Here's the most powerful and well thought out gripper and robotic arm I've seen:
Here's where you can download his STL files for the gripper:
He also made the coolest looking and most powerful arm that can lift 2 KG outstretched:
Here's his CAD files on GitHub:https://github.com/4ndreas/BetaBots-Robot-Arm-Project
Here's a blog where he describes his process in more detail:
He's spent 4 years perfecting this build.
His design got picked up and cleaned up a little by bcn3dtechnologies.
Here is a link to the Opensource project:
and then there is another here:
and one more: