The Mug Marker is a Don McRae's cardboard mug-decorating robot that uses an Eggbot-style EBB controller board and stepper motors to draw precise patterns on your favorite coffee-mug. Lenore from Evil Mad Scientists has a writeup on the design process and the way it performs.
Much like the Mug Plotter on Instructables, it uses the same EBB controller board and stepper motors as the Eggbot, but with linear motion for the pen instead of rotation. However, unlike that version, Don has incorporated the same winch-drive mechanism that we use on the WaterColorBot to provide motion for the linear axis– meaning that it can go fast or slow, with very good accuracy.
Don laser cut the large white winch on the back of the machine from acrylic. It controls a string that pulls the pen carriage back and forth as it rides on a pair of rods.
The pen holder itself slides into some additional bearings, and has a small protrusion on the back that rests on the servo horn, allowing it to be lifted up or lowered down. Like the Eggbot’s pen-lift mechanism, this mechanism only (actively) lifts the pen, which means that it can ride over uneven surfaces, or plot on mugs with variable diameter.