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Is it possible for a DIYer to sharpen the blade on their lawn mower?

If so, how would I sharpen the blade on my lawn mower? It is a reasonably basic push mower but it does have a 'mulching' blade.

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"What'cha doin' with that lawn mower blade Karl?" –  Tester101 Jun 29 '11 at 21:23
    
^^^ beat me to it –  oscilatingcretin Dec 25 '12 at 23:09
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7 Answers

up vote 11 down vote accepted
  • Remove the blade by removing the nut that attaches it to the mower.
  • If there is any rust on the blade, remove it with steel wool or a steel brush.
  • Clamp the blade in a vice, or to the edge of a solid work surface.
  • Run a file down the cutting edge, following the same angle already on the blade.
  • If the blade is badly damaged, you will have to use a bench grinder or have it professionally sharpened.
    • Run the blade perpendicular to the grinding wheel to remove any nicks or gouges (this will give you a straight blunt edge).
    • Hold the blade at the proper angle (the angle already on the blade) and grind the length of the blade until it is sharp.
  • Balance the blade (from the center point) on a nail or on a Blade Balancer (~$5) to check that the blade is balanced.
    • If the blade is not balanced, you will have to remove a bit of the material from the heavier side (Do not remove this material from the cutting edge).
  • Now that your blade is sharpened and balanced, reattach it to the mower.

Sharpening the blade more frequently will make the task quicker and easier, since you won't have to grind as much to get a good edge.

If you do sharpen the blade using a power grinder heat from the process can weaken the blade, so you'll want to quench the blade to prevent overheating. dip the blade in water frequently during the sharpening process to cool it, and wipe it dry before you continue grinding.

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And remember step 0: make DAMN sure the lawnmower cannot start up while you are removing the blade. I have a relative that is missing some fingers because in trying to remove the nut he bump-started the motor. Modern machines have better interlocks about this sort of thing, but pulling the spark plug and blocking the blade with something thick are also good precautions. –  Alex Feinman Jul 1 '11 at 15:29
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If you have a workbench with a vice it is pretty easy. The other answers have already covered the basic techniques. I don't own a bench grinder, but I have used a metal file, and a cordless drill with a grinder tip. Both methods worked pretty well for me. Rarely takes me more than 15-20 min (not counting removing and re-attaching the blade).

Just a few comments that weren't covered in the other answers:

  1. The blade should be "butter knife" sharp, not "razor blade" sharp. If you make the edge too sharp, it will curl up as it gets dinged by tiny pebbles. (A brand new blade from the factory is usually only butter knife sharp.)

  2. Don't change the overall angle of the blade edge while sharpening it. The mfr knew what they were doing when they designed the blade.

  3. Make sure your blade is balanced when you are done. If it is off balance, it will hurt the mower.

  4. The big home improvement stores sell a little "kit" for sharpening and balancing your blade that is pretty cheap. All you need is a drill and a vice.

  5. Some curved mulching blades are difficult to sharpen properly. It may be easier to just replace it.

  6. For a small mower, an OEM replacement blade can be found for $10-$20. So, if you are struggling to sharpen your current blade, don't be afraid to buy a new one. If nothing else, having a spare blade or two around can make life easier.

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A bench grinder, or failing that an angle grinder and a bench clamp, are the tools for this job. A dremel will do in a pinch but it is not designed to take off this much material quickly. You usually want about a 40-degree angle on your average knife blade; A mower blade gets a lot more abuse though, so you should probably go up to 45* or even 50*. The important thing is that there is a sharp, even edge along the base of the blade. If it's rounded, it's dull, and avoid taking too much material off the bottom of a mower blade as that will lessen the "vacuum" effect of the blade passing over the grass, making for a more uneven cut.

What's almost as important as a sharp blade is a balanced blade. Nail a box spike into a stud in your garage wall, and whenever you sharpen the blade, when you think you're done, balance the blade on the spike through its center mounting hole. It should be within a couple degrees of level; if not, the mower will vibrate severely at high throttle. In the extreme, you can wear out the drive axle bearings of the motor prematurely, or even bend the drive axle (at which point the lawn mower is garbage).

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A file will do the job just as easily as a power grinder, and it's less likely you will damage the blade while using it. A grinder is great... if you have one, and are familiar with it's operation. If not pick up an inexpensive file for sharpening the blade. –  Tester101 Jun 30 '11 at 12:08
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Here is a quote on the down sides of power grinders for sharpening blades "They [power grinders] will likely remove too much blade quickly, and they usually grind a concave edge, which will become dull sooner and is weaker than a V-shaped edge. The heat produced by a rapidly turning grinder can also weaken the blade. If you choose to use a grinder, dip the blade in water frequently during the sharpening process, and dry it each time before proceeding. This will cool the metal to reduce overheating and weakening."s: –  Tester101 Jun 30 '11 at 12:10
    
I have never seen ANYONE mess around with a file for this purpose if they had any sort of grinding wheel available, whether bench, angle, dremel or drill attachment. Every time, without exception, that I have either taken the blade to a mower repair shop or done it myself, it's been run against a bench grinder and balanced on a nail. Without exception. I agree you must know what you're doing, but it is not difficult or damaging in the slightest if you have any skill whatsoever with power tools. –  KeithS Jul 11 '11 at 15:26
    
You are right most people do use power tools for sharpening lawnmower blades, but I don't think it's safe to assume the OP has "any skill whatsoever" with power tools. It's safer to give all the options, and let the OP decide what they are most comfortable with. Of course a shop is going to use a grinder, it's faster. And if the blade dulls faster that's more money in their pocket when you come back to have it sharpened again. I was not saying your answer was wrong, just pointing out some of the possible problems that might be encountered with power tools. –  Tester101 Jul 11 '11 at 15:38
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I have even used a course grit sandpaper to give it a quick sharpening. This doesn't work for big nicks and dings, but does ok just to clean up the edge.

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There is certainly an optimal angle, but close is good enough. Just use a dremel or a file. If you keep an edge on the blade, the mower runs so much smoother and it is better for your lawn.

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You can touch up most wear with this: http://www.amazon.com/Dremel-675-Garden-Sharpener-Attachment/dp/B00008Z9ZR It will go through the grinding stones fairly quickly, so buy a couple of spares.

Also note that if it's been a while since the last sharpening, this tool won't do large gouges - for that, you'll need a bench grinder. And as @KeithS says, making sure the blade stays balanced is important.

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A file is quick and effective. Push the file (or pull from the non-handle end) towards the edge of the beveled side of the LM blade. Do this until it is bright and shiny.

This will have created a burr on the flat side of the blade. So remove the burr by pushing the file on the flat side of LM blade towards the blade. Keep the file as flat as possible.

If you do this every few times that you cut your lawn, you can keep the blade sharp, and it only takes 5 minutes.

Been doing this for 25 years. Trust me, it's the best way with the least effort.

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