Most open source robotic hands are limited by the fact they have servos in the forearm, which is the logical place to put them, given the vast space in the arm. However this restricts the wrist motion as the strings attaching the fingers to the servos run through the wrist and if the wrist rotates the strings will pull on the fingers, moving the fingers when they should otherwise be stationary.

Instead by hacking apart and redesigning several sg90 servos I was able to fit all the required servos inside the palm of the hand which allowed me to create a wrist that rolls in a similar motion to that of a humans. Whether or not you decide to build this arm the steps about the servos and how to make them compact and continuously rotate might be of interest.

Counter-intuitively to what Instructables is meant for I do NOT encourage anyone to build this arm. This prototype has several flaws as outlined in the link below. Rather this is documentation of my experience in the hopes it can help others who wish to design a robotic arm avoid the same pitfalls.

If you wish to build this arm by all means do, but just be prepared to troubleshoot and problem solve as you go. I have made all the code and .f3d files available online. If you just want to build a robotic arm that works without problems I would suggest either the InMoov hand or the Flexy Hand (although you will need to redesign the forearm to work with servos).

What are the issues?

Here is a list of issues that need resolving, and are best to keep in mind when building either this project or your own.

Being my first Instructable there may be some sections that are incoherent/ glossed over, if any of the sections don't make sense let me know in the comments below and I will try to clarify and edit them.