Follower arm · Step 4 of 26 · F-010

Fit both horns on motor 1 (shoulder pan)

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Do this

Take the motor you labelled 1.
src
- servos.yaml · follower/shoulder_pan — ID 1 · STS3215 1:345 · Feetech STS3215 servo, 7.4V, 1:345 gear (C001)
- F-002 · check — Each motor answers on its own ID 1–6, bottom joint = 1, gripper = 6; leader and follower motors are labelled and kept apart.
Push the geared
horn
Horn. The metal disc that fits on the end of a motor; printed parts screw onto it. Each motor ships with two, one with gear teeth around its edge for the driven end and one plain disc for the other end.Source: waveshare.com · lerobot/docs/source/so101.mdx#joint-1
onto the
output shaft,
Output shaft. The end of the motor that turns. It has a short grooved stub; the geared horn pushes onto it and is held by one M3x6 screw.Source: mantech.co.za · lerobot/docs/source/so101.mdx#joint-1
the upper end of motor 1, and hold it with one
M3x6 screw.
M3x6. A screw size. M3 means a 3 mm thread, 6 means 6 mm long. It has a machine thread and goes into the threaded holes of a horn; the same size holds each printed part to its horn.Source: mantech.co.za · lerobot/docs/source/so101.mdx#joint-1
src
- orientation note — Output shaft (geared horn) is the upper end of motor 1 (Waveshare).
- check — Geared horn on the output shaft, retained by its M3x6; plain horn on the bottom, no screw.
- waveshare.com — Follower step 2: geared horn on the output shaft (upper end), plain horn at the bottom
Push the plain horn onto the bottom end. It gets no screw.
src
- check — Geared horn on the output shaft, retained by its M3x6; plain horn on the bottom, no screw.
- waveshare.com — Follower step 2: geared horn on the output shaft (upper end), plain horn at the bottom

Done when Geared horn on the upper end with its screw; plain horn on the bottom without one.

waveshare.comFollower step 2: geared horn on the output shaft (upper end), plain horn at the bottom

Orientation: Output shaft (geared horn) is the upper end of motor 1 (Waveshare).

Check: Geared horn on the output shaft, retained by its M3x6; plain horn on the bottom, no screw.

Motor slot: shoulder_pan · ID 1 · STS3215 1:345 · 7.4V servo on a 5V supply; 12V servo on a 12V 5A+ supply (optional variant);

Motor 1 · shoulder pan

The motor you labelled 1: STS3215 motor,

1:345

Gear ratio. The number on the motor's label, such as 1:345. The motor inside turns 345 times for one turn of the output. The follower arm uses 1:345 at all six joints. The leader arm mixes 1:191, 1:345 and 1:147, so check the label before fitting a motor.Source: lerobot/docs/source/so101.mdx#step-by-step-assembly-instructions · SO-ARM100/README.md#sourcing-parts

gear, code C001. details

You need

STS3215 motor, 1:345 gear, code C001Feetech STS3215 servo, 7.4V, 1:345 gear (C001) servo-sts3215-c001-345
approximation3D stand-in
Stand-in shape: the 3D geometry or placement is not measured from the real part.
Qty 7 is the two-arm build (6 follower + 1 leader). A 12V variant with 30 kg.cm stall exists for the follower only and needs a 12V 5A+ supply (README). Includes two horns and a 3-pin cable per the assembly texts; kit contents are not itemised upstream. Geometry: the upstream repo's own simplified body `STEP/SO100/STS3215_03a.step` (Apache-2.0, single solid drawn to the datasheet, D-007 for where they differ) with its fused horn plates cut off and the datasheet ø6 x 3.4 output boss added, posed on the embedded vendor model's transform; the vendor mesh is not redistributed.
details
Horn with gear teeth (drive end)Servo horn, geared (drive side) servo-horn-geared
approximation3D stand-in
Stand-in shape: the 3D geometry or placement is not measured from the real part.
Ships with the servo. "Each servo is equipped with two steering wheels, one with gears and the other without" (Waveshare). Qty = one per servo for two arms. Geometry: drawn from the datasheet Accessories drawing: ø19.95 plate 2.5 thick, ø9 hub to 4.5 overall, ø3.2 centre bore, 4-M3 on ø14; the plate outline is drawn as the circle (the drawing shows a polygon whose side count is not dimensioned) and the 25T spline is not modelled. Placed on the embedded vendor horn's transform; the vendor mesh is not redistributed.
details
Horn without teeth (idle end)Servo horn, plain (idle side) servo-horn-plain
approximation3D stand-in
Stand-in shape: the 3D geometry or placement is not measured from the real part.
Ships with the servo. Not fitted on motor 5 (one horn only) on either arm, hence 10 for two arms. Geometry: drawn from the datasheet Accessories drawing: ø19.95 plate 2.1 thick, ø9 hub to 3.1 overall, ø6.05 centre bore, 4-M3 on ø14; outline drawn as the circle. Placed on the embedded vendor horn's transform; the vendor mesh is not redistributed.
details
M3x6 screwM3x6 screw m3x6 ×1 details
Phillips screwdriver #0 · PH0 tool details
Phillips screwdriver #1 · PH1 tool details

Sources (2)

lerobot/docs/source/so101.mdx#joint-1
waveshare.com (retrieved 2026-09-07) — Follower step 2: geared horn on the output shaft (upper end), plain horn at the bottom