Lesson 5: Assemblies and Joints

In this lesson, you learn about the different types of joints to position components together and how to have them interact with each other.

 

Tutorial: Joints

In Autodesk Fusion 360, joints define the relationships between components in an assembly.

About Joints in Fusion 360

Fusion 360 defines relationships between components by using joints and as-built joints, and joint movement provides degrees of freedom. With other CAD tools, you use a constraint or mate to limit one or two degrees of freedom at a time, then add constraints or mates until you have enough degrees of freedom. In contrast, with Fusion 360 you begin by limiting all degrees of freedom and then select a joint motion type that specifies degrees of freedom. This approach allows you to obtain the required degrees of freedom all at once, in one command.

A joint allows a component to move translationally (back and forth) along the X, Y, or Z axis or to rotate around one of these axes. Each joint uses the number of degrees of freedom needed for the intended motion. When you insert a joint between two components, you choose one of the following types:

  • Rigid. A rigid joint fixes two components to one another. It provides no degrees of freedom.

  • Revolute. A revolute joint has a single rotational degree of freedom, much like a hinge. This joint can rotate around the standard X, Y, or Z axis, or around an edge in the model (a custom axis).

  • Slider. A slider joint has a single translational degree of freedom. It is used for components that slide along one another. Options are similar to revolute joint options, except that components slide along the selected axis rather than rotating around it.

  • Cylindrical. A cylindrical joint provides two degrees of freedom: one translational and one rotational. Components joined with a cylindrical joint always rotate around the same axis.

  • Pin Slot. A pin slot joint also allows two degrees of freedom, but components can rotate around different axes.

  • Planar. A planar joint allows three degrees of freedom. It allows two directions of translation in a plane and a single rotational direction normal to that plane. It is useful for joining two components so they can rotate while sliding across the plane.

  • Ball. A ball joint has two degrees of rotational freedom: pitch and yaw. Pitch allows components to rotate around the Z axis. Yaw rotates components around the X axis.

Create a Joint Between the Shaft and the Gear Housing

In this step, you use the Joint command to create a Revolute joint between the shaft and body of a gear housing.


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  1. If the Data Panel is not open, click Show Data Panel
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  2. In the Data Panel, open 5_Assembly Joints from Projects Samples Workshops & Events Adoption Path ➤ Mechanical Assembly ➤ 5_Assemblies and Joints.
  3. In the Model workspace, choose Assemble Joint.
  4. In the Joint dialog, set Motion Type to Revolute.
    Fusion 360 enables the required degrees of freedom for the motion type. Fusion 360 automatically chooses the Z axis, but you can specify another axis if necessary.
  5. Select the edge of the shaft. The Joint dialog shows the shaft selected for Component 1.
    By default, Fusion 360 offers only one point to connect to: in this case, the center point of the edge.

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    A small icon called the joint origin appears on the shaft. It is the reference point for the joint on this component.

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  6. Select a point on the inner rim of the opening in the gear housing. The Joint dialog shows this selection for Component 2.

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    The shaft moves to the gear housing and is joined in a revolute relationship.

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  7. Click OK.
  8. Rotate the base of the shaft. The rotating flag shows the animation.

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Join the Gear to the Shaft

Learn more about joint origins as you join the gear to the shaft.

  1. Choose Assemble Joint.
  2. Zoom in on the gear and hover over the top face.
    Different faces of the gear offer different points to place a joint origin. These points are called implicit joint origins. They are automatically generated and typically found on a face or edge.
    • A circle denotes a vertex.
    • A triangle denotes a midpoint.
    • A plus symbol denotes a center point.
    • A square denotes the centroid of a face.

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  3. Next, designate a face for the joint origin. Select the face that contains the hole of the shaft without referencing any of the individual points. The face changes color.

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  4. Select the center point (plus symbol) to place a joint origin on the hole for the shaft.

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  5. Zoom in to the shaft and select the center point on the top to place a joint origin there.

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  6. In the Joint dialog, specify the following values:
    • Set Angle to 0.0 deg.
    • Set Offset X to 0.00 mm.
    • Set Offset Y to 0.00 mm.
    • Set Offset Z to 3.00 mm.
      Notice that you can offset the final position of the gear on the shaft in all three axes and revolve around one of the axes. Also, because Fusion 360 automatically flips the gear, you use the Flip option to put the gear in the correct alignment.
  7. In the Joint dialog, set Motion Type to Rigid to remove all six degrees of freedom.
  8. Click OK.
  9. Drag the gear to preview its circular motion on the shaft.

Relate the Connecting Rod and the Block Guide with an As-Built Joint

You used regular joints to move the shaft and gear to their correct positions. Now you can use as-built joints between the connecting rod and block guide components, because they are already in their proper locations.


NOTE As-built joints are available only in parametric modeling, not in direct modeling. You typically use as-built joints in top-down design, where components, including imported geometry, are assembled in place.

  1. Choose Assemble As-Built Joint.
  2. For components, select the connecting rod and the block guide.

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  3. In the As-Built Joint dialog, set Motion Type to Slider.
  4. Select an edge of the connecting rod to specify a direction.

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  5. Select the end of the connecting rod edge. Once this end reaches the block guide, the connecting rod will slide no further.

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Animation shows a preview of the sliding relationship between the two components.

Join the Crank Arm to the Connecting Rod

In joining the crank arm to the connecting rod, you learn how to place a joint origin between two faces when snap points aren't available.

  1. Choose Assemble Joint.
  2. Move the crank arm to view the sides of the two faces on one end.

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  3. Right-click and select Between Two Faces.
  4. Select the upper face.
  5. Select the lower face.
  6. Hold down the Ctrl key (Windows) or Command key (macOS) and hover over the area until you see the center point between the two faces. Select that point for the joint origin.

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  7. Hold down the Ctrl key (Windows) or Command key (macOS) and hover over the cylinder at the end of the connecting rod until you find the center point. Select that point for the joint origin.

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    The crank arm moves to the connecting rod in a rigid relationship.
  8. In the Joint dialog, do the following:
    • Set Motion Type to Revolute.
    • Make sure that the X, Y, and Z axes are set to 0.

Animation demonstrates the revolving movement of the crank arm on the connecting rod.

Join the Crank Arm to the Gear

Complete the linkage between the gear, crank arm, and connecting rod, and then watch the resulting movement demonstrate the open degrees of freedom.

  1. Choose Assemble Joint.
    Alternatively, because you just used the Joint command in the previous step, you can right-click and select Repeat Joint. In Fusion 360, the previous command is always shown at the top of the marking menu.
  2. Rotate the crank arm so that you can view the bottom face.

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  3. Click the edge of the hole on the bottom face of the crank arm to select the center point.

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  4. Click the edge of the hole on the gear shaft to select the center point.

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  5. In the Joint dialog, set Motion Type to Revolute.
  6. Click OK to see the joint take effect.


Now that all the components have been joined, the movement of one them causes the rest to move according to the designated degrees of freedom. Notice that the five joints you have created are shown in the timeline.

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