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Dull mower blades tear grass rather than cutting it cleanly, leaving ragged edges that turn brown and invite disease. Sharpening your blades is one of the simplest ways to improve your lawn’s appearance and health. Here’s how to do it properly.
Why Sharp Blades Matter
A sharp blade slices grass cleanly, like scissors through paper. The cut heals quickly and the grass tip stays green. A dull blade rips and tears, leaving shredded ends that turn brown and create entry points for fungal disease.
Look closely at freshly mowed grass. Clean cuts show a neat, even edge. Torn grass has frayed, ragged tips that discolour within a day or two. The difference is obvious once you know what to look for.
Dull blades also make your mower work harder. The engine strains to tear through grass rather than slicing it, using more fuel and causing faster wear.
Signs Your Blades Need Sharpening
Several signs indicate it’s time to sharpen.
Brown or white tips appearing a day after mowing suggest torn rather than cut grass. Healthy grass should stay green after cutting.
Uneven cut quality, with some areas looking ragged while others are clean, indicates an uneven or damaged blade.
The mower feels like it’s working harder than usual, bogging down in grass it normally handles easily.
Visual inspection of the blade shows nicks, dents, or a rounded cutting edge rather than a sharp one.
As a general rule, sharpen blades at least twice per season, more often if you mow frequently or hit debris regularly.
Safety First
Before working on any mower blade, take essential safety precautions.
For petrol mowers, disconnect the spark plug lead to prevent accidental starting. Tip the mower with the air filter side up to prevent oil flooding the carburettor.
For electric mowers, unplug from the mains or remove the battery entirely.
Wear heavy gloves when handling blades. Even dull blades can cause serious cuts.
Removing the Blade
Most rotary mower blades are held by a single central bolt. You’ll need a socket wrench or spanner that fits, typically 13mm to 19mm depending on the mower.
Wedge a block of wood between the blade and the deck to stop the blade rotating while you loosen the bolt. Turn anti-clockwise to loosen.
Before removing, mark which side of the blade faces down. Installing upside-down means the cutting edge faces the wrong way.
Note any washers or spacers and their order for reassembly.
Sharpening by Hand
A metal file is the simplest sharpening tool and gives excellent results with practice.
Secure the blade in a vice or clamp it firmly to a workbench. Work on the cutting edge, which is the angled face on one side of the blade.
File in one direction only, following the original angle of the cutting edge, typically about 40-45 degrees. Push the file away from your body along the edge, lift, and repeat.
Remove the same amount of metal from each end of the blade to maintain balance. Count your strokes and match them on each side.
You’re aiming for sharp, not razor sharp. A mower blade should have a butter knife edge rather than a scalpel edge. Too sharp and it dulls quickly; too dull and it tears grass.
Using a Bench Grinder
A bench grinder is faster than hand filing but requires more care to avoid overheating or removing too much metal.
Use light pressure and keep the blade moving across the wheel. Don’t hold it in one spot, which causes overheating and can destroy the blade’s temper.
If the metal turns blue or black from heat, you’ve removed the hardness and the edge won’t hold. That section needs grinding back to fresh metal.
Dip the blade in water frequently to keep it cool.
Match the original cutting angle and remove equal amounts from each end for balance.
Checking Balance
An unbalanced blade vibrates, damaging mower bearings and producing an uneven cut. Always check balance after sharpening.
The simplest method uses a nail in the wall. Hang the blade by its centre hole on the nail. If one end drops, that end is heavier and needs more metal removed.
Dedicated blade balancers are inexpensive and more accurate than the nail method. They’re worth buying if you sharpen regularly.
Remove metal from the heavy end by filing the back of the blade (not the cutting edge) until it balances level.
When to Replace Rather Than Sharpen
Some damage is beyond sharpening.
Deep nicks or gouges that can’t be filed out without removing excessive metal mean replacement time. Small nicks can be filed out, but large chunks missing compromise the blade.
Cracks anywhere on the blade are dangerous. A cracked blade can shatter at high speed. Replace immediately.
Significant bending can sometimes be straightened, but a blade that’s been bent badly is usually weakened and should be replaced.
Excessive wear from repeated sharpening eventually thins the blade to the point where it’s no longer effective. If the cutting edge is noticeably narrower than original, replace the blade.
Reinstalling the Blade
Install with the cutting edge facing the correct direction, spinning into the grass (the angled edge leads).
Replace any washers or spacers in their original positions.
Tighten the bolt firmly. Check your mower manual for the specified torque if you have a torque wrench, typically around 35-50 Nm for most mowers.
Reconnect the spark plug or battery before testing.
Professional Sharpening
If DIY sharpening doesn’t appeal, many garden centres, hardware stores, and mobile mower services offer blade sharpening for a modest fee, typically £5-15.
Professional sharpening is worth considering for cylinder mowers, which have multiple blades requiring precise adjustment that’s difficult to achieve at home.
Having a spare blade allows you to swap immediately and get the dull one sharpened at your convenience rather than waiting.
Maintenance Between Sharpenings
Good practice extends the time between sharpenings.
Clear the lawn of stones, sticks, and debris before mowing. Hitting objects causes the nicks and damage that dull blades quickly.
Avoid mowing wet grass when possible. Wet grass clogs the deck and forces blades to work harder.
Clean the underside of the mower deck regularly. Built-up grass traps moisture and promotes corrosion that can pit the blade surface.
Sharpening Schedule
For typical domestic use, sharpen at the start of the season and again mid-summer. Heavy users or those with large lawns may need more frequent sharpening.
Inspect blades monthly during the mowing season. Catching dulling early means less work to restore the edge and better results between sharpenings.
Sharp blades combined with proper mowing height and frequency produce the best results for your lawn.
Sharp blades cut cleanly, but healthy grass looks best. Our Year Round Bundle provides the right nutrition in every season, keeping your lawn in peak condition to show off that perfect cut.
