How do I sharpen lawn mower blades?

Disconnect the spark plug wire (or take the battery out of an electric mower), take the blade off, clean it, and file or grind the cutting edge until it is bright and keen, keeping the original angle. Check that the blade is balanced, then bolt it back on to the torque in your manual. A few basic tools are enough.

Why sharpen at all

A sharp blade slices the grass cleanly. Illinois Extension puts it plainly: Sharp blades cut grass. Dull blades bludgeon, rip, and tear grass blades leaving ragged wounds that are more difficult to heal.^[1] Virginia Cooperative Extension adds that dull blades rip and tear the turfgrass, which promotes disease, causes the shredded leaves to appear tan and ragged, and results in weakened plants.^[4]

Make the mower safe first

The blade is the most dangerous part of the machine. Virginia Cooperative Extension says rotary mower blades rotate at a high speed of 2,000 to 4,000 revolutions per minute.^[2] Its rule for any repair is: "Disconnect the spark or electric plug before troubleshooting or repairing the mower."^[2]

Illinois Extension is specific about the order: before removing the blades, "disconnect the spark plug or for electric mowers remove the battery to prevent any accidental engagement of the blades."^[1] Do that before you tip the mower or put a hand near the deck. Wear thick gloves and safety glasses, because a blade edge is sharp and filings fly.

Read your manual for which way to tip the mower. Most want the air filter and carburetor side facing up so oil and fuel do not run into the wrong places. Mark the blade's underside with paint or a marker so it goes back the same way up.

Take the blade off

Illinois Extension's method is to brace the blade with a board, brick, or metal pipe and remove the bolt holding the blade in place.^[1] A block of wood wedged between the blade and the deck holds it still while you turn the bolt with a socket wrench. The bolt is usually tight, so use a long handle and steady pressure rather than a jerk.

Scrape off the caked grass and wipe the blade clean. Look at it closely. A blade that is bent, cracked, or worn thin at the tips should be replaced, not sharpened.

Sharpen the edge

Illinois Extension lists the tools that work: a bench grinder, grinding stone, or a simple file.^[1] For most people a mill file held in a vise is the easiest to control.

• Sharpen only the cutting edge, which is the angled side at the end of each wing. Leave the flat side alone.
• Follow the factory bevel. Push the file along the edge in one direction, away from your body, and keep the same angle each stroke.
• Take off about the same amount of metal from both ends. A few light strokes on each end is usually plenty, and nicks should come out before the edge is finished.
• If you use a grinder, make short passes and let the blade cool. Heat can ruin the temper of the steel.

You are after a clean, keen edge, not a razor. Mower blades hit sticks and soil, and a finer edge dulls quickly.

Check the balance

An out-of-balance blade shakes the engine and cuts unevenly. Illinois Extension says sharpening kits often come with a cone-shaped balance, and that an unbalanced blade can create a slight wobble that causes strain on a mower and may result in an uneven cut.^[1] You can also hang the blade by its centre hole on a nail in a wall stud. If one end drops, grind a little more from that end's edge until it sits level.

Put it back

Fit the blade the way you marked it, with the cutting edges leading in the direction of rotation and the lifting wing facing up into the deck. Illinois Extension says to use a torque wrench to tighten the bolt to the specifications listed in your mower's user manual.^[1] Reconnect the spark plug wire or battery last, then test the mower on level ground, standing clear.

How often to do it

Virginia Cooperative Extension's lawn guide says to sharpen blades every four to six weeks during the mowing season, and that Zoysiagrass and tall fescue turfs require more frequent sharpening than others.^[3] A lawn that looks frayed or has tan tips a day after mowing is telling you the blade is due.

Stop and call a pro if

• The blade is bent, cracked, or has a deep nick or a worn-through wing. Replace it; do not try to straighten or weld it.
• The bolt will not budge or its head is rounded off. A shop has the tools to free it without damaging the shaft.
• The mower still vibrates after you balance and refit the blade. That points to a bent crankshaft or a damaged spindle on a deck.
• You are not comfortable tipping the mower or holding the blade safely. A shop can do the whole job quickly.

Getting help in Virginia

If you would rather hand it off, read how much mower blade sharpening costs in Virginia and whether to sharpen blades yourself or have them sharpened. Browse all lawn equipment repair answers or see lawn care services services on Loclis.

When you are ready, describe the mower and local small engine shops can quote it.