1/8th thick aluminum

[steelerborn]

I would say it really depends on how heavy your mechanism is on the end of the arm. Having a lot of weight dangling on the end, really increases the torque on the arm.

For instance we were using 1/8th inch aluminum sheet metal for our arm, after it was manufactured we noticed it was overkill. So we redesigned it to be 1/16th inch thickness. It decreased our arm weight by half, around 4-5 pounds total weight now. And by adding smart bends, and clever internal supports, our arm is just as sturdy.

Just test for your weak points and make a good solid design, if your using sheet metal remember that it gains strength with bends.

Hope this helps.