3D render of the Tendon-Driven Robot Hand: a white hand with a graphite palm on a black open forearm, five blue servos with orange spools in a staircase and five red lines running down from the wrist
Free 3D model

Tendon-Driven Robot Hand

A life-size right hand on a forearm. Every finger has three printed bones on hinges of 1.75 mm filament. A braided fishing line runs along the palm side of each bone to the fingertip, through the palm and down the front of the forearm: pull it and all three joints of that finger bend, just as the tendons from the muscles in your forearm bend yours. A round elastic cord along the back of the finger pulls it straight again. Seen from the side the bones meet in a V on the palm side and flat on the back, so the finger stops straight and stops again where a fist ends. In the forearm five micro servos stand in a staircase, each with a spool that winds up one line, and an Arduino in the base plays gestures. Without servos, the lines run under a bar at the foot to five rings that you pull by hand.

  • 28printed parts
  • 10 × 14 × 38 cmassembled (L × W × H)
  • ≈ 0.3 kgfilament in total
  • 150 mmbed length for the forearm, which prints on its back (72 × 150 mm); the palm needs 92 × 119 mm
Download all files ZIP · 0.6 MB

Free for personal use under CC BY-NC 4.0: print it, remix it and share it with credit to Shapekit. Please don't sell the files or the prints.

Take it apart in 3D · Parts · How to build it

In 3D

Turn it, take it apart

Every part as you print it. Pick colours to see what your filament will look like.

The 3D model (2.3 MB) loads when you get here.

Preview colours

Fingers and thumb
Palm
Forearm and base
Spools and rings

Drag to rotate · scroll or pinch to zoom

In the download

20 parts, ready to print

Every STL is already placed the right way up. Download them all at once, or one by one.

Palm

Holds the four knuckles and the thumb. The five lines run through channels inside it from each knuckle down to the wrist; the elastic of each finger comes out of a hole in the back of the hand, where it is knotted. The block under the wrist goes into the forearm.

Size
119 × 92 × 29 mm
Print
On its back, wrist block down too. No supports.
Filament
≈ 61 g
STL 0.5 MB

Index finger, proximal bone

The first bone. Its tongue sits between the two cheeks of the knuckle on the palm.

Size
56 × 17 × 15 mm
Print
On its back, the flat side on the bed. No supports.
Filament
≈ 7 g
STL 92 KB

Index finger, middle bone

Between the first bone and the fingertip.

Size
38 × 15 × 15 mm
Print
On its back, the flat side on the bed. No supports.
Filament
≈ 4 g
STL 90 KB

Index finger, distal bone

The fingertip. The line and the elastic end in the two pockets on the nail, each with a knot.

Size
28 × 15 × 14 mm
Print
On its back, the flat side on the bed. No supports.
Filament
≈ 3 g
STL 0.1 MB

Middle finger, proximal bone

The first bone of the longest finger.

Size
60 × 17 × 15 mm
Print
On its back, the flat side on the bed. No supports.
Filament
≈ 7 g
STL 92 KB

Middle finger, middle bone

Between the first bone and the fingertip.

Size
41 × 16 × 15 mm
Print
On its back, the flat side on the bed. No supports.
Filament
≈ 5 g
STL 91 KB

Middle finger, distal bone

The fingertip, with the two knot pockets on the nail.

Size
30 × 15 × 15 mm
Print
On its back, the flat side on the bed. No supports.
Filament
≈ 3 g
STL 0.1 MB

Ring finger, proximal bone

The first bone of the ring finger.

Size
57 × 16 × 15 mm
Print
On its back, the flat side on the bed. No supports.
Filament
≈ 7 g
STL 92 KB

Ring finger, middle bone

Between the first bone and the fingertip.

Size
39 × 15 × 15 mm
Print
On its back, the flat side on the bed. No supports.
Filament
≈ 4 g
STL 91 KB

Ring finger, distal bone

The fingertip, with the two knot pockets on the nail.

Size
29 × 15 × 14 mm
Print
On its back, the flat side on the bed. No supports.
Filament
≈ 3 g
STL 0.1 MB

Little finger, proximal bone

The first bone of the little finger, a little narrower than the others.

Size
47 × 15 × 14 mm
Print
On its back, the flat side on the bed. No supports.
Filament
≈ 5 g
STL 91 KB

Little finger, middle bone

Between the first bone and the fingertip.

Size
33 × 15 × 13 mm
Print
On its back, the flat side on the bed. No supports.
Filament
≈ 3 g
STL 89 KB

Little finger, distal bone

The fingertip, with the two knot pockets on the nail.

Size
26 × 15 × 12 mm
Print
On its back, the flat side on the bed. No supports.
Filament
≈ 2 g
STL 0.1 MB

Thumb, proximal bone

The thumb has two bones. This one hinges on the thumb's root on the palm, which is turned towards the palm so the thumb closes across it.

Size
46 × 19 × 15 mm
Print
On its back, the flat side on the bed. No supports.
Filament
≈ 6 g
STL 91 KB

Thumb, distal bone

The tip of the thumb, with the two knot pockets on the nail.

Size
34 × 17 × 15 mm
Print
On its back, the flat side on the bed. No supports.
Filament
≈ 4 g
STL 0.1 MB

Forearm

An open frame: a back wall with a rib for each servo, a socket for the wrist at the top and a foot at the bottom. The servos sit in a staircase, each one row lower and one line further to the right, so the five lines run straight down the front without touching. The bar along the front of the foot is for the version you pull by hand.

