The spade bits I made for the 2006 event just didn't hold up. The steel blades rotated out of position and bent, and the nuts and bolts broke, bent, or simply came unscrewed. They worked briefly but couldn't withstand sustained use. I should have:
- used thicker steel,
- ground the cutting edge symmetrically, and
- welded instead of using nuts and bolts.
Lars says he'll try to do just that and make us a nice set of giant spade bits for the 2007 event. With a 3/8" shaft, I think they'll hold up well and will hopefully last for many years.
Another option would be to use commercially available forstner bits, but a set of very large ones would be quite expensive. We prefer to make whatever we can and save our money for the things (giant chainsaws!) that are hard to make.