Keep Your Mower Blade Sharp

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Introduction: Keep Your Mower Blade Sharp

About: I miss the days when magazines like Popular Mechanics had all sorts of DIY projects for making and repairing just about everything. I am enjoying posting things I have learned and done since I got my first ...

Soon it will be time to mow the lawn regularly. A sharp blade on your rotary mower works so much better than a dull and nicked blade. This blade is freshly sharpened. The light/shadow contrast near the end of the blade gives the best view of the new edge in this photo.

Step 1: My Tool of Choice

I get a lot of use from my radial arm saw, including using it as a grinder to sharpen my mower blade.

Step 2: Using a Radial Arm Saw As a Grinder

Remove the blade and its guard. Turn the motor 180 degrees and lock it in position. Put a coarse to medium grit grinding wheel on the arbor.

Step 3: Make a Guide to Support the Blade

A simple upright guide from scraps works very well. The piece clamped to the table is simply glued onto the vertical piece with the "V" cut into it.

Step 4: Align the Guide With the Grinding Wheel

The bottom of the "V" needs to align on center with the grinding wheel. Each side of the "V" is cut at an angle 33 degrees off of the horizontal.

Step 5: Position the Motor and the Guide

The flat part of the blade in the area of its mid-point will ride on the guide. Move the guide so its distance from the grinding wheel allows the blade to slide over the guide without riding up onto a curved section of the blade, and yet the full length of the sharpened portion of the blade can come into contact with the grinding wheel as the blade slides over the guide.

Step 6: Clean the Blade Before Grinding

Scrape the crusted old grass from the sides of the blade so it rides smoothly on the guide.

Step 7: Grinding--part 1

Position the mower blade on the guide and the wheel as shown to sharpen the upper side of the blade. For the best control, grasp the end of the blade with your finger tips to guide it and keep it on the wheel. Only light pressure is needed. Hold the blade firmly down on the guide with the other hand. I like to count the number of passes I make so I can make the same number of passes on both ends. This keeps the blade from getting out of balance. Three or four passes will sharpen most blades, unless they are very dull or badly nicked.

If you suspect your blade is out of balance because someone ground more on one side than the other, you can buy a little cone tool that sets on the tip of a spindle to see how it balances. Although I have one of those, I have not found it completely helpful. I like to put a small bolt through the blade's mounting hole and slip a collar onto the bolt so the bolt fits the hole closely. I secure the bolt with washers and a nut. Then I chuck the bolt into a variable speed drill. If the blade is out of balance, it should gyrate as the drill spins the blade. It does not need to spin very fast. Bring the spinning blade near to an old board with paint on it so the end of the blade begins to thumb the board a little. Stop the drill and see which end of the blade has paint on it. Grind a bit of steel from the end of the blade and repeat the drill test until both ends have about the same amount of paint on them.

When using a radial arm saw as I have described, the sparks will fly away from you.

Step 8: Grinding--part 2

When the upper part of the blade looks good on both ends, turn it over and grind a little on the underside. A couple of passes should do the job.

How often should you sharpen the blade? A lot depends on how much mowing you do each week and if your mower picks up small rocks while mowing. If your lawn is not large and the mower does not kick up small rocks, once a season should be enough. If you keep your blade fairly sharp, it does not take long at all to touch it up a little when it begins to dull.

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    27 Comments

    Do you think it would be accurate enough to grind jointer knives?

    2 replies

    After more thought, I doubt one could keep the edge straight enough over its length without some serious adaptation.

    I have never ground joiner knives. You could make the fixture adjustable.

    Ran across this DIY .. getting ready to try it out . Was looking for a quicker way to sharpen the blades as mine get dinged deeply from gravel and I maintain a cemetery .
    Note : I was starting to set up the " V ' jig and lined out the V on a 33 degree angel . But it did not look right compared to the pic in this DIY . I had to reread it a couple of times before I saw the instruction 33 degrees OFF HORIZONTAL . I was marking it off the vertical center that would line up with the grinding wheel .
    As soon as I get the jig up and see if I can and if this system is better for me I will repost with a review .. Thanks for the idea .
    Duncan
    Indiana USA

    1 reply

    My aim was to sharpen mower blades more accurately so they would be less likely to throw the engine out of balance. I am not sure this method is faster.

    Why go through the trouble? A file is faster (considering setup) and does a better job putting an edge on the blade.

    3 replies

    It is really very, very little trouble. I have always found files slow.

    I agree, Phil B. Thanks for posting this. I mow 15 acres with a 60 inch ZTR mower and your method will work very well for me. I must sharpen my blades at least 6 times per season, so I have 6 sets (18 blades) and sharpen several sets at a time so they will be ready when I need them. If I let the blades get too worn before sharpening, they require too much grinding to be practical. Use a file? Forget it. I do have a Sears radial arm saw and I will try it.

    Thank you for looking. Let me know how it works for you.

    Your Title should be: " How to SHARPEN" your mower blade. I read this thinking it was tips on how to prevent the blade from needing to be sharped as often. Which is a fine instructable, just misled by your title.

    1 reply

    Thank you for the suggestion. I am sorry for the misdirection. I keep the blade quite sharp and never really let it become dull, so it seemed an appropriate title for me at the time. But, I can very much see your point.

    That toll is called a 5 in 1 tool. It is used mostly by painters. The half-round cutout is for scraping/cleaning off the wet paint from a paint roller so you can reuse them. The point is used for glazing windows . The rest is self explanatory. Here in the US a good one is made by HYDE tools.

    what is that thing that you are using to clean the blade? It looko ... interersting :-)...and very useful!

    2 replies

    I would have called it a putty knife, but I was in a home improvement store and saw one for sale.  It was simply called a painter's tool.  Here is a link to much nicer one at Amazon.com.  Thank you for asking.  (I checked your profile, but could not tell in what nation you live.  I am in the USA.  If you are in the USA, I saw them at Lowe's.)

    I'm English, but I live in Portugal, so no Lowes, Home Despot or big chains... instead we have small shops nationwide called "drogarias" where you can buy tools, nails and stuff if it can't be found in one place, there's the shop round the corner that might have it...waaay better than chain-stores, better service too they actually try to help, unlike the guy in the chain store who's just trying to sell stuff...forgive me for ranting.. :-)

    I think that to sharp the mower blade is a very easy task, I do it "in situ" (without removing it) with the hand grinder. I stop the blade with a foot, that is sufficient.

    4 replies

    I expect you use a high-speed hand grinder. I assume you mean you hold the blade in place with your foot to keep the shaft turning while you are grinding the cutting edge. To stop a spinning blade with your foot might separate a few toes from your foot. You probably also remove the ignition wire from the spark plug, perhaps remove the spark plug, too, in order to keep the engine from accidentally firing and starting.

    I have dealt with lawn mower engines on the side now for 15+ years and have NEVER seen one start "accidentally".  Not sure if they even could due to 1) you would have  to spin them up fast enough to generate a spark with the magneto and 2) build compression on a compression stroke. 

    If you pull the plug wire you will NEVER get one started anyhow so pulling the plug is redundant waste of time.

    This is IF one would ever start by just twisting the blade by hand.  Think about how fast the flywheel spins when you pull the start cord.  Unless your underneath pulling on the blade like an old airplane I say it would never start.

    My mower is electric, Phil, I take the necessary precautions. The grinder is 11000 RPM or so. I need all my fingers and toes for now. ;)

    I use an angle grinder myself sometimes but I take the blades off my tractor. The angled part of the blade is on the top of the blade. Be a bit tough for me to get a grinder up there. Then sometimes I finish the blades up on a belt sander too.