SUCCESS! ....with some learning
I got the extension built. I had one pipe inside another, for a total 10 holes going across both sides of a pipe (20 holes counting each side of the pipe). 8 of 10 holes went great. 2 holes ate 4 drill bits and required 2 trips to different hardware stores. There was a serious consistency/quality issue with one end of the pipe. It was nearly impossible to drill through. I was using oil, and had already slowed down my drill press, but that didn't matter. It was hard as heck.
It took one auger hole to become efficient and learn some lessons. I did use clevis pins to make swapping easy. However, that displaced shear pins, so there may be some risk with that. I am in sand and am not digging near my woods-lines (roots), so I figured I'd be ok.
Someone above asked about lifting it out of the hole. When we started on the first hole, we drilled until the auger was buried, then added the extension, then drilled way down, then learned we couldn't pull it back up! That was a hard lesson. I had to dig the dirt off the top of it, then dig down beside it and get dirt of the blade. Lesson learned.
I'm not sure if it's the perfect process, but on the second and third hole, things went very well and rather fast, so we are sticking with it:
- Drill the hole twice with just the bit (no extension), pulling it out and pulling/knocking off the dirt each time.
- Lower the bit into the hole until the top blade is just above ground level, then put a board across the blade to hold it in place.
- Disconnect the head from the bit, raise it, and attach the extension. Doing the head first is key, as you can turn it to match the holes in the bit, whereas the bit is in the hole and turning that would be very hard.
- Lower the auger with extension down and connect it to the bit.
- Drill until the soil goes over the top blade. Raise it and pull/knock off the dirt. Repeat to desired depth.
- Lower into the hole until the top blade is just above ground level. Put a board across the blade to hold it in place.
- Disconnect the extension from the bit. Raise the head, remove the extension.
- Lower the auger and connect to the bit. Pull it out. Hole done.
We got 3 holes done before we got rained out. Doing the math, it looks like it takes 25% of the time (75% less) than digging 2/5 of each hole by hand. It's totally worth the time it takes to mess with the extension.