Just like a lawnmower's lift blade, a mulching blade has a specific job to do. In order for a mulching blade to properly cut and circulate clippings, it must be kept sharp. If you want a well-maintained lawn, then it's essential to maintain the lawnmower's blades. The effort put into keeping the blade sharp will make the difference between a yard that's simply average, and one that stands out.
Disconnect the spark plug. This will ensure that the lawn mower will not start while the maintenance is being performed. Tilt the lawnmower and insert a piece of wood (a 2-by-4 will work well) inside the mower deck to prevent the blade from moving.
Remove the blade. Locate the blade's fastening bolt and use a wrench to loosen and remove it. Make a note as to which side is the bottom of the blade.
Secure the blade in a bench vise grip. Use a file and follow the angle of the cutting edge on the blade to sharpen it. Keep the angle at about 40 degrees. Start sharpening for the top side of the cutting edge. Use light pressure to move the file back and forth across the blade. Make about 50 strokes on the blade to achieve. Use a 4 ½-inch angle grinder if the file does not appear to be sharpening the blade. Place the angle grinder on the blade and follow the angle of the cutting edge. Move the grinder back and forth until the blade becomes sharp.