Summary

Part of a bigger project to reverse-engineer and make a fully 3d-printable circular knitting machine. https://github.com/jonnor/projects/tree/master/idda-knitting-machine

This lets you replace the crank of your circular knitting machine, in case it broke down or you want to attach a motor to it. Can connect a drill to the crank using a 3mm Allen key bit, or a custom motor via the MXL belt pulley.
A complete mod for motorizing these knitting machines is expected soon.

The device this is for is sold for instance by Banggood as "Creative DIY Scarf Hat Quick Knitting Machine Handheld Handwork Weaving Tool for Adult Child"
http://www.banggood.com/Creative-DIY-Scarf-Hat-Quick-Knitting-Machine-Handheld-Handwork-Weaving-Tool-for-Adult-Child-p-1030934.html