Sharpen a Rotary Mower Blade

No matter how much time and effort you spend maintaining the engine on your rotary lawn mower, it will only function as well as the blade allows. It doesn’t matter how often you change the oil, clean the spark plug, or clean the fuel filter, if the blade is dull, chipped, or out of balance you just won’t get a clean cut on your lawn. Maintaining and sharpening your blade should be at the top of the list every spring when you perform your routine checks.

Take a quick inventory of the tools that you have in your shed or garage before you decide how you want to sharpen your blade. There are several ways that you can accomplish the same result. We’ll take a look here at each way as well as some tricks to make sure that your blade is well balanced.

Safety

It probably goes without saying, but in case you’ve never worked on machines with small engines before, you need to drain the gas and unhook the spark plug before you do anything else. It may seem like over-kill on the safety end of things, but mowers can, have, and will start if you spin the blade by hand and the spark plug is still connected. Just like pushing a manual transmission car is enough to get it to turn over when the battery is dead, spinning the blade will create a spark in the mower. Don’t take a chance. Disconnect the plug before you begin.

Wear a pair of heavy leather gloves when you remove the blades. All of the corrosion from last season will most likely be built up around the nut that holds the blades on, and you may have to use a large amount of force to break it free. Since you are dealing with blades that could still have some pretty sharp edges, wear the gloves. Last, safety glasses should be worn for the sharpening of the blades.

Sharpening

Use a Vice

Once you have the blade safely removed then you need to find a way to secure it. If you have a power grinder, then you must have a vise to hold the blade. Do not try to hold it in your hand and use the grinder to sharpen it. Secure the blade in the vise and use the grinder very lightly to knock off any apparent burrs. If you’ve never done this before, it is important that you start out with light passes so that you can get the feel of it. When the grinding wheel spins against the lawnmower blade it will make a pretty good shower of sparks, so don’t let it take you by surprise. Be sure to wear safety glasses.

Power Grinder

After you are comfortable with how the grinder feels on the blade, you can begin sharpening the blade in earnest. You should still be able to see the contour of the blade that it was given in the factory, and you want to keep that same angle on your newly sharpened edge. That angle was specifically designed to function best in your mower and you should maintain it. After you have one side completed, flip the blade over in the vise and repeat the process. Do your best to remove the same amount of metal from both sides. This will mean less work when it is time to balance the blade.

Metal File

You can do a professional job of sharpening your mower blade even if you don’t have a grinder. A metal file will work very well and, with a little practice, give you an edge sharp enough to rival any blade that just came off the factory floor. Secure the blade in the vise and work in much the same way that you would with a power grinder. Use deliberate even strokes to hone the burrs and nicks out of the blade. Once you have a good edge then you can use the file to give it a razor sharp cutting surface. Don’t worry about pushing to hard. Let the file do the work and you’ll be amazed at how quickly the blade begins to look like new.

Balancing

If you are careful and precise when you are sharpening the blade, then the balancing step doesn’t require much at all. Folks get into trouble though, when they remove a significantly larger amount of metal from one side of the blade than the other. When this happens and the blade is reattached to the mower, it causes the mower to buck and jerk. When the unbalanced blade spins at a high rate of speed, it throws the whole machine out of balance.

Picture your washing machine walking across the floor when the load becomes unbalanced. It may not seem like an ounce of metal would make that big of difference, but when that ounce is spinning at several thousand revolutions per minute it makes a really big difference.

Balance Test

To test the balance of your blade you need to purchase a balance tester. These small pieces of plastic have a notch for you to set the blade in. If the blade leans to far one way, then one end is heavier than the other and you need to grind more steel from that end. The balance testers are not very expensive and are highly recommended. Some old timers will tell you to just use a coat hanger, but where safety is concerned, it’s a pretty minimal investment.

Re-Installing

After you are confident that the blades are balanced and are sharpened to your liking then you just need to put your leather gloves back on and bolt them back into the mower deck. After, and only after, the bolt is secure and the mower is flat on the ground you are ready to reconnect the spark plug. Put in some fresh gas and start it up. If you did your job correctly then it should run great. If there is some vibration that wasn’t there before, then you may still have balancing issues that need to be addressed.

TIP: Our expert gardening advisor, Karen Thurber adds, "After the blades have been sharpened, be sure to install the blades in the correct direction. An upside down blade will not cut well. Some blades will have the word "bottom" stamped on them and that is the side that should face the ground. Otherwise, the raised tip, or wing, of the blade should be toward the top of the mower and the sharpened edge will be closest to the ground. Marking the bottom of the blades when you remove them makes it easy to identify and reinstall correctly."

Tackle your lawn this spring with a mower that is running in top condition. Not only will your lawn look better, it will also be healthier, as the grass will have been cut and not torn or broken by a dull, battered blade.