Practice: Gearbox

This is a practice exercise — they've seen the steps in Stage 1A/1B, now they apply them solo. Frame it like a real design review: we have a motor, a ratio, and a box it has to live in. The whole point of a gearbox is trading speed for torque. A NEO free-spins near 5676 RPM, which is way too fast for most mechanisms, so we reduce it. Tell them the three constraints (ratio, footprint, output spec) are non-negotiable acceptance criteria.

Do this on the whiteboard before anyone opens Fusion. Reduction ratio is just driven divided by driver. A 12-tooth pinion meshing a 108-tooth gear gives exactly 9:1. Common mistake: students put the big gear on the motor — that's an overdrive, wrong direction. The motor pinion is always the SMALL gear in a reduction. Warn them: pinions below 12 teeth on a 20DP profile start to undercut and get weak. NEO pinions are often press-fit; for practice we'll use a bored gear on a hex shaft instead.

Center distance is THE number that makes gears actually mesh. Get it wrong and gears either bind (too close) or skip teeth (too far). Formula: sum of teeth divided by twice the diametral pitch. For 20DP that's sum-over-40. Have them compute it for their chosen tooth counts. This number becomes the exact distance between the two bearing-hole centers in their sketch — emphasize that the math directly drives the CAD.

Fusion ships a SpurGear sample script — Utilities tab, Add-Ins, Scripts and Add-Ins, run SpurGear. It's not the FeatureScript custom-feature world of Onshape; it's a script that drops a finished gear body. Module is metric, so convert: 25.4 divided by diametral pitch. 20DP = 1.27mm module. Use 14.5 degree pressure angle to match standard FRC stock (AndyMark/WCP 20DP). After it generates, they sketch a 1/2in hex and cut it through the bore. Common mistake: leaving a round bore — it'll spin freely on the hex shaft and transmit no torque.

In Fusion, Components are the moving units and Bodies are the geometry inside them — this maps to Onshape's Part Studio split. Each gear and shaft needs to be its OWN component so we can put Joints between them later. Biggest beginner mistake: modeling everything as bodies in the root — then nothing can move and you can't add joints. Teach the habit: activate the component (double-click), then sketch and feature inside it.

The plate carries the bearings that hold the shafts at the correct center distance. Use the standard 1.125in bore for a 1/2in hex flanged bearing (REV/AndyMark). The NEO has a 50mm pilot and 3-bolt pattern — pull the real spacing from the REV datasheet or insert the NEO STEP file. Remind them to make TWO plates (mirror the body) so the gearbox is a sandwich; gears can't cantilever off one plate. The 3.000in bore spacing here is the same center distance they calculated.

This replaces Onshape's MKCad library. In Fusion you download a STEP or F3D from McMaster-Carr, REV, WCP, AndyMark, or MAXSwerve and Insert it. Always model around REAL hardware — if the NEO body collides with your plate, you find out now, not at the machine shop. Tell them to rigid-joint inserted parts so they don't float around the assembly.

Onshape Mates = Fusion Joints. Revolute = one rotation axis (perfect for a shaft in a bearing). Rigid = locked together (gear pinned to its shaft). Fusion does NOT auto-enforce gear mesh ratios in standard joints — dragging the pinion won't auto-spin the gear unless you add a Motion Link (Assemble > Motion Link) between the two revolute joints. For practice, adding the Motion Link at the 9:1 ratio is the payoff — drag the pinion one turn, the gear turns 1/9. Common mistake: jointing to the wrong origin so the gear orbits instead of spinning in place.

Acceptance criteria check. Run Inspect > Interference to catch gears clipping the plate or each other — a classic hidden failure. Re-confirm the ratio: count teeth, divide. Measure the output bore to prove it's 1/2in hex, not a leftover round hole. Confirm the plate never grew past the 4in box. This is exactly the kind of self-review a design lead does before a CAD gets approved.