This is a medium-difficulty puzzle that I enjoyed solving many years ago!
The goal is to remove the center ring. No force is necessary.
I designed it primarily in Fusion 360. I printed it with Reprapper grey and Atomic purple PLA.
Note for the original version: I used organic supports just for the tops of the pillars but later discovered they were not necessary.
UPDATED and IMPROVED V2 version:
No support is required! My new unsupported print turned out great! I made a few changes, including the removal of supports (I left the unused paint-on supports sections in case someone still wants/needs to use supports in their specific case), added variable layer height to smooth out curves on/within the pillars, and concentric fill on top/bottom of rings/beads. The entire puzzle prints in less than 4 hours using a 0.4mm nozzle and looks even better! Enjoy!
Additional items for assembly:
A MINIMUM of ~60cm (2 feet) of the free-moving string plus more for the knots. Longer string does not pose a problem other than the possibility of someone creating giant knots while trying to solve it! ;) I used this 4mm paracord as found here on Amazon, but a thinner cord may work better:
https://www.amazon.com/gp/product/B095SVH1GK/?th=1&psc=1
Melt the ends together (as suggested by @FreddyBee_466602), use hot glue, or tie knots to the base anchor and end ring.
Enjoy the challenge and good luck!
The author marked this model as their own original creation.