Good question. It does not need to be a threaded rod. Check the instructions for my JAx Dragon violin for more info. On that one I was even able to use a carbon fiber tube. But a CF rod is probably safer. :-) I did notice that depending on the printer and the CF tube, the workable diameter can vary just slightly (so I made the hole just a hint bigger on the JAx Dragon). The threaded rod can serve as a makeshift file if things get a little too tight. You get the idea. Best of luck and don’t forget to post a Make and share your discoveries.