The Printer robot: An up-close look at the carriage
This is another shot of the Printer robot's carriage up close with the pen inserted. You can see the rubber band behind the wheel. The wheel, along with pieces that slide above and below the railings of the printer, enable the carriage’s movement.
For more about the book, read my Geekend post Geek Gifts 2011: The Unofficial LEGO Mindstorms NXT 2.0 Inventor's Guide.