How To: Sharpen Lawn Mower Blades
Sharp blades all season long will provide the cutting edge needed to guarantee a healthy, green lawn this year.
At the start of growing season, part of the necessary prep every homeowner with hopes of a greener yard tacks (or should tack) onto his to-do list involves the cleaning and sharpening of the lawn mower blades. If the blades are left dull, each afternoon spent mowing only rips and chews the grass. Coarse-cut grass doesn’t just look bad; it’s also more vulnerable to pests, disease, and drought.
While old-fashioned cylinder or reel mowers—those with a cutting bar and multiple-blade assembly that cuts forward as the operator pushes it—likely need professional servicing, the more typical gas- or electric-powered rotary mower can be taken care of in a do-it-yourself fashion. These utilize a single blade that spins horizontally around a vertical spindle—easy to remove and maintain at home. If yours is a riding lawn mower, you’re in luck, too: These machines typically have two rotary blades, which can be removed and sharpened just the same. Read on for how to care for your largest landscaping tool, and you’ll reap the benefits of a clean-cut all season long.
Even a dull blade can mangle your hands, so first disconnect the spark-plug wire and put on a pair of work gloves. Then, flip the mower over, spark-plug side up. Holding one side of the blade to keep it from moving, loosen the blade’s mounting nut using your socket wrench.
Clean the blade with a scraper and assess its condition. If rocks, branches, and other debris have inflicted deep nicks, you’ll want to replace it. Otherwise, gather the tools you need to sharpen it manually or mechanically.
At this point, you have a few options for how to proceed.
- To sharpen the blade manually, clamp it horizontally in a vise and run a bastard-cut mill file along the cutting edge, using a one-way down stroke. Follow the existing angle (usually 45 degrees, but you can refer to your owner’s manual for the ideal angle) until the blade has the sharpness of a butter knife. Aim to remove an equal amount of metal from the cutting edge of both ends of the blade. Tip: Counting strokes may help you keep track.
- A faster technique involves a power drill with a sharpening stone attachment. Clamp the blade—cutting side up—in a vise. Secure the sharpening stone in the drill; the stone’s plastic guide should rest against the rear of the blade to ensure the proper cutting angle. Put on safety glasses, turn on the drill, and move the stone from the center of the blade to the tip.
- The fastest way to sharpen the blade is with a bench grinder. With this method, you’ll need to be careful not to oversharpen the blade or remove too much metal. Oversharpening creates a thin edge, which is easy to damage, and removing too much metal shortens the life of the blade. Hold the blade perpendicular to the rotating wheel as you move it along the cutting edge at the angle of the existing bevel. This throws sparks, so be sure to wear safety glasses. If the blade gets too hot, dip it in a bucket of water to cool it.
Check the blade’s balance before remounting it, because a lopsided blade will wobble and overtax the mower’s engine. Set a blade balancer on a flat surface and rest the blade on the balancer using the center hole. If the blade dips, use the file, sharpening stone, or grinder to remove metal from the back edge—not the cutting edge—of the heavier side toward the end of the blade.
You can also check the balance by hanging the blade on a wall from a nail through its center hole. If the blade tilts to one side, remove material from that side.
Reinstall the blade, reconnect the spark-plug wire, and get to work!
Once your lawn mower blades are sharp, reinstalled, and ready to use, you’re ready to mow—almost. If you’re new to the art of mowing (and even if you’re not) there are a few common lawn care mistakes you need to avoid. Take a look at the video to learn more.