In my mechatronics class, the professor gave us the freedom to choose any project we wanted. My groupmates and I wanted to explore hand movements and gestures, so we chose the game rock, paper, scissors. I was very excited because it gave me the perfect excuse to build a robotic hand. I had built one previously for an 8th-grade science fair project and was looking forward to applying my new knowledge to this topic.
The hand that I had built when I was 11 was very rudimentary. I used a bendy plastic pipe with notches cut out at the joints and a fine fishing line to control closing the finger. Getting these to work well took quite a bit of time. I wanted to make it as realistic as possible, so I used my father's hand as a mold. While this initially seemed like a good idea, the result was much creepier than expected. I put the pipe fingers in the finger holes in the mold and then poured in silicon to make the rest of the hand. It was not the most aesthetically pleasing design, but it was functional. I was very proud of this project, especially since I did all of it in only one week.
For the current project, I could now bring those skills to make something of a much higher quality. I started with the same design that I had done all those years ago, a cable-driven system with elastic. My team got together one evening to start work. I started by drawing a system of finger segments that would be 3D printable. I then made a model out of cardboard for a proof of concept and used it to demonstrate functionality.
I CAD’ed up a more representative model in Solidworks and printed it on my 3D printer in PLA. The first print was very blocky and did not meet my expectations. The pegs could fit together, but it took some force to rotate them. To remedy this situation, I tested different part tolerances and optimized the print resolution to reduce friction. I had to adjust and reprint one problematic joint a couple of times to get it to fit and rotate smoothly. After getting this joint working, I replicated it for the finger and then used it as a reference for all of the other fingers.
The palm was the next part to tackle. The base of the finger joints all had to be attached without interfering with each other. Another challenge was the decision to add a servo for the rotation of the thumb. After completing a working digital model with all the mates working in Solidworks, I made a print of all the parts. This first model was a good start. It looked like a hand and you could use the cables to make the fingers close and open. However, the joints were under-designed. They would mesh and rotate around, but nothing stopped them from just slipping off.
One of my groupmates had the ingenious idea of using the design of the Lego pegs that snap together to keep the finger joints on. A quick edit and test prints to try it out, and it worked! I switched all of the other fingers over to this design.
The last task for designing the hand was an arm that could house all the servos for the fingers. My groupmate did the shell design. I had the difficult task of figuring out how to fit all these servos and sensors into a small space. To accomplish this, I put the servos in Solidworks and designed around them. Next, I added a support structure around each part and printed it to see if it would all fit together.
To my great delight, everything fit together perfectly. From there, I started a final print.
The final print of just the hand parts took a little over 15 hours. I had learned a great deal about 3D printing by this point and most parts needed little to no cleanup.
The final build went together very well. This hand had come a long way since the one I built in 8th grade. It had a wide range of motion and even an opposable thumb. When it came time to add it all together and get the program running, seeing it play scissors for the first time on its own was incredible.