the ground outward. Test each before adding the next so you find the bad one immediately.
- When the assembly is top-down and the layout sketches change often, prefer joint origins on the sketch geometry's components (see Joint Origins) so as-built joints survive an edit.
- Save a version before building a closed loop. If it locks, you can compare against the saved state.
Practice exercise#
- Model, in one session, a base plate, a crank arm, and a pin as three components, with the pin through a hole in the arm and base, all positioned exactly.
- Pin the base. Create an As-built Revolute joint between the arm and the base using the pin center as origin (Simple mode).
- Create a second As-built Rigid joint between the pin and the base.
- Drag the arm and confirm it rotates about the pin without shifting.
- Edit the first joint and add a limit of 0 to 90 degrees.
Expected result: the arm rotates 90 degrees about the pin and stops. Nothing moved when the joints were made.
Stretch challenge#
Model a slider-crank: a ground block with a 60 mm long slot, a crank of 25 mm radius, a connecting rod of 80 mm between pin centers, and a slider block. Position all four parts with the crank at 0 degrees so the slider is at 105 mm from the crank pivot (25 + 80). Use as-built joints only: Revolute (crank to ground), Revolute (crank to rod), Revolute (rod to slider), and Slider (slider to ground). Drive the crank through 360 degrees. Expected result: the slider travels between 55 mm (80 - 25) and 105 mm from the pivot, a stroke of 50 mm, which is twice the crank radius. If the mechanism locks at 90 degrees, one of the pivot axes is not parallel to the others; measure it and correct the origin.
Related#
- Joints and Joint Types
- Joint Origins
- Rigid Groups and Grounding
- Joint Limits and Motion Links