Contact Sets

Overview#

By default, components in a Fusion assembly pass through each other when you drag them or drive a joint. Joints control which motion is allowed, but nothing stops two solids from occupying the same space. Contact sets add collision detection: when enabled between components, motion stops when the geometry touches.

Common uses:

• Stop a vise jaw exactly where it meets the workpiece, instead of guessing a joint limit.
• Prevent a door from swinging through the frame.
• Check whether a mechanism binds before you build it.
• Make a piston stop at the cylinder end.

Autodesk's help describes contact sets as components that "only move when they come into contact with one another": if interference is detected the components do not move, and the method requires more computation than a motion joint. Its example is constraining a component to revolve around a joint without passing through its bracket.

Where to find it#

New contact sets are listed in the Contact Sets folder in the Browser; the selected components are constrained together when contact sets are enabled.

Step by step#

Selective contact sets (recommended)#

1. Choose Assemble > Enable Contact Sets.
2. Choose Assemble > New Contact Set. The New Contact Set dialog opens.
3. In the canvas or Browser select two or more components that should collide.
4. Click OK. The set is listed in the Contact Sets folder in the Browser.
5. Drag a joint-driven component toward its neighbor. It should stop on contact.

Repeat for other groups. For example, create one set for the jaw and workpiece and another for the door and frame.

Enable All Contact#

1. Choose Assemble > Enable All Contact. It analyzes contact between all components in the design regardless of contact sets, and performance depends on the number of components.
2. Drag a joint. Everything now collides with everything else.
3. Choose Disable All Contact to turn it off if the assembly becomes sluggish or if parts that should touch (such as a bolt in its hole) stop movement.

Contact set rules and limits#

• Jointed pairs. Not confirmed in Autodesk's documentation: whether a contact set can include two components that already share a motion joint. Joint limits are the documented way to bound a jointed pair's travel.
• Rigid contacts. Two components with a Rigid joint do not need contact because they cannot move relative to each other.
• Within a rigid group. Members of a rigid group never move relative to each other, so contact between them is wasted effort; leave them out of contact sets (see Rigid Groups and Grounding).
• Geometry. Contact uses actual solid geometry, so a tiny overlap from a press fit or threaded fastener will stop movement immediately.
• Direction. Contact stops the dragged component. It does not push other floating components out of the way unless they are connected by joints.

Options reference#

Fine-tuning contact#

Clearance modeling#

If two parts must have clearance and still be flagged on touch, give them realistic clearance in the model. A 0.00 mm tolerance fit will stall motion immediately.

Selecting only moving pairs#

Not every pair needs a contact set. In a vise, the moving jaw needs contact with the workpiece and maybe the fixed jaw. It does not need contact with the base, which it slides on by joint design. Selecting only moving pairs gives a much smaller test count.

Using proxies#

For detailed parts such as threaded rods, create a simplified collision component and include only that in the set, then hide the detailed one. This is general modeling advice, not an Autodesk-documented workflow, and the extra component can appear in the BOM unless excluded.

Performance#

Contact tests compare solid geometry on every motion step. Cost grows with the number of components and the complexity of each. Practical advice:

• Enable contact only for the checks you are doing, then disable it.
• Use selective sets instead of Enable All Contact.
• Hide or suppress fasteners and small parts that do not need to collide.
• Use rigid groups for sub-assemblies that move together.
• Work with simpler (derived or simplified) versions of complex parts.

See Large Assemblies for broader performance strategy.

Contact sets in a motion study#

Autodesk's help says contact sets limit movement when components are moved, and its tutorial pairs them with joints (Enabling motion using joints and contact sets). How contact interacts with a motion study is not described in the contact set topics; test it on your own design. See Motion Study.

Tips and pitfalls#

Tip: Create contact sets after you have joints and limits working. Debugging both at once is hard.

Warning: Contact sets are not a physics simulation. Per Autodesk, they use physical contact to limit movement and components do not move through each other; no friction or bounce is described.

