Joints and Joint Types

Overview#

A joint in Fusion does two jobs at once: it positions one component relative to another, and it defines how they may move afterward. This is different from the "mate" constraints of some other CAD systems, where you stack individual geometric relations. In Fusion, you pick a connection point on each component (a joint origin or a snap point), pick a motion type, and the software handles the rest.

Fusion offers seven joint motion types, per Autodesk's documentation:

Note: Autodesk describes the Ball joint as rotating around all three axes on a gimbal system (three nested rotations), with no translation. You define Pitch and Yaw; the Roll axis is determined automatically. (The Joint types page lists "1 translation and 1 rotation" in the summary lines for Planar and Ball, which contradicts its own descriptions; the table above follows the descriptions.)

Joints also come with joint origins, joint limits, drive behavior, and can be linked with motion links. Contact sets and motion studies build on them.

Where to find it#

• Design workspace > Solid tab > Assemble panel > Joint, shortcut J.
• As-built Joint: same panel, shortcut Shift+J. See the As-Built Joints page.
• Existing joints: the Joints folder (Relationships > Joints) in the Browser; right-click for Drive Joints, Edit Joint, Edit Motion Limits, Lock Motion, Suppress, Animate Joint, Animate Joint Relationships, Go to Home Position / Set as Home Position, Select Components and Delete. Joints sit in the Browser under Relationships > Joints.
• Drive Joints: Assemble panel > Drive Joints to move joints by exact value.

Step by step#

Making a revolute hinge#

1. Make sure each part is a separate component. Pin the one that should stay fixed (right-click > Pin; see Rigid Groups and Grounding).
2. Press J. The Joint dialog opens with a Position tab and a Motion tab; Component 1 prompts for a snap point.
3. Click a snap on the moving part, such as the center of the hinge pin hole. A joint origin triad appears.
4. Fusion prompts for Component 2. Click the matching snap on the fixed part, such as the center of the corresponding hole.
5. The moving part jumps so the two origins coincide. In Joint Alignment use Flip if the part is on the wrong side, Offset X/Y/Z to add spacing, and Angle to rotate it into place.
6. Open the Motion tab, set Type to Revolute, and confirm the Rotate axis (X, Y, Z, or a Custom Axis).
7. Optionally set Limits (see Joint Limits and Motion Links).
8. Click OK. Drag the moving component with the mouse; it should swing about the pin only.

Making a slider#

1. Press J and select the snaps on the rail and the carriage.
2. On the Motion tab, choose Slider. The slide axis is shown as an arrow; use the Slide axis selector (X, Y, Z or Custom Axis) to choose the direction.
3. Set limits and click OK.

Which snap to pick#

• Revolute: circle center or arc center on each part.
• Slider: any point on the sliding face; the origin needs to be on the right plane for the carriage to ride the rail, so use Between Two Faces joint origins if the rail has width.
• Rigid: any two corresponding points, such as corner vertices or hole centers.
• Ball: sphere centers.

Options reference#

Position tab#

Motion tab#

Joint behavior details#

Grounded and floating components#

The Joint tool typically moves the first component you select (Component 1) to the second; if needed Fusion relaxes existing relationships or grounded status so it can move. Selection order also sets the positive direction of motion. If neither component is connected to anything, animation is hard to predict, so pin one component first.

Ground to Parent#

Ground To Parent (right-click a component in the Browser) defines an inherent rigid relationship between a child and its parent so they move together; the child returns to its initial position. The first component in a design, the first child of any component, and pasted components are grounded to parent automatically. Patterned and mirrored components cannot be grounded to parent. When you create a joint, a first-selected component that is grounded to parent is automatically ungrounded from parent. Pin is different: it fixes the component at its current location in the top-level coordinate system and adds a Pin feature to the timeline.

Lock and Unlock#

Right-click a joint > Lock Motion to freeze its location. You can still edit the joint and its limits, but you cannot move it until you unlock it. Use Suppress to make a joint temporarily unavailable, as if deleted, which helps troubleshoot joint conflicts.

Animating and driving#

• Animate Joint (right-click joint): animates that joint only, ignoring other joints and relationships.
• Animate Joint Relationships: animates the joint in context with other joints, grounded components and contact sets.
• Drive Joints: Assemble > Drive Joints. Choose a joint and enter an exact value (Rotate, Slide, Axis, Normal, Pitch or Yaw fields, depending on type) or drag the manipulator handles.
• Go to Home Position / Set as Home Position: the home position is where the joint was created.

