How to Sharpen Lawn Mower Blades & Knives

A rotary lawn mower uses a blade with two cutting edges that spin beneath the mower deck.

Rotary lawn mowers, whether powered by a gasoline or electric engine, use a balanced blade with a sharpened cutting edge across the last 4 to 6 inches of the two leading faces of the blade. Behind the cutting edges, the blade is angled upward, so that after cutting the grass, the grass is propelled upward toward a chute and either out into the yard or into a catcher. When the blade becomes dulled, you can sharpen lawn mower blades and knives using a bench grinder and a simple jig.

Things You'll Need

• Socket set
• Bench grinder with adjustable tool rest
• 6-inch piece of 1-by-1-inch angle iron
• Bucket of cool water

Instructions

1. Turn off the lawn mower and allow the engine to cool.

2. Disconnect the spark plug wire from the spark plug and secure the wire away from the spark plug to prevent contact.

3. Turn the mower on its side, then remove the blade by loosening the bolt in the center of the blade with a socket set.

4. Lift the blade guard over the grinding wheel of a bench grinder out of the way. With the grinder turned off, place the lawn mower blade flat on the tool rest, with one of the cutting edges against the grinding wheel. Loosen the tool rest's set screw and adjust the angle of the tool rest until the cutting edge of the blade is flat against the grinding wheel. Tighten the set screw to lock the tool rest in position.

5. Pull the blade away from the grinder, then turn on the grinder. Put the blade back into position on the tool rest and begin grinding the cutting edge of the blade evenly by sliding it along the tool rest. Grind in two- to three-second increments to avoid overheating the metal. Should the metal become too hot to touch, dip it into a bucket of cool water for a few seconds before returning to grinding. Keep grinding until the edge is sharp and without any nicks in the cutting edge.

6. Turn the blade over and repeat the previous step with the other cutting edge.

7. Place a piece of angle iron on a work table, with the V of the angle iron pointing upward. Center the lawn mower blade onto the angle iron, with the center bolt hole directly over the bend in the angle iron. If both cutting edges are cut evenly, the blade should be balanced and level on the angle iron. If the blade dips to one side, regrind the cutting edge of the heavier side until the blade is balanced.

8. Reattach the mower blade to the mower and tighten the bolt. Set the mower on all four wheels, then reconnect the spark plug wire to the spark plug. Start the mower and make a test cut. The mower should cut cleanly and evenly.

Tips & Warnings

• When using a bench grinder, wear appropriate clothing and safety glasses to protect against flying sparks.

References

• Grounds Maintenance: Sharpening a Lawn Mower Blade