That's Handy! - Making A Working Model Hand
By Mike on Sunday, February 27, 2011, 20:22 - Permalink
The objective of this project was to make a life-sized humanoid hand that can be controlled with wires or cables - and hopefully to achieve fairly realistic finger movements and grasping action.
To begin, I needed a real human hand as a model.
I drew around it onto a piece of paper, also marking the positions of all the joints.
That's all. Thank you, hand.
I covered the paper with a flat sheet of clear polythene, then pinned it down to a wooden board.
Using ordinary silicone bathroom sealant, I traced a line around the outline of the hand (on top of the polythene.
Once it's cured, the silicone will peel away from the plastic layer cleanly.
I filled in the outline with more silicone, squeezed in as a pattern of zigzags, so as to create a fairly thin layer across the whole area of the hand.
Once the whole hand was covered, I pressed on five plastic drinking straws.
These are in more or less the same position as the bones in a real human hand - and they will partly serve the same purpose - to provide rigidity.
They will also form channels for the control cords (analogous to tendons in the real hand).
I left the first layer of silicone to dry overnight - it's best not to create anything too thick with this stuff, as it would then take a really long time to fully cure.
The next day, I cut the tip off the spout of the slicone applicator, allowing it to produce a thicker bead - which I then applied along the back of each finger. This thicker section will give the fingers their spring and substance.
Once everything was fully cured, I trimmed everything up so it looked a bit nicer, then cut channels across the fingers, and through the straws, on the palm side of the hand at each joint position.
Taking a closer look at those joints - the palm layer is removed, along with a section of the straw, but leaving the thick bead below intact.
It makes the whole thing look a bit skeletal - I guess if I was going for greater realism, I might stretch a latex glove over the whole thing.
Finally, I threaded a piece of strong, thin cord down each of the straws, tied to a small bead at the fingertip, to prevent it pulling through.
And that's it - the hand is ready for testing...
Video footage of the first test of this model hand is at the top of the page.
I've seen tutorials on making a similar hand to this one elsewhere on the web - such as this one - but I wanted to see if it could be refined a bit - and I wanted something that didn't include any paper in the construction.
What I Might Do Differently
If I was to make another one of these, I think I might try to do it so that it requires less cutting for the joints - this could be achieved by creating the finger pads (between the joints) as separate pieces on the first layer, then laying the straws across them.
This model hand is based on a real human hand - but it's not really brilliant at picking up or gripping. If I was building this as a prosthetic, or as part of a robot, I might design something more functional, even if it ended up less hand-like.
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