Generally speaking, more payload is a Good Thing, as is more reach. Thing is, both of them increase the amount of torque needed at the two main arm joints (think shoulder and elbow). When you price steppers, higher torque means more money, and more amps, which means the drivers cost more, and it's not perfectly linear. If you can stay under 2A or so, you can use drives like the Pololu ones at around $15 each. Go to 3A, and you're spending $50-$70 for eBay specials or Gecko drives that are good for 3.5-4A. Go above that, and you're getting into $100+ for the drive, and $70 or more for the motor. It would be easy to hit a price target of $2500 or so, but in the long run I'd like to get this under $1000 in quantity, which means peeling the Lincoln heads off every penny.

So to keep the cost down, the first round design uses 425oz-in NEMA23 motors for the two "heavy" joints, with reduction gearing to get the effective torque to where my calculations say it needs to be. It's a balance as the gearing adds non-trivial costs of its own, but I'm going with it for now.

I decided to start by building these two joints so I could do a (literal) load test to find out if my torque calculations were correct. The video linked to the right shows the arm lifting some very pricey calibrated loads that total about 36 ounces, which exceeded the target of 32. This gave me enough confidence to continue on to one of the most mechanically-challenging parts, the wrist, which needs to incorporate three axes of motion in a 5" cube.