This project is a sequel to my first instructable: http://www.instructables.com/id/Arduino-Robot-for-lowest-cost/
No need to learn code and mess with processing files. So easy your grade schooler can create their own designs!
1) Foam board
2) fishing line or other string
3) Arduino with Scratch 4 Arduino interface
4) 2 continuous rotation servo motors - I used GSW S35
5) miscellaneous K'nex parts. I love prototyping this way. The video shows the fifth iteration of the pen holder.
Things I learned along the way:
A slant of approximately 10 degrees seems ideal
By using a smaller shaft I got better resolution/repeatability. Since I was using continuous rotation servos I used time to control distance moved by each axis. I may talk to the guru's at Citilab and see what would work better.
Connecting to the servo horn can be tricky. I super glued wood dowel to a horn that I screwed to the horn mounted on the servo shaft.