Don McRae recently stopped by our shop last week to show off his homebrew CNC project, the “Mug Marker” — a wonderful little cardboard robot that can draw on mugs.

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.

Underneath the pen carriage, the opening of the mug fits onto a three-jaw coupler on the motor shaft, and the base of the mug is held against a rubber faced spring loaded plunger. Small copper tubes are used as bushings to allow the coupler pieces to rotate inward or outward to allow mugs of differing diameters to fit on. A little silicone on the surface of each of the three parts provides a gripping surface, and the upcurved lip keeps the mug from sliding too far.

Looking down through the pen carriage, you can see the mug below held in the coupler.

The chassis of the machine is made from cardboard, either hand or laser cut to slot together, and held together very cleverly with pins. The whole machine is put together from a combination of off the shelf parts and found materials, many of which are laser cut for the correct shape. For software, Don uses the Eggbot Inkscape extensions with very reproducible results.

Thanks to Don for bringing it by and letting us take pictures!