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How to Sharpen Rotary Lawnmower Blades

Posted By Duncan Idaho On September 24, 2009 @ 4:44 pm In Garden & Outdoors | No Comments

Summer’s coming and you know what that means – barbecues, the beach and… weekly lawn mowing. It’s a chore, but it’s gotta be done, so you might as well get the best-looking result you can for your effort.

Sharp Cuts Better

A dull lawnmower blade will tear the tops of your grass and leave a ragged, uneven cut that looks amateurish and damages the grass leaf more than is necessary, making it more susceptible to disease.

For a smooth, bowling-green result you need to sharpen your blade regularly. In summer, with its increased mowing demands, rotary lawnmower blades should be sharpened once a month as the soil and sand in grass will quickly dull them.

Do it Yourself

Sharpening your lawnmower blades is not difficult and will only take 15 minutes of your time. Here’s how:


Disconnect the sparkplug wire – you do not want the mower accidentally starting.


Drain the petrol tank – you’ll be turning the mower on its side and any petrol left in the tank is going to spill all over the mower and your driveway. You’ll see a rubber pipe running from the base of the tank, just disconnect this and bleed the petrol into a bucket.

Removing the Blade

Turn the lawnmower on its side. Rotary lawnmower blades are generally connected to the drive mechanism with a single nut between two guide pins. Take note of the blade’s orientation – which way the cutting edges face and which way up the blade sits – as the blade must be reattached this way.

Undo the nut with a spanner or wrench and remove the blade. Similar to removing a wheel from a car, you may find the blade rotates when you try to undo the nut. To prevent this, wedge the blade with a block of wood.

Tip: if the nut is difficult to move, fitting a length of pipe over the wrench handle will give you extra leverage.

The Sharpening Process

The blade will be dirty and covered with grass cuttings. Clean it with a wire brush and, if you want to go that extra mile, wipe it down with WD-40.

The areas against which to apply your sharpening tool will be immediately obvious as lawnmower blades are generally bevelled on one side only. All you have to do is follow this existing bevel.

Clamp the blade in a vise, take a fine-toothed metal file and file the cutting edge (following the bevel). Use long, smooth strokes and work from the centre out.

When you’ve finished sharpening, check the underside of the blade. Sharpening any metal blade often produces a “feather-edge”, or fine burr of metal, below the edge of the bevel. Remove this with a stroke or two of your file.

Balance is Everything

Next, and this is very important, so do not ignore it, check the balance of your blade. Support it at the centre and check that it lies straight and even. Lawnmower shops actually sell small cone-shaped lugs specifically for this purpose, but any central support will do – the handle on your vise, for instance.

Lawnmower blades spin at high speeds and if unbalanced can damage the drive mechanism and become dangerous. If your blade does not balance evenly, simply file away a little more metal from the heavier side and recheck.

Enjoy Your Lawn

You’re done! Just reattach the blade (making sure it conforms to its original orientation), reattach the sparkplug, fill the tank with gas and get out in that garden. Your newly sharpened lawnmower blade will give you a smooth, professional cut that you admire while the steaks sizzle and the beers pop.

Learn more about sharpening lawnmower blades in this video.

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