How to sharpen a lawn mower blade

Reader Bart C writes…

Are you or have you done an article on how to properly sharpen a lawn mower blade?

We have not. But we will… right now. Thanks Bart.

OK. So there are two components to getting this particular job done right: Sharpening and balancing.

So let’s take a look at the whole process from start to finish. And for simplicity’s sake, we’ll be referring to a walk behind mower.

TOOLS REQUIRED FOR THE JOB:

For this job, you’ll need the following tools and equipment:

Safety Glasses

Work Gloves


A 12″ flat metal file or hand grinder

A Blade Removal Tool, Bench Vice, or two adjustable C clamps.

A well lit, sturdy work surface.

tools

A standard socket or wrench set

REMOVING THE BLADE

dangerBefore you put your hands on that blade, I want you to disconnect the spark plug wire! You start twisting that blade around and you’re gonna start the engine like one of those old WWI fighter planes.

Jpeg

Once the spark plug is disconnected, put on your gloves and lets take a look at the blade.

Most folks tip the mower over on its side. That’s fine. But what I like to do is to angle the mower backwards, keeping the rear wheels on the ground and the spark plug pointing upward.

And, if you can place the mower on an elevated surface, such as a work bench; you may find it even easier to accomplish this particular task.

mower access

When the mower is tipped backwards, secure it in place.

Your blade will be held on with one or more bolts (depending on the brand of mower).

Jpeg

It’s important to have the correct size socket to remove the bolts. Stripped out bolts are a pain to remove and can be costly to replace if you have to have a technician remove it.

no woodNote: I’ve seen folks jamming a block of wood into the discharge port to “secure” the blade. Folks, I don’t like it and I’ll tell you why.

First off, if you’ve disconnected the spark plug, the engine will not start. Second, if you do end up wrestling with the blade bolt, you could end up bending the blade or damaging the blade adapter.

If you need to prevent the blade from turning, consider holding onto it with your free hand. If you’re not comfortable with that, or if the blade bolt is being stubborn, consider picking up a snifty blade removal tool or perhaps applying a couple of c-clamps (below) to either side of the blade to prevent if from moving.

DECK UNDERSIDE

Remove the bolt(s) the blade and the blade adapter if applicable. A good way to avoid misplacing the bolts is to thread them back in their respective holes.

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INSPECTING THE BLADE

To conduct an effective inspection, you may find it necessary to clean the blade with a wire brush. Inspect the blade for wear and damage.

Here’s what you might find:

Nicks: Nicks are to be expected and can be dealt with when sharpening the blade. It’s the deep ones with the little cracks running from the nick that mean its time to get a new blade. If you’re not sure, take a plain, old number two pencil and color in around the nick. Now wipe off the graphite and lets see if any cracks stand out.

Bends: If the blade is bent, you’ve hit something “hard”. Like, real hard. The true hazard lies not in the fact that the blade has bent but rather the possibility that something else may have bent as well. Something like the engines crankshaft. If the blade is bent, replace it, but have the crankshaft inspected before using the mower again.

Jpeg

Breaks: If you’ve hit something hard enough to break the blade…dang. If you’re not going to dig up whatever it is you keep hitting, then get some blaze orange spray paint and mark it! If the blade is broken…anywhere, replace it. And you should probably get that crankshaft inspected before starting the mower again.

Jpeg

Cracks or Fissures: These can result from erosion or weaknesses in the steel. Any signs that the integrity of the metal has been compromised means replace the blade. Believe me, you don’t want that metal flying off of the blade at 3000 rpms.

Erosion:  The whole reason for needing to sharpen the blade is erosion. And the level of erosion may differ greatly depending on the type of soil your grass grows out of. Sandy soil? Hang it up, chief. You’re poor blades never stood a chance. But when we talk about erosion here, what we’re really looking at is not the cutting edge, but the area of the blade opposite of it. As particulates pass over the surface of the blade, they begin to wear down and thin out the steel behind the edge. Slowly, but surely the metal will thin and disappear. And by disappear, I mean fly away like tiny little throwing stars. Any sign of rear blade erosion and its time to replace the blade.

