I've got good sharp-enough carbide-tipped teeth (for the rocks I invariably hit) but I still had to table that project for when the clay softens up with a bit of moisture.I was Rural King yesterday picking up a water trough and looked at their new post hole diggers and augers thinking of this topic. All had twist points and blunt teeth. They might dig in the valley but not on my clay hillsides. My old Ford 3pt phd will dig anything you can dig by hand, thru the toughest clay, shale and rocks. Screw point and sharp teeth. Biggest problem getting stuck by drilling down too fast. You only do this once hopefully to learn your lesson. Blisters help you learn faster. I go slow and watch the bit feed. Only speed up to sling the spoils off the flights to clear the hole.
I remember the bins of different cutter teeth and points at Co-op with knowledgeable people to help you. Not so much now. Use the right tool the right way.
More recently I had to dig twelve 60" deep 12" diameter holes for a solar array (ground mount), and I rented a bobcat for it. Good move; it's got good down force (enough to pick up the front of the bobcat when I pushed too hard). I still ended up having to use my digging bar and sledgehammer when I got to too strong a layer of rock, but luckily that later was only about a foot deep... And then finally there's the manual post hole digger to clean out the foot of loose dirt the auger left behind at 5' depth - squat & jump time!
We don't go to CrossFit - we just do FarmFit
