How to Sharpen a Lawn Mower Blade. Keeping your lawn mower's blade sharp is an important part of yard maintenance. When you mow a lawn with a sharp blade, it helps the grass stay healthy. Dull blades rip the grass and make it more susceptible to disease, while sharp lawn mower blades cut grass evenly and quickly.
Remove the spark plug wire on the front of the engine. With the spark plug wire removed, the engine won't start. This prevents injury while you are sharpening the blades.
Detach the blade from the bottom of the lawn mower. Use a wrench to loosen and remove the nut and bolt holding the blade in place.
Sharpen the blade with a file following the angle of the edge. If the angle is worn down it should be at 45 degrees.
File each side of the mower blade the same number of times. If the sides are filed differently the blade be off balance.
Re-attach the blade to the bottom of the lawn mower. Secure the nut and bolt to hold the blade.
Balance the blade with a blade balancer. If the blade is not balanced, file a little of the heavy side and try again. The lawn mower vibrates if the blade is not perfectly balanced.
Replace the spark plug wire and try out your new, sharp lawn mower blade.