This is a tracked robot I put together based upon the treads from LoialOtter's NerfTank. It uses an ATMEGA328-PU microcontroller and a L293NE 4 Channel Half-H Bridge Driver to drive the motors. It has two bumper sensors which are made out of brass wire and melted into the chassis. The chassis also has adjustable idler wheels to achieve the correct tension on the tracks.
The materials used in this robot are PLA, electronics (veroboard, wires, IC's etc), motors and brass wire.
The brass wire I used is 1mm in diameter. It's used to link all the treads and make the bumpers. Make sure the edges of all the treads are smooth to avoid them catching on the wheels.
The motors used are these cheap things: http://www.rapidonline.com/Electrical-Power/120-1-Inline-motor-and-gearbox-81728. They are attached to the robot using an M3 screw at the top, and two PLA rivets in the middle.
The numbers of each part are as follows: