Of all of the nutcrackers that I have encountered the best have been the wooden ones that consist of a 'cup' body with a wooden threaded 'bolt' plunger that screwed into the side of the cup to crush the nut between the end of the plunger and the inside of the cup. The advantage this design has over lever based nut crackers is that once the nut's shell fails it is very easy for a lever based nut cracker to go on and crush the rest of the nut, while the screw plunger versions do not.

This nutcracker was designed for FDM printing in PLA without any support.

A detail that I wanted in this design is that the load bearing face of the screw thread should be as perpendicular to the load as practical. A more traditional steeply sloped face on this side of the thread would increase the friction under load and that would be undesirable in a nutcracker.

Testing various 'cup' designs eliminated the possibility of printing with the threaded hole orientated vertically; the end always cracked along the layer lines (even when printed solid). This suggested that the 'cup' had to be printed horizontally, and so with the threaded hole in an orientation that would suggest that its upper surface would need supporting. To avoid this I have re-designed the threaded hole in the body so that its upper surface is 'bridged' and so does not need support. The 'cup' body also has internal hollows (that is, the model has more than one shell) that are intended to increase its rigidity.

So far I have tested this by cracking 1kg of mixed nuts (Walnuts, Hazelnuts, Brazil nuts, and Almonds). The nut cracker survived this testing and showed no signs of damage on the surfaces that contacted the nuts. Obviously the Almonds offered most resistance to being cracked and one of these prompted me to lubricate the thread, which I did by rubbing some candle wax on the thread (candle wax has the advantage in this application that it is a good enough lubricant and is not inherently sticky, and so does not attract/accumulate (nut) dust in the way that any oil would). Once lubricated no other almonds were a problem to crack.

I would strongly recommend not using this (or indeed any other plastic) nut cracker to attempt to open Macadamia nuts. The forces required to open them are too high and remember that when PLA shatters it shatters into very sharp fragments.

Printing

The nutcracker is in three parts; a body, the threaded plunger and a handle to go on the end of the threaded plunger. The plunger is designed to fit tightly into the socket in the handle; it should stay in place if firmly tapped down into the handle with a rubber or wooden mallet (or, say, the end of a rolling pin), however, if some Cyanoacrylate (CA or 'super') glue were first applied to the inner surface of the socket in the handle the plunger would really never come out again.

The 3D models are orientated in the STL files in the way that they should be printed. The body need to have its flat underside flat on the print-bed. The plunger needs to have its narrow end flat on the print-bed in order for the threads not to need support (this may suggest printing with a brim for stability). In principle the handle could be printed the other way up, but prints fine in the orientation that it is defined in the STL file.

The one I tested was printed in (standard) PLA using:-

0.4mm Nozzle
0.2mm layer height
15% infill (rectilinear)
3 perimeters
4 bottom layers (0.8mm)
4 top layers (0.8mm)

-and the total nutcracker weighed ~110g (yes that is more than 10% of a role of filament).

If anyone thinks that they would like to guarantee that theirs was stronger than this I would recommend increasing the number of perimeters before increasing the infill percentage.