How to Safely Sharpen Lawn Mower Blades

Sharpening your lawn mower blades regularly is crucial for maintaining a healthy, green lawn. Dull blades tear and shred grass rather than cutting it cleanly. This causes damage to the grass and leaves it more susceptible to disease and pests. Sharpening the blades helps ensure a clean cut and allows the grass to heal properly.

While sharpening the blades is important, it must be done safely to avoid injury. Lawn mower blades are very sharp and spinning them improperly when sharpening can cause serious cuts. Follow these tips to safely sharpen your lawn mower blades and keep your lawn looking pristine.

Materials and Tools Needed

Sharpening lawn mower blades requires just a few essential items:

  • Block of wood – Use a thick block of scrap wood to elevate the mower and keep the blade still while sharpening.
  • Gloves – Wear thick work gloves to protect your hands when handling the blades.
  • Eye protection – Use safety glasses or goggles for eye protection from metal shards.
  • Hammer – Use a regular claw hammer to remove and reinstall the blade bolts.
  • Wrench – An adjustable wrench or appropriate sized socket is needed to hold the blade bolt while loosening and tightening.
  • Screwdriver – A flat head screwdriver can help pry off caps covering the blade bolts.
  • File – Use a 10-12 inch mill bastard file to sharpen the blades.
  • Rag – Have a cleaning rag to wipe off metal filings when complete.

Disconnect the Spark Plug

Before starting any work on the mower, the spark plug must be disconnected to ensure the engine does not start accidentally. Taking this preventative measure eliminates the chance of the blades spinning while your hands are near.

Follow these steps to safely disconnect the spark plug:

  • Locate the spark plug on the engine block. It will likely have an ignition wire connected to it.
  • Use the wrench to remove the nut securing the ignition wire. Pull the wire off the spark plug.
  • Next use the wrench or socket to unscrew the spark plug itself. Completely remove it from the engine block.

With the spark plug detached, the mower cannot start. You may now proceed with safely removing the lawn mower blade for sharpening.

Jack Up the Mower

The next step is safely elevating the mower to access the blades on the underside. Lifting just one end is sufficient to work on the blade.

Here is how to properly jack up the mower:

  • Place the block of wood under the front end of the mower deck near the blade. This serves as your jack stand.
  • Position a sturdy floor jack under the mower near the block of wood. Raise the jack just high enough to lift the mower deck off the ground when the wood block is under it.
  • Once raised, slide the block of wood under the elevated mower deck so it rests securely on top.
  • Lower the jack and remove it from under the mower. The wood block keeps the end safely lifted to access the blade.

With the front end supported on the block, you can now remove the blade for sharpening.

Remove the Lawn Mower Blade

With proper preparation complete, it’s time to get access to the blade for sharpening. Use caution when removing as the blades are extremely sharp. Follow these steps:

  • Locate the blade bolts securing the blade to the undercarriage of the deck. Most mowers use 2-3 bolts per blade. There is often a cover over the bolts that may need prying off with a screwdriver first.
  • Using an adjustable wrench or the appropriately sized socket, hold the bolt stationary while using the claw hammer to turn and loosen the nut. Loosen but do not fully remove the first bolt.
  • Repeat the loosening process on the remaining blade bolts, leaving all still partially threaded in. This prevents the blade from dropping once the final bolt is removed.
  • Now fully unscrew the final bolt and carefully remove the blade from the mower deck. Maintain control of the blade as the weight can cause it to drop once detached.

With the old blade removed, it can now be sharpened or a newly sharpened blade can be reinstalled.

Secure the Blade for Sharpening

Once detached from the mower deck, the blade needs to be properly secured during the sharpening process. There are two main options to keep the blade still and safe:

Use a Vice

  • Clamp the blade upside down in a sturdy vice, using pads or rag to protect the cutting edges.
  • Ensure the vice is tightly secured to a stable workbench.
  • Position the blade so the area to sharpen extends just beyond the edge of the vice jaws.

Use Blocks of Wood

  • Place two thick blocks of wood spaced apart on a flat surface.
  • Lay the blade upside down between the blocks to bridge the gap.
  • The blocks hold the blade steady and elevated for sharpening.

With the blade immobilized by either method, it can now be sharpened safely.

Sharpen the Blade

With preparations complete, it’s time to sharpen the blade. This requires passing a mill bastard file across the cutting edges at the proper angle. Follow these steps:

  • Put on protective gloves and eye wear. Have the rag for cleaning nearby.
  • Take the mill bastard file and place it against one end of the blade edge. Maintain a 25-30° sharpening angle between file and blade.
  • Using steady, even pressure, draw the file forward along the entire length of the blade edge. Apply moderate downward pressure as you file.
  • Lift the file away and reposition it back at the starting point. Repeat the filing motion. Do 8-10 full length passes per blade edge.
  • Flip the blade and repeat the filing on the other cutting edge. Perform 8-10 passes to sharpen the other side.
  • Use the rag to wipe away any metal filings collected on the blade when finished.

