If you're more electrically than mechanically-inclined like me, then assembling a frame for your robot will probably be the most daunting part of this project. Even though I do have some machine shop skills and access, with the competition's time constraint, I decided it would be best to focus on what I'm best at, electronics and code, and find an easy way to get the actual arm together.
Luckily for us, lynxmotion
Their servo erector set
parts include everything we need to put together a simple, straightforward mechanical backbone to out arm. Note that the servo brackets and metal horns will only fit Hitec servos, but this can be changed with a little bit of machining to suit almost anything you find on Hobbyking.
Take a look at the bill of materials I've included in the last step to see which parts we bought, and adjust according to your needs. The main types of parts you'll be buying are servo brackets, C brackets, aluminum tubes, tube end connectors, and metal servo horns.
The result looks something like (Picture 1
As for the gripper, two options exist - I say got for both!
An electromagnet uses an electric current (see: Biot-Savart Law) to induce a megnetic field that can then be used to pick up metal objects, such as the nails and batteries in the competition area. It is generally easier to get working than the mechanical gripper, but needs to be wound by hand on a laminated core from something like an AC induction motor from a microwave fan in order to work effectively. Sloppily winding an electromagnet on an iron nail might work for picking up the nails, but won't do for something as heavy as the batteries.
ii. Mechanical Gripper
A mechanical gripper usually uses another servo motor to open and close a set of jaws that pick up an object. The lynxmotion little gripper is good for our arm, and is compatible with the rest of the lynxmotion erector set.
Where to buy: Lynxmotion Robotics (http://www.lynxmotion.com/)
Servo Erector Set Parts (See BOM)
Little Grip Set
Also get (for the electromagnet):
AC Induction Motor from Microwave Oven
28 Gauge Enameled Wire