Edit: My new design "Better grain auger" is a much nicer design. There are considerable improvements needed to this design, so I restarted from scratch.

Edit: Redesigned hopper and saddle. 11/29/16

This is a working grain auger. Completed, it is 18" long, and 1.75" diameter. This works just great with popcorn or small beans. Hopefully there will be more parts coming to build a full working toy grain handling system.
You will need 3 of the auger flightings, which need a bit of epoxy before sticking together, and 2 wheels. Everything else requires a single print of each part.

Edit: deleted obsolete info.

This was created in Onshape, and is public. Search for Grain Auger, and feel free to customize as you wish. Have fun printing, and happy farming.