Rigid Groups and Grounding

Overview#

Not every relationship in an assembly needs a full joint. Sometimes you simply want several components to travel together as one unit, or you want one component to stay put. Fusion provides:

• Pin: fixes a component in place in the active design. It is pinned at its current location relative to the top-level coordinate system, and a Pin feature is added to the timeline. (Autodesk's current help uses Pin and Unpin; earlier material called this Ground.)
• Ground To Parent: defines an inherent rigid relationship between a child and its parent, so they move together; a sub-assembly can be fixed internally yet still move as a whole.
• Rigid Group: locks the relative position of the selected components; they are treated as a single object when you move them in the canvas or apply joints.
• Rigid joint: a joint of type Rigid; zero degrees of freedom, positioned by joint origins.

By default, components in assemblies are flexible: they move freely until you define relationships such as joints, and each instance of a component is flexible independently of other instances. Choosing among them is mostly about intent. A rigid group says "treat these as one lump during motion". A rigid joint says "this part sits here, exactly, relative to that part".

Where to find it#

Tip: If you do not see Rigid Group, press S and search "rigid".

Step by step#

Pinning#

1. Expand the Browser and right-click the base component.
2. Choose Pin. The component is pinned to the default component in the active design and a Pin feature appears in the timeline.
3. To release it later, right-click > Unpin; an Unpin feature is created.

Pin the component that anchors the assembly: a baseplate, frame, or housing. Autodesk's tip: for predictable results, pin one main component and then create joints between it and other components. (Pinning a second part in an independent assembly is general practice to avoid; Autodesk does not describe conflicts.)

Ground To Parent#

1. Insert a sub-assembly as a component (for example, a gearbox containing a housing, shaft, and gears).
2. Expand the sub-assembly in the Browser.
3. Right-click the housing component inside it > Ground To Parent.
4. The housing is now fixed relative to its parent, so they move together; the parent itself is free to move in the main assembly (or be pinned or jointed there).

Per Autodesk: the first component in a design, the first child of any component, and any component you Copy and Paste or Paste New are automatically grounded to their parent. Internal and external components can be grounded to parent; patterned and mirrored components cannot. Ground To Parent overrides Rigid Groups, Move, Pin and Capture Position on that component, and the component moves back to its initial position. When you create a joint, the first component you select is automatically ungrounded from parent; the second keeps its status. In a Manufacturing Model, ground-to-parent relationships are overridden so you can arrange the components.

Rigid Group#

1. Choose Assemble > Rigid Group.
2. In the canvas or the Browser, select two or more components.
3. Optionally check Include Child Components to include all child components nested within the components you select (this keeps sub-assemblies intact).
4. Click OK. A Rigid Group feature appears in the Timeline.

Now drag any one member of the group; the others follow. If you move or apply a joint to one of the components, all of them move together.

Editing a group#

• In the Timeline, right-click the Rigid Group feature > Edit Rigid Group, then add or remove components in the selection set and click OK.
• Delete the feature in the Timeline to dissolve the group (general Fusion behavior).
• Autodesk recommends creating a separate Rigid Group for each instance of a sub-assembly, because each instance is independently flexible by default.

Note: not confirmed in Autodesk's documentation: whether a component can belong to more than one rigid group.

Options reference#

Which tool should I use?#

Rigid group vs rigid joint#

• A rigid group does not position anything. It locks the current relative positions of the selected components.
• A rigid joint can pull a part into place by origin and can be edited with offsets and angles.
• Joints are listed in the Joints folder; a rigid group is a feature in the Timeline.
• A rigid group is easier to maintain on large, already-correct assemblies. A rigid joint is more reliable when a part may be replaced, because the relationship is defined by geometry rather than a frozen position (general guidance). Note that a rigid joint can only be applied between two components.

