How to Safely Sharpen Your Lawn Mower Blade
When your lawn mower blade gets dull, it has to be either replaced or sharpened. Some newer mower types have blades that should not be sharpened and if the old blade is bent or broken it should absolutely be replaced for a new one.
You should never try to sharpen a blade on the mower. It is almost impossible to do and can be dangerous.
To prevent the engine from accidentally starting when I work under the mower deck, I always remove the wire from the spark plug and bend it away so it can not come into contact with the spark plug. Manually turning the mower blade could ignite the engine, even with the fuel valve in OFF position.
You might have to empty the fuel tank before you tilt the mower to remove the blade. You should at least turn the fuel valve to OFF position.
Now, tilt the mower on the side with the fuel tank facing up. Find a suitable wrench for the blade bolt. Wear a thick glove, holding the blade stable while you loosen the blade bolt.
The blade bolts are usually threaded in opposite direction with the blade revolution. To loosen the bolt You should move the outer end of the wrench towards the cutting edge of the blade.
Here we have the blade removed and the bolt with two washers. This blade has no support holes and is only held in place by the big washer that is concave and builds up a great pressure when the bolt is tightened.
You should mark which side is down on the blade and then clean it carefully before you start to sharpen it cause you have to balance it afterwards and any buildup gives you incorrect balance.
This blade is very dull and has dents from hitting stones on the lawn. It should actually be replaced, but I decided to try if I could sharpen it and use it one more month.
I put the blade in a vice and use a grinder to sharpen it. The edge should be made about 30 deg from horizontal. Don’t put too much pressure on the grinder. Just make a slight cut in each pass and repeat it several times. The cutting edge of the blade is relatively short, all inside the oval circle on the picture.
If you need to remove lot of metal to make a good edge, the blade becomes very warm. You should allow the blade to cool down between passes to avoid hardening.
When you finish sharpening the blade has to be balanced. Make sure that the center hole of the blade is clean and apply little bit of oil to it for lubrication. Put a screwdriver in the vice or just hold it horizontally and place the blade on it as shown.
If the balance is correct the blade stays like that. If one end is heavier the blade will turn and the end facing down has to be ground more and the balancing repeated.
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