Solving Two-part Metal Puzzles: Theory and Practice

Wouter M. Koolen



A brief look at two metal take-me-apart puzzles.


This is a quasi-serious post about solving two-part metal puzzles.


The puzzle in this section is the “Cast H&H” puzzle by HANAYAMA. It is rated 5-star difficulty out of the maximum 6.

I got this really nice puzzle from Tim van Erven. It is made of metal. Its two parts can move relative to each other through shaped holes and slits.
Just beautifully made.
And after fiddling with it for a while, exploring the possible moves, it suddenly gave. I still do not know exactly how large the state space is. But I can consistently take it apart and put it back together. Here you also see why it is called H&H.


The puzzle in this section is an Extra Plus Xianhua Top Security bike chain padlock.

Then “reality” handed me a very similar puzzle. Two weeks ago my bike lock wouldn’t open anymore. Possibly due to internal rust. The key would turn only a quarter turn. Challenge accepted. Lock spray did not help. Neither did striking the top loop from within with a hammer.
I made it worse by snapping off the key, by trying too hard to force it to turn. I was able to pry out the broken-off key shaft with the pointy tweezers in my jeweller’s screwdriver set. I do not have a spare key.
 Two weeks later, by sheer serendipity this youtube video showed up in my recommendations. Using a small variation of the method in the video, I was able to force the lock open in about 10 minutes. I used a wrench as a lever supported atop combination pliers and a hammer. In the video some metal part breaks. Here nothing snapped as far as I can tell. I forced the top loop past the play in the bolts.

With a little bit of practice, I think one could open this lock on the street without drawing (much) attention. I plan to find a sturdier successor.