Status as of 11/25/17: The Original "Flexible Hand Mockup" is worth a look. It seems to work better without bowden tube than version 2, and doesn't require as much force to actuate. I added stabilization towers to the STL. This will need to be modeled in FreeCAD before changes can be made. I'm still conflicted as to which way to go with this. The idea works, though. Note that the scale is off on the Mockup so it needs to be scaled up by 2540%.
11/18/17: Test Finger Added and started working in FreeCAD. Hope my scatterbrain modeling habits don't drive anyone up a tree.
I have printed this with NinjaFlex SemiFlex and it is very promising.
There is a potential here for one roll of SemiFlex to print up to 6 hands and the filament itself can be used as the string for actuation, although I would recommend a stronger non-flexible string. I envision this being something that could be made on a reprap in Africa, for example, without a trip to the hardware store (which may not be an option). It works best with pieces of bowden tube (2mm ID, 4mm OD PTFE [Teflon] tube) inserted into the routing holes for the string. Since bowden tube is a necessary component of a printer that would be capable of printing this filament, I feel it safe to assume that it would be available in a remote village that had a 3D Printer that could handle SemiFlex.
I'm planning to go with elbow articulation for this since it will need a bit more 'throw' and force than a nicely jointed hand. My hope is this would allow kids "to carry their own water," I hope it is strong enough to carry a bucket of water. I'd like them to be able to catch a tennis ball, etc. as well.
Anyone in need of a proven design should look at Gyrobot's Flexy-Hand 2. https://www.thingiverse.com/thing:380665
He also has a one piece printable hand, https://www.thingiverse.com/thing:439484
Thanks for looking!
2 Perimeters, 15% infill. 4 top layers, 0 - 3 bottom layers. With a .2mm layer height I used .6mm for extrusion width and .7mm for infill width. Infill is very important because it is what provides for the flexibility. Once the 'filament tube' exists the finger, the remaining knuckle is mostly hollow space and allows for articulation.
Print slow, 10mm/sec for small perimeters, up to 20mm/sec for perimeters and infill. I used 15mm/sec for perimeters and 15mm/sec for infill. Be mindful of travel move speed as well, since some of these moves are inside the perimeter of your finger and you want to avoid jostling the finger around.
My SemiFlex extrusion settings for the Prusa i3 with an E3D V6 and Greg's extruder are: 235C, 40C Build Plate Temperature (Glass with MagiGoo), Lift Z 0.2mm, 3.5mm retraction at 35mm/sec. I also have it set to retract even when doing moves inside the permieter, so you get the Z lift during in-permiter travel moves to avoid the finger moving around.