I'm probably a machinist's worst nightmare when it comes to this. I had to drill and tap 1/8" ip hole through 1/4" material that was a scroll shape. Taking a 11/32 hole first and then tapping this was killing me. After a test piece I went as far as a 3/8" hole. The tap was about worn out yet in such a way I could take advantage of the thicker threads. Used a chordless drill with an extension that fit the end of the tap. 17 holes later and made some decent time. You can see the tap compared to a new tap (right) that this went through. Probably not the best example but definitely one about using what you have to get where you want.

Colorado Artist Blacksmith - Dragon Forge LTD