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How to sharpen a lawnmower blade

by Jackson White

It is important to always have the best tools at your disposal when you are going out to work – and for a lawn owner, it is essential to keep your lawnmower blades sharp and in optimal condition.

As a general rule, you should sharpen your lawnmower blades at least once every year, though this may seem unnecessary to some homeowners. You might see it that way too – after all, the blades will still cut efficiently when they are slightly dull, right?

However, this is not actually the case, because dull blades are a bad idea to use on your grass. They will actually have an uneven and rough result when you use them, which leave behind ragged grass blades that are more vulnerable to pests and diseases. However, sharp blades will cut the grass cleanly and evenly, which leaves your lawn looking healthier and more attractive.

The good news is that sharpening your blades is not a difficult task, as there are a few ways you can do it. These include using a rotary tool, bench grinder, angle grinder, or hand file. These methods are great, but an even faster method is using a sharpening stone and drill to bring some life back to your blades.

Preparation

Disconnect the power source and spark plug

The last thing you want to happen is injuring yourself while sharpening your blades, so make sure you start by disconnecting its power source and spark plug before you begin the process of disassembling the machine.

You can do this through removing a prominent wire on the engine’s front or side from its metallic mounting. Once you remove this, the engine will not start – because it is the main power source.

When you are handling this, ensure you protect yourself by wearing eye protection and heavy gloves, even after disconnecting the spark plug.

Turn the mower on its side, and make sure the carburetor is facing upwards

Turning the lawnmower on its side will allow you to access the blades. Despite this, you must turn the mower over as carefully as possible, because its construction will lead to engine oil spilling into the air filter and carburetor if you do it carelessly. To avoid this from happening, these components need to face upwards, not downwards.

The air filter and carburetor are usually in a plastic casing on the side of the engine; although you can check the manual if you are unsure. If you need additional protection from spillage, you can transfer the oil into a separate container or run the mower until its gas is over.

Mark the blade side that faces downwards

This is a common mistake that people make when they are sharpening the blade, because they later install it upside down. This will lead to the blades not cutting grass at all, regardless of their sharpness, so ensure you mark the downward-facing side before removing it.

Immobilize the blades, and then loosen the fasting bolt

There are several ways to block any movement of the blades, which include wedging a strong wooden block between the mower deck and blades to help them remain steady; and noting the orientation and placement of any spacer plates or washers as you remove the blades.

Clean the blades of rust and clippings

Once you block the blades and remove them, you should use this chance to clean them up of grime, grass clippings, rust, and so on.

Sharpening

Sharpening by hand

Clamp the blades

Through using a steady clamp or a vise, secure each blade that allows you to sharpen it easily.

Use a file

To sharpen, run a metal file along the blade’s cutting edge. Make sure you move from the inside of the blade to its outer edge, until it produces a clean edge. You can then switch to the other edge.

Follow the factory bevel angle as you sharpen

The usual case is a 40 to 45-degree angle, although this might be different in some cases – check the manual for more details.

Once sharp, reinstall the blade

Spray a sealant/lubricant like WD-40 on the main bolt to prevent rusting, and then reinstall the blades, the washers, then the bolt. Ensure you orient the blade accurately, and use a wrench to tighten the bolt.

Using a machine

Using a bench grinder

This can work if the blade has some minor cracks or dents, or if you do not want to sharpen by hand. An ordinary bench grinder will work, as you only need to move the blade back and forth against the grinder’s wheel.

Using a belt sander

This is another alternative, and it uses the same principle as the grinder. Note though that you will need to invert the sander so that the belt faces upwards.

To prevent overheating, quench the blade

The sharpening process can lead to very intense friction, and very hot blades. The heat is bad news, because it will weaken or warp the blade over time, which makes it important to quench it regularly. As it becomes hotter, dip it in a bucket of water to cool it, and then wipe it dry before continuing to sharpen it.

 

Different types of sharpening lawnmower blades

Method of sharpening
How it works
Hand filing
Easiest of the three, and best for minor sharpening. Use of a flat hand file and you sharpen from the inner to outer edge.
Bench grinder
Fastest method, but tends to produce noise and send sparks into the air.
Sharpening stone and hand drill
Works similar to a hand file, but is more accurate. However, it is difficult for beginners to use.

 

Conclusion

The process of sharpening your blades can seem like a daunting task, but it is important to perform – even more so if you do the mowing tasks regularly.

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