How to use the new Tangent Constraint in Assemblies
- Brad Tallis
- Feb 27
- 5 min read
Creating complex movements in Fusion used to be a difficult task for many designers. If you had a part that needed to slide and rotate through a strange shape, standard joints often made that task difficult. You might have tried to use pin-slot joints or contact sets, but these sometimes didn't give you the result you needed. Now, there is a better way to handle these designs. A new tangent option inside the constrain components tool makes complex motion much easier to set up. This tool allows parts to follow paths that were once very hard to simulate. In this post, we will look at how to use this tool for drawers, cams, and other moving parts.
The Problem with Traditional Joints
In the past, many users relied on as-built joints to create motion. For a simple pin in a straight slot, a pin-slot joint works fine. You select the two parts and define the direction of travel. However, things get challenging when the slot has curves or turns.
If your design has two pins in one slot, standard joints often struggle. They do not always recognize when a pin should hit a wall or turn a corner. You might see your parts passing through each other. To fix this, some people use contact sets. While contact sets work, they put a lot of stress on your computer. They try to calculate every single touch point, which can make your model lag. The new tangent constraint offers a faster, cleaner solution.
Getting Started with Constrain Components
To use this new method, you need to look at the Constrain Components menu. This tool is different from the standard joint menu. It allows you to create relationships between parts based on their physical faces.
Before you start adding complex movement, you must set a base. In most cases, you want your parts to stay on the same plane. You can do this by following these steps:
Open the Constrain Components tool.
Select the flat face of your moving part.
Select the flat face of the fixed part it slides against.
Set the constraint type to Planar.
Once you click okay, the moving part will stay flush against the back face. It can still move up, down, left, and right, but it won't pull away from the surface. This is the perfect starting point for adding a tangent constraint.
Setting Up the Tangent Constraint
The tangent constraint is the secret to smooth, complex motion. Imagine a part with a round pin that needs to stay inside another part's curved slot. Here is how you set that up:
Re-open or edit your constraint set.
Select the tangent option from the menu.
For the first geometry, select the cylindrical face of the pin.
For the second geometry, select any face inside the slot.
When you first do this, the pin might jump to the face you picked. If you try to move it now, it will act like a basic pin-slot joint. It only knows about that one face you clicked. If the pin reaches the end of that specific face, the constraint might break or act weird. To fix this, you need to use a special setting...
The Power of the All Faces Option
The most important part of this new tool is the All Faces checkbox. When you select one face of a slot, Fusion usually only cares about that one surface. But a slot is made of many faces that connect to each other.
By checking All Faces, you tell Fusion that the pin should stay tangent to the entire loop of the slot. This means as you drag the part, the pin will rub against the top, the bottom, and the curved ends of the slot. It treats the entire internal boundary as one continuous path.
Handling Multiple Pins for Realistic Rotation
Many mechanical parts use two pins to control how a part rotates as it slides. For example, a specialized drawer might slide out straight and then tilt down at the end. To do this, you need to constrain both pins.
After you set up the first pin, add a second tangent constraint for the second pin. Pick the cylinder of the second pin and the corresponding face in the slot. Again, make sure to select All Faces.
Sometimes, the pin might jump to the wrong side of the wall. If the pin is outside the slot instead of inside, use the Flip button. This toggles which side of the face the tangency is calculated on. Once both pins are constrained to all faces, the motion becomes very realistic. You can grab the part and watch it slide, pivot, and tuck away exactly like a real physical prototype.
Advanced Motion with Barrel Cams
Another great use for the tangent constraint is a barrel cam. A barrel cam is a cylinder with a groove cut into it. An arm or a "follower" sits in that groove. As the cylinder spins, the groove pushes the arm up and down.
In the past, this was very hard to animate in CAD. With the new tangent constraint, it is simple:
Select the cylindrical end of the follower arm.
Select one face of the cam groove.
Enable the All Faces option.
Now, as you rotate the barrel cam, the arm will follow the groove perfectly. It will even roll to the other side of the groove if the motion changes direction. This is a massive time saver for anyone designing engines, automated machinery, or complex toys.
Important Tips for Smooth Performance
While the tangent constraint is powerful, it does require some computer power. It performs live calculations as you move your mouse. Because it is figuring out the math on the fly, you should follow these tips for the best experience:
Move the parts slowly: If you drag your mouse too fast, the software might not be able to keep up with the math. This can cause the part to "lose" the constraint or jump to a weird position.
Check your geometry: Ensure your pins are slightly smaller than the slots they live in. This gives the math a clear "tangent" point to work with.
Limit other joints: Try not to have too many conflicting joints active at once. The cleaner your constraint list is, the faster the motion will be.
This tool essentially acts like a high-speed version of "Enable All Contact." It focuses the contact calculation only on the surfaces you choose. This makes it much more stable than turning on contact sets for the whole model.
Conclusion
The new tangent option in the Constrain Components tool is a major upgrade for Fusion users. It removes the frustration of trying to map out complex paths with simple joints. Whether you are building a sliding drawer or a complex barrel cam, this tool gives you the power to see how your design will work in the real world.
By using the All Faces setting, you can ensure your parts stay exactly where they belong. It provides a level of realism that was previously very hard to achieve.
Have fun learning Fusion and trying out these new mechanical relationships!
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