My equipment includes three glass flower vases I bought at Goodwill for 69 cents apiece. A thin stick and some small spring clamps hold the tangs out of the acid. Drain cleaner provides sulfuric acid to etch the files and a saturated baking-soda solution neutralizes the acid. The third vase has a fresh-water rinse. The file card (lower left) cleans file teeth and shop-made tools of copper, brass, and steel remove the debris the file card leaves behind.photos and video by the author

My equipment includes three glass flower vases I bought at Goodwill for 69 cents apiece. A thin stick and some small spring clamps hold the tangs out of the acid. Drain cleaner provides sulfuric acid to etch the files and a saturated baking-soda solution neutralizes the acid. The third vase has a fresh-water rinse. The file card (lower left) cleans file teeth and shop-made tools of copper, brass, and steel remove the debris the file card leaves behind.

Files don’t get the attention that planes and chisels receive, but they are cutting tools too work best when they’re sharp. Because files are made of hardened high-carbon steel and their teeth are numerous and small, the easiest way to sharpen them is with an acid. I’ve used vinegar—acetic acid—and drain cleaner—sulfuric acid. The acids react with the iron in steel, removing metal from the surface. A dull cutting edge is rounded over with wear, and as the metal dissolves, the radius of the curve diminishes and the edge gets sharper. Before putting acid to work, the file needs to be cleaned. Anything stuck between the teeth will prevent the acid from getting to the metal. A wire brush will get some of the debris left by the work, but a file card, with its short, stiff wire bristles will do a better job.
I work files with my vise holding a block of wood that I've run through the table saw to make a raised lip along to restrain files set on the flat and a couple of grooves to hold files on edge.

I work files with my vise holding a block of wood that I've run through the table saw to make a raised lip along to restrain files set on the flat and a couple of grooves to hold files on edge.

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