Your rotary lawn mower blade will become dull over time because the grass slowly wears down the sharp edges. When the mower blade becomes significantly dull, the ends of the grass blades will shred when you mow them, and quickly brown out in warm sun. Shredded ends also allow disease to easily enter your grass. Sharpen the mower blade at least once a year to keep your grass green and healthy. If you run over sticks and other debris frequently, sharpen the blade more often.
Disconnect the spark plug wire from your rotary lawn mower. Grasp the plug wire and pull it free from the spark plug.
Lift the lawn mower and lay it down on its side. Locate the mower blade, which is connected with a single bolt in the middle.
Put on a pair of gloves. Slide a wrench that fits your mower over the bolt. Grab the blade with one hand and turn the bolt counterclockwise with the other.
Loosen the bolt all the way and remove it. Pull off the mower blade and clean it with a wire brush. Scrub it until all the dirt and debris are removed.
Place the blade in a table vise and clamp it down tight. Examine the blade. You will notice that it has two beveled edges--one on each end of the blade. These are the cutting edges you will be sharpening.
Turn on the angle grinder and push it along the beveled edge parallel to the blade. Match the existing angle of the bevel and apply just enough pressure to remove a little material with each pass. Repeat this until the edge is smooth, looks new and has no nicks. Repeat the process on the other edge.
Remove the lawn mower blade from the vise and support it in the middle with your finger to check the balance. A balanced blade will not droop to either side. If it droops, grind off a little more material from the side it droops toward. Repeat this process until the blade is balanced.
Place the blade back on the lawn mower and tighten the bolt with your wrench. Set the mower upright and reconnect the spark plug wire.