by Christopher Cunningham
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.
If there is still debris stuck fast in the gullets between teeth—paint, epoxy, and aluminum are my usual culprits—a piece of copper pipe with one end flattened or a piece of brass will dislodge it. Pushed parallel to the file’s teeth, the copper or brass will wear away, creating a saw-tooth-like edge with tips that reach down into the gullets. For very stubborn debris, piece of mild steel, heated and hammered thin and flat, is the most effect cleaning tool I’ve found. I used a piece of 1/4” steel rod; a big ungalvanized nail could work too.
White vinegar is often recommended as the acid for sharpening files. The type you’ll find for kitchen use is 4-percent acetic acid. There’s also a 6-percent cleaning version that you’d find at the store among household cleaning products. Even the stronger version is very slow at etching a file. I found that after three days it would begin to make slight progress. The sulfuric acid that is available as drain cleaner is much more aggressive and can do the work in about an hour. The Clean Shot Drain Opener I bought at a home improvement store is 93 percent sulfuric acid. It contains “metal inhibitors,” but they clearly don’t prevent the acid from sharpening files. This high concentration of acid makes the product very harmful if it gets on you or almost anything else, so it requires handling with care. I wear glasses, the full face shield I use for lathe work, rubber gloves, and an apron.
The acid is too strong to use undiluted; 10 percent acid/90 percent water is strong enough. Never pour water into acid. It can splatter and raise a little cloud of vapor because of a quick exothermic reaction. You’ll notice that a bottle of sulfuric acid will feel heavy. It is almost twice as heavy per unit of volume than water. When poured into water, the 10-percent solution will noticeably warm up, which is why I prefer glass containers to plastic. Keep away from the vapor rising from it by working outside or wearing a respirator.
Prepare a baking soda solution to neutralize the acid after the file has been etched. Mix baking soda until the water can dissolve no more and soda begins to accumulate at the bottom.
Before you immerse the file, check how sharp it is by pressing a fingertip against the teeth and slide it toward the tang. With a dull file, you’ll feel some resistance but won’t feel the teeth catching the skin.
Submerge the file in the acid solution and give it a gentle swirl to dislodge any air bubbles. The chemical reaction will quickly produce very small bubbles of hydrogen. The acid does the work in about 60 minutes. To check the progress, I remove the file from the acid and rinse it in water. I do the same fingertip test and when I feel the teeth catching, the sharpening is done.
Dipping the file in the baking-soda solution will neutralize the acid. The residual acid on the file will produce a flurry of carbon-dioxide bubbles. Rinse the file with fresh water and dry it with a heat gun, hair drier, or a blast of air from a compressor.
Some do-it-yourselfers spray a lubricant such as WD-40 on the file to prevent rust, but I find that gets pretty messy and the lubrication might make the teeth slide across the work rather than cut into it. Rubbing chalk or, my preference, soapstone over the teeth will assure the file is dry and won’t rust, and then help prevent clogging when it’s being used.
To dispose of the sulfuric acid solution, I fill a 5-gallon bucket half-full of water and pour the solution in. Next, I slowly pour the baking-soda solution into the bucket. It creates a lot of froth as carbon dioxide bubbles up from the reaction. The mix is still likely to be acidic, so I sprinkle in baking soda, straight out of the box, until the frothing stops. It can take a whole 1-lb box. At that point, the mix in the bucket is no longer acidic and the remaining by-product of the reaction is a solution of sodium sulfate, a neutral salt. It is safe to pour down the drain—it won’t harm plumbing—and isn’t an environmental hazard. The sodium and sulfate ions in the solution are, according to the source linked above, “ubiquitous in nature.”
While you’re devoting some attention to sharpening your files, consider grinding the teeth off one side. Some flat files come with toothless safe edges to assure you’re only working one side of an angle. I use square files for cutting through-mortices in the gunwales of Greenland kayaks. With cutting faces on all sides, it’s difficult to tune the corners accurately.
By making one face smooth I can concentrate on one mortice surface, knowing the adjoining surface will be unscathed. I remove the teeth with a bench grinder and a 1×30 belt sander/grinder. As an added benefit, the intersection of the safe surface and the cutting surface will cut a sharp, precise corner. The file’s original edge cuts a slightly rounded corner.
Christopher Cunningham is editor of Small Boats Monthly.
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