Last time, I tested the mechanism which moves the fingers, and I identified a solenoid which looked suitable for powering the mechanism. This time I print out the rest of the fingers, and design a mounting bracket for the fingers and the solenoids.

I’ve deliberately not done much research into how to build robotic hand – I think there’s a possibility that I’ll become too influenced by other people’s good ideas, and end up just building a replica of someone else’s project (which I don’t want to do). This approach has the advantage that even though I’ll make more mistakes, I’ll still end up learning more things (which is a slightly strange sounding advantage, but anyway). This time I make a few mistakes, and run into a few dead ends.

I’ll finish the post with a few lessons learned and things I’ll plan to do in future posts.

Re-designing the knuckle

Last time, I mentioned that I wanted to re-design the knuckle so that it could be bolted onto any mounting brackets, rather than acetone welded. This makes it easy to move or replace the knuckle unit. The picture below shows my redesign – using the splitting tool, I cut a hexagonal shaped hole into the knuckle, which allows an M3 nut to tightly fit. When I come to build the mounting bracket, it’s an easy task to extend a 1.8mm radius hole through the bracket, so the knuckle can be bolted on.

Designing the rest of the fingers

I based the other fingers on the one I designed last time – I aimed to make them approximately the same proportions as fingers on my own left hand. The proximal phalange was slightly tricky to adjust as I had used a curve for the outer edge, so it wasn’t possible to just cut a section out to shorten it. However, Autodesk 123D’s scale tool was useful here – I scaled in only one direction, shortening the proximal phalange by 10%. I then used the slicing tool to cut the holes again (as these will have become slightly enlongated during the scale).

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I printed out each of these fingers, shown below.

printed fingers

Designing the mounting bracket

One of the things I’ve noticed about other similar projects (in the deliberately limited research that I’ve actually done) is that the fingers usually seem to be set out to be perfectly parallel, which I don’t think reflects how a real hand looks – the fingers are at a slight angle to each other. I decided to try setting my mounting bracket out to reflect this, and see what I learn. I set the index finger out at 10 degrees to the middle finger, and the ring finger and little finger to be 5 degrees to the middle finger in an opposite direction.

I designed and built an initial mounting bracket, which I’ve shown below.

test jig for knuckles

I ordered more solenoids, and extended the design of this bracket to allow them to be attached to the bracket and the fingers. This looked alright in theory – the bracket seemed to be a bit wide, so I decided to use the middle sized solenoid from last time to control the little finger.


One other problem which became obvious pretty quickly was that the solenoids were too big to allow them to be set out in line with their respective finger. I set them out parallel to each other, figuring that that I could print a linkage with a mounting hole at a 10 degrees or 5 degrees angle, which would allow the mechanism to move.

Arranging the fingers on the mounting bracket in Autodesk 123D showed that they would fit without clashing into each other – each of the fingers appears in one colour this time because I combined all the parts into a single part to help with Autodesk 123D’s memory usage on my computer. If you do this, make sure to save this as a copy as you can’t easily undo the operation.

I printed the bracket out using a wide brim on my printer as something with a large surface area is likely to warp quite easily.

After attaching the fingers to the mounting bracket (shown below) they all fitted reasonably well, although I’ve noticed that some of the alignment is slightly off. This misalignment is because I printed the proximal phalanges in two separate pieces, and they’ve not been acetone welded in a perfectly symmetrical way. This means that the opposing bolt holes are slightly misaligned, and that small error is greatly magnified over the length of the finger. I think for version two I’ll try printing these phalange pieces out in one go.

assembled hand

So at this point, it was starting to become really obvious that this wasn’t going to work – the solenoids are far too big, and also too heavy (I didn’t even bother installing the smaller solenoid for the little finger).

Another problem that occurred to me was that whereas the solenoids will close the four fingers, they’re either on or off – which means that when power is applied to the solenoids, they’ll snap shut – not great for picking up something fragile. Finally, the solenoids really drain power quickly.

So next time, I’m going to go back to doing some research and development – I’m going to investigate whether a small servo would be suitable to power the mechanism, instead of the solenoids.