I am considering making one. This baby is kind of a wonderful project. When reading the provided documentation, I have a few questions:
a) why mix digital and analog servos? Metal gear analogs should be good enough. And maybe that would solve the issue of not controlling the servos directly from the Raspberry (and save a lot of money...).
b) is that much torque (20 kg !!!) necessary for the rotation servos, especially with the new low friction stickerless cubes which are now on the market?
c) given the various sizes of cubes on the market (from 55 to 65 mm), which is the size you have designed the robot for? Do the grippers need to hug the cube very closely or should there be some slack (small play)?
d) after rotating the bottom layer, the gripper needs to disengage, but the cube has a trend to fall through the side grippers. What is the solution? Make smaller grippers or adjust the side servos so they maintain some pressure to keep the cube in place (or both)?
e) I see there is a license fee (need to buy a key). Please state upfront the cost of a lifetime license, as I do not want to invest that much time and money without a global view of how much it costs. And I do not like the idea of an annual fee (what happens if you go out of business one way or another?).