Taking that lesson to its extreme you realize there's a rounding effect where seemingly no matter how carefully you hold the file, the edges of the face are cut

Taking that lesson to its extreme you realize there's a rounding effect where seemingly no matter how carefully you hold the file, the edges of the face are cut down more than the center. To combat this you might discover you can use the end of the file (like the last tooth) as a scraper. You can use a gauge block and some blue ink to discover the high spots and repeatedly work them until it's flat. Congratulations! You've just discovered how to make a lathe bed.