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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.
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.