When you sharpen a lawn mower blade, it can become unbalanced if you remove too much blade material from one side. Because the blade spins on the engine shaft from a bolt mounted through the center of the blade, if the blade is heavier on one side it will rotate unevenly. The first sign of an unbalanced blade is rattling or vibrating under your mower deck. An unbalanced blade may eventually result in damage to the lawn mower, the blade, or both. You can balance your lawn mower blade with a metal file.
Move the lawn mower to a flat, level surface. Disconnect the spark plug cable from the lawn mower so the mower won't start by accident.
Lay the mower on its side to get a better look at the blade. You will see that it is connected to the engine shaft with a single bolt in the middle of the blade.
Slide an adjustable wrench onto the bolt and grip the blade with a gloved hand. Use your other hand to turn the bolt counterclockwise until it comes out. Pull the blade off the mower.
Set the lawn mower blade on the tip of your finger. Put your finger in the exact center of the blade.
Examine the blade to see whether it tips down toward either side. The side to which the blade tips down is the heavier side. You will need to remove metal from this side to balance the blade.
Take a metal file and file the blade on the heavier side. Follow the existing contour of the blade edge with the file to ensure the blade remains sharp.
Balance the blade on the tip of your finger and check that it's level again. If it's not, file a little more material off and check again. Repeat until the blade sits level on your finger.
Put the blade back on the lawn mower, turning the bolt clockwise. Set the lawn mower upright and push the spark plug wire back on the spark plug.