New: the Cycloid Drive 25:1. Free STL + STEP files for a 3D-printed NEMA17 gearbox.Get the free files
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Exploded 3D render of the Cycloid Drive 25:1: motor, base with ring pins, cam, two discs, output carrier and cap3D render
Free mechanism pack / HS-MEC-001

Cycloid Drive 25:1

A 3D-printable 25:1 cycloidal reducer for a NEMA17 stepper. Measured on our bench, free to download.

FreePersonal, non-commercial licence · v1.0.0
Download free (ZIP)

STL + STEP, fit-test coupon, BOM, PDF assembly guide and firmware example in one ZIP. No sign-up.

Ratio25:1
HousingØ85 mm
InputNEMA17

You print the parts and buy the bearings, dowel pins, screws and motor. Figures on this page were measured on our bench with PETG-CF prints; your results depend on your printer, filament and parts. Torque has not been measured yet.

01 / The download

What’s in the pack.

  • Printed parts as STL and STEP: housing base, housing cap, cycloidal disc (print two), cam and output carrier
  • The full assembly as a STEP file
  • 18-piece fit-test coupon (STL)
  • Bill of materials for the bought parts
  • Illustrated PDF assembly guide (11 pages)
  • No-library ESP32 / Arduino test sketch for a TMC2209 driver
  • README, licence and changelog

Printed parts

Printed parts and quantities
Housing base1
Housing cap, with a small debossed Harbe Works mark1
Cycloidal disc (two identical)2
Cam1
Output carrier1
Fit-test coupon18 pieces
02 / Measured on our bench

Numbers we measured.

PETG-CF prints, measured on the Harbe Works bench, October 2026.

Measured specifications
SpecValueHow we got it
Reduction ratio25:1Output encoder moved 1.000° per commanded degree over about 10 turns
Repeatability1.32 arcmin0 → 90°, repeated 10 times
Lost motion57 arcmin meanWith the R2 discs in this pack (71 arcmin on R1)
Output speed tested10 to 150 °/sRan clean across the range
SizeØ85 × 44.9 mmGearbox without the motor; bench units printed in PETG-CF
InputNEMA175 mm D-shaft
TorqueNot measured yetWe will publish a figure only once it is measured
03 / Bill of materials

Parts to buy.

The full BOM, with specs, is in the ZIP. Links go to the exact parts we used. All parts we use →

Bought parts
PartQtyNoteBuy
Hardened dowel pin, 4 × 20 mm3226 ring pins + 6 output pinsAmazon ↗
684ZZ bearing122 per output pinAmazon ↗
6802-2RS bearing2Pressed into the discsAmazon ↗
6808-2RS bearing2Pressed into the capAmazon ↗
NEMA17 stepper motor, 5 mm D-shaft1Tested with StepperOnline 17HS19-2004S1Amazon ↗
M3 socket-head cap screws (4 × M3×6, 6 × M3×16)10From an M3 assortment kitAmazon ↗
TMC2209 stepper driver1To run it; any NEMA17 driver worksAmazon ↗
100 µF ≥ 35 V capacitor1Across the driver motor supplyAmazon ↗
Optional: MT6701 magnetic encoder + diametric magnet1Encoder bracket comes in a later versionAmazon ↗

As an Amazon Associate I earn from qualifying purchases. It costs you nothing extra and supports the free files.

04 / Print notes
  1. Material: PETG-CF (our bench units) or PETG. Use a hardened nozzle for PETG-CF.
  2. Turn slicer X-Y hole compensation OFF. Use the fit-test coupon, not compensation, to check your printer.
  3. Print the 18-piece fit-test coupon first, in the same filament and settings, and check the fits before printing the full set.
  4. Import in millimetres at 100% scale. Never scale the parts: the bearing seats and pin holes are sized for real hardware.
  5. Print two of the disc file; the two discs are identical.

Tested with

We built and measured our drives with these parts. Other motors and bearings have not been tested.

  • StepperOnline 17HS19-2004S1 NEMA17 stepper (5 mm D-shaft)
  • 684ZZ, 6802-2RS and 6808-2RS ball bearings
  • 4 × 20 mm hardened steel dowel pins
  • PETG-CF prints of every printed part
05 / Assembly

Step by step.

The PDF guide in the ZIP has the full instructions for each step.

  1. 3D render of assembly step 1: press the 26 ring pins into the base
    1. Press the 26 ring pins into the base · 3D render
  2. 3D render of assembly step 2: bolt the nema17 under the base floor
    2. Bolt the NEMA17 under the base floor · 3D render
  3. 3D render of assembly step 3: press a 6802 bearing into each disc
    3. Press a 6802 bearing into each disc · 3D render
  4. 3D render of assembly step 4: slide the two discs onto the cam
    4. Slide the two discs onto the cam · 3D render
  5. 3D render of assembly step 5: grease, then drop the cam stack onto the motor shaft
    5. Grease, then drop the cam stack onto the motor shaft · 3D render
  6. 3D render of assembly step 6: press the output pins in and add the 684 rollers
    6. Press the output pins in and add the 684 rollers · 3D render
  7. 3D render of assembly step 7: lower the output carrier into the discs
    7. Lower the output carrier into the discs · 3D render
  8. 3D render of assembly step 8: press the two 6808 bearings into the cap
    8. Press the two 6808 bearings into the cap · 3D render
  9. 3D render of assembly step 9: fit the cap with 6 × m3×16 and spin-test
    9. Fit the cap with 6 × M3×16 and spin-test · 3D render
06 / Licence

Free, with a few rules.

Free for personal, non-commercial use. Print it, build it, modify it for yourself.

Do not re-upload or redistribute the files. To share it, link to harbeworks.com/cycloid instead.

Selling printed parts or assembled drives is not included.

Read the full licence summary

07 / Follow the build

See it run.

Instagram @harryson.stl YouTube · Harbe Works
08 / Questions

Good questions.

Something else? Email help@harbeworks.com.

Do I need a 3D printer?

Yes, or access to one. Every custom part is printed; our bench units were PETG-CF on an FDM printer. You also buy the bearings, dowel pins, screws and a NEMA17 stepper listed in the bill of materials.

Which motor should I use?

We tested it with a StepperOnline 17HS19-2004S1, a NEMA17 stepper with a 5 mm D-shaft. Other NEMA17 motors have not been tested with this drive, so check the shaft and mounting against the STEP files before you buy.

How much torque does it handle?

We have not measured torque yet, so we do not quote a number. The measured figures are ratio, repeatability, lost motion and the speed range above.

Can I sell prints or assembled drives?

No. The free pack is for personal, non-commercial use only. Please do not sell prints or kits, and do not re-upload the files; link people to this page instead.

Does it include an encoder mount?

Not in v1.0. The MT6701 encoder and magnet are optional, and the encoder bracket comes in a later version.

Why is it free?

It is the drive from our Instagram reel, and we would rather you build one. If you do, tag @harryson.stl so we can see it.

Ready to build one?

Free files, measured on our bench. Print the fit-test coupon first.

Download free (ZIP)
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