Hardware

A 12-DoF, three-finger hand for about $200 in parts. Printed finger structures, aluminium joint housings, and twelve of the same hobby-grade serial bus servo.

3D model

Open a model, then drag to rotate, scroll or pinch to zoom, and right-drag to pan.

Rendered views of the hand and its parts. The interactive 3D viewer opens shortly.

Specifications

Layout	Three identical thumb-like fingers, arranged radially
Degrees of freedom	12 (4 per finger)
Joints per finger	IP, MCP, and a two-axis CMC base
Finger length	162 mm (links 62 / 50 / 49 mm)
Actuators	12 × Feetech STS3215, magnetic encoders
Servo loop	200 Hz, < 5 ms command latency
Transmission	Direct linkage
Mass	≈ 1.8 kg
Pullout strength	62.5 N
Fingertips	Interchangeable: rigid wedge, soft cleaning tip
Force sensing	Optional three-axis module per fingertip (release R2)
Cost	≈ $200 hand, + ≈ $170 per sensing fingertip

The base hand works on its own for about $200. Force sensing is an add-on: each fingertip adapter takes either a plain tip or the sensing module below.

Joint layout

Every finger has the same four joints: an interphalangeal (IP) joint, a metacarpophalangeal (MCP) joint, and a compound carpometacarpal (CMC) joint with two axes, CMC1 and CMC2.

Kinematic diagram: three fingers, each with IP, MCP and a ball-joint-like CMC, meeting at the wrist — Kinematic layout of the three fingers

Render of the hand with the IP, MCP, CMC1 and CMC2 rotation axes marked on one finger — Rotation axes on one finger

Fingertip force sensor

Optional add-on, part of release R2.

Each sensing fingertip carries three miniature S-type strain-gauge load cells, set in an equilateral triangle between two rigid plates. Their sum gives the normal force; a calibrated linear map of the three readings gives the two tangential components.

The fingertip tool mounts on the outer plate and is mechanically decoupled from the load cells, so swapping a rigid wedge for a soft cleaning tip needs no recalibration.

Output	Three-axis force: normal and two tangential
Load cells	3 × S-type, 0–5 kg each
Calibration fit	R² > 0.986 on every axis
Aggregate error	0.349 N RMSE
No-load noise	0.0034 N mean, 0.0129 N max (std)
Peak load seen in tasks	63.1 N normal
Cost	≈ $170 per fingertip

Rigid wedge fingertip mounted on the force-sensing module

Wedge-shaped rigid fingertip mounted on the sensing module.

Fingertips

Two interchangeable fingertips mount on the sensing module. Because the tip is mechanically decoupled from the load cells, swapping one for the other needs no recalibration.

Downloads

Geometry is provided as neutral STEP and printable STL. Links go live with release R1.

Licence. Hardware files (STEP, STL, drawings, bill of materials, assembly guide) are released under CC BY-NC-SA 4.0: free to build, modify and share for non-commercial use, with attribution and under the same licence. Code and simulation models are released under the MIT licence. For commercial use, contact the corresponding author.

Assembly STEPFull hand, single finger and fingertip sub-assemblies
Download
Printable STL setOne file per part, with quantity, orientation and print settings
Download
Machining drawingsDimensioned PDF drawings for the aluminium parts
Download
Bill of materialsEvery part with quantity, price and purchase links
Open
Assembly guideStep-by-step, with photos and video
Open
Simulation modelsURDF, MJCF and USD — see Software
Repository
Minimal driverServo ID setup, zero calibration, joint read and write in Python
Repository

Bill of materials

The main groups are below. The full list with part numbers, prices and suppliers ships with R1.

Group	Item	Qty	How to get it
Actuation	Feetech STS3215 serial bus servo	12	Off the shelf
Structure	Finger links and palm	—	3D printed from the STL set
Structure	Joint housings	—	Machined aluminium, from STEP and drawings
Fingertips	Rigid wedge tips, soft cleaning tips	3 + 3	3D printed; sponge or tissue wrap for cleaning
Electronics	Servo bus adapter, power supply, wiring	—	Off the shelf
Sensing (optional)	Three-axis fingertip module: 3 miniature S-type load cells and acquisition board	3	Release R2

Assembly

Print and orderPrint the STL set, send the aluminium parts out for machining, order servos and electronics.
Set servo IDsGive each of the twelve servos its bus ID with the driver's setup tool.
Build one fingerIP, MCP and the two-axis CMC base. Repeat three times; the fingers are identical.
Mount fingers to the wristRadial layout, then route and secure the bus cables.
Fit the fingertipsWedge tips for prying and scraping, soft tips for wiping.
Calibrate zeroRun the zero-offset routine once, then the first-motion check.

The detailed guide, with torque values and photos of every step, is published with R1.

Leader device

Demonstrations are collected with a passive replica of the hand that has the same joint layout and link geometry. Its joint angles map one-to-one onto the follower after a per-session zero-offset calibration, and commands stream at 15 Hz.

Files for the leader are part of R1; the teleoperation software and live force display follow in R2.