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One of those "I really must get around to..." questions, but what is the best way to sharpen a lawn mower?

My own lawn mower is a fan blade (i.e. flat blade on a vertical axis), but for maximum usefulness an answer should also include the cylindrical blade types (i.e. traditional transverse axis cutter with helix segment blades wrapped in a cylinder) and any other common types as well.

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Could you please post a photo of a "fan blade". I can add one for a "flat blade" if you wish (just let me know). I think of a photo of each would add to the question... –  Mike Perry Oct 5 '11 at 15:24
    
fan blade/ flat blade are the same thing. It rotates in a horizontal plane. I'm not sure about finding the cylindrical one - I'll try to find one online. –  winwaed Oct 5 '11 at 15:30
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Oops! my bad (too many tabs open at the moment is my poor excuse) I meant "fan blade" (rotary mower) & "cylindrical blade". It's probably just me, but "vertical axis" in "flat blade on a vertical axis" isn't making sense to me at the moment... –  Mike Perry Oct 5 '11 at 15:45
    
I always just pull the blade off the machine and use an angle grinder, maybe there's a better way? Takes maybe 15-30 minutes, depending on how hard it is to get the bolt off. (sigh reading this reminds me that my mower is sorely in need of a new blade... lots of dings from rocks that don't grind out well...) –  bstpierre Oct 5 '11 at 17:04
    
@MikePerry: The axis is the direction it rotates around. A merry-go-round also has a vertical axis. The cylindrical mowers (eg. the old roller types that put stripes in the grass) have a horizontal axis at right angles to the direction of motion. –  winwaed Oct 5 '11 at 22:15
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2 Answers 2

up vote 11 down vote accepted

First let me say, I'm no expert when it comes to sharpening lawnmower blades, I've done it on the odd occasion -- only on rotary mower blades, never on cylinder or reel mower blades.

I do sharpen my own hand-tools, things like wood chisels, plane blades, etc and use sharpening stones to do those, but when it comes to lawnmower blades I've personally found it's more cost effect to pay a "professional" to do it...

Cylinder or Reel mowers

Reel mower NaturCut Classic mower

For cylinder or reel mower blades it's my understanding (and experience) it makes the most sense to take them in and get them sharpened by a "professional" who specialises in lawn tools.

I mainly say the above because even my dad who was a foundryman by trade and done everything himself around the home, would spend his money to get his cylinder lawnmower blades sharpened once a year. Then through the course of the lawn cutting season he would regularly hone the blades himself via a combination of fine metalwork files and sandpaper.

Via a web search I see you can buy reel mower sharpening kits, I honestly have no experience with them (nor do I know anyone who has used such a kit), therefore I can't comment on how good or bad they might be.

I have no affiliation with the below links, am just posting them as examples of what I found on the web:

Rotary mowers

Craftsman lawnmower Troy-Bilt lawnmower

For rotary mower blades you have a number of ways (tools) you can choose to sharpen those blades. Which method you use pretty much comes down to personal choice, each method has pluses and negatives associated with them...

  • Fine metalwork files and sandpaper (old school).

    • Personally I think this method is best used for regular honing ie Keeping the blade edge sharp through the lawn cutting season.
  • Handheld sharpening stones.

    • Again probably best used for regular honing...
  • Handheld angle grinder and a fine metalwork file (for removing burrs).

    • Takes a steady hand and more experience (skill, a light touch) than other methods to achieve good results -- mainly because it's very easy to remove too much material quickly...

    • Personally I think this method is best reserved for blades that have (large) nicks in them, use the grinder to remove those nicks, then finish the job, hone the blade edge, with a fine metalwork file (or handheld sharpening stone) and sandpaper.

    • Also keep in mind, the heat generated by the action of the grinding disc can have a negative effect on the blade itself...

  • Bench-top grinder and a fine metalwork file (for removing burrs).

    • Very similar to the above handheld angle grinder method in its pros and cons. That said, I believe it's the chosen method used by most "professionals" that offer lawn blade sharpening services.
  • Specialist lawnmower blade sharpening attachments, available for a variety of different power-tools eg (I have no affiliation with the below links...)

Recommended (safety) equipment if doing it yourself

  • Sharpening tools from from one of the above methods (or from a method I haven't listed above).

  • A scrap piece of 2"x4" (50x100) timber, used for wedging the blade in place when removing it from the lawnmower.

  • Suitably sized socket rachet or spanner ie Correct size to remove bolt(s) holding the blade on.

    • Optional -- correct size spark-plug socket.
  • Bench-top vise.

    • Or a suitable clamp (eg G-clamp) you can use to clamp the blade to a flat work surface.
  • Blade balancer (basically a small cone) or a large nail/screw driven ⅔ (fixed securely) into a wall.

  • Heavy-duty work gloves.

  • Safety glasses.

  • Handheld (stiff) wire brush.

  • Some sort of cleaning cloth/rag.

  • Can of silicon based spray.

General lawnmower blade sharpening procedure

  • Remove the wire from the spark-plug. Some recommend then removing the spark-plug, personally I don't think it's required to-do-so, but if you want to play it extra safe, remove the spark-plug as well.

  • Now the lawnmower needs to be "laid on its side" so the blade can be removed. The trick I use (prevents petrol/gas from leaking out of the tank), is to tilt the lawnmower backwards and place a heavy toolbox on the handle to prevent the lawnmower from coming back down. See below image (click to enlarge)...

Lawnmower titled backwards

  • Put on the heavy-duty work gloves.

