How to sharpen lawn mower blades using a bench grinder

Keeping your lawn mower blades in optimal condition is a lot more important than you might think. Regular mowing can put a surprising amount of wear-and-tear on your blades, causing them to dull quickly. Consequently, your lawnmower rips and pulls the blades of grass instead of cutting them cleanly. This results in ragged tears that not only weaken the grass but increase the risk of fungal growth and other plant diseases.[1] This is why it is important to keep your blades reasonably sharp at all times. 

The good news is that blade sharpening is a fairly simple and straightforward task that anyone can learn. Bench grinders are by far the best ways to sharpen lawn mower blades, possibly with a few swipes on a sharpening stone to finish them off. But as you will see in this article, there are quite a few options available for getting your blades back to optimal condition, to keep your lawn looking great.

1. Get a clear working area

Every time you have to sharpen something, always clear out your workspace first. I can’t emphasize this enough. A clean and clear working area will let you work more efficiently and reduce the risk of accidents. 

Make sure you have a flat and level surface to work on. If you are using a grinder, make sure that it is secure and properly supported. 

Put away any flammable materials, including rags, gas cans, and solvents. It is also a good idea to secure loose objects that could get caught in the grinder. This goes for your hair and clothing as well.

2. Remove the blades

The process of removing the blades differs according to the type of lawnmower you have. With ride-on and push lawn mowers, it is advisable to remove the spark plug first. This will ensure that the motor doesn’t start accidentally. 

It is also often necessary to tip the mower over on its side. This prevents the oil and fuel from dripping onto the air filter, so you don’t get a cloud of smoke the next time you start up your lawnmower.

Inserting a short length of 2×4 timber between the blade and the deck this will help keep the blade in place. You can then loosen the bolt easily with a long-handled wrench and remove the blade.

Ride-on mowers

When working with a ride-on mower, it is advisable to jack it up first to have more room to maneuver. A standard car jack should work well in most cases, or you could use a couple of cinder blocks to prop up the front of the mower. 

Once your mower is raised, look for the mounting nut keeping the blade in place. You will have to turn this counterclockwise to remove the blade.

Electric lawn mowers

The process for removing the blades from a push mower is similar. Use the correct size of wrench to turn the bolt counterclockwise to loosen it.  Keep in mind that the blade will probably turn along with the wrench, so put on your work gloves and use your hand to keep the blade in place.

Push mowers

Are you seriously still using one of these? Okay if you only have a very small area, they can be a bit of fun and a bit of a workout! Of course just like any blade they can be sharpened. Just like an electric mower, you can use the same tools like a bench grinder to sharpen the blades. Taking of the blades should be fairly easy, just start undoing the bolts.

3. Get your tools ready

Get all your sharpening equipment together before you get started. Make sure that your bench grinder is positioned securely and won’t wobble while in use. Now would also be a good time to inspect your grinder thoroughly and ensure that it is in proper working condition. 

Pay particular attention to the power cord and the ground terminals. If the cord is frayed or broken, or if the ground terminals show signs of damage, fix or replace them before you use your grinder. 

Get your sharpening stone ready and make sure that it is free of cracks and chips. Have some water handy as well to cool the blade in case it overheats.

Finally, make sure to have the necessary personal protective equipment (PPE) such as safety goggles, earplugs, and work gloves.

4. Sharpen your lawnmower blades

The best way to sharpen lawn mower blades by far is to use a bench grinder and a sharpening stone or file to finish the job. This combination will give you much better results than any single tool.

Using a bench grinder

Bench grinders will help you sharpen your lawn mower blades quicker and easier than any other tool. They are especially useful for restoring heavily-damaged blade that would be impossible or difficult to fix with a file or smoothening stone. 

Bench grinders are better suited for sharpening the blades of rotary push motors. Unlike riding mowers, these are usually light enough to flip over so you can remove the blade easily. 

To sharpen your blade, you will have to hold them against the grinding wheel at the proper angle. You will need to wear earplugs to protect your ears against the loud noise that the grinder will make, and safety goggles and work gloves to protect your eyes and skin against the sparks. 

Ease the blade to the grinding wheel in the direction that the wheel is spinning. This will remove a bit of material with each pass. Keep going until you have sharpened all the dull spots along the cutting edge, and the nicks have been removed. You can then turn the blade over and sharpen the other side. 

