If you want to build a robotic arm from scratch then study the successful arms that are sold on the robotic sites, and DIY arms displayed on youtube. I find the below "Axis palletizing robot" to be an interesting design beyond the standard servo designed arm. I'm tempted to buy one just to accurately trace the arm geometry and pdf prints. Don't know if the arm actually works other than youtube videos, but the price is better than the usual servo based designs costing twice as much.