Practice: Indexer

Frame the indexer as the 'conveyor belt' between intake and shooter. Most rookies forget the indexer entirely and wonder why their robot jams. Stress: the indexer's job is reliable hand-off, not speed. Ask students what game piece they're handling — a note/disc routes differently than a cube or ball. The whole subsystem lives or dies on consistent compression on the piece.

Reinforce the habit from earlier stages: ALWAYS make a new component and activate it before modeling. Set compression as a user parameter (typically 0.25–0.5 in for notes) so they can tune grip in one place. Show Modify > Change Parameters. If they model loose bodies in the root, joints get messy later.

The path sketch drives everything. Spacing between the grippers must equal piece diameter minus compression so the piece is squeezed, not free. Tight bends cause jams — demo with a curve that's too sharp. Tell them to physically trace where the piece goes before drawing. This is the single most important sketch in the subsystem.

All powered shafts should be 1/2in hex — standard FRC. Drive roller carries the pulley; idlers just guide. Use a rectangular or circular pattern to space rollers along the curve. For belt routing, the belt wraps the drive pulley and idlers. Common mistake: forgetting bearing bores in the side plates for the hex shafts. Use 1/2in hex bearings (e.g. FR8 or REV/WCP hex bearings).

Belt routing must stay tangent — the belt leaves a pulley in a straight line. Add a tensioner on a slot so they can take up slack (real belts stretch). Have them check pulley center distance against a real HTD belt length (belts come in fixed tooth counts). Two-sided indexers mirror everything — use Mirror feature, not re-modeling. Polycord is the simpler alternative for beginners.

This is the Onshape-Mates-to-Fusion-Joints translation. Plates and standoffs that never move = Rigid joints. Each spinning roller = Revolute joint about the shaft axis. Use 'Drive Joints' to animate and confirm rollers spin the right way. Mistake: applying joints between bodies inside one component — joints go between components, so each roller may need its own sub-component.

Indexers don't need much power — a NEO 550 on an UltraPlanetary, or a Kraken if sharing motors. Aim for a moderate roller surface speed; too fast and it throws the piece before the shooter is ready. Reduction gives torque to overcome a stuck piece. Mention #25 chain or a 5mm HTD belt as the transfer from gearbox to the drive roller.