Joints & Mates

Open by reminding students that everything they've built so far has been static geometry. The moment you have an arm, an intake, or a swerve module, you need motion. Tell them frcdesign.org teaches this with Onshape mates — we do the exact same ideas with Fusion Joints. The concepts transfer; the buttons differ.

This is the number-one beginner trap. Students try to joint two bodies and the tool refuses or behaves weirdly. Demo: select a body, right-click, Create Components from Bodies. Stress that components are the LEGO bricks — bodies are just shapes inside a brick. Activate a component by double-clicking it before editing its geometry.

Compare to Onshape where the first inserted part is auto-fixed. In Fusion you do it yourself. Ground the chassis or the part bolted to the robot. Common mistake: grounding nothing, so the whole assembly floats and drags when you grab a part. Or grounding everything, so nothing can move. Ground exactly one thing — the frame.

The Joint command asks for two snap points: Component 1 then Component 2. Show snapping to a bolt-hole center and to a hex-shaft center. Tell them Fusion previews the origin as a little triad — if it lands in the wrong spot, the joint will be off. Tab key cycles snap options. This is the fiddliest part; have them slow down here.

Key distinction students miss. A normal Joint will yank the second component to the first snap point — surprising if you'd carefully positioned it. As-Built Joint says 'lock them exactly where they are now, just define the motion type.' For FRC, As-Built is perfect when you've inserted a REV or WCP STEP file already mated by geometry.

Show them grabbing the arm and dragging — instant feedback, the payoff moment. Then set limits so it can't swing through the floor or the bumpers. This is how you catch a mechanism that crashes into the frame before you cut metal. Onshape calls these mate limits; same idea. Encourage testing every revolute and slider this way.

Students panic when they drag a part and it 'won't go back' — teach Revert Position. Capture Position is for documenting poses: scoring config, stowed config. Warn them: editing geometry while a captured position is active can cause weird results — revert before editing sketches. It's the Onshape 'positions' equivalent.

This replaces Onshape's MKCad library. Show downloading a STEP from REV Robotics or grabbing a McMaster-Carr bolt, then inserting it. Inserted vendor parts arrive as components already — perfect for As-Built or hole-snap joints. Tip: insert as a STEP for geometry you'll joint to; the hole centers snap beautifully to hex shafts and bolt patterns.