ahh, yes, another great idea. Basically a spiral "screw" that your yarn would follow along with the groove. Pretty much a rotating "slide." I can see it working in one direction, trying to picture how it reverse, though. A double-helix perhaps?
As far as whether this discussion belongs here if you go purely mechanical, I am not sure. I am an old-school engineer that doesn't believe that a microcontroller is always the answer. You will find me suggesting mechanical methods all over, but not because I am anti-micro. I just think people way over-use stuff like the arduino to do things that really don't need it and tend to overcomplicate things.
I try to adhere to the KISS principles. You already have a crank providing driven rotation. You do not intend to motorize that. Adding a motor that needs to be synchronized to that crank requires sensors, limit switches, and other stuff as well as having to build mechanisms anyway....