This is a remix of Skip Meetze's excellent Gripper Hand. It incorporates a rack and pinion thumb actuation that translates a circular motion to a straight one which then compresses a flexible filament "spring" such as TPU or Ninjaflex. PLA is fine for everything else. This is a terminal device that screws into a gauntlet or complete forearm as needed.
Note: Files updated 3/31 for minor alterations, better fits.
Note: Improved Spring! Better action, wider range. See misc. notes.
Note: 9/2/18 Updated thumb with mount added for a flexible pencil strap to be installed. Very useful for pens, utensils, etc. that might otherwise be difficult to hold with spring pressure.
This thing was made with Tinkercad. Edit it online https://www.tinkercad.com/things/7lVQiLWJ0kh
I print the new improved spring at 100% infill, on its side, with 3 wall perimeters(.20). The palm can be printed either upright or on its side but needs support either way. However, do NOT print supports inside the hand as you won't be able to remove them completely from the spring and rack channel.
The screw holes are 3mm; thumb pivot is 35mm long and the universal fitting is a 1/2"FT bolt. Both regular and large hands use the same spring and rack.
Thumb and rack need to be mated under slight spring tension with thumb fully closed against fingers when inserting the 3x35mm pivot screw. The rack and thumb pieces should move freely before final assembly. The spring should snap the finger closed on smaller movements but may be a tad slower when fully opened to grasp, say, a travel mug.
Be sure to use a grommet or two o-rings between the gauntlet or forearm when attaching the device. This prevents adding too much pressure and possibly screwing the nut out of the attachment. I also add CA (super glue) to nuts and bolts after they are fitted for additional build integrity.