A rotary-blade power mower cuts grass by slicing it off at high speed. The blades on many move around 2,500 revolutions per minute. But that high speed becomes ineffective if the blades are dull or nicked. Fortunately, keeping them sharp is easy and takes only a few minutes with the proper tools and technique.
Since the task will require turning over your mower to remove the blade, you may have to drain the gas depending on your model. Some can be tipped over without leakage, others can't. In any case, at least disconnect the spark plug temporarily. That's particularly important for those models with an auto-starter. The chances are very low that the mower can be started accidentally, but make them zero anyway.
Turn the mower deck over and wedge a block of wood between the blade and the underdeck to keep the blade from turning. Put on a pair of leather gloves and grab an adjustable wrench or one of the right size for your mower.
Remove the blade and clean off any grass and dirt residue. While you're at it, use a putty knife or scraper to remove built up grass from the underdeck area as well. Then clamp the blade into a bench vice for manual sharpening. Examine the blade for nicks. Depending on the size of any nicks present, you may have to use a larger or coarser file. For most cases, a medium to fine metal file is fine.
Oil the mower blade lightly with fine machine oil. Then take the file in both hands and run it across the beveled edge in a smooth motion from the center to the edge away from your body. Take care to keep the file at the same angle as the bevel as much as possible. For those without the skill, special angled clamps are available.
Don't file the flat edge, unless it's necessary in order to smooth off a nick that has pushed metal out, making it not flat at that spot.
Make the same number of strokes on each side of the blade, with the same pressure. The goal here is to keep the blade balanced, having the same amount of metal on each side of the nut hole. Out-of-balance blades cut less well and cause excessive vibration on the mower.
The ideal mechanism to sharpen a mower blade is an electric bench grinder, but if improperly used they can destroy your blade quickly. It's essential to clamp the blade at just the right angle. Many grinders make this simple, using attached clamps.
If you have to do it free hand, make sure you have a very steady hand. Otherwise, you'll grind too much off the blade at once. That will either ruin the blade, or at best make it off-balance. With the proper clamps or technique you can complete the task in seconds. Always wear safety goggles, of course. Flying bits of metal can ruin eyesight for life.
If it only takes a few minutes to re-sharpen, the effort is worthwhile, since you'll save a trip to the store. Sometimes, though, it's simply less trouble to replace the blade.