Check that both cutting edges feel equally sharp by gently running your gloved finger across them. Repeat additional strokes if needed to obtain a smooth, razor-sharp edge on both sides.

Balance and Inspect the Blade

Prior to reinstalling, the blade must be checked for proper balance and inspected for damage:

  • Balance the newly sharpened blade on a screwdriver shank or use a commercially available balancing tool. Grind off any small excess metal from the heavy side until blade is perfectly balanced.
  • Thoroughly inspect for any cracks, gouges, or deformations in the metal. If present, the blade must be replaced.
  • Ensure the mounting holes are intact and not elongated. Use a gauge pin to verify proper sizing.
  • Confirm that the blade is flat across its entire length with no warping present. Lay on a flat surface and check for gaps under blade.

Only reinstall if the blade checks out perfectly. An unbalanced or damaged blade can break apart when operating at high speed and poses serious danger.

Reinstall the Sharpened Blade

Once properly balanced and inspected, the sharpened blade is ready for reinstallation under the mower deck. Follow these steps:

  • Return the mower deck to the raised position with front end securely on block of wood.
  • Place the sharpened blade onto the mower spindle aligning with screw holes. Take care as the blade edges are now extra sharp. Wearing thick gloves is highly recommended for protection.
  • Insert the blade mounting bolts through the holes and thread them partially in with your fingers first.
  • Finish tightening with wrench or socket, securing each bolt a few turns at a time until snug.
  • Repeat tightening pattern twice more to ensure an evenly torqued fit. Do not overtighten.
  • Reinstall any protective bolt covers that were removed initially.
  • Carefully lower mower deck by removing wood block and jack. Blade change is now complete.

Reconnect Spark Plug and Test

As a final safety precaution, reconnect the spark plug prior to testing the sharpened blade:

  • Insert spark plug into engine block by hand until seated fully.
  • Attach ignition wire to end of spark plug and tighten securing nut using wrench.
  • Pull the starter cord to verify proper operation without blade spin first.
  • If engine starts normally, proceed to engage the blade and perform test run on a paved surface. Ensure the blade does not wobble and cuts evenly while spinning.
  • Turn off, disconnect spark plug wire, and recheck blade bolts for tightness after the test.

With the blade sharpening complete, you can continue enjoying a healthy lawn thanks to cleanly cutting blades. Just remember to regularly inspect and sharpen the blades every few months or as needed to maintain performance. Follow these safe sharpening techniques and you’ll keep your mower cutting like new.

Frequently Asked Questions About Sharpening Lawn Mower Blades

Sharpening lawn mower blades does require caution, but these common FAQs help clarify the process:

How often should you sharpen mower blades?

Ideally lawn mower blades should be sharpened every 25 hours of runtime or minimum once per year. More often is needed with heavy use on thick or sandy grass.

Can I sharpen blades while still on the mower?

It is not advisable to sharpen them while still attached. Removing allows proper securing and awareness of hands at all times near the sharp edges.

What tools are needed to sharpen blades?

A mill bastard file, scrap wood blocks, hammer, wrench, screwdriver, gloves and eye protection are the basic tools needed. A vice or blade balancer are optional extras.

Should I just buy new blades instead?

If damaged or extremely dull, new blades may be needed. But properly sharpened blades provide the same cut as new for much cheaper if still in good condition.

What are signs the blades need sharpening?

Torn, frayed grass tips after mowing or the mower leaving uncut patches are signs of a dull blade needing sharpening.

Can I sharpen lawn mower blades with an angle grinder?

Yes, angle grinders with a grinding disc can sharpen blades. Take care to evenly grind each side while avoiding excess heat.

What is the best sharpening angle for mower blades?

Aim for a 25-30° angle between the file and blade edge when sharpening. Maintaining this angle along the full length of the cutting edge is key.

Should both sides or just one side be sharpened?

Always sharpen both the top side and underside cutting edges of the blade equally. This maintains balance and optimal cut.

Sharpening lawn mower blades periodically is essential for maintaining healthy grass. Follow these safety tips and techniques to properly sharpen dull blades. Be sure to regularly inspect blades and sharpen when needed to keep your lawn looking its best all season long.


Sharpening lawn mower blades is an important regular maintenance task for the health of your lawn. But it must be done safely and properly to avoid potential harm from the sharp spinning blades. This article has provided a comprehensive guide to safely removing, securing, sharpening, balancing, inspecting, reinstalling and testing your mower blades.

The key is taking preventative safety steps such as disconnecting the spark plug, wearing thick gloves and eye protection at all times when handling the blades, and properly immobilizing the blade during sharpening. Maintain the proper 25-30° sharpening angle as you carefully file each blade edge with 8-10 passes of a mill bastard file. Then balance, thoroughly inspect for damage, and reinstall the blade properly before testing.

Follow these tips and you can safely sharpen your lawn mower blades yourself and avoid the cost of replacement or professional sharpening. Your lawn will thank you through the return of clean, healthy cuts from your finely sharpened mower blades. Just be sure to regularly inspect and sharpen every 25 hours of mowing time to maintain that crisp, precision cut all season long.