Edge tracing wheel.

This was one of my early designs, I've just tweaked it a bit.

I've added a pen clamp instead of using rubber bands.
Also added is a larger wheel in case it doesn't run freely
This was made to use for tracing the edges of paper or wood etc to increase or reduce the dimensions, the wheel follows the edge and the pen marks the new line.

Print all the parts, assemble as in the pictures (V groove in axle aligns in pen holder)it should be a good push fit, but glue if needed.
Use split rings t