Cathy's Lucky Fin V2 - Prosthetic Hand - Bowden / Push-Pull Variant

Description

<h2>Outline</h2> <ul> <li> This prosthetic hand model has no elastics for finger return (push pull / Bowden principle)</li> <li> Outside of the tendon cables, anchors and the screw for the tensioner, everything is 3D printed</li> <li> Remixed from a previous design, this is a revision to a model that I’ve been working on for my daughter</li> <li> The biggest difference with respect to the previous design is that there’s a single tensioner for the tendons from all 4 fingers. Furthermore, all the cables are routed down the center of the wrist so lateral movement (radial & ulnar deviation) don’t cause the fingers to bend</li> </ul> <h2> Specifics of this design </h2> <ul> <li> The hand works on the Bowden Cable principle, where the cables that pull the fingers closed also push the fingers back open</li> <li> Fingers have single channel for the cable</li> <li> Fingers offset with respect to one another (not all parallel)</li> <li> No hardware required for the fingers, pins are incorporated in the phalanges and they click into place</li> <li> The Gimbal mechnism allows an additional degree of movement - Radial & Ulnar deviation</li> <li> The design is made for using crimps for terminations (I'm using #1 single barrel fishing crimps) </li> <li> The Distal & Proximal phalanges are in 2 parts: One to be printed in PLA (files named solid) and the other in rubber (either in rubber using the mould or printed with flexible filament)</li> </ul> <h2> Tendons </h2> <ul> <li> This hand is designed to use push pull actuation so strings that work with the other e-Nable prosthetic designs won’t work here</li> <li> After plenty of experimentation, 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</li> <li> A company called Aquateko makes fishing leader cables with this material at reasonable prices and they come in a range of sizes https://www.aquateko.com/collections/buy-knot2kinky-leaders</li> <li> As a reference, 25lb single strand cable for a hand scaled to 65% worked fine. So it would be best to go with a higher gauge</li> </ul> <h2> Grip</h2> <ul> <li> The fingertips could be printed directly with flexible filament. The filaments that I tried weren’t tacky enough IMO</li> <li> 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) https://www.smooth-on.com/products/mold-star-15-slow/</li> <li> The proximal joints have an alternate version with a slot running across for an additional point on the finger for grip. The idea is to insert O-ring cord or rubber band</li> </ul> <h2> Assembly</h2> <ul> <li> The phalanges have slots to slide the pins - align, push and they click into place</li> <li> Similarly, the Gimbal is spread slightly and slid onto the palm till the pins on the palm pop into place</li> <li> The Gauntlet is inserted into the gimbal and the printed pin goes in from the top. The cables are routed through this pin so make sure that the slot is pointing in the right direction</li> <li> 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</li> <li> The tendon cables 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</li> <li> Routing the cables is easier in 2 parts. First at the palm behind the proximal phalange towards the wrist and once its out, route the other end of the cable towards the finger tip</li> </ul> <h2> Printing</h2> <ul> <li> The parts print well in the orientation as loaded in Thingiverse</li> <li> Parts to be printed with supports – Gauntlet, Palm, Gimbal</li> <li> Everything else prints fine without supports</li> </ul>

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