How to Sharpen Lawn Mower Blades

Any cutting tool needs sharpening periodically and lawn mower blades are no exception. A dull blade will rip up the grass blades, leaving ragged tears that weaken the grass and promote disease. With a freshly sharpened blade you’ll immediately notice a better quality of cut and you may find that you’re able to mow faster as well.

How Do I Know When the Blade needs Sharpening?

The best answer to when it’s time to sharpen is when you notice the cut deteriorating. But, at a minimum, inspect your blade at the beginning of each mowing season. You are looking for nicks and burrs and, in some cases, you may find irreparable damage (chunks of blade missing, or perhaps a bent blade). If that’s the case, it’s time to put on a new blade.

Do I Have to Remove the Blade?

Sharpening a blade while it is on the mower is difficult and we don’t recommend it. You will get much better results by removing the blade. There are several advantages to removing the blade:

Clamping the removed blade in a vise will give you the best angle for sharpening and the most control over the process. When sharpening the blade on the mower you are restricted in the movement and follow through of the file. And it’s even harder, if not impossible, to use a grinding wheel or other grinding tool.
Cleaning the underside of the mower is a lot easier with the blade removed. Grass clippings and all manner of gunk accumulate and harden under a mower deck. With the blade out of the way, you can use a wire brush, scraper, hose (or all three…whatever it takes) to clean it all out without risking a cut. A clean underbelly generates better airflow and “lift”, and combined with a sharp blade, it will produce a higher quality cut.
With the blade removed, you will have the opportunity to balance the blade as well. More on that below.

Step By Step

Get out your owner’s manual. It likely has instructions for removing and sharpening, and you should review it since not all mowers are alike.

Safety and tools. Protect your hands with work gloves. A blade in need of sharpening has jagged edges that can produce a nasty gash. Wear safety glasses (metal filings are potentially dangerous irritants). And work shoes (if the blade slips from your hand you want your feet protected). While you can use a grinding wheel, this job is easily done with a flat metal file.

Prepare the mower. If it is a gas mower, disconnect the spark plug and ignition wire so there is no possibility of the blade roaring to life. Drain the gas tank so there is no leaking when you tilt the mower to get at the blade. If it is an electric mower just remove the battery.

Tilt the mower on its side. If it’s a gas mower, the air filter should be facing up.

Tilting the mower on its side is more stable than raising the front.

Remove the blade. This is usually a matter of removing a single bolt. Grasp the blade firmly with a well-protected hand or use a piece of scrap lumber (2x4 is good) to immobilize the blade. If the blade has not been removed for a long time you need to use a lubricant (such as WD-40) to loosen the nut.

Removing the blade prior to sharpening.

Sharpen the blade. With the blade mounted in a vise, match the angle to the bevel of the blade and make strokes in one direction only, moving from the center of the blade toward the tip. Don’t make more strokes than necessary to remove the visible nicks and burrs and produce a sharp edge.

Move the file from the center toward the end of the blade.

Balance the blade (this can only be done with a removed blade). Ideally, your blade should weigh the same on each side of the center hole. While there are blade balancing tools available you can also use a simple hack: tack a light nail in a vertical surface, then hang the blade on the nail from the center hole. A perfectly balanced blade will rest in a horizontal position. If one side sinks that means it is heavier, and you may want to file more on that side until you can get the blade to balance. A balanced blade will run more smoothly and create better airflow than an unbalanced one.

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Last updated: 11/14/2022