Size
150 × 72 × 42 mm
Print
On its back wall. No supports: the ribs stand up, the openings in them are short bridges.
Filament
≈ 61 g
STL 0.2 MB

Spool × 5

Fits over the servo horn, its two arms clipped to 20 mm. A full pull of a finger takes in 33 mm of line, which is 164° of a 23 mm spool; the thumb needs 89°.

Size
28 × 28 × 9 mm
Print
Front face down, the horn recess up.
Filament
≈ 19 g
STL 67 KB

Base

Holds the Arduino, the breadboard and the power jack. The forearm stands on it with four M3 screws, the servo cables come in through the slot under it, and the rings rest on the front.

Size
110 × 100 × 32 mm
Print
Upside down, its top on the bed.
Filament
≈ 74 g
STL 96 KB

Base lid

Closes the base from below with four M3 × 8 screws. The slots let warm air out of the box.

Size
105 × 95 × 2 mm
Print
Flat.
Filament
≈ 27 g
STL 61 KB

Pull ring × 5

Only for the version without servos: tied to the end of a line that runs down the forearm and forward under the bar on its foot. Pull a ring and its finger closes; let go and the elastic opens it.

Size
28 × 22 × 4 mm
Print
Flat.
Filament
≈ 6 g
STL 33 KB
Also needed

Parts you buy

  • Five SG90 or MG90S micro servos (MG90S has metal gears and lasts longer), each with its two-arm horn and screws. Not needed for the version you pull by hand.
  • An Arduino Nano (or Uno), a mini breadboard of 170 holes and some jumper wires.
  • A 5 V power supply of at least 2 A with a 5.5 × 2.1 mm plug, a panel jack for it (11.5 mm hole) and a 470 to 1000 µF capacitor. The servos must not run from the Arduino's own 5 V pin.
  • About 3 m of braided fishing line (PE), 0.4 to 0.5 mm, for the five tendons.
  • About 1.5 m of round elastic cord, 1 mm (beading elastic), to pull the fingers open.
  • A few centimetres of 1.75 mm filament for the 14 hinge pins.
  • M3 screws: 2 × M3 × 16 for the wrist, 4 × M3 × 10 with 4 nuts for the forearm on the base, 4 × M3 × 8 for the lid. Some superglue.
Print settings

Slicer settings

0.20 mmlayer height
3walls
15%gyroid infill
PLA or PETGmaterial

For a 0.4 mm nozzle. Filament amounts are estimates for these settings.

Assembly

How to build it

Print the parts

Print the bones and the palm on their backs, the forearm on its back wall, the base upside down and the spools front face down. Nothing needs supports. Run a 2 mm drill bit by hand through the holes in the tongues (they must turn freely on the pin) and a 1.8 mm bit through the cheeks (the pin must be snug there).

Hinge the fingers

Push the tongue of a bone between the cheeks of the one before it and push a piece of 1.75 mm filament through. Cut it flush on both sides and touch each end with a hot soldering iron, or a drop of superglue on the cheeks only, so it cannot walk out. Every joint must swing freely and stop when the finger is straight.

Thread the elastic

Tie a knot in the elastic, pull it through the back channel from the fingertip so the knot sits in the pocket on the nail, and on into the palm until it comes out of the hole in the back of the hand. Pull until the finger stands straight with some force, knot it there and put a drop of superglue on both knots.

Thread the lines

Do the same with the fishing line through the palm side channel, starting at the second pocket on the nail. It comes out at the bottom of the wrist. Leave 40 cm on each line.

Fix the hand on the forearm

Put the block under the wrist into the socket on top of the forearm and screw it with two M3 × 16 from below, before the thumb's servo goes in, which covers the screws.

Fit the servos

Put each servo through its rib from the front, shaft outwards, and screw its tabs to the rib with the screws that came with it. Run the cables through the window behind it and down the back of the forearm to the slot in the foot.

Wire the base

Put the breadboard with the Arduino in the base and the jack in the round hole at the back. Servo signal wires to D3 (thumb), D5 (index), D6 (middle), D9 (ring) and D10 (little); servo red to +5 V and brown to GND of the jack; the jack's GND also to a GND pin of the Arduino; the capacitor across the jack. Load robot-hand.ino from the ZIP with the Arduino IDE.

Fit the spools and tie the lines

Type tie in the Serial Monitor: every servo goes to its open angle. Clip the arms of each horn to 20 mm, press it into the back of a spool and screw it onto the servo. Lead each line down the front, onto the front of its spool's groove, through the tie hole and knot it with the finger straight and the line just tight. Screw the forearm to the base with four M3 × 10 and the nuts underneath, and close the lid.

Let it move

Plug in the power supply: the hand plays its gestures over and over. In the Serial Monitor you can type open, fist, thumbsup, point, peace, rock or wave, or five numbers from 0 to 100 for how far each finger closes. If a finger opens where it should close, change its sign in DIR at the top of the sketch.

Or pull it by hand

Leave out the servos and the electronics. Lead each line down the front of the forearm, behind the bar at the foot and forward under it, and tie a pull ring to its end so the rings lie on the front of the base. Pull a ring and its finger closes; let go and the elastic opens it.

The servos pull hard: keep your fingers out of the spools while it runs, and unplug it before you work on the lines. Small parts: keep them away from young children.

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Open the makers