When your mulching mower is no longer mulching, or there is a major vibration coming from beneath the mower deck--or even if it has been damaged in some way by hitting a foreign object--it's time to replace the blade. Mulching blades are fairly inexpensive and can be found at virtually any hardware store. You will need the make and model number of your lawn mower for the correct sized blade, and plan on using a little muscle to complete this task.
Flip the mower onto its side. This will give you access to the underside of the deck.
Locate the bolt in the center of the deck that holds the blade in place. This will be directly beneath the actual mowing deck, attached to the drive shaft of the motor which will be centered to balance the rotation of the blade.
Position your piece of wood between the blade and the deck and wedge it in this position. This will act much like inserting a wrench between the spokes of a bicycle and completely stop the blade from any rotation as you unscrew the center bolt.
Attach your wrench to the center bolt. Use the box or closed end of the wrench. Common sizes are 9/16 and 5/8 inch, but sizes vary from different manufacturers.
Turn this bolt in a counterclockwise direction. It may take some muscle to get it started, so don't be afraid to turn it hard.
Loosen the bolt and keep turning it out until it falls into your hand. Tap the blade slightly with your wrench, and it will fall from the mounting. A mounting washer or two may also come off with the blade, so keep track of those and how they went on.
Replace the new blade exactly the same way that the old blade came off and tighten the bolt securely.