Degrees of freedom#

A fully constrained part in an assembly must have no unwanted DOF. Check by dragging; any movement indicates a joint that is looser than you intended. Rigid joints remove all six DOF. A revolute leaves one. If you chain joints (revolute then slider), the remaining DOF accumulate through the chain.

Over-constraint#

You can create conflicting loops: a closed kinematic chain (four-bar linkage) where joints cannot all be satisfied. The usual symptom is parts snapping to odd positions or a joint that will not move. Use as-built joints for loops, and use Suppress on joints to find the conflict.

Tips and pitfalls#

Tip: Always pin one component first. It gives the assembly an anchor and avoids unpredictable moves.

Tip: Create joints in assembly order: base first, then the part attached to it, then parts attached to that. A tidy tree is much easier to debug.

Warning: Joints belong to the parent of the components being jointed. If you create joints while a sub-component is active, the joint may live in that sub-assembly rather than in the top level. Check the Joints folder location in the Browser.

Warning: Joints relate components, not loose bodies. Convert bodies to components first.

Tip: For repeated identical items (bolts), use joint origins on the part and the receiving hole. Autodesk also provides Duplicate With Joints for copying components together with their joints.

Warning: Modifying geometry used by a snap can break the joint. Prefer joint origins on stable references.

Tip: Use Flip and Angle in Joint Alignment before accepting rather than redoing the joint. Keeping the fewest degrees of freedom makes the assembly faster to test.

Worked example: choosing joint types for a desk lamp#

A lamp has a base, a lower arm, an upper arm and a head. Choose by asking what motion each connection really has.

A Ball joint gives three rotations (a gimbal of three nested rotations) with no translation. You choose Pitch and Yaw axes and Roll follows automatically; each degree of motion can take its own Minimum/Maximum/Rest in Joint Motion Limits (a Motion selector picks which one). Limit all of them if you want to stop the head spinning about its own axis.

Degrees of freedom budget#

A free rigid body in space has 6 degrees of freedom (3 translations, 3 rotations). A joint removes some of them:

• Rigid removes 6, leaving 0.
• Revolute and Slider each leave 1.
• Cylindrical and Pin-Slot each leave 2.
• Planar leaves 3.
• Ball leaves 3.

A four-bar linkage with four Revolute joints and a ground link should end up with 1 degree of freedom. If Drive Joints shows no motion, the loop is over-constrained; if the linkage flops freely, there are too few constraints. Counting like this on paper before building is faster than debugging after.

Failure modes and fixes#

Workflow advice#

• Join in order from the grounded part outward and test each joint with Drive Joints before proceeding.
• Use the joint with the fewest freedoms that represents reality. A Planar joint where a Revolute plus a Slider are needed will let the part wander.
• Check motion at the extremes: drive to the minimum and maximum and look for interference.
• Keep a short "joint log" in your head: name each joint after its role ("Hinge_Left") by renaming it in the Browser, so errors are readable.

Practice exercise#

Build a door hinge.

1. Create components "Frame" and "Door". Pin Frame.
2. Add a Revolute joint at the hinge pin location with the Z axis vertical.
3. Open the Motion tab and set limits of 0 to 110 degrees.
4. Drag the door; confirm it stops at both ends.
5. Add a second hinge (a second joint origin pair) at the lower hinge position. Observe that Fusion handles two hinges only if the axes align; note what happens if they do not (see Joint Limits and Motion Links for debugging tips).

Expected result: a door that swings between closed and 110 degrees about a vertical axis.

Stretch challenge#

Build the lamp from the table above using only Revolute joints and limits. Place a 25 mm pin diameter at each joint. Drive the lower arm to 20 and 80 degrees and measure the head height at each; with a 200 mm lower arm the pivot height changes by 200 x (sin 80 - sin 20), about 129 mm. Expected result: the head moves about 129 mm vertically between the two limit positions (confirm with Inspect > Measure), and the mechanism has 3 independent degrees of freedom you can drive separately.

Related#

• Joint Origins
• As-Built Joints
• Rigid Groups and Grounding
• Joint Limits and Motion Links
• Contact Sets
• Motion Study