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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.
Do you think it would be accurate enough to grind jointer knives?
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.
<p>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 .<br>Note : I was starting to set up the &quot; 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 . <br>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 . <br>Duncan <br>Indiana USA </p>
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.
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: &quot; How to SHARPEN&quot; 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.
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!
I would have called it a putty knife, but I was in a home improvement store and saw one for sale.&nbsp; It was simply called a painter's tool.&nbsp; Here is a<a href="http://www.amazon.com/Red-Devil-6251EZ-Multipurpose-Painter/dp/B00004YNM2/ref=sr_1_1?ie=UTF8&amp;s=hi&amp;qid=1274891973&amp;sr=8-1" rel="nofollow"> link</a> to much nicer one at Amazon.com.&nbsp; Thank you for asking.&nbsp; (I checked your profile, but could not tell in what nation you live.&nbsp; I am in the USA.&nbsp; If you are in the USA, I saw them at Lowe's.)<br />
I'm English, but I live in Portugal, so no Lowes, Home Despot or big chains... instead we have small shops nationwide called &quot;drogarias&quot; 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.
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 &quot;accidentally&quot;.&nbsp; Not sure if they even could due to 1) you would have&nbsp; to spin them up fast enough to generate a spark with the magneto and 2) build compression on a compression stroke.&nbsp; <br /> <br /> If you pull the plug wire you will NEVER get one started anyhow so pulling the plug is redundant waste of time.<br /> <br /> This is IF one would ever start by just twisting the blade by hand.&nbsp; Think about how fast the flywheel spins when you pull the start cord.&nbsp; Unless your underneath pulling on the blade like an old airplane I say it would never start.<br />
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.
Thanks for the tip. I have never seen a jig for a lawnmower blade and always used an angle grinder and this is a great way to get more use out of the saw.
Thanks for your comment. A radial arm saw offers a lot of versatility at a much lower price than a Shopsmith and takes very little space. If you have a Sears Craftsman radial arm saw, you may be interested in an Instructable I did to compensate for wear in some of the yoke's indexing holes. See: <a href="https://www.instructables.com/id/Sears_Radial_Arm_Saw_Egg_haped_Indexing_Holes/">https://www.instructables.com/id/Sears_Radial_Arm_Saw_Egg_haped_Indexing_Holes/</a> If you have a Sawsmith radial arm saw, you will be interested in another Instructable I did. See: <a href="https://www.instructables.com/id/Sawsmith_Radial_Arm_Saw_Enlarge_Arbor_Hole_on_a/">https://www.instructables.com/id/Sawsmith_Radial_Arm_Saw_Enlarge_Arbor_Hole_on_a/</a> <br/><br/>I have in mind to do two or three more Instructables related to radial arm saws. Apparently they have lost popularity to compound miter boxes. Still, there are a lot of them around waiting for someone to use them right.<br/>
I also have a power miter and it is great since it is very portable but it is only capable of cutting angles and is limited to narrow lumber as it was designed. The radial arm saw is much more versatile in that it can cut angles in 24" width stock and I have ripped 4X8 sheets of 3/4" plywood. I really like the radial arm saw for cutting the dado and the rabbit for cabinet construction. The table saw is not as fast when cutting a dado or a rabbit and you cannot see the results until your finished and turn over you material. If the blade is out of adjustment or dull and chipping the material it is not noticeable until your finished, so using a test piece is advisable. The radial arm saw has historically intimidated potential users with its open blade design. I will attest to the fact that a table saw is the most dangerous wood cutting machine in the shop and requires extreme attention to safety from the blade and the force of a kickback, which is capable of punching a 2x4 through a chain link fence after flying 10' past your hip. "WARNING" do not stand directly behind your material while pushing it through the table saw blade. I have seen kickbacks that almost killed the user. The radial arm saw has no hidden agenda. I have seen very good radial arms for sale as low as $25 for a barely used Powercraft 10" with a new blade. I bought that one and, many are available for $150. You can buy a new Craftsman for about $400 if you want it in the box ready for assembly.
I do not have a power miter box or a table saw. The few miters I need I cut on my radial arm saw. I did make a miter attachment to make the job faster. I had not thought about problems with dadoes on a table saw. Even with my radial arm saw I avoid standing in line with the blade. I have heard of carbide tipped blades throwing a tooth. I keep my hands away from the blade's path when crosscutting. I have heard the blade can catch and force the motor carriage back toward the operator.
In addition to the things I listed in my first response to your comment, take a look at how to use your radial arm saw in place of a joiner. See: <a href="https://www.instructables.com/id/Joint_Boards_without_a_Jointer_1/">https://www.instructables.com/id/Joint_Boards_without_a_Jointer_1/</a> (if you do any furniture making).<br/>
step 7 doesnt sound too safe how do you stop the blade, with your hand?
I trust you are referring to the side comment I made about checking a blade for balance with an electric drill. Believe me, as soon as you release the trigger switch on the drill, the blade stops. It really does not go around very fast. For one thing, a drill is too underpowered to make it go fast. The second thing is that an out of balance blade will make the drill in your hand gyrate too much to hang onto at very many rpm's. It is all VERY self-limiting. And, I think over the last 30 years I have had to do this only once.

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