I wrote a short post previously mentioning that I was going to try to build a robotic hand with my 3d printer. I don’t have any grand plan for how to achieve this – I’m going to think a little bit ahead, experiment, and see how things go!

What’s a 3d printed finger going to look like?

The first thing I did was examine the structure of a hand.

Hand
Source: https://en.wikipedia.org/wiki/Hand

I decided that for the printed version, I would make the distal phalanges and intermediate phalanges into a single piece. I also thought that I could use an internal linkage from the knuckle to the intermediate phalange which would allow the finger to curl when the proximal phalange rotates around the knuckle – which only requires rotation around one point.

I won’t judge you if you didn’t read that last paragraph too closely.

I decided to draw out my first attempt a single finger in some CAD software – as usual, I chose to draw this in Autodesk 123D design, and I’ve included some screenshots of this design below.

It’s difficult to explain how this mechanism works using words – a video is probably a better medium for this. I’ll try to explain this below, and colour-code the biology phrases to help clarify which parts they relate to in the diagrams above.

  • The knuckle will be bolted to the proximal phalange, and this in turn will be bolted to the intermediate phalange.
  • There is an internal linkage from the knuckle to the intermediate phalange. This prevents the parts from rotating freely, but it should ensure that when the proximal phalange is rotated that the intermediate phalange will rotate also.

So the next step was to lay these parts out flat and then 3d print them.

  • I had to split the intermediate phalange into two parts which were mirror opposites of each other to simplify the print.
  • Similarly for the proximal phalange, I split the parts (although they aren’t perfect mirror images, I decided to make it asymmetric so that it would be more difficult to match them incorrectly).
  • These opposing parts needed to be stuck together – glue would be fine, but acetone welding is easier.

The photo below shows the printed mechanism – first of all in an un-tensed configuration (meaning lying flat):

flat

The photo below show the mechanism when it’s tensed (meaning rotated around the knuckle). You can see how the top part (intermediate and distal phalanges) starts to bend around as well because of the internal linkage.

upright

The next step is to think about how I can mechanically power this – I’ve ordered some solenoids, as I can use another linkage to translate the solenoid’s linear motion into a rotational movement around the knuckle.