How to Sharpen a Lawn Mower Blade

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If you‘re a professional landscaper, you probably sharpen blades weekly, but the rest of us may only think about it after the mowing season has ended. However, your lawn will better off if you sharpen the blades based on their performance: if you see the tips of the grass are jagged, the blade is so dull that it’s ripping off clippings, leaving your lawn open to infection. Here’s how you can get your blades performing the way they should.

Accessing the Blade

First, disconnect spark plugs. This will keep the engine from starting on accident, whether the starter is bumped, or pushing on the blade kicks the engine over.

On riding and wide area walk-behind mowers, raise the deck to the highest position. On walk-behind mowers, tilt the mower on its side with the carburetor facing up.

A blade removal tool clamps onto the edge of the deck, trapping the blade. This is the easiest, safest way to secure the blade when loosening the bolt. If you don’t have this tool, wedge a wood block between the blade and the deck to keep the blade from spinning.

Loosen the blade bolt. As you take off the blade and surrounding components, pay attention to their order and position. Some models will only have a washer on the blade bolt, while others will have a separate blade adapter that will slide off of the spindle. If the washer is curved, take note of its direction.

Some blades are clearly marked so you can tell which direction they should be installed. To make this easier, mark the underside of the blade with some paint or a bright permanent market so you can tell at a glance which side should be facing out when you put it back on the mower.

Inspection

Take a moment to check the blade for signs that it needs to be replaced. Mower blades are curved to create a vacuum inside the mower chamber; mulching blades having a higher curve than side discharge blades. If there are any other bends, then the blade should be replaced. The blade should also be replaced if there are signs of cracking. Eventually, severe wear will make the blade unusable. If it has a thin trailing edge, it’s worn out.

Sharpening

The blade needs to be as sharp as a butter knife to cut grass effectively. Sharpening it more won’t make it cut better, it will just wear down the cutting surface faster.

If you’re sharpening by hand, you’ll need a 10 inch or longer mill bastard file. Yes, that’s really what it’s called: a bastard cut has a medium grain that’s perfect for sharpening mild steel. Clamp the blade in a vise to keep it steady. The file will only sharpen when it moves across the blade from the tip of the file to the handle. By wearing gloves, you can hold onto the handle and the tip of the file, making it easier to maintain steady pressure and angle. Sharpen from the top side of the edge, moving toward the front of the cutting surface.

Blades can also be polished with a bench grinder. The principles are the same, but unless you’re certain of your skills, it’s best to rely on a tool that’s less likely to eat away the blade surface if you make a mistake.

A blade sharpening kit will include a sharpening stone designed to fit an electric drill. Clamp the blade in a vise and make several passes with the sharpener, sliding the blade between the stone and the backing plate.

Mulching blades sometimes have curved edges. These angles can be sharpened with smaller files, an angle grinder, or a rotary tool.

Balancing

For the best results, use a balancing tool. The center of the blade sits on top of this cone, letting you see the balance of the blade in three dimensions. Sand off the metal on the corner that points down. The blade is balanced when it sits level.

You can do a less accurate test by hanging the blade by a nail. As before, sand off the side that hangs low, and install the blade once it sits level.

Reinstallation

Install the blade in the reverse order you removed it. When installing the blade bolt, first tighten it down by hand, then tighten it with a wrench, and finally, use the torque wrench to tighten the bolt to manufacturer’s specifications. If the bolt isn’t torqued correctly, the bolt may unscrew if it’s too loose or it can snap off if it was overtightened.

Get What You Need for Your Mower from Shank’s

Do you need a new blade for your mower? Shank’s Lawn is a dealer for most mower brands including Woods, Scag, Exmark, Yard Machines, MTD, and Murray, so we probably have what you need in stock.
Visit our shop at 4900 Molly Pitcher Highway in Chambersburg, PA, one mile east of I-81 via Exit 10. Not in the area? We ship parts and accessories across the U.S. and Canada. To see what we offer, go to www.shankslawn.com.

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