A strain-wave (harmonic) gearbox that bolts onto a standard NEMA 17 stepper, prints on a bog-standard bed slinger, and costs about $4 of hardware per unit. Tested to 3.3 N·m.
| Reduction | 20:1 |
| Tooth profile | Cycloidal, module 0.8 |
| Measured torque | 2.1–2.6 N·m @ 0.7 A · 3.0–3.3 N·m @ 1.4 A (peak 3.5 N·m) |
| Efficiency | 48–58% |
| Backlash | measurement in progress |
| Motor | Any NEMA 17 with a 5 mm shaft (tested: OMC 17HE12-1204S, 26 N·cm, 1.2 A) |
| Lubrication | Super Lube synthetic PTFE grease |
| Printed parts | 10 pieces across 9 unique parts, including 2 shear pins |
| Fasteners | 8 × M3×0.5 × 6 mm button head screws per drive |
| Hardware cost | ~$3.96 per actuator |
| Version | v1.0 (RevA, 2026-08-09) |
| License | CC BY-SA 4.0 |
Testing is ongoing. The numbers above come from the load-cell runs in
docs/torque-test-report.html. We're in the middle of revising the test method, so expect these figures to be updated. Read the honest caveats in Known behaviour before you build.
Three parts do the work. A wave generator, an ellipse riding on 11 loose 5 mm balls in a printed cage, pushes a flexible toothed ring (the flexspline) into an oval, so its teeth engage a rigid ring (the circular spline) at just two points. The circular spline has two more teeth than the flexspline, so one turn of the motor walks the output around by two teeth. That tiny slip per revolution is the 20:1.
New to strain-wave gearing? We wrote a plain-language explainer: robrotics.web.app/learn.
The tooth profile came out of our own free generator, the harmonic maker. You can use it to make profiles for your own drives.
Not included in the BOM, bring your own:
- A NEMA 17 stepper with a 5 mm shaft. We tested with an OMC StepperOnline 17HE12-1204S (42 × 42 × 30 mm, 26 N·cm, 1.2 A, 4-wire) under closed-loop FOC control; an open-loop stepper on a basic driver will behave differently.
- 4 × M3 screws to bolt the motor to the interface plate. These are separate from the 8 × M3×0.5 × 6 mm screws that hold the drive together, which are in the BOM.
- A soldering iron for the heat-set inserts, and hex keys.
Filament: PLA for everything except the flexspline, which must be PETG.
⚠️ Do not print the flexspline in PLA. It flexes on every single revolution. PLA has almost no fatigue life in that duty and will crack.
Machine-readable source: bom.json.
| # | Item | Qty | Pack | Pack cost | Per actuator |
|---|---|---|---|---|---|
| 1 | M3 heat-set inserts | 12 | 100 | $9.99 | $1.20 |
| 2 | M3×0.5 × 6 mm button head, stainless | 8 | 100 | $7.69 | $0.62 |
| 3 | 30 × 42 × 7 mm bearing | 1 | 10 | $16.39 | $1.64 |
| 4 | Set screw | 1 | 50 | $5.69 | $0.11 |
| 5 | 5 mm steel bearing balls | 11 | 200 | $7.20 | $0.40 |
| 6 | Super Lube synthetic grease | 1 | — | — | — |
| $46.96 buy-in | $3.96 each |
One tube of grease lasts many builds, so it isn't counted in the per-unit cost.
Parts are sold in packs, so the first actuator costs about $47 in hardware and every one after that costs about $4, and you'll have enough left over for eight more.
Affiliate disclosure: the purchase links in
bom.jsonand on our website are Amazon Associates links. If you buy through one, Robrotics earns a small commission at no extra cost to you. Every part listed is what we actually used; the links don't change the recommendation.
| Part | Qty | Material | File |
|---|---|---|---|
| Circular spline (base) | 1 | PLA | hd17-circular-spline-base |
| Circular spline (output) | 1 | PLA | hd17-circular-spline-output |
| Base preloader | 1 | PLA | hd17-base-preloader |
| Output preloader | 1 | PLA | hd17-output-preloader |
| Interface / motor mount | 1 | PLA | hd17-interface |
| Wave generator | 1 | PLA | hd17-wave-generator |
| Ball cage | 1 | PLA | hd17-cage |
| Flexspline | 1 | PETG | hd17-flexspline-petg |
| Shear pin | 2 | PLA | hd17-shear-pin |
Both shear pins are required. They carry shear load across the output joint directly, so the connection doesn't have to rely on friction from the preloaded screws to resist it. The screws clamp; the pins take the sideways load.
