Cathy's Lucky Fin - Prosthetic Hand - Bowden / Push-Pull Variant by Rhadamanthys76

Last crawled date: 3 years ago

Outline
This prosthetic hand model has no elastics for finger return or hardware (tendon anchors and screws for tensioning excluded)
It uses the palm from the Flexy Hand 2 with different fingers and the remaining parts I already designed in the Flexy Hand Reloaded remix
Video of the printed hand (https://youtu.be/FpLIcS0G04E)
This is a specific variant (Left Hand thumbless) since its a model I've been working on for my daughter
Specifics of this design
The hand works on the Bowden Cable principle, where the cables that pull the fingers closed also push the fingers back open
Fingers have single channel for the cable
Fingers offset with respect to one another (not all parallel)
No hardware required for the fingers, pins are incorporated in the phalanges and they click into place
The Gimbal mechnism allows an additional degree of movement - Radial & Ulnar deviation
The design is made for using crimps for terminations (I'm using #1 single barrel fishing crimps)
The fingertips are in 2 parts:
One to be printed in regular PLA
The second flexible portion provides grip without sacrificing the shape of the finger
Tendons
This hand is designed to use push pull actuation so strings that work with the other e-Nable prosthetic designs won’t work here
I've found Nickel Titanium cables to be the best for this purpose. They’re incredibly flexible, kink resistant and work very well as bowden cables
A company called Aquateko makes fishing leader cables with this material at reasonable prices and they come in a range of sizes http://www.aquateko.com/category_s/21.htm
I used the 25lb single strand cable for the hand I made (scaled to 65%)
Grip
The fingertips could be printed directly, however I only have access to Filaflex and its not tacky enough to provide grip. I've made the cuts straight to allow for printing in case there are any other filaments that could be used
The other option is to use the provided mould. The mould is made to use "pourable" material. I have experimented with Smooth-on's Mold Star 15 and had excellent results, the tips come out really soft and tacky (Shore Hardness of 15A versus Filaflex @ 85A)
Silicone works brilliantly for this application and its extremely durable, the only down side I found to this particular brand was that the color cant be changed, its a greenish cyan color - If anyone knows of better pourable silicone for this, I'll be happy to hear suggestions
The proximal joints have a slot running across for an additional point on the finger for grip. The idea is to insert O-ring cord stock here. A piece of rubber band should work too
Assembly
The phalanges have slots to slide the pins - align, push and they click into place - keep in mind that there needs to be slight flex for this to work - I tried printing the fingers with 30% infill and they wouldnt flex enough to slide the joints in. 5% infill works great
Similarly, the Gimbal is spread slightly and slid onto the palm till the pins on the palm pop into place
The Gauntlet is inserted into the gimbal and the printed pin goes in from the bottom
Each tendon has a crimp at one end that sits in the groove in the fingertip - glue or melted PLA keeps it in place well
The tendon lines are routed through the fingers, palm, and terminate in another crimp that resides in the tensioner - same as fingertip, glue or melted pla to keep the crimp in place
Its easier to begin terminating the cable at the tensioner and routing the cable from the back of the hand to the tip
I still need to finish building the hand and get my daughter to start using the hand. There will be other changes coming once I get to see how it works out for her
Update 15-Nov-2016, List of changes
Palm: If you notice in the video, lateral movement (radial & ulnar deviation) produces a slight bending of the fingers
This movement has to do with the position of the tendon where it passes the pivot point above the wrist
The range of motion of the finger has to do with the distance of the tendon from the same pivot point
New Alternate Palm has the tendon channels reversed (index and middle tendons pass to the left of the pivot point and vice versa for ring and little) Also the index tendon channel passes furthest from the pivot point - With this, Radial deviation will cause the index finger to bend and allow a pinch grip
Modified Gauntlet to eliminate need for housing (tubing for tendon support), tendons have more support printed
Modified the proximal joints o-ring groove to a dovetail shape for easier installation
Uploaded all the CAD files for the parts (SW 2014)