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. Half again the reduction of our HD20 in a thinner package, using the same screws, the same bearing and the same motor face.
| Reduction | 30:1 |
| Tooth profile | Cycloidal, module 0.55, 60-tooth flexspline / 62-tooth circular spline |
| Measured torque | not yet measured, see Test data |
| Efficiency | not yet measured |
| Backlash | not yet measured |
| Motor | Any NEMA 17 with a 5 mm shaft (same interface plate as the HD20) |
| 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 (RevB, 2026-08-30) |
| License | CC BY-SA 4.0 |
⚠️ This revision has not been tested on a load cell yet. There is no torque number, no efficiency number and no backlash number for the HDP30, and we would rather ship the files with that gap stated plainly than quote you the HD20's figures and let you assume they carry over. They don't: the tooth module dropped from 0.8 to 0.55, so the teeth are smaller and the engagement is different. Testing is queued; this page gets the numbers when the numbers exist.If you build one and measure it before we do, please open an issue. We'll credit you.
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. With a 60-tooth flexspline, that tiny slip per revolution is the 30:1.
New to strain-wave gearing? We wrote a plain-language explainer: robrotics.web.app/learn.
The HD20 got 20:1 out of a 40-tooth flexspline at module 0.8. The HDP30 keeps the same two-tooth difference but packs in 60 teeth at module 0.55, which buys half again the reduction at roughly the same pitch diameter, and lets the whole stack get thinner, hence pancake.
Six parts were re-cut for the new profile: the flexspline, both circular splines, the wave generator, the ball cage and the output preloader. The interface plate, the base preloader and the shear pins are unchanged, and so is every piece of hardware: same 11 balls, same 30×42×7 bearing, same 8 screws, same 12 inserts. If you have already built an HD20, your leftovers cover this one.
What you give up is the test data. The HD20 is the measured design; the HDP30 is the newer one.
Not included in the BOM, bring your own:
- A NEMA 17 stepper with a 5 mm shaft. The interface plate is unchanged from the HD20, which was validated with an OMC StepperOnline 17HE12-1204S (42 × 42 × 30 mm, 26 N·cm, 1.2 A, 4-wire) under closed-loop FOC control.
- 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. This matters more here than on the HD20, not less. Module 0.55 teeth are smaller, and the wall they sit on is thin.
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 | hdp30-circular-spline-base |
| Circular spline (output) | 1 | PLA | hdp30-circular-spline-output |
| Base preloader | 1 | PLA | hdp30-base-preloader |
| Output preloader | 1 | PLA | hdp30-output-preloader |
| Interface / motor mount | 1 | PLA | hdp30-interface |
| Wave generator | 1 | PLA | hdp30-wave-generator |
| Ball cage | 1 | PLA | hdp30-cage |
| Flexspline | 1 | PETG | hdp30-flexspline-petg |
| Shear pin | 2 | PLA | hdp30-shear-pin |
Both shear pins are required, and they're the same part printed twice, so there's only one file. 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 HD20 was printed on a Bambu Lab P1S with Bambu PLA (and PETG for the flexspline), and the HDP30 uses the same settings:
- 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.
cad/step/ STEP, for slicing and for CAD work
cad/dxf/ The four tooth profiles, straight out of the harmonic maker
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.
There are no pre-sliced files in the repo. Import the STEP into your slicer and use the print settings above, so the geometry you print is always the current one. A convenience 3MF mesh export is attached to the release for slicers that won't take STEP. Note that it doesn't include the two shear pins, which are STEP-only.
📹 Quick assembly run-through: instagram.com/reel/DclcvVig55a
That one is fast. A full, properly paced build video is in the works and will go up on youtube.com/@robrotics and be linked here. Written step-by-step instructions are being put together alongside it.
Two ways in, depending on how deep you want to go.
Change the tooth profile. Every profile in this drive came out of our own free generator, the harmonic maker. This link opens it with the exact settings used for the HDP30, so you can nudge one number and export a new DXF:
Open the HDP30 profile in the harmonic maker →
The four DXFs it produces are checked into cad/dxf/ if you'd
rather start from ours.
Change the mechanics. 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.
The HDP30 hasn't been characterised yet, so this section is honest about which column each item comes from. The material-level items carried over from the HD20 apply to any printed drive of this family and are worth reading before you build. The performance items are HD20 numbers and are listed only so you know what kind of behaviour to expect, not what to expect from this drive.
Applies to this drive:
- PLA creeps under sustained load. Holding a heavy static load for hours will slowly deform the circular splines.
- Unit-to-unit variation is real. Two HD20s of the same design differed by up to 18% at the same current. Printed gearboxes are not precision parts, and smaller teeth won't make that better.
- Output spline preload is the biggest tuning knob. On the HD20, tightening it further took one unit from 2.37 to 2.55 N·m. Expect it to matter here too.
- Torque drops after running in. One HD20 measured 2.32 N·m fresh and 2.08 N·m after further running. Plan around the run-in figure.
Not yet known for this drive: torque, efficiency, backlash, thermal behaviour, and whether the module 0.55 teeth hold up as well as the 0.8 teeth under load. That last one is the open question this revision exists to answer.
Found something we haven't listed? Please open an issue and include your version, filament, and printer.
None yet. The load cell rig that produced the HD20's torque report hasn't been run against this revision. When it has, the report will be linked here and on robrotics.web.app/actuators/hdp30.
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.