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

thingiverse

Outline This prosthetic hand model features a unique design without elastics for finger return or hardware, excluding tendon anchors and screws for tensioning. It utilizes the palm from the Flexy Hand 2 with custom-designed fingers and existing components from the Flexy Hand Reloaded remix. Video of the printed hand is available here: https://youtu.be/FpLIcS0G04E This design is a specific variant, a left-hand thumbless model, tailored for my daughter. The specifics of this design include: The hand operates on the Bowden Cable principle, where cables pulling fingers closed also push them back open. Fingers have a single channel for the cable and are offset relative to one another, not all parallel. No hardware is required for the fingers; pins are integrated into the phalanges and click into place. The Gimbal mechanism allows an additional degree of movement - Radial & Ulnar deviation. The design uses crimps for terminations (I'm using #1 single barrel fishing crimps). The fingertips consist of two parts: one printed in regular PLA and a flexible portion providing grip without sacrificing the finger shape. Tendons This hand is designed to use push-pull actuation, so strings compatible with other e-Nable prosthetic designs won't work here. I've found Nickel Titanium cables to be ideal for this purpose - they're incredibly flexible, kink-resistant, and perform well as Bowden cables. Aquateko offers fishing leader cables made from this material at reasonable prices and in various sizes (http://www.aquateko.com/category_s/21.htm). I used the 25lb single-strand cable for my hand, scaled to 65%. Grip The fingertips could be printed directly; however, I only have access to Filaflex, which isn't tacky enough to provide grip. The cuts are made straight to allow printing with alternative filaments if available. Another option is using the provided mold. The mold is designed for use with "pourable" material. I've experimented with Smooth-on's Mold Star 15 and had excellent results; the tips come out soft and tacky (Shore Hardness of 15A versus Filaflex @ 85A). Silicone works exceptionally well for this application, but its color can't be changed - it's a greenish cyan hue. If anyone knows of better pourable silicone for this purpose, I'd appreciate suggestions. The proximal joints have a slot running across for an additional point on the finger for grip. A piece of rubber band or O-ring cord stock can be inserted here. 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, but they wouldn't flex enough to slide the joints in. 5% infill works great. Similarly, the Gimbal is spread slightly and slid onto the palm until 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. Update: 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 the 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 CAD files for the parts (SW 2014). Print Settings Printer Brand: MakerBot Printer: MakerBot Replicator 2 Rafts: Yes Supports: Yes Resolution: 0.15 Infill: 5 Notes: STLs oriented for printing; the gauntlet looks like it could print straight up, but the part is too weak when printed in that orientation. 5% infill and 2 shells worked fine for me.