I was having trouble deburring the aftmost holes in the flanges of the rudder ribs. There was just no way to fit my deburring tool inbetween those flanges. So I made this tiny custom deburring tool from a nail. It's small enough that I can fit it between the flanges of the rib and stick the shank through the hole. Then I chuck the shank with my cordless drill and very slowly make a deburring turn or two. It took about 10 minutes to make with a hacksaw and a file.