This is a tool for plying yarn. It's called a "Handy Andy". It's used for Andean Plying. Basically you make a yarn cake that can be pulled from both ends. This gives you two string to ply together. There are instructional videos on Youtube on how to use this thing.
After printing this, you will need a small dowel. I'm using a 1/4 inch wooden dowel about 5 inches long. (6mm dowel, 130mm long)
Do not glue the dowel. It is meant to be removed during the use of this tool. The dowel should be a little loose