Mechanism CAD
Workshops 1 and 3 run on a real arm or a real flywheel on a bench. This page has the 3D models, the parts lists, and the build notes for both. Teams attending in person can skip it, and so can a team that already has a mechanism with a TalonFX on it.
- Nothing installed. The models spin in the browser.
- To build one: a 3D printer, a saw, and hex keys.
- The CTRE hardware from Prerequisites: Kraken X44, CANivore, CANcoder.
- Bench space for the arm to turn a full circle without hitting anything.
Both builds start from the same base: 1x2 aluminum tube, T-slot extrusion, and a two-stage WCP rotation gearbox. What bolts to the gearbox output is the only real difference between them.
Build one, not both. The two parts lists overlap by about two thirds, and Workshop 1 tunes one device at a time. Printing and assembly take two or three build nights, so start before the week you need it.
- About $860 in new parts
- One motor, one through-bore CANcoder, and a 9 inch tube on the output shaft. The tube is short enough to clear the bench through a full 360 degrees, so a wrong sign or a bad gain spins the arm instead of crashing it.
- About $800 in new parts
- Same base, no encoder, two 4 inch compliant wheels on the output. It stays inside its own footprint, and the parts list ends with a tennis ball.
The two models
Drag to orbit either model, and scroll to zoom. The buttons underneath jump to a fixed angle.
Flywheel Mechanism
The gearbox output carries a compliant wheel on each end of the shaft. There is no encoder on this build. The TalonFX has a rotor sensor of its own.
Arm Mechanism
A 9 inch aluminum tube clamped to the gearbox output, with a through-bore CANcoder reading the joint angle. That encoder is the main thing the flywheel list does not have.
| File | Opens in | Use it to |
|---|---|---|
.stl | PrusaSlicer, Cura, Bambu Studio | Slice and print a part as drawn. |
.stp | Fusion 360, SolidWorks, Onshape | Change a dimension before you print. |
| Onshape | A browser, nothing installed | Measure a part or check a fit. |
Bill of materials
Both lists are complete down to the CAN terminating resistor. Open Show Details to sort, filter by vendor, or print a shopping list for whoever orders parts.
All new, the arm comes to about $860 and the flywheel to about $800. Most of that is CTRE hardware. Tick Recycle CTRE Parts and 3D Print for $5 Total and the arm lands near $170.
Most rows come from West Coast Products, with the extrusion and T-nuts at Tnutz and the collars, resistor and Wago nuts at AndyMark. The Kraken ships from CTRE, and the arm hub from ThriftyBot. Place all five orders the same day. Nothing stalls a build like waiting a week on a 19 cent resistor.
Arm Mechanism - Bill of Materials
Cost-Saving Tip: Many parts listed below can likely be built from scrap material or parts you already own from previous projects, significantly reducing the actual cost of this mechanism.
Shooter Mechanism - Bill of Materials
Cost-Saving Tip: Many parts listed below can likely be built from scrap material or parts you already own from previous projects, significantly reducing the actual cost of this mechanism.
Printing the parts
Seven rows on the arm list are marked printable, and the gearbox and its gears are four of them. Printing them instead of buying saves about $150, and hands you the part of the mechanism most likely to fail.
Print the gears in something stiffer than PLA. A Kraken X44 behind a two-stage reduction strips PLA teeth, and a stripped tooth makes no noise. The mechanism stops reaching its target and nothing tells you why.
Printed gearbox plates need the four flanged bearings and the long #10-32 screws further down the list. The bought kit includes those. Those are the rows people skip.
- Print one tube plug sleeve and test fit it before anything else. The plugs are cut for .125 inch wall and the tube is .0625 inch, so the sleeve makes up the difference.
- Measure that first print against the model. A slicer that read the file in the wrong units hands you a part that looks correct on its own.
- Print the gearbox plates next, then dry-fit the bearings before anything else goes in.
- Print the gears last. Reprinting one is cheap. Reprinting four because the plate spacing was wrong is not.
Assembly notes
None of this is in the CAD. Build in this order and you will not have to take the mechanism back apart.
- Cut the tube to 9 inches and the extrusion to 10 inches first. WCP ships tube in 48 inch sticks, and Tnutz will cut extrusion to any length you ask for.
- Slide all eleven T-nuts into the extrusion before the end cap goes on. They are 10-32. Adding a twelfth later means pulling the cap.
- Build the gearbox on the bench, not on the mechanism. It should turn by hand with no gear noise before it bolts to anything.
- Set the hex collar clamps so nothing rubs the CANcoder. The collars on the list carry a ridge for that. A shaft walking sideways into an encoder reads as a dead sensor.
- Bolt the base to a bench or a steel plate. Not to a folding table.
- Wire the CAN chain late, with the terminating resistor at the far end from the CANivore. The two-slot lever nuts hold it.
- Land the battery leads dead last, with the battery in another room. The 12 gauge cable goes into the WAGO lever nuts once its alligator clips are cut off. Strip only enough to seat the wire, because bare copper on a battery lead will weld a wrench to your frame.
Bolt it down before power
A Kraken X44 behind this gearbox will drag a bench that is not fixed down. Bolt the base before the arm sees power. The bench arm needs no stops because it clears a full turn. A reused or longer arm that cannot clear the bench needs a physical stop at each end of travel. Workshop 1 applies voltage by hand in Tuner X, and an inverted motor drives straight into whatever is in the way.
Check your work
Run this with the battery disconnected. Every line is cheap to fix on a build night. Finding one during Workshop 1 stops four people instead of you.
Before Workshop 1
- The base is bolted down and does not shift when you lean on it.
- Arm: it turns a full circle by hand without touching the bench, with no binding and nothing rubbing the encoder. An arm that cannot clear lands on a stop you built at each end, not on the gearbox.
- Flywheel: it spins freely by hand and coasts for a second or two.
- One CAN chain runs from the CANivore through every device and ends in the resistor.
- Battery leads landed, polarity checked, no bare copper.
Write the gear tooth counts on tape and stick it to the extrusion. The arm list builds 8:60 then 16:60, about 28:1. Workshop 1 never asks for it, because the arm's CANcoder reads the output shaft directly. A mechanism that reads only the motor's own sensor does need it. Counting teeth through an assembled gearbox is miserable.