Mulching lawn mowers use a rapidly spinning rotary blade to cut your lawn. The mulching blade chops up the grass clippings very finely and redistributes them into the lawn. This returns nutrients back to the lawn. After mowing the grass numerous times, the mulching blade will begin to dull. The dull blade allows fungus and other diseases to get into the grass due to an unclean cut. When you sharpen a mulching blade on a lawn mower, you will keep your grass and lawn mower in great condition.
Turn off the lawn mower and disconnect the sparkplug wire.
Turn the lawn mower on its side and remove the lawn mower blade. A bolt holds it to the engine shaft. Remove this with an adjustable wrench and the blade will come free.
Clean the blade by brushing it with steel wool or a steel brush. This will remove rust and other debris from the blade.
Put the lawn mower blade in a table vise. This will keep it stable while you sharpen it.
Sharpen the mulching blade, using a grinder or a metal file. Follow the angle of the current edge. File it down until there is a nice, smooth edge and you no longer see pits.
Remove the mulching blade from the vise after all edges are sharpened. Support the blade in the middle and check for balance. A blade is balanced if it doesn't tip toward any direction. If it tips toward a certain direction, remove a little more metal material from that side and check for balance again.
Put the blade back on the lawn mower and tighten the bolt down with your adjustable wrench.
Set the lawn mower back upright and replace the sparkplug wire.