When To Replace or Sharpen Mower Blades?

You could easily argue that the cutting blades are the most important part of any lawnmower. Though you might be surprised to hear that there are many different types of blades used throughout the wide spectrum of lawnmower blades on the market today.

Some you can sharpen yourself three or four times before they simply run out of available material and need to be replaced. Others might need to be completely replaced two or even three times in a single season. Knowing when it’s time to replace your lawnmower’s blades can depend on the mower, the type of blades, and some obvious signs of a problem.

How To Tell When Lawn Mower Blades Are Dull

There are a few obvious signs that your lawnmower blades are dull, dinged or chipped. One of the most salient is when stray strands of uncut grass left behind after you make a pass. Though this could also happen if you are cutting the grass a little bit too fast, or your lawn is a little too wet to be mowed. If your lawn is reasonably dry, and you’re not racing through each pass, then patches of stray grass being left behind likely mean that your blades are dull.

Other Signs Of Dull Lawn Mower Blades

  • Ragged Torn Blades Of Grass
  • Clumps Of Grass Left Behind
  • Section of Grass Are Flattened Or Knocked Down Rather Than Cut
  • Rust On The Surface Of Metal Lawnmower Blades

If you have heavy-duty metal cutting blades, you might be able to sharpen them yourself with a fine-toothed mill file, a rotary tool sharpening bit, or a bench grinder. Though with most metal lawnmower blades you can only do this three or perhaps four times before you simply start to run out of viable material and you need to replace them.

With thinner metal lawnmower blades and plastic lawnmower blades, dullness typically demands replacement. With some mowers like this ignoring, the dull blades could cause excess wear and tear on the lawnmower and potentially damage sensitive internal components over time.

Signs Of Damaged Lawn Mower Blades

Rocks, sticks, and even a small toy accidentally left in the lawn can present a major hazard for even the most robust metal lawnmower blades. When the blades strike these objects they can be dented, dinged, chipped cracked or even fractured. Worse still, in a severe incident, the central drive shaft for the blade can be bent or knocked out of balance.

Sometimes you will feel the effect of a damaged lawnmower blade immediately. This is usually a strange vibration or a distressing noise that happens right after hitting something. In a moment like this you need to immediately stop the lawnmower, as it’s likely a sign that something is damaged or out of balance. Ignoring the problem and continuing to mow could cause severe damage and even injury.

If you have a gasoline-powered lawnmower, you should let it cool down for 10 to 15 minutes before performing a visual inspection of the cutting deck, cutting blade, pulleys, and driveshaft.

How To Safely Inspect Lawn Mower Blades

Once your lawnmower is cooled down, you can use the following steps to safely inspect your lawnmower blades. Remember this is a safety-first situation, and you need to just as careful as you are thorough.

  • Step 1: Put on safety glasses and gloves. Then disconnect the power source, by either unplugging the cord, removing the battery, or disconnecting the spark plug wire. You might also want to drain the fuel tank if your lawnmower has a gas engine.
  • Step 2: Lift a riding lawnmower on a jack or gently lift the push mower’s cutting deck. Make sure it is secure so you can use both your hands.
  • Step 3: Use clean shop rags or paper towels to wipe away any grass clippings or other residue.
  • Step 4: Visually inspect the blade looking for obvious signs of chips, dings, dents, cracks or fractures.
  • Step 5: Gently rotate the cutting blade. Listen for any sounds or changes in orientation that might suggest the drive shaft has been bent or damaged.
  • Step 6: Inspect the cutting deck for dents, rust, and damage. Sometimes the cutting blades can strike a damaged section of the cutting deck causing unforeseen problems.
  • Step 7: Check any pulleys or drive belts to make sure they aren’t cracked, damaged, or frayed. If they are stiff, try lubricating them.

When A Damaged Lawn Mower Blade Needs To Be Replaced

Sometimes it’s hard to tell if a lawnmower blade needs to be replaced or if sharpening away the imperfection will solve the problem. Signs that the blade is damaged beyond repair include:

  • Unnatural noises when the lawnmower cutting blade is engaged
  • A crack in the metal or plastic of the blade
  • A chip in a plastic blade
  • A ding or a chip in a metal blade larger than the diameter or a penny
  • Any piece of material missing from the tip of the blade
  • A crack in a metal or plastic blade

If you notice any of these signs of damage, it’s likely time to completely replace the lawnmower’s blades.

How To Safely Replace Lawn Mower Blades

If you’re careful, removing and replacing damaged or overly dull lawnmower blades is relatively easy. You’ll need to source the correct replacement blades for your lawnmower, which you should be able to find in the owner’s manual or order online. You’ll also need a pipe wrench of a robust box wrench to loosen the nut holding the blade in place.

  • Step 1: Just like in the inspection process, you need to put on safety glasses and gloves
  • Step 2: Disconnect the lawnmower’s power source by disconnecting the spark plug wire, or removing the battery.
  • Step 3: Carefully turn a push mower on its side or lift a riding lawnmower with a sturdy lawn tractor jack. It’s important to make sure the air filter and carburetor are pointed up.
  • Step 4: Use a piece of duct tape, permanent marker or spray paint to mark the top and bottom of the cutting blades. If you need to you can then use this as a reference when you install the new blades.
  • Step 5: Find the nut that connects the cutting blade to the mower deck. Use the correct size wrench to turn it counter-clockwise and loosen until the mower blade comes free.
  • Step 6: Use a dry shop rag or cloth to gently wipe the drive shaft clean. Lightly grease it with a penetrating lubricant like WD 40.
  • Step 7: Fit the replacement mower blade in place and double-check with the original that you have it in the right orientation. Then tights the nut to secure it in place.


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