Scout is a printable tribot designed for teaching the Boy Scout robotics merit badge. http://www.scouting.org/scoutsource/BoyScouts/AdvancementandAwards/MeritBadges/mb-ROBO.aspx

I've marked this as a derivative of MiniSkyBot because I was inspired by it and because Scout still uses the wheels from that project.

The frame itself is a complete redesign with similar but different goals.

Goals:
Provide a kit that meets the merit badge requirements with parts totaling < $50 USD.

-Minimum printed parts
-Minimum vitamins
-Accommodate an arduino diecimila or nano.
-use a 9V battery.
-Flexible enough to allow boys to improve and extend the design.

The frame is a single printed piece. The tabs for holding the servo motors can be tapped with a 4mm tap to avoid the need for nuts.

The top holes can be tapped with a 3mm tap to screw on an arduino without nuts.

The bracket will hold a 9v battery with just friction. The rear battery bracket has a friction fit cup for a ball bearing caster.

Large holes in the bracket minimize the amount of plastic needed and allow wires to be routed up to the arduino.

Cost
Since affordability was a major consideration, here's a run down of the final project cost.

The plastic cost is just the cost of a roll divided by the number of kits we built. There was lots of plastic left over and the boys are using it to design improvements.

Frame & Wheels $7.86 (plastic from Makerbot)
Tires (2) $2.36 (Hardware store o-rings)
4mm bolts (4) $0.84
wheels screws (4) $0.96
Servos futaba (2) $8.38 (purchased new in bulk on ebay)
Arduino Nano $18.99 (purchased new on ebay)
Ultrasonic $3.90 (purchased new on ebay)
9v connector $0.81 (Radio Shack)
DC power plug $1.86 (Radio Shack)
9V battery $1.25
Total: $47.21