I hate to disagree with flail whisperer Leonz but IMO, unless you are running a commercial mowing operation or maintaining your private putting green, there is little need or economy in sharpening flail knives. Yes flail knives dull with use but the hassle and time in sharpening the buggers just ain't worth it. Unless you buy an expensive device that can be used pretty much only for sharpening flail knives in situ, you need to remove all 50-100 of them anyways. That by itself is a major PITA (unless you have one of the few flails where a single rod controls a dozen or more blades). For me it would be a 4-5 hour job just to remove and replace all 56. Plus time to grind them all. Most air wrenches don't have the power to back off nuts from damaged and dirt encrusted bolts. Manual wrenching with cheaters is what I usually need for at least half. Putting them back on is just about as hard unless you replace them with new. Unless you only finish cut, the knives are going to lose their fine edge within a few mowings anyways. I just treat mine like a bush hog and accept that the grass will be more "ripped" than cut. From 10 feet away I can't tell the difference in grass cut by rough flail versus standard Craftsman lawnmower with fresh blade either. I replace blades when the clevises break (more common with Caroni metric clevises). Otherwise the blades do their work with a dull edge. They still cut fields and brush just fine.
