1. Gather your tools
First, start by gathering everything you'll need for this project. In addition to the lawn mower, you'll need:
- A wrench and screwdriver to remove the blade
- A bench clamp
- A rag
- A penetrating cleaner
- Heavy gloves
- Eye protection
- A mask
- A file or grinder
Safety gear is always important when working with sharp (or soon-to-be-sharp) items. Wear heavy gloves to protect your hands. Eye protection and a mask aren't strictly necessary, but they will help you avoid breathing in metal particles or getting them in your eyes while you're working so are strongly advised.
2. Disconnect the spark plug
You should always disconnect the spark plug first before working on a mower. This will ensure it doesn't start accidentally while you are trying to remove the blade. Spinning the blade around manually can accidentally start it up, which is the last thing you want!
3. Tip the mower on its side
You'll need to tip your mower on its side to get to the blade. Always tip it away from the air filter and carburetor. If you tip it the wrong way, fuel and oil can leak into your filter and carburetor and can damage them.
4. Remove the blade
When you're getting ready to remove the blade, you should first mark which side is facing down. This will help you re-install it properly later. Some spray paint or a piece of duct tape should do the trick. Once your blade is marked, unscrew the bolt that is holding it in place. You may need some penetrating cleaner or lubricant to loosen it.
5. Clean the blade before sharpening
If you don't clean your blade before sharpening it, the accumulated rust, grass, and dirt can make it difficult to get enough abrasion to file or grind away the metal. Wiping the blade down with a rag may be enough, but if your blade is in bad shape, you can use a penetrating cleaner.
6. Clamp securely
Once your blade is clean, place it securely in a bench clamp. Don't try to sharpen your blade without a clamp; you'll need both hands to do the sharpening and trying to brace the blade with a body part could result in cuts and injuries.
7. Sharpen with a file or grinder
If Using a File:
A hand file is the best sharpening tool to use, especially if you don't have much experience with a grinder. Once your blade is secured, you'll need to file with the angle of the bevel. This is usually a 40- or 45-degree angle. Push the file from the inside edge of the blade toward the outside edge, applying enough pressure to feel the abrasion between the file and the blade.
Most blades need about 50 strokes to get them sharp enough. You want your blade to be butter knife sharp when you're done, not steak knife sharp. A blade that is too sharp will dull quickly.
If Using a Grinder:
A hand grinder is much faster than a file, but it's easier to ruin the angle of the blade or damage the metal through overheating.
If you have some experience with a grinder, though, the process is similar to using a file. Simply run the grinder along the edge of the blade following the bevel until your blade is roughly as sharp as a butter knife. If the metal starts to overheat, wait for a few minutes or run some water over it to cool it down.
8. Balance the blade
Always check the balance of your blade before putting it back on your mower. The process of removing metal to create a sharper edge can leave a blade unbalanced. An unbalanced blade on your mower can cause the engine to run rough and can even damage the bearings.
To check the balance, hammer a nail into a wall and thread it through the hole in the center of the blade so your blade looks like a plane propeller. If one side or the other drops toward the ground, more material needs to be removed from that side. You'll know your blade is balanced when it can hang evenly with both sides of the blade horizontally level.
Once your blade is cleaned, sharpened, and balanced, re-attached it to your mower, making sure the side you marked earlier is facing down. Screw the fastening screw as tight as possible. A loose screw can make a mower hard to start and will mess with the way your engine runs.
Only reattach the spark plug once your blade is securely fastened and your lawnmower is right-side up.
10. Mow with confidence
Go try out the sharpened mower. You should notice that the mower is cutting grass more evenly and easily. If you hear any rattling or other loud noises that you didn’t notice before, turn it off immediately and repeat steps 8-9 above. If you notice that the mower is still cutting unevenly or poorly, try sharpening the blades once more or consider taking it to a professional or your local big box retailer for help.