Lawnmower blades become dull over time due to rocks and other objects the blade may hit. Keeping a sharpened lawnmower blade ensures evenly cut grass. Sharpening your lawnmower blade does not require extensive knowledge or tools. If your blades become dull, you do not need to replace the lawnmower blade. Keeping all grass clippings away from the underside of the lawnmower helps to keep the blades sharp. When working with sharp lawnmower blades, take precaution to avoid injury.
Drain the gas from your lawnmower by carefully pouring it from your gas tank into a container. Disconnect the spark plug wire. Turn your lawnmower upside down on a flat surface.
Wedge the lawnmower blade into an immobile position with your scrap piece of wood. Grasp the other end of the blade with your hand (wearing gloves) and loosen the three centre bolts on your lawnmower blade with your adjustable wrench. If the bolts stick and do not loosen, tap the wrench with your hammer.
Remove the blade from the lawnmower. Inspect the blade for any damage. If your blade is bent, do not attempt to straighten it, as this may cause serious injury. Replace any bent lawnmower blades.
Place the lawnmower blade in your vice with the sharp end facing up. Sharpen the lawnmower blade along the original angle with your medium-rough flat file. File the blade in one direction towards the edge of the blade. File both edges equally.
Verify your lawnmower blade balances horizontally by placing a screwdriver upright in your vice and balance the centre hole of the blade on the tip of the screwdriver. Check the balance one way and then flip the blade over and check the other side of the blade. If your lawnmower blade does not balance evenly, file metal from heavy end while avoiding filing the newly sharpened edge. Continue until your lawnmower blade balances.
Reinstall your lawnmower blade making sure the lift wing (bent piece on each end) point towards the deck of the lawnmower. This allows the lawnmower blade to cut grass properly.
Reattach the spark plug wires and refill the lawnmower with gas.