My father had one of these that was handed down from his grandfather, used on their farms. It's the best post hole "digger" I have ever used. I remade it using "modern" parts.
It's a 6" post hole auger made from readily available stuff. Right handers will probably want it to turn clockwise (as shown), left handers may want it to dig going counter-clockwise. Typically people find it easier to pull on the handle with their preferred hand (rather than push away). You can add additional pieces of pipe to make it go deeper. (If you do, be sure to use pins so you can back the auger out without unscrewing the pipe sections!)
- Auger came from an electrical supplier (for stuff like telephone / power distribution). It was 8 inches. I cut it down to 6. You may only need 4-5" for average poles. Smaller is easier / faster to use. - Pipe is cut to my shoulder height. 3/4" ID thick wall steel (1" OD).
- Pipe fitting is 3/4" steel (1" OD). Avoid cast iron, it's hard to weld
- T fitting is 3/4" ID (1" OD) and 2". The 2" part is a bit big and I'd get a smaller one to better fit the wooden handle next time.
- Point is a cheap 3/4" ratchet hand tool extension. The square drive part has almost all been cut off (so it looks like a big nail).
- Wooden handle is rough round stock from a hardware store. It's been cut so it's just about shoulder width.
WARNING: Welding can be dangerous. If you don't know how, get training or don't do it.