I really like Itai Nahshon ring puzzle (https://www.thingiverse.com/thing:3311600). While I was solving it, I had the idea of making a version with multiple rings interlocking like a chainmail, with the string weaved around. I didn't even knew if the puzzle would be solvable, so I made my first prototype out of cardboard. I decided to go with a simple version, only 3 x 3 holes, 5 rings. The configuration of the ring reminded me of a celtic knot, which eventually became the name of this puzzle.
My first version was too small, the finger could not get through the holes to pinch the string. So I made a larger version.
Customizer note: It's customisable, but the Customizer app is using an old version of OpenSCAD which doesn't support partial rotate_extrude. If you want to customise it, I would recommend to download the scad file and customise it using OpenSCAD.
Purple and white
You will need to print:
1 x celtic_ring_puzzle_base.stl
2 x celtic_ring_puzzle_bead.stl
1 x celtic_ring_puzzle_ring-cut.stl
If you like challenges, you can try to print the celtic_ring_puzzle_ring-uncut.stl instead. You will need supports, and it's probably a good idea to use a raft for that one.
You will also need a string of about 1.5 meter. The string needs to be flexible enough to be easily bent on itself. I'm still looking for a way to calculate the minimum length of string needed. The actual length is closer to 1m.
Follow the matching picture for each step.
Pass the string in one of the bead and tight a knot. The knot should hide in the bead, and it should be large enough to prevent the bead from slipping out.
Step 1: Pass the string from one corner to another, in diagonal. Leave about 5cm of string loose. That will help with the weaving in later steps.
Step 2: Pass the string in the 4 middle holes, as shown in the picture. I have added semi-transparent lines to help understand where the string is going underneath.
Step 3: Place the celtic knot on top of the string, as shown on the picture.
Step 4: Pass the string underneath, from the top middle hole to the right middle hole, as shown with the semi-transparent white line. Pull the string to make a loop in the top-right corner of the celtic knot.
Step 5: Pass the string on top of the celtic knot, in the loop, then back in the top middle hole.
Step 6: Pass the string underneath, from the top middle hole to the left middle hole, as shown with the semi-transparent white line. Pull the string in the bottom left hole of the celtic knot to make a loop, and pass the string through the loop, then in the bottom middle hole.
Step 7: Pass the string underneath, from the bottom middle hole to the bottom right corner. Pull the string in the remaining 3 holes of the celtic knot to make loops.
Step 8: Pass the string on top of the celtic knot, through the 3 loops and in the top left corner hole. Attach the remaining bead to prevent the string from going back through the holes.
Cutting the string
Follow the following steps to cut the string to its minimal length. This will make the puzzle slightly harder to solve by limiting the number of solutions.
Step 9: Move the bead along the right hand string to where you think the minimum should be. This will allow you to calculate the string length without accidentally cutting it too short.
Step 10: Move the left hand string to the opposite diagonal corner of the puzzle. Move the loose string to the right hand string by passing through the whole puzzle. The string needs to be almost tight in the puzzle.
Step 11: Move the right hand side string to the opposite diagonal corner of the puzzle. Tight a knot in the string and cut the extra.
NOTE: This is one of many possible configurations. The solution will basically stay the same no matter how you wire it. Feel free to experiment with it.
If you want some clues about how to solve this puzzle, look at the solution for Itai Nahshon ring puzzle (https://youtu.be/0juqeATz29I). This one is more complex, but the solution is basically the same, just pushed to the next level.
If you really want a solution, I will make a video and put a link here.