I'd like to design my own robotic arm. I'd like it to be roughly twice the size of AR2/3, approx. 1.2 m. I'd like to have accuracy of a few tenths of a millimeter. When I calculated what angle it corresponds to, it turned out I'd need gearboxes with <0.1 arcmin backlash. Even if obtainable, they'd cost a fortune.

How come, then, AR2/3 robot has so good accuracy using cheap hobbyist parts? Why doesn't it go "flapping around in the breeze, wiggle wiggle wiggle yeah", as some might say? Am I missing something?

I understand it's not such a big problem when parts are under constant tension in the same direction. In that case the backlash doesn't come into play... But what about rotating the arm around horizontal axis so it passes the vertical? Wouldn't the end effector just fall freely a few millimeters (based on a few arcmin backlash) when the gravity starts pulling in the other direction?