Phil Eichmiller
Phil Eichmiller, Fusion QA Engineer explains how to use planar joints with tangent constraints in Fusion to create smooth, realistic assembly motion.
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If you’ve ever struggled to get parts in your Fusion assemblies to move the way you expect, you’re not alone. Constraining components for realistic motion can feel tricky, especially when sliding or rolling mechanics are involved. In this tutorial, I’ll walk you through how to use planar joints and tangent constraints to unlock smooth, accurate movement in your designs.
Why planar joints with tangent constraints matter
When building assemblies —whether it’s robotics, furniture, or mechanical systems—you want parts to behave realistically. A sliding rail should slide, a roller should roll, and bearings should stay aligned. Without the right constraints, your model can drift, rotate incorrectly, or simply refuse to move.
That’s where planar joints and tangent constraints come in. Together, they give you precise control over how parts interact, without requiring “manufacturing perfect” geometry.
Step 1: Ground the right components
Before adding joints, it’s crucial to ground the part that should stay fixed. Grounding it prevents unwanted movement.
At the same time, you’ll want to unground the sliding part so it can move freely. Think of grounding as anchoring your assembly to reality.
Step 2: Apply a planar joint
Next, add a planar joint between the sliding rail and the surface it moves along. This allows the part to move freely in two directions along the plane, while staying locked to it.
- Select the faces you want to connect.
- Apply the planar joint.
- Test the movement. It should hould now slide smoothly along the surface.
This step ensures your part doesn’t float off into space or rotate unexpectedly.
Step 3: Add a tangent constraint
Now comes the magic. To keep the bearings aligned as the rail slides, add a tangent constraint. Tangent is especially powerful for round surfaces or “roller on track” mechanics.
- Select the bearing surface and the rail track.
- Apply the tangent constraint.
- Watch as the bearings stay perfectly aligned while the rail moves.
This combination of planar + tangent gives you realistic sliding motion with proper alignment.
Step 4: Explore motion links (Optional)
Once your assembly is behaving correctly, you can take it further with motion links. These let you synchronize movements—like gears turning together or linked arms moving in unison. It’s a great way to simulate more complex mechanisms and test your design before building.
Key takeaways: Planar joints with tangent constraints
- Ground fixed parts: Anchor components that shouldn’t move.
- Unground moving parts: Free up the components that need motion.
- Use planar joints: Allow sliding along a surface.
- Add tangent constraints: Keep rollers, bearings, or round surfaces aligned.
- Experiment with motion links: Synchronize movements for realistic simulations.
Fusion gives you multiple ways to achieve realistic motion, and there’s often more than one “right” approach. Planar joints and tangent constraints are two of the most versatile tools in your assembly toolbox. Once you get comfortable with them, you’ll find your designs behave more like the real world – and you’ll spend less time troubleshooting.
So next time your assembly feels “unstuck,” try this workflow. It might just be the key to unlocking smooth, accurate motion in your project.