How to sharpen lawnmower blades—safely
The goal is to cut grass, not yourself
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With spring on the move, it’s time to get your landscaping equipment ready for the season. Before the lawn cutting season starts is the perfect time to make sure that your lawnmower blades are sharp and balanced.
A dull blade tears the grass, which can lead to uneven portions of your lawn, or worse, disease and fungus taking hold in your lawn. To keep your lawn pristine, sharpen your blades at the beginning of the season, and potentially a few times during the season, if you notice that the grass is starting to tear rather than cut cleanly.
Fortunately, it’s a quick, easy process that doesn’t require many tools.
Tools you’ll need
Here’s how to sharpen lawnmower blades
Step 1: Remove the blade
For safety, anytime you work on your lawnmower, disconnect the rubber spark plug boot. This guarantees that the engine can’t accidentally fire and injure you.
When the boot is removed, tip the lawnmower straight backwards with the handle flat on the ground, to access the blades.
The lawnmower blade will be held on with one or two bolts. The hard part of loosening them is that the entire blade will spin with them. Sometimes you can just break the bolt free by holding onto the blade with a gloved hand.
However, if that doesn’t work, you can jam a piece of wood into the lawnmower deck to stop the blade from turning as you loosen the bolt.
Mark the top or bottom of the blade with a Sharpie so that you remember how it needs to go back on. It’s easy to accidentally reinstall the blade upside down.
Step 2: Clamp the blade securely
The best tool for this is a good vice mounted to a solid work bench. Put the middle of the blade (where the bolt holes are) in the vice and secure it. If you don’t have a vice, you can clamp the blade securely to a table or workbench as well.
Step 3: Sharpen the blade with a metal file
The blade should already be shaped at the correct angle, around 30 degrees. Hold your file against the blade at the existing angle, and keep steady, consistent pressure as you file in one direction, not back and forth. Many metal files only cut one way.
Keep filing until you have a clean, smooth surface on one side, and you’ve removed all the nicks and dents from the blade. When you have finished the top, file the burr off the back of the blade.
You aren’t looking to create a razor-sharp edge, which will be much more likely to chip and break against sticks and rocks. Instead, try to get the blade as sharp as a butter knife.
Step 4: Check the blade for balance
Because a blade spins so fast on a horizontal plane, the balance is very important. If one side is heavier than the other, you’ll get a lot of excess vibration, which will start to damage the machine.
Many companies sell dedicated blade balancers, which are great. However, you can balance a blade just as easily with a nail in a post or stud—I use a nail in a beam in my basement.
Put the center hole of your blade on the balancer or nail and let go. If one side dips, then that side is heavier. Take it down, remove some more material with your file, keeping the blade sharp, and then check the balance again until the blade stays level.
Step 5: Reinstall the blade
Once the blade is sharp and balanced, reinstall it with your ratchet, making sure to put it on right-side up. Again, you can use a piece of wood to lock the blade in place to tighten the bolts if you need to.
Don’t forget to put the spark plug boot back on, or you’ll spend some frustrated minutes trying to figure out why your mower won’t start.
All that’s left is testing it out! Mow some of the lawn and make sure that the grass is getting cut cleanly rather than torn. Resharpen as needed throughout the season, particularly if you accidentally hit debris or rocks.
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