Fixing Component Position with Rigid Joints

1. The Anchor: Grounding a Component

Before we can join components, we need to establish a fixed point of reference. Imagine building a car; you don't build it while it's floating in mid-air. You start with the frame held stationary on a jig. In Fusion 360, this concept is called grounding.

When you ground a component, you lock its position relative to the design's origin. It cannot be moved. This component becomes the anchor for the entire assembly.

To ground a component:

1. In the Browser on the left, find the component you want to fix.
2. Right-click on it.
3. Select Ground.

You will see a small thumbtack icon appear next to the component's name in the browser, indicating it is grounded.

The following video explains the importance of grounding a component before you start adding joints.

2. The Fusion 360 Joint Philosophy

Your mechanical engineering background gives you an intuitive grasp of concepts like degrees of freedom (DoF). In many older CAD systems, you would constrain a part by progressively removing these DoF—for example, by mating one face to another (removing 1 DoF), then an edge to an edge (removing more DoF), until the part is fully constrained.

Fusion 360 uses a different, more efficient approach. You start by removing all 6 degrees of freedom (3 translational, 3 rotational) and then you re-introduce the specific motion you want (e.g., rotation for a hinge).

A Rigid Joint is the simplest case: you lock the components together and introduce back zero degrees of freedom.

3. Applying a Rigid Joint

Let's put this into practice. The Joint command (keyboard shortcut: J) is your tool for this. When you create a joint, you are defining a relationship between two components by selecting a Joint Origin on each one. A Joint Origin is a specific point and orientation that you want to align.

Here is the general workflow:

1. Ground your base component.
2. Press J to activate the Joint command.
3. The first component you select will be the one that moves into position.
4. Hover your mouse over a face of the moving component. You will see several potential Joint Origins appear as small disc-like icons.
5. Pro Tip: To select a specific origin on a face without it disappearing as you move the mouse, hold down the Ctrl key (or Cmd on a Mac) to lock the face. You can then freely select the exact point you want (e.g., the center of a hole, a corner).
6. Click to select your first Joint Origin.
7. Select the second Joint Origin on your stationary (grounded) component.
8. The first component will move to align the two origins. The Joint dialog box will appear.
9. In the dialog, go to the Motion tab and select Rigid. You will see a brief shaking animation, indicating that the parts are locked together.
10. Click OK.

The following video demonstrates this process clearly, including the crucial tip for selecting joint origins.

4. Hands-On Practice

Let's build a simple assembly using the skills from this and the previous lesson. We'll create a simple L-bracket.

1. Create Two Bodies:
   • Start a new design.
   • Create a sketch on the bottom (XY) plane. Draw a 50 mm x 100 mm rectangle. Extrude it up by 10 mm to create your first body.
   • Create a second sketch on one of the short 50 mm x 10 mm end faces.
   • Draw a 50 mm x 50 mm square. Extrude it by 10 mm. Crucially, ensure the Operation in the Extrude dialog is set to New Body.
2. Create and Name Components:
   • In the browser, find the Bodies folder.
   • Right-click the first body and choose Create Components from Bodies. Rename the new component Base.
   • Right-click the second body and do the same. Rename this component Upright.
3. Ground the Base:
   • Right-click the Base component in the browser and select Ground. The thumbtack icon will appear.
4. Join the Upright:
   • Move the Upright component by clicking and dragging it away from the Base so you can see them separately.
   • Press J to start the Joint command.
   • Component 1 (the one that moves): The Upright. Hover over the face that will connect to the Base. Hold Ctrl and select the Joint Origin at the center of that face's bottom edge.
   • Component 2 (the stationary one): The Base. Hover over its top face. Select the Joint Origin at the center of the corresponding edge where the Upright should attach.
   • The Upright will snap into place.
   • In the Joint dialog, make sure the Motion type is set to Rigid.
   • Click OK.

Try dragging the Upright component. You can't. It is now rigidly locked to the Base. Congratulations, you've created your first joint!

5. A Quick Note: As-Built Joints

You may also see an As-Built Joint command in the Assemble menu. The difference is simple:

• Joint: Use this when your components are not in position. It moves the first component to the second and then applies the joint.
• As-Built Joint: Use this when your components are already modeled in their final positions. It applies a joint without moving anything.

We will use As-Built Joints more when we explore top-down design techniques, but for now, the standard Joint command is your primary tool.

Conclusion

You've taken a significant step forward today by locking two separate components into a fixed assembly.

Key Takeaways:

• Every assembly should have one grounded component to act as a fixed anchor.
• The Joint command (J) in Fusion 360 defines both position and the type of motion in a single step.
• A Rigid Joint removes all 6 degrees of freedom, locking two components together.
• The first component selected for a joint is the one that moves into position.
• The As-Built Joint is used when components are already positioned correctly in your model.

In our next lesson, we will move beyond static connections. You'll learn how to use Revolute and Slider joints to define single-axis motion, opening the door to creating functional designs with moving parts like hinges and sliding lids.