How to sharpen a lawn mower blade

Know-How Notes – How to Sharpen a Lawn Mower Blade

Nicked and dull lawn mower blades make an already dreaded task even hard to accomplish. Instead of slicing through the grass, a dull blade rips the grass. This slows down the engine, leaves more uncut grass in the yard (which looks cut but pops back up in a few hours), and makes you work twice as hard. Don’t let a dull blade keep you down, grab your grinder and we’ll show you how to sharpen a lawn mower blade!

Disassembly

Removing the blade is fairly easy. First, remove the spark plug from the engine to prevent accidental ignition. Then lay the mower on it’s side and unbolt the blade using the wrench. You may need to use a 2×4 to stop the blade from turning. Mark the top or bottom of the blade so that you know which side is up. If you install the blade upside down, it won’t do much good.

The best way to sharpen a lawn mower blade is to remove it. You may need a 2x4 or a helper to hold the blade while you unbolt it from the spindle.

The best way to sharpen a lawn mower blade is to remove it. You may need a 2×4 or a helper to hold the blade while you unbolt it from the spindle.

 

Make sure you note the way the blade was positioned on the spindle. It must be reinstalled the correct way.

Make sure you note the way the blade was positioned on the spindle. It must be reinstalled the correct way.

Sharpening

Unlike sharpening a kitchen knife, there is no specific angle that you need on your lawn mower blade, just as long as it is smooth and sharp. There are two methods to getting this done- hand file or power grinder. Flat blades have short cutting edges and are not curved. These can easily be sharpened with a file. If you have big nicks in the blade, then you will probably want to move on to the power method, as it can take some time to work a nick out of the blade by hand. The grinder method uses a hand-held 90-degree grinder with a grinding wheel or flap disc mounted. I prefer a flap wheel as it cuts quickly without gouging the metal or leaving a rough finish. The more smooth the edge is, the better it will cut.

A flap-wheel works really well for this task. It won't overheat the blade and it leaves a smooth edge, unlike a grinding disc.

A flap-wheel works really well for this task. It won’t overheat the blade and it leaves a smooth edge, unlike a grinding disc. 

Locate any nicks in the blade. You will want to mark these so you can find them as you go. If the blade it heavily nicked, you might need to flatten the edge first to remove the nicks and get a good surface to sharpen.

Nicks in the blade should be smoothed out before sharpening.

Nicks in the blade should be smoothed out before sharpening.

 

We used the grinder to remove the nicked edges.

We used the grinder to remove the nicked edges.

Pro Tip- Wear safety goggles to protect your eyes. Safety first.

Work the grinder over the edge at roughly a 45-degree angle. This will leave plenty of meat to back up the edge so it does not dull as fast. You only need a “butter knife”-like sharpness, anything more will dull quickly.

Next, the blade was sharpened with the grinder on roughly a 45-degree angle. This will let the blade cut cleanly through the grass without the edge being so thin that any debris will nick it.

Next, the blade was sharpened with the grinder on roughly a 45-degree angle. This will let the blade cut cleanly through the grass without the edge being so thin that any debris will nick it.

 

This is the sharpened edge of the blade. Note the upswing on the rear edge of the blade, this is the fan edge that creates the lift which pulls the grass up into the blade.

This is the sharpened edge of the blade. Note the upswing on the rear edge of the blade, this is the fan edge that creates the lift which pulls the grass up into the blade.

Putting It Back Together

We sprayed the blade with Mower Guard graphite. This should make the blade slippery so that grass does not stick to it.

We sprayed the blade with Mower Guard graphite. This should make the blade slippery so that grass does not stick to it.

Once the blade is sharp, you can check the balance of it by hanging it on a nail in the wall. The blade should lay out flat and not swing one side down. If one side is heavier, the imbalance will throw off the engine and the mower will shake. You can always balance the blade by grinding a little material off the heavy side. Be sure to check you blade for bends.  A warped blade will make the mower shake and not cut very well. When the blade gets thin or bent, it is time for a replacement. Only use the correct blade for your mower, they are not universal.

Once the blade is sharp and balanced you can bolt it back on the mower. We also took this opportunity to give the mower deck a good cleaning.

Once the blade is sharp and balanced you can bolt it back on the mower. We also took this opportunity to give the mower deck a good cleaning.

Stay safe and happy mowing!

Check out all the lawn & garden products available on NAPA Online or trust one of our 17,000 NAPA AutoCare locations for routine maintenance and repairs. For more information on how to sharpen a lawn mower blade, chat with a knowledgeable expert at your local NAPA AUTO PARTS store.

about author

Jefferson Bryant

A life-long gearhead, Jefferson Bryant spends more time in the shop than anywhere else. His career began in the car audio industry as a shop manager, eventually working his way into a position at Rockford Fosgate as a product designer. In 2003, he began writing tech articles for magazines, and has been working as an automotive journalist ever since. His work has been featured in Car Craft, Hot Rod, Rod & Custom, Truckin’, Mopar Muscle, and many more. Jefferson has also written 4 books and produced countless videos. Jefferson operates Red Dirt Rodz, his personal garage studio, where all of his magazine articles and tech videos are produced.

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