Tip: Name sets for the purpose: "Contact_Jaw_Workpiece".

Warning: If the dragged part will not move at all, check whether it is already overlapping a neighbor in the contact set. Move it manually away or temporarily disable the set.

Tip: Use Disable All Contact when you do not need contact analysis, since it adds computation (general guidance).

Tip: Interference check (Inspect > Interference) is the right tool for static overlap. Contact sets are for dynamic motion.

Worked example: sizing the stop on a clamp#

A toggle clamp has a handle on a Revolute joint (limits 0 to 80 degrees) that pushes a plunger on a Slider joint (limits 0 to 30 mm) toward a 12 mm thick workpiece. You set the slider limit by eye at 30 mm, but the real stop is the workpiece face.

1. Enable Contact Sets and create one set containing the plunger and the workpiece (the workpiece is Rigid-jointed or grounded to the base).
2. Drag the handle from 0 degrees. The plunger advances and stops when its tip touches the workpiece, say at 18.4 mm of slider travel, even though the limit allows 30 mm.
3. Read the actual stop with Inspect > Measure between the plunger tip and its starting position, or look at the joint value in Drive Joints.
4. Now replace the 30 mm limit with 18.4 mm only if you want the limit to hold without the workpiece present. Otherwise leave the limit generous and let contact do the work. Typically you do both: the limit keeps the mechanism sane in the empty state, and the contact set gives the precise stop with a part clamped.

Note the interaction: the handle can still be dragged to 80 degrees by an input that tries to force it; the contact set stops the dragged component, so the handle rests at whatever angle corresponds to 18.4 mm of plunger travel.

Choosing how many sets: counting pairs#

Autodesk notes that the number of components affects performance. As general arithmetic, the number of pairs tested grows roughly with the square of the component count. Enable All Contact on 40 components means up to 780 pairs (40 x 39 / 2). A selective set of 3 moving parts against 4 neighbors needs 3 x 4 = 12 pairs plus the 3 pairs inside the moving group, so 15, about 2% of the total. That is the whole argument for selective sets. Before you press Enable All Contact on anything beyond about a dozen components, write down which parts actually move and which parts they could realistically hit.

Failure modes and fixes#

Reading collisions rather than just stopping#

Contact sets tell you that something collides, but not by how much. To know how much space you have, combine them: drive the joint to the stop, then use Inspect > Interference or Measure to read the gap. For a drawer closing against a back panel, if the drawer stops 3 mm from where you expected, the contact set has just found a real 3 mm discrepancy in your model, not a bug. Fix the model and rerun.

When not to use contact sets#

• For parts that touch by design (a pin seated in a bore): the permanent contact blocks motion.
• For checking static clearance across a whole assembly: an interference check is cheaper.
• For anything needing force, friction or bounce: Fusion's contact is kinematic only.

Practice exercise#

1. Build a vise: fixed jaw (grounded) and moving jaw on a Slider joint along the screw axis, plus a 20 mm cube workpiece (grounded or Rigid to the fixed jaw).
2. Set slider limits from 0 to 100 mm, deliberately larger than needed.
3. Enable Contact Sets and create a set containing the moving jaw and the workpiece.
4. Drag the moving jaw toward the workpiece; it should stop exactly at contact.
5. Delete the contact set and confirm the jaw now passes through the workpiece.

Expected result: the jaw stops at the workpiece face with precise contact rather than at the arbitrary joint limit.

Stretch challenge#

Extend the vise: add a second workpiece 25 mm wide beside the first and a stop block, build two contact sets (jaw with workpieces, jaw with stop), and compare the drag stop positions. Then estimate cost: with 12 components, Enable All Contact tests 66 pairs, your two selective sets test far fewer; note the difference in drag smoothness. Expected result: the jaw stops at 20 mm with one workpiece, and 25 mm when you swap workpieces, with no changes to the joint limit.

Related#

• Joint Limits and Motion Links
• Motion Study
• Rigid Groups and Grounding
• Large Assemblies