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.
Provide a kit that meets the merit badge requirements with parts totaling < $50 USD.
-Minimum printed parts
-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.
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
Print the frame bracket.
Tap the bracket holes.
Print two wheels. Mount the servo horns to the wheels. Put on o-rings or rubber bands for traction.
Route wires up to the arduino.
add a battery.