Project Guide

How to Sharpen Lawn Mower Blades

How to Tell If You Need to Sharpen Your Mower Blade
A person checking lawn mower undercarriage during maintenance.

Clean cuts with a sharp mower will promote good lawn health by preventing lawn disease from getting a foothold. Time and use can make lawnmower blades dull. Make it a habit to regularly inspect the mower blades and the grass after mowing. There are often signs that it is time 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.

Follow the steps below to learn how to sharpen lawn mower blades with ease.

Remove and Clean the Blade
A person preparing a lawnmower for blade removal.
  • First, always disconnect the power source for the mower and take off the spark plug wire.
  • Put on 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.
  • Find the nut that holds the mower blade to the deck. Use a wrench to loosen the nut until the mower blade releases. 
  • Take a dry rag or microfiber cloth and clean the blade before sharpening it. 
  • If the blade is exceptionally dirty or caked, spray it with some penetrating oil and scrub with a stiff brush. 

Tip: To avoid re-installing the lawn mower blade upside down, mark the bottom area of the blade with spray paint or a permanent marker.

Sharpen the Blade
A person sharpening a lawn mower blade.

Mower blades have a 3- to 4-inch cutting edge on opposite sides of the blade. Clamp the mower blade into a vise or onto a worktable with the first cutting edge facing up. Examine the blade and decide if it is better to sharpen it or replace it. If your blade has dents or cracks, replace it. Blades that are damaged won’t respond to lawn mower blade sharpening. 

Lawnmower blade sharpening can be done either by hand with files, chisels and punches, or with a machine like a bench grinder. Only sharpen to a “butter knife” level of the blade. Trying to achieve a razor-sharp blade will make it dull more quickly. You’ll have to replace the blade more often.

Sharpen by Hand:

  • Sharpen the edge of the mower 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. Do not saw back and forth. You should be able to feel the file teeth on the blade as you stroke. If you do not, 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 vice. Turn it over to the other edge and repeat.

Sharpen with a Drill-Powered Blade Sharpener: 

  • Clamp the mower blade into a vise.
  • Put on protective eye gear in case of sparks.
  • Secure a blade sharpener into a standard drill. (A blade sharpener will have a round grinding stone with a beveled edge, a guide piece and 1/4-inch shank.)
  • Pull the drill trigger to power it. Then, 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 will automatically give 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 “butter knife sharp” 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, then turn it over to the other edge and repeat.

Sharpen with an Angle Grinder:

  • Clamp the mower blade and don safety gear, as noted. 
  • 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, then turn it over to the other edge and repeat.

Tip: Always wear gloves and long sleeves when working with grinders and blades.

Check the Blade Balance
Two types of lawn mower blade balancing tools.

Mowers run smoother when the blade is balanced. Sometime, sharpening will remove more metal from one end, causing the blade to not be balanced.

  • Check the balance by supporting the blade under its center. You can suspend it from a rod or dowel clamped to the worktable. You could also hang it from a wall nail. (For more precision, purchase a blade balancer and add it to your mower maintenance tool kit.)
  • Hold the blade horizontal, then let go. It should remain level.
  • If the blade drops down in either direction, it means that the blade is unbalanced.
  • Take a few strokes off the heavier end to remove some metal, then retest.

Reinstall the Blade
A person tightening a lawn mower blade.
  • Wipe the area under the mower clean before reinstalling the mower blade.
  • Look for the mark you made earlier. This should make it easy to find the right blade position. 
  • Use your wrench to tighten the bolt. Make sure the mower blade is secure.
  • Reconnect your spark plug wire and your power source.

Maintain Your Mower Blade
A person performing lawn mower maintenance in a garage.

Now that you know how to sharpen lawn mower blades, use your lawn mower 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 tend to 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 will translate differently, depending on how often you mow and how long it takes. Calculate how long it takes you to complete a mowing session, then multiply by the number of times you mow in a given time period. This will help you figure out how often your mower blades will need to be sharpened.

For example, if your yard takes 30 minutes to complete and you typically mow twice a week, your blade will reach its 20-hour limit in 20 weeks (or 40 cutting sessions). This mean you will sharpen your blades roughly two to three times a year.

It may be a good idea 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.

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