A large hollow threaded bolt to enable the reuse of carboard spools with spool refills. Simply pull the two sides away from the center of your empty cardboard spool, you should be left with the sides as pictured.

Now put one either side of your refill and run the bolt through and hand tighten the nut. Everything should self center. now it is ready to be used. And it saves you from printing a new whole spool

Settings- PLA at 20%, 4 walls and Top/Bottom layers