Another device allowing to easily draw the central axis of an object up to 7 cm wide. This design is based on a dual rack-and-pinion movig in opposite directions, the pinion staying at the center of the object.

It prints well on a resist printer, I would be curious to see how it prints on a filament printer. Once printed, you can assemble it this way:

• Insert the two J-shaped parts in each-other

• Slide them up to the stop to get a circular hole

• Insert the gear and gently spread the Js

Designed with OpenScad, source code there: https://github.com/bluesyann/openscad_shapes