Finger, Robotic, Modular, pull string actuated

thingiverse

Here are some modular finger joints that I designed that worked extremely well. The goal was to replicate the movement of a finger within its space, and they seem incredibly robust while staying together seamlessly. These finger joints utilize Hillman Fasteners extension spring 540213, #71, 5/32" x 15/16", .016" wire, which I obtained from True Value Hardware Store. The finger joints snap together in their out-of-joint position. To create a pull string, I used some weed wacker twine and tied it to the finger tip's angled tube, securing it with a 6-32 set screw deep within the finger tip. You'll need to cut some threads using a 6-32 tap. There are multiple sizes available, which should be sufficient for creating a realistic hand if needed. I've included a knuckle that you can adapt into a hand. The finger joints come in two different joint angles: a 60-degree angle for the finger tip and a 90-degree angle. The finger tip is designed to work with either angle. I attempted to integrate these finger joints with my string pulley servo winch, but it lacked sufficient pull using cheap servos. However, I believe that you can't create a device small enough yet strong enough to pull five strings within the confines of a forearm. My servo winch will function effectively with 1/8" nylon air brake tubing and the weed wacker line inside for remote operation. Hopefully, someone can incorporate this design into a usable hand. The springs fit perfectly in their holes, but if the spring shaft breaks, a little model glue does the trick. A paper clip wire end helps get the spring into its hole. These finger joints may seem insignificant, but they worked incredibly close to replicating a human finger, with all joints moving together and providing some pushback when using thicker weed wacker string. If you modify the scale, you'll need to adjust the spring size accordingly. I have smaller finger sizes already drawn if needed for little fingers. These designs are patterned from my hand, which I've attached along with a picture of hand bones and a trace of my hand with joint measurements. I was really excited to experiment with these modular finger joints, and hopefully, they'll spark an idea in someone else.