You could design a set of robot hands using the joints center of rotation, so that they would rotate properly, then once the modeling is complete, you can skin the robot hand by just flooding each robot hand part with the correspondent joint, which is very fast to do, and you won’t be doing any advanced skin weight paint.
Alternatively you could also avoid the skin weight completely, design the robot hand, then import each finger section into UE4, and attach each one to a joint socket, and hide the hand.
Being mechanical parts, the skin weight is very simple, so I suggest you to go witht he first option 