The tested units were printed on a Bambu Lab P1S with Bambu PLA (and PETG for the flexspline), using:
- Arachne variable-width wall generator
- One extra wall loop for strength
- Seam position: random, on every part, so a single seam line doesn't stack up into a weak spot or a visible ridge on the round surfaces
- Supports on the output circular spline and the interface. Everything else prints unsupported.
No custom temperatures, no special bed prep.
There's also a Multi Jet Fusion nylon variant of the flexspline, printed through PCBWay. Nylon has far better fatigue life than PETG, so it should last considerably longer. Ours cost $24.89 for five pieces, about $4.98 each, before tax, shipping and any import duty, which on a small order can easily cost more than the parts themselves.
⚠️ The MJF variant is a matched set of three parts: flexspline, cage, and wave generator. All three have different geometry from the standard versions, so use them together. Mixing an MJF flexspline with the standard cage and wave generator will not fit correctly.
Only the flexspline actually has to be made in nylon MJF, because that's the part fighting fatigue. The MJF cage and wave generator have their own geometry, but you can still print them yourself in PLA. So a service bureau order can be just the flexspline, and you print the other two from the MJF set at home.
If you order from PCBWay, ask for the parts unpainted. Dyeing or painting adds cost and puts a coating on the tooth flanks and the bore, which is exactly where you don't want extra material on a part this dimensionally fussy.
MJF files live in cad/step/mjf/.
cad/step/ STEP, for CAD work and remixing
cad/step/mjf/ STEP, MJF nylon matched set
docs/ Load-cell torque test report
photos/ Build photos
bom.json Bill of materials (source of truth)
Grab the packaged bundles from the Releases page rather than cloning, because the STEP files are large.
📹 An assembly video is on the way and it'll be posted to youtube.com/@robrotics and linked here.
Written step-by-step instructions are being written up alongside it.
The full parametric model is public on Onshape:
Copy it to your own workspace and change what you like: a different motor face, a different output interface, a different ratio. If you build a variant, we'd genuinely like to see it: open an issue or tag @robrotics.
Read this before you conclude you assembled it wrong.
- Torque drops about 10% after running in. A freshly printed unit measured 2.32 N·m; the same unit after further running measured 2.08 N·m at 0.7 A. This is expected, so plan around the run-in figure, not the fresh one.
- Output spline preload changes everything. Further tightening the output spline took one unit from 2.37 to 2.55 N·m. It's the single biggest tuning knob you have.
- Unit-to-unit variation is real. Two units of the same design differed by up to 18% at the same current. Printed gearboxes are not precision parts.
- Efficiency is 48–58%, so roughly half your motor torque becomes heat. Size the motor accordingly.
- PLA creeps under sustained load. Holding a heavy static load for hours will slowly deform the circular splines.
- The 1.4 A figures are above the test motor's rating. The 17HE12-1204S is rated 1.2 A, so the 3.0–3.3 N·m numbers were measured over-driven. Treat them as a short-burst ceiling, not a continuous rating. At 1.4 A that motor gets hot.
Found something we haven't listed? Please open an issue and include your version, filament, and printer.
docs/torque-test-report.html is a standalone
page with the full load-cell traces: 7 configurations, ~1500–3000 samples each,
100 mm lever arm, raw counts scaled to N·m, with a no-gearbox baseline for the
efficiency maths. Open it in a browser.
CC BY-SA 4.0. Use it, change it, sell it. Just credit Robrotics and share your derivatives under the same license.
No warranty. Printed parts fail. Don't put this anywhere a failure could hurt someone.