Sharpening a Lawn Mower Blade




Introduction: Sharpening a Lawn Mower Blade

This instructable will lead you step by step to sharpen a lawn mower blade. It is important to keep your blade sharp, for it decreases the time spent mowing as well as helps with overall lawn care. A dull blade will rip the tops off your grass, leaving it vulnerable to disease, while a sharp blade provides a clean cut that helps the plant recover quickly. Depending on the tools you use and how bad the blade is, this can be done in anywhere from 15 to 40 minutes. Sharpening is a simple task, even for a novice. It may take a few times to master however. Yet by doing this by yourself, you will reduce the time to cut your yard, while providing a better cut, as well as keeping your total maintenance cost down.

Step 1: Gather Materials

These are some materials you may need for this project. Because there are various ways to accomplish this project with different tools read carefully.
• Clamp  (If using a grinder this may not be needed.)
     Table Clamp OR C-Clamp
• Safety Glasses (Really only necessary if using grinder.)
• Ratchet (Or some kind of wrench)
• Screwdriver (Or something to clean the blade.)
• Bolt & Nut (Optional)
• Leather Gloves
• Block of Wood
• File
• Grinder
The file and grinder are not both required. Can be done with one or the other, or both.

Step 2: Get to the Blade

The first step to gain access to the mower's blade.

1. Remove or disconnect the spark plug.
Warning: There is a chance the motor is at the top of its compression stroke and with a little bump could turn over causing the blade to spin under power.

2. Locate the carburetor
The throttle cables run to it. Another reference may be the primer, or little bulb, that you push before you start your mower.

3. Turn the mower on its side with the carburetor up.
Caution: Gas cap could have an air hole in it so you may leak gas.

Step 3: Remove the Blade

1. Get the ratchet and leather gloves and/or the block of wood

2. Grip the blade with your hand or wedge the wood in between the deck and blade
Warning: Removing the blade will require a lot of force which could cut your hands.

3. Remove the nut holding the blade
If you cannot do it with the ratchet your using try a bigger one or a cheater bar (a long bar that goes over the wrench).

4. Mark the bottom of the blade
Some way or another mark the bottom of the blade. If put on the incorrect way it will not function properly or damage the mower

Tip: It is a good idea to put the mower back on its wheels while you sharpen the blade.

Step 4: Assess the Blade

1. Remove built up grass
I use the screwdriver but anything you have around can be used to clean it off.

2. Look over the blade
It is important to look at the blade now, as you may need or be better off just getting a new blade. As seen in the picture you will have some minor nicks as well as some larger ones. If you just have minor nicks and one or two larger nicks you are okay. However, if you have a lot of larger nicks, a crack, or a bent blade you need a new blade.
CAUTION: A cracked or bent blade can break during use and cause severe damage to the mower and possibly injure the operator.

Step 5: Sharpen the Blade

For both methods make sure the stroke of the grinder or file is from the top of the blade to the bottom. This will provide the best cutting edge.

Using a Grinder
1. Put safety glasses on
WARNING: Pieces of metal can possibly fly at your face and eyes

2. Grasp the blade firmly in your hands (or clamp depending on grinder)

3. Align the angle
It is important to keep the angle that is already on the blade. Manufactures have found the angle used to be the most effective for both cutting and keeping an edge.

4. Grind
Take small amounts off at a time. DON'T jam the blade in the grinder or you will affect the tempering (heat treat which makes the blade strong).

Using a File
1. Clamp the blade

2. File
It is important to keep the angle that is already on the blade. Manufactures have found the angle used to be the most effective for both cutting and keeping an edge.
Tip: When using a file, you should preform only one stroke (push OR pull).

You do not need to get every nick out of the blade, or make it razor sharp. Go until most minor nicks are gone and you have decent edge on the blade.

Step 6: Balance the Blade

There are countless ways to accomplish this. There is actually a tool specifically for this, however here are two ways you can do it with stuff you may have on hand.

Balancing with Bolt & Nut

Place bolt through center hole of the blade

2. Secure the nut at least 1/2" up the bolt

3. Place the bolt on its end (shown in the picture)
The smaller the bolt the better the results. However you will need a washer or something to stop the nut from going through the hole. Personally once I get it to stand on end I preform step 4 but it is optional. However if one side droops it is unbalanced.

4. Spin the bolt and blade
Once it is going let go and see if it will stay standing while spinning. If one side droops it is unbalanced. If it doesn't droop the blade is balanced enough.

