Practice: Pivot

Open with real 7558 examples: last year's arm, an intake flip-down. Tell students that 90% of FRC 'mechanism CAD' is just bolting tubes to a pivot done correctly. Today they build one from a blank design — no library shortcuts. The skill that transfers is the axle-and-bearing pattern, not this specific arm.

Insist on inch units up front — FRC stock is imperial (1/2in hex, 2x1 tube, #10-32 hardware). Make them create real components now, not loose bodies. Common mistake: students model everything in the root component and can't make joints later. Two components = two things that can move relative to each other.

2x1 aluminum tube is the FRC default arm member — real wall is 1/16in (0.0625). Tell them to ACTIVATE the Arm component first (it goes bold) so the body lands inside it. If they forget, the body ends up in the wrong component and joints break. A shell is fine for practice; in a real design they'd insert WCP/AndyMark tube stock.

0.875in (7/8) is the standard FRC flanged bearing OD — both the 1/2in hex bearing and the 3/8in bore bearing share this OD. Stress 'center it': use a midpoint constraint or symmetry, don't eyeball. If the hole isn't centered, the arm pivots off-axis and looks broken. Cut through BOTH walls so the bearing is supported on both sides — that's the whole point.

Walk through Insert > Insert McMaster-Carr — paste a flanged bearing part number, download the STEP, it drops in. Or use a REV/WCP bearing F3D. Two bearings, one per side, flanges facing outward so they seat against the tube wall. This is the dead-axle pattern: bearings ride IN the moving part. Common mistake: one bearing only — then the arm cantilevers and wobbles.

Shaft goes in the Frame because it's dead — it never moves. 1/2in hex is the FRC workhorse; 3/8 round is used for lighter free pivots. The shaft must extend past both bearings so the frame plates can clamp it. Have them double-check which component the shaft body lives in — wrong component is the #1 reason their joint won't behave.

The frame plates are the 'bearing support' the title promised — they hold the dead shaft and transfer all pivot loads into the frame. Leave ~0.02-0.04in clearance between plate and arm so it doesn't bind. In a real robot these plates bolt to a 2x1 frame rail or a gusset. Emphasize: load path goes arm > bearing > shaft > plate > frame.

In Fusion, Onshape 'mates' are Joints. Press J. Snap the joint origin to the bearing bore center / shaft axis — use the circular edge so it auto-centers. Pick Revolute (one rotational DOF). Then grab and drag the arm to confirm it spins about the right axis and nothing passes through the frame. If it spins about the wrong axis, the joint origin snapped to the wrong edge — redo it.

Two parts: (1) Edit Joint Limits to cap the swing so the animation respects travel. (2) Model a REAL stop — a bolt head, a 1/4in block, or the frame edge — at the exact angle the joint maxes out. The limit and the physical block must agree. Common mistake: setting joint limits but no physical stop, so the real robot has nothing to hit. Drag the arm to both limits and verify contact.