Rotary lawn mowers utilize a rapidly spinning single blade to cut the grass. The blade cuts the grass by shearing the grass blade in two. Each time you cut the grass, the blade comes in contact with immense numbers of grass blades. Over time, the cutting action slowly dulls the blade, and encounters with sticks and small stones speed the process. Sharpen a rotary lawn mower blade at least once a year. This is a fairly easy process that can be completed quickly.
Turn the lawn mower off and disconnect the sparkplug wire. You don't want the lawn mower starting when you are removing the blade.
Turn the lawn mower on its side. Prop a 2-by-4 board between the blade and the mower housing. This will keep it from spinning when you remove the blade.
Loosen the bolt that holds the lawn mower blade on to the engine shaft of the lawn mower, using your adjustable wrench. Unscrew the bolt completely and remove the blade. Set any washers aside that came off with it as you'll need these later to put the blade back on.
Take your blade over to your table vise and clamp it in. The vise will keep the blade steady while you work on it.
Take a metal file or an angle grinder and begin to sharpen the blade. You'll notice that only part of the lawn mower blade has an edge. Follow the natural contour of the blade. Don't take too much off at once; the blade should already be close to being sharp if it's cutting your grass. You just want to clean it up to give it a nice clean, pristine edge.
Remove your blade and place it on a plastic balancer. If it tips in either direction, remove it and put it back in the vise. Remove a little extra metal from the side that tipped down. Check it again and repeat this process until it's perfectly balanced.
Put your lawn mower blade back on the lawn mower. Tighten the bolt and set the lawn mower upright. Reconnect the sparkplug wire.