This is a fun little puzzle that isn't too challenging to solve. This puzzle goes by many names. The most common names are Trick Bolt, Eiffel Tower Bolt, and Cross Threaded Nut and Bolt Puzzle. The object of the puzzle is to remove the washer. It looks simple enough; but with all puzzles, there is a trick.
I started with a purchased version from Amazon. I was pretty happy with the look and feel of the puzzle. It has a nice weight and I like the bronze finish. The 3D print version is slightly shorter and includes a modification to the cut line (improvement).
I've included all STL files (last step) for those interested in printing. I've also included the STP files in case you would like to make modifications. As you will notice, I haven't printed it yet. I will give a free Instructables membership to the first person that prints the puzzle and posts a picture in the comments.
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Step 1: Metal Version
When you look at the bolt, you don't see anything strange. You can tighten the nut but it won't reverse. The trick is that the bolt is cut into two pieces which are covered by the nut. As you try to reverse the nut, the threads don't completely line up. This mismatch prevents the nut from unthreading.
The trick is to spin the nut to tighten. Next, apply pressure to the end of the thread (nut side). This will lock the threads in place. Continue holding pressure while reversing the nut. You will eventually need to place your finger inside the nut to get the last few turns. Once you have passed the cut line, remove the nut and top portion of the thread.
See video for the solution.
Step 2: 3D Print
I started by downloading a bolt STEP file from McMaster-Carr. Although a bolt isn't that hard to model, I didn't want to spend the time figuring out the threads. This part is 5/8" with 11 TPI. Next, I realized something that I didn't like about the metal version. When the nut is fully tightened, you can see the cut line. This partially gives away the trick. To solve that, I extruded the shank up a little to reduce the number of exposed threads.
Step 3: Bolt Cut - Part 1
This was a little tricky. If you plan to model your own, pay attention to the cut lines. When mated, you shouldn't see a discontinuity across the second threaded section.
Step 4: Bolt Cut - Part 2
The second section of the thread.
Step 5: Bolt Assembly
Note that the nut will fully cover the cut line when tightened.
Step 6: Remaining Parts
The nut is a 5/8" with 11 TPI. The washer size is arbitrary - it just needs to fit over the bolt.
Step 7: Full Model
Although shown in different colors, I suggest printing both pieces of the bolt the same color to not give away the trick.
Step 8: STL Files
As I said in the introduction - I haven't printed the puzzle. I've actually never tried to get printed parts to thread. If you have a tap and die set available, that might be a good option to clean the threads. I also found that garage door lube works very good to reduce the friction on my other 3D printed parts. Anyway, if it works or not, please post a picture if you tried to print it. At the worst case, you will have a 3D printed bolt as a conversation piece.
Thanks for viewing!