How To Sharpen a Power Lawn Mower Blade
Sharpening a lawn mower blade does not require the same skill and precision as sharpening fine wood chisels, so don't be intimidated. A dull lawn mower blade results in the grass being torn rather than cut. The problem with that is that the grass is more susceptible to disease as a result. It also tends to make the tips of the grass turn brown. Your lawn mower has to work harder and so do you. The poor cutting edge means that your lawn mower can't cut as quickly and you may find yourself having to mow the same spot more than once to get the job done.
Sharpening a lawn mower blade isn't difficult, nor is it expensive should you decide to have it done professionally. In either case, you will have to remove the blade.
Disconnect the spark plug wire and remove the spark plug. You don't want the mower to start up while you are removing the blade. Be sure to drain the gas tank or run the mower dry before tipping it up on its side. Wedge a 2 x 4 or similar block of wood between the blade and the deck to hold the blade in place. Remove the bolt(s) that secures the blade to the mower and remove the blade. Make sure to note the order of the nut and washers (if any) as you take them off.
Inspect the blade for damage and excessive wear. If you find any cracks or large bits missing, then just replace the blade. Also, the blade should have a 45 degree bevel on the cutting edge. If this bevel has disappeared, then just replace the blade, don't try to restore the bevel by sharpening.
Since the motor turns in only one direction, the blade only needs to be sharp on the forward facing edge. Because the blade is rotary, the sharp edge will be on opposite sides at either end.
If the bevel is visible, sharpen the blade by following the existing angle. You can sharpen the blade with a file, sharpening tool, grinding stone or a bench grinder. Always wear safety goggles when sharpening tools. The goal is to hold the blade or the tool to match the angle of the bevel and then draw the tool over the blade to bring the dull edge back to sharp. Because this isn't a precision blade, don't worry too much about getting it perfectly sharp and don't worry if you can't keep the angle completely consistent down the edge of the blade.
Place the blade in a bench vise to hold it securely. Run the file along the length of the blade while also drawing the file from the thicker part of the blade toward the sharp edge. Essentially you are filing the blade diagonally. Drawing it the other way, from the edge to the thicker part of the blade, will result in poor sharpening and excessive unevenness.
Before returning the blade to the lawn mower, you must check the blade for balance. An out of balance blade will cause excessive vibration and eventually damage the lawnmower.
Hang the blade on a nail, parallel to the ground. If the blade is unbalanced, one end will drop downward. The end that drops down is heavier. Sharpen the heavy end to remove some metal and then retest for balance. Once balanced, it can be reinstalled on the mower.
Make sure the blade's edge is facing toward the ground; it can be easy to install the blade upside on some mowers. Replace all the fasteners and tighten them securely.
While you have access to the underside of the mower, clean out any grass build-up. Wet grass build-up causes rust.
Remove the wooden block, restore the spark plug, reconnect the spark plug wire and refill with gasoline. Your mower is ready to go.