Balancing with Screwdriver

1. Clamp screwdriver so it overhangs the edge of a table
This can be done with any other straight, round, and narrow object or even a nail in the wall.

2. Place the blade on the screwdriver
Repeat this step a few times as you may not center it each time. If one side droops it is unbalanced.

If the blade is not balanced, then one side is heavier than the other. Therefore you must take a little more material off the heavy side with the grinder or file until it passes the balance test.

Once it is balanced it can be put back onto the mower.

Step 7: Reattach the Blade

1. Place the blade back on the mower
Make sure the mark you made earlier is on the bottom so it is on correctly.

2. Screw nut back onto the bolt

3. Tighten the nut with force
Warning: Blade is now sharp and could easily cut you.
Use the leather gloves or block of wood.
Tip: Most mowers operate so as the mower runs it actually tightens the bolt.

You now have a sharpened blade. The toughest part of this will be either sharpening or balancing depending on the condition of the blade. However this will ensure a better cut for the rest of the mowing season. It is recommended to sharpen the blade at least once a season,but depending on how much you mow you may want to do it more. The more times you do, the less sharpening it will require. The best thing, however, is you didn't pay someone to complete a simple task.

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    6 years ago

    Very useful and practical instructible. I sharpen my blade at least once each season, but have never known how to properly assess the balance. Thanks!


    6 years ago on Step 7

    Very helpful! Wanted to do this but no one I knew, had done it. My one friend said his blade not so much cut but antagonized the blades of grass so much that they volunteered to just bend over. I hadn't known about the safety details nor the balancing. Thank you for this!!


    10 years ago on Step 7

    In my experience (I am only 22 but have been mowing lawns for 16 years) you should never sharpen your blade.
    Lawn mowers are made to work just like strimmers, using the high speed, they swosh the grass of, if you know what I mean.
    My sharpening the blade your are just going to make the blade not last very long, and make it get more nicks in it. The ideal, is a nice rounded edge.

    But that's just my 2 cents.


    Reply 8 years ago on Introduction

    This is about as wrong as wrong can be.


    Reply 9 years ago on Introduction

    You MUST be kidding, MPLangan. Blades must be kept sharp to do a decent mowing job. Nice Instructable Coder08. Thanks for including all the necessary safety tips. A lot of bad things can happen when changing or sharpening a blade. My neighbor cut his hand installing newly sharpened blade & got 12 stitches at the emergency room to close the cut at the base of his thumb. Leather gloves or blade clamp would have prevented it.


    Reply 10 years ago on Introduction

    For proper maintenance of your lawn, a sharp blade is required.A dull blade does not cut the grass, but rather rips the grass. By not providing a clean cut, your lawn is more open to disease, which can lead to a brown yard. A clean cut allows the grass to heal properly, while leaving your lawn looking much nicer.
    The design of the blade is directly related to how a lawn mower operates however. A mulching mower has a different blade than a mower that throws the grass into a bag; however they both have a specific bend that causes lift at high speeds, in order to draw the grass up toward the blade. These bends are on the back of the blade.
    Now if you sharpen the blade incorrectly, you can decrease the life of the blade. However, the initial cutting edge of a blade, when brand new, is the ideal edge. Research has been done that proves that angle provides the best cut to edge retention. Therefore if you sharpen your blade to look like a razor blade, you will get a very nice cut, but the edge will wear quickly, and is much more likely to get nicks in it. While if you go to a more vertical edge it will keep its shape better but not provide as well of a cut.
    So it is good to sharpen your blade as long as it is done properly.


    10 years ago on Step 7

    I must disagree. Look at the resulting cut from a "nice rounded edge".
    The grass is shredded rather than cut cleanly, resulting in bruising and die-back. I'll take a sharp blade any time for lawn grass. If you are just cleaning up a rough area, it probably doesn't matter, but a sharp blade puts less load on the motor.
    Just my 2-cents worth.


    10 years ago on Step 3

    4 words.

    Rub Ber Mal let.

    And don't hit the drive with anything, you'll just break it. Get one of those fancy Crescent wrenches.


    10 years ago on Introduction

    Good set of instructions you have made for changing the blade on a lawn mower, I do the same thing but I also change the oil and clean out the fuel tank and change the air filter when doing a blade sharpening (makes for less mess and the whole mower gets serviced in the process.)
    Again great job on the documentation