How to Sharpen a Lawnmower Blade?

Only when the blade is properly sharp will you get a clean cut when mowing the lawn. How to sharpen the blade of your rotary mower yourself.

Like any tool, a lawnmower needs care and maintenance. The heart of it needs special attention – the blade. A sharp, fast-rotating lawnmower blade precisely cuts the tips of grass blades and leaves an even cut. Frequent use and inevitably running over sticks or stones wear down the metal of the lawnmower blade and gradually dull the blades. As a result, the grass is no longer cut neatly, but chopped off raggedly, leaving severely ragged interfaces. They dry out, turn unsightly gray and form an entry point for diseases.

So the cut pattern is a good indicator of when the knives need a new sharpening. As a rule of thumb, you should sharpen them at least once per season – preferably before the new season begins.

Sharpen lawn mower blades yourself: Steps in brief

• Remove blade and clean roughly.
• Fix the blade bar in a vice.
• With coarse file remove old burrs, sharpen the new cutting edge.
• Rework cut edges with a fine file.
• Make sure that the knife remains balanced.

Have lawnmower blades sharpened by a professional

If you don’t feel confident sharpening the lawnmower blade yourself, you can simply take the entire lawnmower to a professional workshop for maintenance – the sharpening or replacement of the blade is usually included. Alternatively, a professional sharpener can be hired: scissor and blade sharpening shops, tool manufacturers, and also DIY and garden centers offer sharpening services for a small fee. In this case, however, you will have to install and remove the lawnmower blade yourself.

How to sharpen a lawnmower blade yourself?

If you have a little practice and the appropriate tools, you can sharpen the lawn mower blade yourself. Unlike kitchen knives, the mower’s coarse blades are not very sensitive and do not need to be sharpened to razor sharpness. Straightening the cutting surface and restoring the cutting angle is quite sufficient here. Compared to household knives, the steel of the lawnmower knife is rather soft so that it does not splinter when it hits a stone. Therefore, the knife can be easily sharpened by hand. Deep notches in the cutting edge, which entail such small accidents, do not need to be completely removed.

Caution: Self-sharpening usually also voids the manufacturer’s warranty on the knives.

However, this is only effective for a very short time on wear parts anyway. Nevertheless, with new lawnmowers, first read the warranty conditions before you do anything yourself!

Preparing and removing the blade

If you have decided to sharpen your lawnmower blade yourself, you must first remove it from the mower. It usually sits directly on the crankshaft on older walk-behind rotary mowers. Newer, well-equipped lawn mowers today have a blade clutch. It decouples the blade from the drive and only establishes the power connection when the corresponding lever on the guide bar is pulled and held. In any case, make sure that the device cannot start accidentally by turning the blade. Therefore, for gasoline mowers, you must first disconnect the spark plug connector. Electric mowers must be disconnected from the mains and, in the case of battery mowers, the battery must be removed. Then carefully lay the lawnmower on its side.

Caution: Always store gasoline lawn mowers with the air filter facing upward to prevent gasoline or oil from leaking in, and place a thick piece of corrugated cardboard underneath in case fuel does leak out.

Then, wearing protective gloves, loosen the blade bar from the mower according to the manufacturer’s instructions. As a rule, the screws have a right-hand thread, which means that they are loosened counterclockwise.

However, there are also manufacturers who use screws with a left-hand thread – so take a look at the instructions for use beforehand. Stuck bolts can be loosened with a little penetrating oil, which is best left to soak in overnight, and a few careful blows with a hammer on the bolt head – never hit too hard, otherwise, the crankshaft bearings or the blade clutch could be damaged.

Tip: It is best to use a box wrench or ratchet with a matching socket to loosen the retaining screw (screws).

An open-end wrench slips off easily, which can lead to injuries.

Tip: To prevent the crankshaft from turning when loosening the screws, it is best to wedge the cutter end to the inner housing wall with a suitable hardwood wedge.

Also, be sure to remove and store any washers so that they can be replaced in the proper sequence after sharpening the lawnmower blade.

What tools do you need for sharpening?

To sharpen a lawnmower blade yourself, you will definitely need a vice in which you can securely fix the blade bar. This way you will avoid injuries and you will be able to adjust the sharpening angle well. For sharpening itself, experts recommend hand files of various thicknesses, because with them you can get an exact grind. Inexperienced sharpeners, in particular, should definitely use a hand file, as the sharpening process is slow and controlled and mistakes can be corrected more easily. Professionals additionally process the lawnmower blade after rough grinding with a grinding file, which makes more precise work possible. Perfect sharpness is then achieved with a whetstone.

If you are afraid of strenuous manual work or have an extremely worn blade in front of you, you can also use power tools. A wet grinding machine with a clamping device and speed control is the first choice here. Alternatively, there are grinding finger attachments for various multitool devices, but they require some practice. An angle grinder is unsuitable for sharpening lawn mower blades. It cannot be guided precisely, removes a lot of material at once, and heats the blade very strongly in the process due to the high speeds. Excessive temperatures cause the soft steel to “burn”: it then turns black and loses its elasticity. We do not recommend dry, fast-rotating whetstones because they also generate a lot of heat.

Sharpening lawn mower blades: Step by step

After removal, the lawnmower blade should first be roughly cleaned so that the cutting surfaces can be easily identified. Then clamp the blade bar horizontally in the vice with the blades pointing upwards.

Caution: Lawnmower blades may only be sharpened from above, the underside remains untreated.

Maintain the specified angle as closely as possible when sharpening. Now use a coarse file to remove old burrs and other damage and sharpen a new cutting edge into the sides of the blade. Use a finer file or a grinding file to finish the cut edges.

Make sure that the same amount of material is removed from the left and right sides of the knife bar so that the knife remains balanced. You can check this by placing the knife bar with the center hole on a screwdriver or centered on a mandrel or small elevation. If the blade bar leans to one side, a little more material needs to be removed on that side. If the lawnmower blade is out of balance after sharpening, an imbalance will occur during subsequent mowing due to the high speed, which can lead to increased wear on the crankshaft bearings. In addition, the device vibrates very strongly.

When both sides are sharp again and the blade is balanced, the cutting edges are honed away from the cutting edge with a grindstone. Finally, the blade can be cleaned of rust stains with a wire brush. Reinstall the lawnmower blade in the mower with the wings facing up and the flat, unsharpened side down.