Rigid groups and motion#

Autodesk says that when you apply a joint to one component of a rigid group, all of them move together, and that you can add joints between components in rigid groups to define motion between sub-assemblies. A revolute joint on any member therefore rotates the entire group. Avoid putting two components in a rigid group when a motion joint also connects them: the group already fixes them (general reasoning).

Performance benefits#

Autodesk recommends a Rigid Group for three or more components with no relative motion, instead of a rigid joint between each pair, because it reduces the number of features to compute in the Timeline. See Large Assemblies.

Tips and pitfalls#

Tip: Pin one main component first. A fully floating assembly can drift when you drag, and every later joint is relative to a moving anchor.

Tip: Name rigid groups by function: "Rigid_Arm_Assembly". The default "Rigid Group1" becomes a puzzle at 12 groups.

Warning: Unpinning the base while joints exist lets the whole assembly float. Check the Browser for the Pin state.

Warning: If components will not move, check whether two parts are both pinned and joined, or whether a rigid group spans a moving joint.

Tip: After building a mechanism, you can lock the whole mechanism for a render or export by creating a rigid group of all its parts. Delete the group to restore motion.

Warning: A new component is not automatically part of an existing group; edit the Rigid Group feature to add it.

Tip: Use Rigid Group after inserting components or sub-assemblies that should stay fixed relative to each other (Autodesk's tip). See Capture Position for keeping or reverting moves.

Worked example: a motor sub-assembly inside a machine#

A motor module has a motor, a mount plate, four screws, a coupling and a shaft guard: eight components that never move relative to each other, but the whole module slides on a rail in the machine.

1. Inside the module, put the components in the correct positions relative to each other (use joints or direct placement).
2. Select all eight and create a Rigid Group. Now they behave as one body in the solver.
3. Add one Slider joint between the group's mount plate and the rail carriage. Because the other seven parts ride along, you need one joint, not eight.
4. For the machine frame, Pin the base. Every other part is positioned relative to it.

Count the cost saved. Eight free components joined individually to the mount plate would need seven Rigid joints, each one solved on every drag. The group replaces seven constraints with a single rigid body, which is why large assemblies respond faster.

Which grounding for which case#

Failure modes and fixes#

Rigid Group vs Rigid joint: a practical way to choose#

Choose a Rigid Group when you have many parts that form one unit and you want a single object to joint onto something else. Choose Rigid joints when the relationship is one-to-one (a rigid joint applies between two components), when you need to see it as a named relationship in the Joints folder, or when you may later want to change it to a movable joint. A group is best hidden in a sub-assembly after it is finished; joints are best where the relationship may change.

Checking the state#

Open the Browser and Timeline: Pin and Rigid Group features appear in the Timeline, joints in the Joints folder. Before delivering a file, scan for components with no joints, no pin and no group; they are floating and will move if someone drags the model. A quick test is to select all and drag one part: nothing unintended should move.

Practice exercise#

1. Create a base plate and pin it.
2. Add an arm component. Add a Revolute joint between arm and base.
3. Insert two M5 bolts (see McMaster-Carr and Standard Content) and place them in holes on the arm. Do not joint them yet.
4. Open Rigid Group, select the arm and both bolts, and click OK.
5. Drag the arm; the bolts should travel with it.
6. Edit the Rigid Group feature in the Timeline and remove one bolt. Observe that it no longer follows.

Expected result: the group moves as one body about the revolute joint, and membership is controlled by editing the Rigid Group feature.

Stretch challenge#

Build a rail-mounted tool carriage: a base (pinned), a rail on a Rigid joint, a carriage on a Slider joint (0 to 300 mm), and a tool head made of six components in a Rigid Group riding on the carriage. Drive the slider to 300 mm and confirm all six head parts move together. Then count constraints: one Slider and one joint between the group and the carriage, instead of one Slider plus five Rigid joints. Expected result: the tool head moves as a unit, and you can explain why the group version is one body to the solver.

Related#

• Joints and Joint Types
• As-Built Joints
• Capture Position
• Large Assemblies
• McMaster-Carr and Standard Content