How to Sharpen a Lawn Mower Blade

Sharpening the blade on a lawn mower is considered routine maintenance. If a lawn mower blade is dull, the machine will not cut cleanly, leaving the grass ragged and more vulnerable to damage from disease and sunlight.

The first step to sharpen a lawn mower blade is to make sure the engine on the lawn mower cannot start while you are working on the machine. Disconnecting the spark plug wire will ensure that the motor cannot start unintentionally.

Next, remove the blade. To keep the blade stationary while you remove the blade nut, place a block of wood between the mower deck and the blade or use a c-clamp to hold the blade. Remove the blade nut that secures the blade to the lawn mower. If the nut is hard to turn, use penetrating fluid to loosen the bolt and use a wrench to turn the bolt.

Once the blade is free, place it in a clamp or vise to hold it steady while you sharpen it. Using a file, sharpen the edges by moving the file in an upward motion along the edge of the blade, filing with the angle of the blade, not against it. Do not use a sawing motion, and try to file the same number of strokes at the same angle as you move around the blade so that the blade stays in balance. Use the file to clean off any jagged edges, or “burrs,” that have formed on the flat side of the blade, but don’t sharpen that side of the blade.

The blade may still show some nicks after sharpening. A few nicks are acceptable, and the blade will still cut properly, but a blade that shows numerous nicks, even after sharpening, should be replaced.

After sharpening, test the balance of the blade. A blade that is out of balance can cause the lawn mower to vibrate, which can damage the equipment. Inexpensive commercial blade balancers are available from hardware stores, or you can place a screwdriver or a dowel through the blade’s mounting hole and turn the blade. If the blade moves smoothly, it is in balance.

Before remounting the blade, clean off any built-up grass or debris from the bottom of the mower. Replace the blade using gloves to protect against the newly sharp edges, and reconnect the spark plug wire.

If it's time for a replacement, head on over to our lawn mower blades section.