I am a complete nutcase about this puzzle!
I don't know where to begin with. OK, let's follow my usual categories, this time in a logical order:
The goal of "Cast Nutcase" is to take the small nut out of the bolt. The small nut can freely move inside the bolt, you can hear it and see it through the two holes on both sides of the bolt. But the holes are so small that the nut cannot pass through it. So, you have to find out how to move the other four pieces until the nut comes out.
There is also a second goal to the puzzle - you can disassemble all pieces and put them back together in two ways. One way is as the starting position - the two big nuts which rotate around the bolt read "Nut-Case". The other way of assembling is making the two nuts read "Case-Nut".
You will be surprised when you do both challenges of the puzzle.
Actually, I was indeed surprised to read very different reviews and opinions about this puzzle. Some people did not like it, others think it is a great puzzle. In the next paragraphs I will try to stress why I like it and find it one of the best Hanayama puzzles.
Before that, for idea, I give it a rating of 6. Created by Oskar van Deventer, I think this is a bright example of his masterworks! Great job, Oskar!
"Cast Nutcase" is looking very different than the rest of the Hanayama Cast puzzles. It looks as if it is made of another material. With the predominant green color, which has red-pink and blue hues, and hard surface, I think its appearance is very pleasant and unique.
If you look further, you will find that its overall design is magnificent! Every detail of the bolt, every part of the nuts and their sizes are so perfectly made that the whole puzzle looks like a masterpiece!
Some people say that their copies of the puzzle are not strictly fit. The nuts are loose and this spoils the solution. But mine is pretty much stable and follows (I am sure) the intended design. That is a great reason for me to respect Oskar and this work of his.
If you look even further more, you can find the difference and similarities in the pieces. There are actually 4 interlocked pieces and one loose (the small nut inside). I think that is why the theme of the puzzle is "interlocking".
When disassembled, you will see two pairs of identical (almost) pieces. But when put together, they might not seem identical, not even when you know how they are designed. At least for me this is the case. I always imagine how they should look like, but when doing any movements on the assembled puzzle, it seems my imaginations are not completely correct. Very intriguing! So brain-teasing!
That is why I give it 6 for implementation.
There are different opinions about what the difficulty of "Cast Nutcase" is. I think this just proves that people are different and everyone accepts difficulty differently. What is simple for some people could be hard for others, and vice versa. For me, this puzzle is correctly labelled by Hanayama at level 6 (from 1 to 6). The fact that the structure is half-visible leaves the solver with their brain and/or luck. And still, if one solves this accidentally, they can choose to stay unenlightened by trying to solve it back again without inspecting the details of the nuts. Or, they can understand why the solution works and go for the second challenge the puzzle presents - assembling it, so that the two nuts read "Case-Nut".
For me, putting the nuts in "Case-Nut" is easier than putting them in "Nut-Case". That is why I think they chose the starting position to be the second variant and leave the easier one with no visual reference for the solver. Otherwise, the disassemble would have been much easier and the assemble of the other version would have been more difficult. At first, I though that namely the first variant is the intended solution, until I solved it and found out I was wrong.
So, even now, when knowing how the solution works, I still have difficulties in applying it. Especially the one for the starting position of the puzzle (the "Nut-Case" version).
For difficulty, I give it 6.
I came up with an idea of the solution after seeing the picture of the puzzle several times on the list with Cast puzzles. Then, when I got it in my collection, I went for that idea. It appeared I was almost right - only the correct inner structure of the two nuts was not completely as I suspected.
It took me a lot of time to find the correct solution. I tried to use every hollow I could find. I turned the puzzle in every possible direction, used light to see through that hollows and try to find out what the inner sides of the nuts look like. I even almost solved the puzzle once, but I was too quick and the pieces went back to interlocked position.
When I finally solved it (by luck, the first time), I was exalted. I immediately went for the second assemble of the puzzle. And even I got the details of every piece and how its position should be according to the rest, I still had difficulties in doing the solution on my next tries. And it has been so up till now.
So, with everything said so far, I give it here a rating of 6 as well.
And my final rating for "Cast Nutcase" is 6! It is one of the best puzzle I have and I would recommend it to anyone (of course, if you are not a beginner in such puzzles, otherwise some obstacles might spoil the fun for you).