  • With the lawnmower "laid on its side" or tilted back, wedge the scrap piece of timber in between the blade and the cutting deck so the blade is unable to spin.

  • Remove the bolt(s) holding the blade on.

  • Put on the safety glasses.

  • With the handheld (stiff) wire brush clean off as much bits and pieces (gunk) as you can. Then take the cleaning cloth and wipe off the blade.

  • Securely clamp the blade in the bench-top vise or clamp it down to a flat work surface using an appropriate clamp.

    • Obviously the above step isn't required if using the bench-top grinder method of sharpening.
  • Sharpen the blade using your chosen sharpening method. Take your time, remove as little material as possible, do your very best to sharpen the blade to the manufactured angle.

  • Take the fine metalwork file and remove any burrs that might be present on the backside of the blade.

  • When you've finished sharpening the blade (both ends), remove from the vise (or clamp).

  • Before putting the blade back on the lawnmower you need to "balance" it. If the blade isn't balanced:

    • You will get an uneven cut.

    • Vibration of the blade will put excessive wear and tear on your lawnmower.

    • Also a blade that is vibrating is a serious accident waiting to happen.

  • To "balance" the blade, ether place it on the blade balancer or hang it on the large nail/screw, via the bolt hole in the middle of the blade.

    • If the blade is balanced it will not tip one-way or the other.

    • If the blade isn't balanced it will tip down on one side (the heavy side), that side of the blade needs to have a little bit of material removed.

      • Back to your chosen sharpening method to remove a little bit of material from the heavy side.

      • Recheck the blade to see if it's balanced...

  • Once the blade is balanced, lightly spray the whole blade with the silicon based spray, then wipe off any excess with the cleaning cloth.

  • Bolt the blade back in place, making sure you put the blade back the correct way up.

  • Don't forget to reconnect the spark-plug (or put it back in if you took it out).

The below videos may prove helpful/useful:

If you wish to read about my "annual" lawnmower maintenance ritual, you can do so here on SE: What's an organic way to discourage crabgrass from a large “lawn”?


Good luck! and I hope the above proves somewhat helpful/useful...

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Good answer. Just wanted to add that one of the best investments I made for blade install/removal was one of those mower blade clamps. They are perhaps $10 and work quite well on all of my mower decks with the exception of my tractor's deck where it isn't quite big enough to reach to the center blade. In that case, I wedge some wood as you mentioned. –  itsmatt May 24 '13 at 14:45
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In the absence of a better answer, here's what I do on my walk-behind 24" "flat blade" / vertical-axis mower:

  1. Before doing anything else, remove the wire from the spark plug. You don't want it to start accidentally.
  2. Wear heavy gloves.
  3. Turn the machine on its side. Gas tank up, so it doesn't leak. (Ideally, the tank is empty when I do this.)
  4. Wedge a board against the blade so it doesn't turn. Remove the nut holding the blade onto the motor.
  5. Handle carefully, the blade may be sharp enough to cut your hands even before you sharpen it.
  6. Clamp the blade, top side up, onto a sturdy surface (work bench) so one side is hanging off the bench.
  7. I use a 4.5" angle grinder with a general-purpose metal grinding wheel (it works for me, but there may be a more appropriate wheel selection).
  8. Maintain a consistent angle so you get a regular edge. I work from the center to the outer edge, though I don't know that it matters.
    • (My lawn is a little rough, and we use the machine like a bush hog, so it's got tons of dings from rocks, stumps, and sticks. You can get aggressive with the grinding to smooth out the dings, but at some point you've got to call it good enough.)
    • Don't overwork it in any one spot. The edge should be even.
    • You should be able to follow the existing angle, maintaining what's there.
  9. Unclamp, turn 180 degrees so the other side is hanging off the bench, reclamp, sharpen the top side as above.
  10. In sharpening the top, you've created a burr along the bottom edge. You want to remove it. Unclamp, flip upside down. Holding the grinder horizontally, make a quick pass from center to edge, just to remove the burr. Unclamp, flip 180 degrees, repeat on the other side.
  11. Securely bolt it back onto the mower. You don't want the blade flying off while you're mowing!
  12. Replace the spark plug wire.
  13. Test for sharpness by making a pass over part of your lawn. Examine the grass blades -- they should be cut, not torn. The quality of the cut should be even across the entire width of the mower.

Some thoughts:

  • I've heard it can be done with a belt sander. As jmusser mentions in a comment, it could be done with a bench grinder. I've never used either method. (Separate answers on this question for either technique would be great.)
  • If you have a riding mower with multiple blades, you'll want to repeat the sharpening on the secondary blade.
  • If you have a riding mower, you can't just flip it on its side to remove the blades! Carefully jack up the machine, and support it with proper blocks when working underneath.
  • Mulching blades have contours designed to recut the grass so it ends up in tiny pieces. My technique probably leaves something to be desired with respect to the mulching blade, but it makes a better cut after I sharpen, so I go with it. Suggestions for improvement are certainly welcome!
  • While you've got it flipped/jacked, and your hands are already dirty, inspect the underside of the machine. Belts worn? Parts missing/broken? Oh, and hey, check the oil. Was the spark plug fouled when you pulled off the wire? Should you clean the air filter? Y'know... maintenance!
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Very comprehensive! I would think a belt sander would wear out quickly. The bench grinder would need care - e.g. heavy gloves to hold the blade with. –  winwaed Oct 6 '11 at 12:49
    
I'm guessing that if your blade isn't too worn, a belt sander would do ok -- you wouldn't need to grind off much. But IMO an angle grinder is the ideal tool. –  bstpierre Oct 6 '11 at 14:27
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