The blade doesn’t necessarily have to be razor-sharp. I recommend sharpening it to a .030 inch to .045 inch edge.

Finish with a sharpening file or stone

For the best results, I recommend finishing up with a file or a stone. Think of your grinder as a “broad strokes” tool that gets rid of the big burrs and nicks and fixes the dull edge of your blade. You could then finish up the job with a file or sharpening stone, which provides more precise control. Using a grinder and a file or stone will give you better results than a grinder alone, and it will be much quicker than using just a file.

5. Re-fit your blades

Reattaching your blade is an equally important part of the process. You will first have to make sure that the blade is properly balanced to reduce vibrations that could possibly damage the blade shaft or bearings. 

Balancing lawn mower blades is pretty easy. All you have to do is drive a nail into a stud and place the blade on top of it. If the blade tips and falls, it means that you need to shave a bit more material off the side that tips down. When the blade remains level on the stud, you will know that is balanced. 

When reattaching the blade to your lawnmower, tighten the bolt by hand. Don’t worry about tightening it too much. With most lawnmowers, it is practically impossible to do, so over-tightening is the least of your worries. It is more important to ensure that you don’t under-tighten the bolt, which will result in a loose blade. This, in turn, could throw the engine’s timing off and make it difficult to start the mower.

Other tools that can be used

If you don’t have a bench grinder, there are other options for sharpening your lawnmower blades including:

• Cordless/corded angle grinders  
• Sharpening stones 
• Sharpening files

Corded angle grinder

A corded angle grinder is a good choice for sharpening lawn mower blades. These tools have grinding disks that spin rapidly, sharpening dull blades much more quickly and more efficiently than any metal file or sharpening stone. You do have to exercise more caution when using an angle grinder because the wheels spin so quickly. With these tools, it is easy to take off too much material at one pass, which could ruin your blade. You can avoid this by applying only light pressure and keeping the grinder moving constantly.

Cordless angle grinders

A cordless angle grinder is an even better option for home use, as it does away with the power cord. This means that you can sharpen your blades anywhere you want–even outdoors–which is often a more feasible option when sharpening the blades of a riding lawnmower. Plus, you don’t have to worry about the cord getting the way or having to have a power outlet nearby. 

As with corded grinders, the same rules apply when using a cordless angle grinder: go slow and don’t take off too much material on every pass. You could even use a grinding attachment on a small rotary tool, which will keep you from removing too much metal at once.

Sharpening stone only

If all you have handy is a sharpening stone, you could still put this to use for sharpening your lawnmower blades. However, they are best suited for situations wherein your blades do not have any chips, and they aren’t too dull. In all other scenarios, it would be best to use a grinder first and then finish up with a sharpening stone.

For best results, secure the blade in a vise then push the sharpening stone smoothly away from the blade’s center. You may also want to use a balancing tool to prevent blade wobble when you reattach the blade to your lawnmower.

Sharpening file only

You could also use a metal file to sharpen your blades, but again, only if they aren’t too dull and if they don’t have any large chips. With a fresh 10-inch file (commonly called a “mill bastard”), you can get your blade back to optimal condition with as little as 50 strokes. 

When using a file, I recommend sharpening from the top of the cutting-edge. This will ensure a longer-lasting edge. As with a sharpening stone, I suggest securing the blade with a vise before filing it.

Other options

If you don’t have any means to sharpen your blades or you would rather not sharpen them yourself, you can either take your blades to a professional sharpening service or replace them with new blades. 

In any case, it is a good idea to check the condition of your blades before you take further action. Follow the directions above for how to remove your blades and check for nicks and burrs along the cutting edge. If you can’t tell whether or not your blades need sharpening just by looking at them, you can take them to the hardware store and compare them against a new blade. 

In most cases, you can get your blades back to working order by sharpening them. But if your blades are too far gone for sharpening, it may be time to replace them. If that is the case, always use only the same type of blade or the ones recommended in your lawn mower’s operation manual.

Conclusion

As you can see, there are quite a few ways to sharpen your lawnmower blades. The available options range from quick-and-dirty metal files and sharpening stones to angle grinders, bench grinders, and even grinder and file/sharpening stone combinations. You can certainly try out which method appeals to you the most. For my money, I recommend using a cordless grinder to get rid of the large burrs and nicks and then finishing up with a file or sharpening stone. I find that this gives me the most control over the sharpening process and the best results.