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Sharpening a lawn mower blade is something that we'd recommend doing at least once a year. If you tend to cut your lawn more than once a week, you may have to sharpen your blade two or three times during the course of a cutting season. Also, if you're one of those people who enjoys flinging rocks and other debris across your yard, you might want to take a look at your blade to make sure that it's still in good condition. Knicks and dings in the blade can reduce cutting performance and can reduce the effectiveness of your blade.
Disclaimer (because we have to): We recommend that sharpening be performed by a trained technician, however, we know that's not always going to happen. We are not responsible for any accidental damage to you, your lawn mower blade, your machine or your environment (house, garage, shop, etc.) if you decide to sharpen your own lawn mower blade. Please make sure that you wear proper safety tools, such as safety glasses and gloves. Use a clean work environment and do not allow the sparks that come off of the machine to land on anything flammable. Always follow recommended safety guidelines for your grinder and any other equipment you're using -- we want you to be safe and to start and finish a project with the same appendages you started with! We don't want "This Isn't How To Sharpen A Lawn Mower Blade" reply videos on YouTube!
Here's a quick recap of the video:
How to Sharpen a Lawn Mower Blade
After you've put gathered and equipped yourself with the correct safety tools for sharpening your blade (safety glasses, thick gloves (ones that go at least in between your hands and your elbows) and something to catch the sparks in, you're ready to sharpen. For this video, we used an RBG blade grinder. You can also use air powered and electric powered drills with sharpening tools, as well as several other hand sharpening tools for this job.
If you're using a blade grinder similar to ours, use the flat bottom edge of the blade to be your guide
Find and set the "cutting edge" face of the blade. Then, set the angle of your grinder to match the pitch of the blade angle.
Only sharpen the angled portion of the blade. You'll want to follow the same angle that is already marked on your blade. Make sure you sharpen the entire angled portion, from the tip of the blade all of the way into where the angling stops. Don't take it any further -- you'll risk damaging the blade or hurting yourself.
Make test cuts before fully sharpening so you can make sure you're sharpening the entire face of the blade. Sharpen the entire face of the angled portion of your blade and check that it's evenly sharpened -- don't leave any bare spots above or below (as you can see in 2:20 of the video).
If you notice knicks or dings in the blade, make even passes until they are removed. If you notice burrs in the blade, lightly pass the backside of the blade along the grinder to remove them.
Don't keep the blade in one location for a long period of time as this damages the tempering of the blade. This will damage the effectiveness of the blade.
When you're done with one side, flip the blade around and sharpen the other side.
Inspect your work as you go -- don't assume everything's going great as long as there's sparks coming out of the grinder!
Balancing Your Lawn Mower Blade
Using a magnetic blade balancer, place the center hole of the blade firmly against the tapered cone. The cone allows the blade to only go down a certain amount but the magnet allows it to spin freely if one side is heavier than another. Hold the blade so that one end of the blade is flat as seen in the video. If one end swings down, most likely that side is heavier and needs to be sharpened a bit more to remove the extra weight.
Double check the heavier side by flipping the blade 180° and checking to see which side is heavier. If the same side flips down, you've found which side needs a little metal removed. Take the blade over to the sharpener and run it through the grinder a few times, then return to the balancer
Know that this process can be time consuming and quite annoying. For this video, it took Pete 5 times to get this blade balanced correctly, but we only showed one extra sharpening.