This instructable describes how to make a robotic hand from a scrap of aluminium sheet, a piece three-ply wood, a length of duct-tape, and some hat-elastic.

Three-ply is not required if you have a 3D printer ... unfortunately I don’t.

The hand was constructed to test some ideas. The construction techniques may be of interest to others.

The estimated cost of this project is $20.

Images:

The cover photo shows the completed hand
Photo 2 shows the hand holding a small bottle

Step 1: Parts List

The following parts were obtained locally:

1 only piece of aluminium sheet (or stiff card ?)
1 only roll of fabric duct-tape
1 only scrap of three-ply (3D printer optional)
1 only length of “hat-elastic”
1 piece of nylon fishing line
21 only M3 x 20mm nuts and bolts.

Step 2: Theory

A sheet of thin aluminium is cut out in the shape of a hand and separated at each of the joints.

The pieces are then reassembled and held together with duct-tape. The tape, which forms a hinge action at each of the joints, may be reinforced by an additional layer of duct-tape.

The aluminium parts and duct-tape serve three purposes:

The duct-tape forms the hinges.
The aluminium sections preserve the hand-shape against sideways movement and
They hold the hand together during assembly.

The top layer of wood has vertical cuts at each of the joints. A vertical cut means that the fingers remain straight when tensioned. Each of the wooden pieces have two 2mm holes drilled length-wise for the tension-cords.

The lower-layer of wood, with the exception of the finger-tips, have 45-degree cuts at each of the joints. Again each of the wooden pieces have two 2mm holes drilled length-wise for the tension-cords.

Joints with adjacent 45-degree cuts will bend 90-degrees. The finger-tips, which have vertical cuts, are restricted to 45-degrees which is a more natural look.

The matching top and bottom sections form a sandwich with the aluminium templates. Each sandwich is held together with a 20mm M3 nut and bolt.

The resting state of the fingers will be straight if the elastic is threaded through the holes on top of the hand and the nylon tendons are threaded through the holes underneath.

I have chosen to thread the hat-elastic through the underside of each finger which means their natural resting state is curled. This has two significant advantages:

The fingers automatically adjust to the shape of any object it holds.
Power is only required to open the fingers ... once closed the fingers look after themselves.

Step 3: Create a Template

Method:

Trace an outline of your hand onto paper. Convert each finger to straight lines (photo 1)
Stick the paper to a thin sheet of aluminium (photo 2)
Cut out the hand-shape (photo 3)
Label, then separate, the finger sections (photo 4)

Step 4: Cut the Fingers

Method:

Cut a finger-width strip of 3-ply (photo 1)
Cut the 3-ply strip into matching length finger sections (photo 2)
The 45-degree angles are best done with a mitre-box (photo 3)

Notes:

The top layer of wood has vertical cuts at each of the joints.
The lower-layer of wood, with the exception of the finger-tips, have 45-degree cuts at each of the joints.

Step 5: Drill the Finger Pieces

Method:

Drill two length-wise holes in each finger section. (photo1 and photo 2)

Notes:

A metal jig makes the job easy.
Watch your fingers !!

Step 6: Assemble the Fingers

Method:

Clamp the matching finger-sections together before drilling (photo 1 and photo 2)

Notes:

Work outwards from the knuckle
Butt the top section of each finger against the last section before drilling. This will ensure that the finger opens straight.

Step 7: Attach the Thumb

The thumb caused me a few head-aches.

In the end I attached the thumb to the hand by means of a custom bracket (photo 1).

To do this I removed a small section out of the hand.

Notes:

The thumb angle, and position, is such that the tip of the thumb and index-finger are able to touch.
Two straight sections of 3-ply are used for the knuckles.

Step 8: String the Hand

Method:

Thread a loop of hat elastic through the lower finger sections and tie them behind the lower knuckle. The fingers should be able to straighten but the tension must cause the fingers to close.
Loop the nylon lengthwise through the top two holes in the fingers and tie a knot. For convenience I have attached mine to some fishing swivels.
Do the same for the thumb except that the hat-elastic should be on top causing the thumb to straighten.

Step 9: Summary

This instructible shows how make a robot hand from “wood-and-string”. [1]

The hand works reasonably well and tests the following concepts:

The use of a flexible hinge.
The use of hat-elastic to make the fingers curl at rest. This simplifies programming ... you only need to open the fingers ... the hat-elastic takes care of the rest.

The estimated cost of material for this project is around $20.

Note:

[1]

A 3D printer would simplify construction

Click here to view my other instructables.