Now that is a good one Curt ! I usually just cleaned them out with a wire file brush. Once they dulled up, I tossed them in the scrap heap. I never really thought about re-sharpening them. This trivia question puzzles me greatly. There are so many types of file teeth arrangments, from fine to coarse, single straight pitch, to the same in dual cross cut pitch. I can't even imagine how sharpening them is done. I would love to hear the answer to this one for sure. I feel kind of stupid after near 40 years in the machine trades.