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Once you have multiple parts, you need to define how they connect. Joints describe relationships: rigid, rotation, translation, slider. Defining joints lets you simulate motion and check for interference.
Joint types.
Use these three in order. Each builds on the one before.
In one paragraph, explain joints in CAD.
Walk me through defining a revolute joint in Fusion 360.
I want to simulate a 5-link kinematic chain. How do I structure?
JOINT TYPES (common across Fusion / OnShape / FreeCAD):
RIGID:
Parts don't move relative to each other.
Use: parts permanently joined (welded, glued, screwed together).
REVOLUTE (PIN):
Parts can rotate around one axis.
Use: hinges, knobs, wheels on shafts.
CYLINDRICAL:
Parts can rotate around an axis AND slide along it.
Use: telescoping tubes, ball bearings.
PRISMATIC (SLIDER):
Parts can translate along one axis only.
Use: drawer slides, lift mechanisms.
PLANAR:
Parts can rotate around one axis and translate in the plane perpendicular to it.
Use: tracks, slide mechanisms with rotation.
BALL JOINT:
Parts can rotate around 3 axes.
Use: ball-and-socket joints.
DEFINING JOINTS:
Step 1: Select two parts.
Step 2: Pick faces or edges on each to define the connection.
Step 3: Choose joint type.
Step 4: Set parameters (e.g., 'allow rotation' = ±90°).
EXAMPLE: PRINTED HINGE.
Body A: a barrel with a 5mm hole.
Body B: a pin 4.8mm diameter.
Joint: REVOLUTE.
Axis: Pin center.
Reference: Pin meets hole.
Rotation: free (-180° to 180°).
Now: in CAD simulation, the pin rotates in the hole.
EXAMPLE: TELESCOPING TUBES.
Body A: outer tube (cylinder with internal hole).
Body B: inner tube (smaller cylinder).
Joint: CYLINDRICAL.
Axis: tube center.
Reference: inner tube inside outer.
Translation: 50mm range.
Rotation: free.
Now: simulating the inner tube extending or retracting.
EXAMPLE: SLIDING DRAWER.
Body A: cabinet frame.
Body B: drawer.
Joint: PRISMATIC.
Axis: pull-out direction.
Translation: 0-200mm.
Rotation: locked.
Now: simulating the drawer sliding in and out.
INTERFERENCE CHECKING:
After defining joints:
- 'Animate motion' to verify nothing collides.
- 'Interference detection' to find any overlap.
Catches design errors before printing.
POSE FOR DRAWING:
Some CAD: set the assembly to specific positions for drawings.
Save assembly poses: 'closed', 'open', 'half-open'.
DESIGN INTENT:
Joints capture design intent.
A pin in a hole = revolute joint.
Without the joint defined: just two bodies in space.
JOINTS IN DIFFERENT CAD:
Fusion 360: 'Joint' command. 'Joint origin' for precision.
OnShape: 'Mate' command. Multiple mate types.
FreeCAD: 'Constraint' in Assembly3 workbench.
PRACTICAL TIPS:
- Define joints early in assembly design.
- Use 'Joint origin' (in Fusion) to mark the exact point of connection.
- Test motion virtually before printing.
- If a joint doesn't make sense (parts won't fit), redesign.