The bottom line is to make a tip for the earthen drill, it is assigned the main breakdown force. The tip is wrapped in the ground and pulls the main knife (from the saw blade and so on).
I made such a tip from the screw of a meat grinder. Works - EXCELLENT! 5+ Wraps into the ground very easily, you do not need to press on top, it goes by itself, how much you will twist. To pull, you need to make a revolution or half turn back. How is done, look at the photo.
I myself made a drill for trial drilling, to search for water. Just welded the screw to the pipe. So you can drill a well up to 10 m.