A Midsummer's Sharpening: How to Sharpen Your Lawn Mower Blade

For a more effective cut, keep your lawn mower blade sharp!

  • lawn mower
    The sharpness of a lawn mower blade affects performance of the machine and appearance of the lawn.

  • lawn mower

Sharpening your lawn mower blade is part of routine maintenance. It’s less complicated than you might think, and you’ll only need a few simple tools. If the tips of grass look ragged and brown after you mow, the problem may be a dull mower blade. Instead of being neatly cut, the grass is beaten to pieces.

Before you begin this project, think about safety. Just like when you pull the starting cord, moving the blade turns the engine, which could start. Disconnect the wire from the spark plug to avoid an unintentional start. It’s a good idea to wear leather gloves when removing the blade, but don’t try to hold the blade with one hand while working a wrench with the other. Instead, wedge the blade against a 2-by-4 to prevent it from turning while you remove the nut that keeps it in place. Before you start sharpening the blade, clamp it securely in a vice.

Choose the Right Tool

There are several tools you can use for sharpening the blade: a file, a bench grinder or a hand-held grinder. Electric grinders are available at most hardware stores for under $70. They will likely remove too much blade quickly, and they usually grind a concave edge, which will become dull sooner and is weaker than a V-shaped edge. The heat produced by a rapidly turning grinder can also weaken the blade. If you choose to use a grinder, dip the blade in water frequently during the sharpening process, and dry it each time before proceeding. This will cool the metal to reduce overheating and weakening.

A coarse-toothed file is probably the best choice. You can find one for under $10 usually. They’re easy to use, especially if you’re uncomfortable with power tools. Run the file toward the sharp edge of the blade. It removes material as you push it (not when you pull it back). Lift the file after each stroke; don’t “saw” with it. Follow the same angle as you see on the blade, and don’t sharpen the flat side. But you should run the file lightly over the flat side to remove the “burr” that forms as you sharpen.

Keep Your Balance

Try to take an equal amount from both ends of the blade to keep it in balance. If it’s not balanced, it will make the mower vibrate. To check for balance, hang the blade on a nail in the wall or from a screwdriver to see if one end is heavier than the other. You also can use a blade balancer, which is a small cone-shaped tool designed just for this purpose. If the blade isn’t balanced, file or grind the end of the side that is too heavy.

When you attach the blade, the wings at the ends of the blade should be up toward the deck (not toward the ground). These lift the grass clippings so they’re chopped as you mow. If the blade is upside-down, the sharp edge won’t lead as the blade turns.

Bill Eckert
8/25/2011 5:16:52 PM

The initial warning about the spark plug wire is technically correct and TERRIBLY WRONG!!! that wire has been in the same position for X number of years and when removed will move right back where it "belongs"....before you EVER go under a rotary mower REMOVE THE SPARK PLUG and PUT IT IN YOUR POCKET!!! (USN Safety Weenie, Retired!)

Frank Mulcahy
6/23/2009 4:08:36 PM

SAFETY. make sure the plug lead is removed. There have been lots of accidents occur with accidental start while removing entangled rope from the cutting disc, same with blade removal. Be careful with this.

6/19/2009 3:38:50 PM

Great writing on the sharpening process. I applaud all efforts to get people to keep their tools sharp. I'm a maniacal honer and you can see my stuff at www.theperfectedge.com. The lawnmower blades here in New England are especially tough to keep sharp due to all the rocks in the soil. They come up every year from the frost.

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