How to Sharpen Lawn Mower Blades
Time Required: Under 2 hours
Sharp lawn mower blades are necessary for a good-looking lawn. Mowing with a dull blade can create a ragged cut, turning grass brown. Knowing how to sharpen mower blades keeps your grass green and healthy. Your mower will last longer, too.
This guide will teach you how to sharpen lawn mower blades. You’ll learn how to tell if the blade is dull and fix it. With a few tools and some practice, you’ll be sharpening a lawn mower blade with ease in no time.
Clean cuts with a sharp mower promote good lawn health. A properly mowed lawn helps prevent lawn disease from getting a foothold. The cut grass also returns nutrients to your lawn so it grows even stronger.
Over time, lawn mower blades get dulled by usage. Regularly inspect the mower blades after mowing. When they’re looking worn, you'll need to remove the blade from the lawn mower and hone the edges.
In general, sharpen twice per season or after 25 hours of use to keep your blades in good shape. Also, sharpen the blades after hitting rocks. Stone can easily dent or nick the blades.
The lawn itself can also hint when you need to sharpen your mower blades. If it’s not recovering well after mowing, that’s a clue.
Look for these signs that you may need to sharpen the mower blades:
- Dents or nicks in the mower blades
- Uneven grass height after cutting
- Grass blades look torn instead of sliced
- Brown, frayed grass edges
Once you’ve determined that you need to sharpen the blade, it’s time to remove the blade from the lawn mower.
Before you begin, always disconnect the power source for the mower. Take off the spark plug wire as well. This protects you from any surprise hiccups of power. Follow these steps to take out the lawn mower blade:
- Put on work gloves and protective eyewear for safety.
- Drain the gas tank to avoid fuel spills.
- Carefully turn the mower on its side. Be sure that the air filter and the carburetor are facing up.
- Mark the bottom of the blade with a permanent marker, grease pencil or spray paint. That way, you can install the sharpened blade right-side up later.
- Find the nut that holds the mower blade to the deck. Use a ratchet or socket wrench to loosen the nut until the mower blade releases.
- If it’s tough to grip the nut, get more leverage. Brace the blade with a piece of scrap wood and clamp the blade still. You should be able to remove the nut now.
- Clean the blade with a dry rag or microfiber cloth before sharpening it.
- Sometimes the blade is unusually dirty or caked with grass clippings. When that’s the case, spray it with some penetrating oil. Let it sit as directed, then scrub the debris away with a stiff brush.
Next, gather the tools to sharpen lawn mower blades. Mower blades have a 3- to 4-inch cutting edge on opposite sides of the blade. Check to see if the blade is dull or damaged.
- Clamp the mower blade into a vise or onto a worktable with the first cutting edge facing up.
- Examine the blade. Decide if it is better to sharpen it or replace it.
- If your blade has dents or cracks, replace it. You can’t sharpen a damaged lawn mower blade.
- If your blade is just nicked or bent, try sharpening it.
Lawnmower blade sharpening can be done several ways. You may already have some of the blade sharpening tools you need:
- Sharpening a lawn mower blade by hand uses metal files, chisels and punches.
- Sharpening a blade by machine uses a blade sharpener, bench grinder or angle grinder.
Only sharpen the blade so it’s as sharp as a butter knife. The edge shouldn’t cut your finger. A razor-sharp blade dulls more quickly. You’d have to replace the blade more often after constant resharpening. A gently sharpened and shiny edge is the goal.
Safety Tip: Always wear gloves, long sleeves and eye protection when working with grinders and blades. Wear ear protection if you’re using a machine method of sharpening.
Sharpen a Lawn Mower Blade by Hand:
- Put on protective eye gear and gloves. You want to be shielded from sparks and metal shavings.
- Sharpen the edge of the blade using a 10-inch file or grindstone, keeping the file at roughly a 45-degree angle.
- Sharpen from the top side of the cutting edge.
- Push the file in one direction along the blade. Don’t saw back and forth.
- As you’re filing, you should be able to feel the file teeth on the blade. If you don’t, apply a little more pressure on the file.
- Follow the angle of the blade edge. Even out any rough spots.
- Most blades can be sharpened with less than 50 strokes of the file.
- Finish one edge of the mower blade, then release it from the vise.
