Mechanical finger

I'm a bit surprised I never posted about this sooner. When I was in high school, back around 1990, I designed and built prototypes of a human-shaped finger for a robot. At the time there were already some human-shaped mechanical hands, but I was disappointed by their Erector Set bulkiness and openness. I imagined they would pinch human skin and damage all sorts of things humans are used to interacting with. I was trying to think of a way to solve that problem and came up with the idea of constructing a solid finger out of a sandwich of layers of parts. I built a prototype out of balsa wood and then another out of plastic. Below are some pictures I just took of them and the design sketch for the plastic one.

I guess I didn't think much of them because, as the years went by, so many great innovations have come about in this area. Still, I think there's merit in this approach.

This is incredibly lightweight. The balsa wood one weighs a few grams.  They each weigh as much as a solid piece of the material they are carved from. Because the hinges are as large as the fingers' diameters, they are very study, so the fingers don't flex from side to side at the joints. Being solid means they resist compression fairly well.

There are few moving parts: just three finger segments, three axles, and two cables. No complex pulleys to deal with or the like.

A mechanical engineer might object to the friction that can come from the large hinges. I was worried about this when I build them but was surprised at how little friction there is. I may have put some powdered lubricant in the balsa one, but I put nothing in the plastic one.

Probably the greatest weakness of what I built is the cables. The balsa one's cables are made from bundles of sewing thread and the plastic one's are ordinary twine. I envisioned a production version being made from plastic and having cables made from thick fishing wire. I imagine this could easily be made from aluminum or another metal and use steel cables.

One other shortcoming is that the cable runs are exposed on the bottom when the finger is extended. Although they are very small slots, they still can be pinch points or places where dirt can collect and foul the machine. When I build these I imagined there would be a skin covering the mechanisms.

One nice thing about this design is that it lends itself well to sculpting. You can probably see that I whittled the outside of the balsa one to give it a human finger shape. It's unusually thin, but I expected I could have added more layers to fatten it out to human proportions. Although the diagram doesn't show this, the various layers have different diameters for the hinge parts to account for the curvature of the fingers.

Again, it's not much to speak of, but I thought it might be nice to post about this here. But it's also a reminder that I made this over half my life ago. So much time has passed and I really haven't done much of the research I had hoped I would in that time. It's a bit sad to think this modest creation might be the pinnacle of my creative efforts in robotics.


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  2. You have an interesting design here. I appreceate the simplicity and would like to use some of your ideas in my own project.

    I lost my left index finger and thumb to a tablesaw about 6 months ago. Since then I have started researching and designing a set of functional prosthetics. I now have a Makerbot Replicator 2X 3D printer that I use to make prototypes. I would love to collaborate with you and get some more details from you. Maybe we can make something really usefull.


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