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I once read that lawnmower blades should be sharpened approximately after 25 hours of use. (Of course this can obviously vary and the 25 hours isn't universal truth but rather a rough approximation.)

What is the primary reason this sharpening should be done?

Is it because the lawnmower blade is expected to hit numerous rocks during those 25 hours, and the cutting angle of the blade gets dented quite many times, and the sharpening is merely dent removing (by removing enough material to create a new cutting angle), i.e. different than sharpening of a knife?

Or is it because the grass the lawnmower blade hits somehow makes the blade less sharp? I find this somewhat hard to believe for two reasons. Firstly, because grass is so soft and steel is so sharp. Secondly, because I felt the cutting angle of my new lawnmower and it doesn't feel anything like a knife would feel, it's not sharp at all, it's very dull and could never damage your fingers at rest (but spinning at 2800 rpm it could do quite nasty damage to your fingers).

So let's suppose I use my lawnmower only in grass areas that I know contain absolutely no rocks. I set the lawnmower cutting height to the highest possible value to avoid hitting anything other than grass. Let's suppose I succeed in this and the lawnmower blade only hits grass during its lifetime, not anything else. Would the lawnmower blade still require sharpening after 25 hours of use?

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  • Do your mower blades not rust?
    – Chenmunka
    Sep 6, 2022 at 16:32
  • With electric lawnmowers, paeticularly need to sharpen Sep 7, 2022 at 11:35

2 Answers 2

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Grass, itself, dulls cutting edges, without hitting any rocks. If you hit rocks, you should sharpen sooner, and take measures to avoid hitting the same rock ever again.

One reason is that grass has "rock" in itself. This article claims 2-5% silica (quartz) content in typical grasses. Silica (Mohs 7) is harder than steel (Mohs 4-6.5, and the 6.5 {file hardness} would shatter as a mower blade.)

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As mentioned in the earlier answer, any mower use, even in long grass without rocks, will dull or blunt the blades. If you do as you suggest, then you may not need to sharpen the blades every 25 hours, instead every 50 or 100 or 200 hours.

Worth noting that sharper mower blades will slice through vegetation, such as grass, with greater ease, which has the following benefits…

Blades

  • less friction / resistance, especially in thicker grass,
  • more consistent cutting speed,

Motor

  • less strain on the motor,
  • prolonged motor life,
  • lower fuel consumption.

All these benefits translate to less money spent by you and less time spent mowing.

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