Simple Gearbox

Anchor the 'why' before any CAD. Ask the class: what happens if you bolt a wheel straight to a NEO? It spins fast but stalls instantly under load. Gear reduction multiplies torque at the cost of RPM. A common FRC first stage is roughly 4:1 to 10:1. Today we model ONE stage only — input pinion, output gear on a hex shaft.

Diametral pitch (DP) is the tooth-size standard — 20DP is the FRC default for VEXpro/WCP gears. Pitch diameter is the imaginary circle where two gears effectively roll. Two gears mesh ONLY if they share the same DP and pressure angle. Have students compute PD for a couple tooth counts so the next slide's center distance makes sense.

This is the Onshape 'Variables' equivalent — in Fusion it's User Parameters under Modify > Change Parameters. Set these BEFORE sketching. Demo typing the centerDist expression so it computes live. Common mistake: students hardcode dimensions, then can't change tooth counts later. Parameters make the gearbox reconfigurable — the whole point.

Fusion ships the SpurGear script under Utilities > Add-Ins (Scripts and Add-Ins). It takes module, not DP — module = 25.4/DP, so 20DP = module 1.27mm. Generate each gear as its own component (right-click > New Component first, or rename in the browser). The 1/2in hex bore on the output gear is critical — that's the FRC standard shaft. Don't forget to bore the pinion to the motor's output (NEO/Kraken splined or round).

Reference the centerDist PARAMETER in the dimension, don't type a number — pick the dimension field and type 'centerDist'. Plate thickness for FRC gearboxes is usually 1/8in (0.125in) or 0.090in aluminum. Add bearing pockets if using flanged bearings (1/2in hex bearing OD is ~1.125in). Students often forget the second plate — you need TWO, mirrored, to sandwich the gears.

Standoffs are the spacers that hold the two plates apart at a fixed distance. The gap must clear the gear thickness plus a little (about 0.030–0.060in total). Too tight and gears bind on the plate; too loose and shafts wobble. You can insert a real standoff via Insert > Insert McMaster-Carr Component, or model a simple tube. This mirrors inserting an MKCad part in Onshape — in Fusion it's Insert > STEP/F3D.

This is the Onshape 'Mates' equivalent — in Fusion use Assemble > Joint. Ground the back plate (right-click > Ground). Standoffs and plates get RIGID joints (no motion). Each gear gets a REVOLUTE joint so it can spin. Pick the joint origin at the bore center. Demo dragging the output gear — if joints are right, it rotates cleanly. Common mistake: forgetting to ground something, so the whole assembly floats.

Use Inspect > Section Analysis to slice through the gear pair and eyeball the mesh. Teeth should nest like a zipper at the pitch line. If they overlap (red interference) or float apart, your center distance is wrong — go back to Change Parameters, not the sketch. Inspect > Interference gives a hard yes/no. This is the payoff for parameter-driving everything: one fix updates the whole model.