Drive sprocket for beaded cord, designed to be used in a Polargraph machine.

It comes in two parts. Should probably just snap together, but if not, a dab of glue sorts it out. This is designed to be used with a switch close to the outside of the rim of the sprocket that will detect an extra-big bead and can be used to do automatic calibration. It has a hole for a 5mm shaft with a slot cut on one edge - there's a "tooth" that will probably also work with a flat on the shaft.

• Updated 2012-10-07 This has a longer square section, and a thicker top layer.