How to Sharpen Mower Blades With an Angle Grinder

Just like kitchen knives and razors, the blade of your lawn mower gets dull with regular use. Sharp mower blades are essential to achieve a clean, uniform cut and maintain the health of your lawn.  An angle grinder with the correct grinding disc will repair damage and extend the life of your mower blade.

Sharpening Mower Blades With an Angle Grinder

While it’s possible to sharpen mower blades by hand using a ten inch file or grindstone, it’s a tedious and time-consuming job. If you have an angle grinder in your tool chest, use that instead to save yourself time and energy. 

Know When It’s Time to Sharpen Mower Blades

At minimum, plan to sharpen your mower blades at least twice a year. Pebbles and rocks kicked up by the mower ding and nick the surface of the blade, dulling it over time. If your blades are forced to cut thick grass, it will dull even faster.

You’ll know your blades need sharpening or replacement when mowing the lawn makes it look worse. Ragged, uneven grass that quickly turns brown is a sure sign that the mower blade is tugging and tearing at the grass rather than cutting it cleanly. 

If you prefer to schedule maintenance for your tools, a good rule of thumb is to sharpen mower blades after every 20 to 25 hours of cutting time. 

Choose the Right Angle Grinding Disc to Sharpen Mower Blades

To safely use an angle grinder, you must choose a compatible disc. The size and speed of the grinding disc need to be compatible with the angle grinder. The material and grit must be appropriate for sharpening steel mower blades. 

Size and Speed

Smaller angle grinders spin at faster speeds, larger angle grinders spin more slowly. Using an incorrectly sized disc can cause injury, so make sure to double check the size on the packaging before attaching a new disc to your angle grinder. Confirm compatibility by checking the maximum RPM listed on the disc. It should equal or exceed the maximum RPM of your angle grinder.

Material

Lawn mower blades are usually formed from low carbon, high alloy steel. Different grinding materials interact with steel in different ways. Two of the most common materials for grinding discs are aluminum oxide and silicon carbide.

Aluminum oxide abrasives have a larger minimum grain size than silicon carbide which allows more efficient and precise sharpening of steel alloys. Therefore, aluminum oxide discs are preferred for sharpening steel mower blades. 

Grit

The abrasive material embedded in the disc is called ‘grit’. Smaller numbers indicate large abrasive particles.

• A 10 grit grinding wheel will remove lots of metal very quickly, leaving a rough surface behind.
• A 120 grit wheel will take longer, remove less metal, and leave a smoother surface.
• For sharpening mower blades, a 60 or 80 grit wheel is ideal. 

Remove the Mower Blade

Before you can remove the blades from your mower, you must ensure your safety.  Disconnect power to the machine and remove the spark plug wire. Put on gloves and eye protection. You should also drain the fuel tank into a UL listed container to prevent fuel spills when you tip the mower over. Use a siphon hose to easily extract the fuel. 

Tilt the mower onto its side.  The air filter should be facing up. Use a permanent marker to mark the bottom of the blade. This will be helpful when it comes time to reinstall the blade and you can’t remember which side is which. 

Most mower blades are roughly rectangular in shape, and have two beveled cutting edges. The cutting surfaces are usually three to four inches long, and are located on opposite sides of the blade. Use gloves when handling a mower blade to protect yourself from cuts. The blade is attached to the deck by a retaining bolt. Use a wrench to remove the nut, then ease the blade off of the bolt.  

Sharpen the Blade

1. Don appropriate protective gear. Goggles, safety glasses, or a face shield will protect your eyes from projectiles should the grinding disc shatter. A leather woodworking apron or other fire-retardant clothing makes it less likely that a spark will ignite into a flame. 
2. Clamp the blade to your workbench using a C-clamp or vise. Ensure the first cutting surface is facing toward you, and that there is enough room to maneuver the angle grinder without hitting the clamp. 
3. Use a rag or microfiber cloth to clean any debris off the mower blade. Inspect the mower blade. Nicks and dings can be removed with the grinder. Dents and cracks cannot be repaired; the blade must be replaced. 
4. Attach the grinding disc and connect the angle grinder to an appropriate power source.  Turn the angle grinder on and allow the disc to get up to speed. (It should take about 60 seconds.)Holding the angle grinder with both hands, position it so that the grinding disc is at the same angle as the beveled cutting edge. This is usually 45 degrees. 
5. Without applying pressure, pass the grinding disc over the cutting surface. Grind a little at a time, checking your work frequently to avoid overheating or removing too much metal from the blade. Keep the grinder moving. 
6. Stop when the metal is bare and clean, with no visible dings or nicks. Avoid over sharpening the blade . A razor-thin edge is less resistant to rocks and will become dull faster than a blade that is ‘butter-knife sharp’.
7. Switch off the angle grinder. Allow the disc to come to a complete stop before setting it down. Unclamp the blade, flip it over, and refasten it. Repeat the above process to sharpen the other cutting edge. Disconnect the angle grinder from its power source and remove the blade from the clamp or vise. 

While you have the blade off the mower, consider balancing it as well.

Reattach the Mower Blade

Once both cutting edges have been sharpened, return the blade to your lawnmower. Slide the blade onto the bolt, making sure the mark you made earlier is visible. Screw on the nut, and tighten with a socket wrench. Tilt the lawnmower back onto its wheels, and refill the fuel tank. 

It is possible to sharpen your mower blade without removing it from the lawn mower.

Conclusion

Regular sharpening of mower blades makes cutting your lawn easier and more efficient. This routine task can be easily and safely accomplished with an angle grinder. The best disc for sharpening mower blades is a 60 or 80 grit aluminum oxide grinding wheel that matches the size of your grinder.