A simple corkscrew nutcracker for hazelnuts. It could be scaled up to crack larger nuts - which wasn't my interest since I only need a nut cracker to crack the very small hazelnuts that the kids bring home from playgrounds.
The screw and the box are aligned that way that the weaker z-layer bonding is avoided for cracking the nuts. My first box, which was simply printed from bottom to up, cracked for that reason. This alignment seems to be quite stronger.
The -175 files are a scaled version that should work for walnuts up to a size of 4.5cm.
I haven't tried this though, since my printer is not large enough to print this.