Top-Level Assembly

Frame it: every student has been CADing a subsystem in isolation. The top-level assembly is the reunion. Stress that 90% of design mistakes — parts intersecting, no room for wiring, a motor hitting a tube — are caught HERE, on screen, where fixes are free. Show last year's robot top-level file if you have one.

This is the #1 Fusion confusion for beginners coming from Onshape. In Onshape a Part Studio holds parts and the Assembly holds instances. In Fusion, ONE file holds everything, and the Component is the magic word — only components can be jointed and counted in the BOM. Demo: a body that won't joint, then convert it, then it joints. Components also let you activate one to edit it in place.

A shared coordinate system is the whole point. Decide the convention as a team and write it down: many FRC teams put origin at the center of the frame perimeter on the ground plane, X forward. Demo Right-click drivetrain > Ground (the pin). Grounded = the anchor everything else mounts relative to. Common mistake: students import a subsystem and it lands 3 meters away because its origin didn't match — that's why convention matters.

Two paths: Insert into Current Design (a copy you can edit) vs Derive (a live link — edits upstream flow down). For a top-level assembly, Derive is powerful because when a subsystem owner updates their file, your robot updates too. Warn them: derived components are read-only in the top-level; you edit the source. Bring parts in ONE at a time so the tree stays clean.

Onshape calls these Mates; Fusion calls them Joints. Same idea, different workflow: in Fusion you pick TWO points (component 1, then component 2) and they snap together, then choose motion type. Rigid for anything bolted (the vast majority of robot mounts). Revolute for an intake that folds, slider for an elevator stage. Common mistake: forgetting to ground the drivetrain first, so the whole robot drifts when you joint. Demo As-built Joint too for parts already positioned.

Tie it to reality: FRC robots are built on 2x1 and 1x1 box tube (often WCP, AndyMark, or REV extrusion) with standard hole patterns. When you mount a subsystem, its bracket holes must line up with the frame holes — that's a real constraint, not just visual. Demo aligning a bracket to a tube face and rigid-jointing. Mention the build team has to actually bolt this, so bolt access matters: if a wrench can't reach, redesign.

This is the payoff slide. Inspect > Interference is Fusion's built-in collision checker — it lists every pair of parts that overlap. Real robots fail inspection or break because two subsystems fight for the same space. Also have them check the bumper zone (FRC rules) and frame perimeter. Inspect > Center of Mass for stability — a top-heavy robot tips. Common mistake: declaring 'done' without ever running interference.

Onshape Variables become Fusion User Parameters (Modify > Change Parameters). Teach them to drive the frame size from a single parameter so changing the robot footprint doesn't mean editing 40 sketches. FRC reality: motors and gears are metric (5mm HTD, 20DP), tube and many COTS parts are imperial — Fusion lets you type '26 in' even in a mm document. Be explicit about units or you'll get a 26mm robot.