A dull mower blade doesn't cut grass blades— it rips through the leaves leaving jagged edges that turn brown and makes the grass vulnerable to disease. A sharp blade shears off excess grass and produces a cut tip that heals faster and imposes less stress.
How often the mower blade needs sharpening depends on the size of your lawn, how often you mow, and how much debris your lawn normally contains (fallen branches under a silver maple tree definitely takes the edge off a mower blade). You should plan on doing the job at least every 4 — 6 weeks.
Professional mowers normally sharpen their blades every day at the end of their shift. So that's about 8 hours of mowing.
Just buy a new blade each time it needs sharpening. Blades aren't that expensive and it may be just easier for some to replace.
Have the blade sharpened by a professional mower mechanic.
Sharpen it yourself.
A combination of the above.
ALWAYS DISCONNECT THE SPARK PLUG WIRE BEFORE WORKING ON THE MACHINE. IN REMOVING THE MOWER BLADE IT'S VERY EASY FOR THE MOWER TO START ON IT'S OWN!!!
Drain the gas and disconnect the spark plug wire in your power mower so the motor doesn't turn over while you're working.
Tilt the mower on its side, and wedge a block of wood between the blade and the mower deck to keep the blade from turning. You can also buy a device called a Blade Buster that locks the blade in place while you work on the mower.
Clean built-up debris from the underside of the mower deck with a putty-knife.
To make the nut removal easier, squirt a few drops of penetrating oil on the nut that holds the blade to the engine shaft. Give it 15 minutes to work its way into the threads.
Use an adjustable wrench or socket wrench to remove the bolt from the center of the blade. Turn the nut in the same direction you would to remove a jar lid.
Mark the blade on the bottom so you can install it properly again. Also note the sequence of washers and clamps as they come off the shaft. Set them aside in that precise order so they can be reassembled properly.
Clamp the blade in a bench vise or with a block of wood and a c-clamp.
Check both sides of the edges for nicks. Remove the nicks using a flat medium file. Lightly file the flat side of the cutting edge to remove any nicks or burrs, keeping the file as parallel as possible. DO NOT TRY TO PUT A BEVEL ON THE FLAT SIDE OF THE BLADE.
Move the medium-toothed file toward the cutting edge with smooth, even strokes. Counting the number of strokes you make, follow the original bevel of the blade closely.
Make the same number of strokes on each edge. If you take more metal off one side than the other, the blade will be out of balance. Out-of-balance blade cuts unevenly and causes excess mower vibration that may damage the engine.
Test the balance by resting the blade on a dowel or the handle of a screwdriver. If one side points up, sharpen the other until the blade lies flat. (Or use a blade balancer, available at garden centers and hardware stores.)
Keep 2 — 3 extra blades on hand and have them all professionally sharpened during your spring tune-up. Then swap them out using a regular maintenance schedule based on the 8 hour rule.
Cracked, badly nicked or bent blades should be replaced. DO NOT TRY TO STRAIGHTEN A BENT BLADE. DO NOT USE A BENT OR CRACKED BLADE.
Never touch the blades of any power mower until the spark plug is completely disconnected. Just a few drops of gas in the tank could be enough to make the mower's engine kick over when you move the blade.
Use extreme caution in reinstalling a sharpened blade. If possible use work gloves, a slip of the wrench when tightening can cause a serious cut.