Lawn Mower Blade Erosion

Worn Lawn Mower Blade

SHARPENING TOOLS

Hand File

Flat Hand File

A hand file works, but it can be time consuming. And like the hand grinder, there is a knack to getting it right. But some folks swear by it and that’s that. So, file on..”filer”. Don’t let anyone stand in your way!

But the fastest way to sharpen a lawn mower blade is with a bench grinder.

Bench Grinder

Bench Grinder

If you have a bench grinder, you’ve just saved yourself some time. Once again, with the progression of the grinding wheel moving against the edge, pass the blade evenly and slowly across the wheel. Once again, take your time.

Hand Grinder

Hand Grinder

Hand Grinder in Action

My personal favorite is the hand grinder. It’s fast, powerful and there’s a bit of an art to using it. Plus, all those sparks flying all over the place is just plain cool. Not everyone can use the hand grinder effectively; it takes practice.

SHARPENING YOUR BLADE: A PRIMER

The majority of cutting is done with the outer inch of the blade. Because of this, a curve will eventually form on the cutting corner. You can grind the edge of the blade flat, but you’ll be greatly reducing the life expectancy of the blade. Best to keep the blade sharp as possible and monitor the deterioration of the outer edge. When we start looking like a butter knife, it’s time to replace it.

First, we want to secure the blade in place. If you’re fortunate enough to have a bench vice, lock it in there at an angle that allows you complete access to the cutting edge.

Lawn Mower Blade in Vise

Bench Vice

If you don’t have a bench vice, you may consider locking the blade to the work bench with adjustable “C” Clamps. It’s always a good idea to use a nail or bolt to hold the center hole in place as well. This will help prevent slipping.

bench blade clamp

Using “C” Clamps

Working towards the edge, you’ll want to take your time. Try to match the original angle of the edge (if there’s enough of the original edge left to match).

Blade on Bench Grinder

Remember, we’re not going for a razors edge here. When it comes to how much to remove: less is more. The thinner the edge, the faster the blade will wear out.

CLEANING UP THE EDGE

Grinders will often leave a thin, curled “bur line” on the back edge of the blade.

blade slag file

The good news is that its easy to remove. Your flat file is a great way to knock the bur off.

LAWN MOWER BLADE BALANCING:

Installing an imbalanced blade will very quickly reveal which parts of your mower / engine are weak and ready to break when it starts shimmying and shaking like a teeny-bopper. So, before we put the blade back on the mower, we want to make sure that its balanced.

Simple

Lawnmowerpros.com offers affordable blade balancers, like the 
OREGON 42-100 BLADE BALANCER
. This handy little device will help ensure that your blade is balanced before re-mounting it to your mower.

 

 

 

New Fangled

wall mounted blade balancer

We encourage you to go to lawnmowerpros.com and order a blade balancer. This nifty little guy will mount right there on the wall of your shop or sit right there on your work bench. You just set the conical point in the center hole. The magnets grab hold. If the blade stands firm, it’s balanced.

If not, it’s back to…the..to the (snicker)…to the grind stone.

HA!

Nailed it!

Basically, you just grind a little more off of the heavy side until they match. No science to it. Just trial and error.

Old School

Get yourself a big old nail and drive it into your wall. Hang the blade on it and see if it tips. If it does, grind grind again. If not, we’re ready to re-install it on your mower.

RE-INSTALLING YOUR MOWR BLADE(S)

Do what you did earlier in reverse. But before you bolt that bad boy back on, this is a great time to take a look at the state of your under carriage. I’m talking grass build up here, kids and it ain’t pretty.

So, maybe we run on over to the tool box? Yeah? Maybe…we grab the old paint scraper or putty knife…hmm? Maybe we go knock some of that nasty old grass and stuff off of the deck…mmm? Maybe?

Sure.

So.. sharpening your blade. One of the few, simple things to do to extend the lifetime of your mower and the health of your lawn.

Easy, peasy…try not to sneezy!

Until next time, stay sharp!

-Bill

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Bill Brown

Bill Brown

Bill is the head of content creation for the LawnMowerPros Blog and DIY section. He’s been in the Outdoor Power Equipment Industry for years and he’s still learning new things everyday. You can often find him creating featured articles, DIY guides, videos, graphics and much more.