- Turn it over to the other edge and repeat.
Sharpen a Lawn Mower Blade with a Drill-Powered Blade Sharpener:
- Put on protective eye gear in case of sparks. Gloves and hearing protection are ideal as well.
- Clamp the mower blade into a vise.
- Insert a blade sharpener into a standard drill. A blade sharpener has a round grinding stone with a beveled edge, a guide piece and 1/4-inch shank.
- Pull the drill trigger to power it.
- Place the grinder over the edge of the mower blade. The flat guide should be against the rear of the blade. The cutting edge should slot into the bevel. This automatically gives the right sharpening angle.
- Move the stone back and forth along the blade edge with medium pressure.
- After four or five strokes, check the mower blade edge. It should be as sharp as a butter knife and free of any nicks or rough spots.
- If needed, make one or two more passes with the sharpener.
- Finish one edge of the mower blade.
- Turn the blade over. Sharpen the other edge.
Sharpen a Lawn Mower Blade with an Angle Grinder:
- Put on your safety gear. This includes work gloves as well as eye and ear protection.
- Use a vise to clamp the mower blade still.
- A flap disc is a good choice for the angle grinder.
- Align the blade of the grinder with the cutting edge of the mower blade.
- Move the grinder slowly back and forth against the edge of the mower blade.
- Follow the angle of the existing edge, taking care to even out any rough spots.
- Finish one edge of the mower blade.
- Repeat the process by turning it over and grinding the other edge.
When the blade looks complete, check the sharpness. Grab a blade of grass or a piece of paper and pull it over the blade. It should cut easily.
Mowers run smoothest when the blade is balanced. However, sharpening sometimes removes more metal from one edge than another. This makes the blade unbalanced.
It’s important to fix a blade that was imbalanced during sharpening. If it wobbles now, it will wobble on your mower too. An unbalanced blade may damage your mower. The vibrations alone are annoying, but the bearings or blade shaft might actually get destroyed.
Follow these steps to check the balance on your sharpened lawn mower blade:
- Support the blade under its center, like a scale. A screwdriver is an easy tool to use. Insert the screwdriver tip through the hole in the blade. The blade should balance on the tip or drop to balance on the handle.
- Hold the blade horizontal, then let go. It should remain level.
- If the blade drops in either direction, it means that the blade is unbalanced.
- The side that dipped down is heavier. Take a few strokes off that end to remove some metal, then retest.
For more precision in judging blade balance, purchase a blade balancer. It’s a good addition to your mower maintenance tool kit. It’s a cone-shaped tool that has a spindle for the blade.
After sharpening the lawn mower blade, reattach it. It’s also an excellent opportunity to clean the bowl around the blade.
- Before reinstalling the blade, wipe the area under the mower clean. Dig out any mud and grass clippings that have caked under there too. A paint scraper or putty knife works well for this.
- Check your blade for the grease pencil or paint mark you made earlier. That’s the bottom of the blade.
- Insert the blade back onto the bolt. The marked side should face you as it did before.
- Use your socket wrench to tighten the nut. Make sure the mower blade is secure. It needs to be extremely tight for your mower to start smoothly.
- Reconnect your spark plug wire and your power source.
- Refill the gas tank.
- Roll the mower outside of the garage or shed to avoid fumes.
- Check to see that your mower starts smoothly.
Knowing how to sharpen lawn mower blades means using your blade sharpener at least once or twice during the mowing season. This keeps your mower in good shape and helps to maintain a healthy lawn. However, if you mow more often, the blades will need more attention.
On average, a mower blade should be sharpened after every 20 to 25 hours of use time. This depends on how often and how long you mow. Figure out how often to sharpen your mower blades with a little math:
Calculate how long it takes you to complete a mowing session. Then multiply that length of time by the number of times you mow per week.
Here’s an example:
- You mow your yard in 30 minutes, twice a week. Multiply 30 minutes by 2 sessions. Your total is 60 minutes.
- Your blade will reach its 20-hour limit in 20 weeks. That’s 40 cutting sessions.
- You’ll need to sharpen your blades two or three times a year.
It’s best to keep an extra mower blade on hand. That way you can swap a fresh blade in when one needs to be sharpened. Keeping a rotation going will make sure each blade lasts a little longer.