Soon it will be time to mow the lawn regularly. A sharp blade on your rotary mower works so much better than a dull and nicked blade. This blade is freshly sharpened. The light/shadow contrast near the end of the blade gives the best view of the new edge in this photo.
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Step 1: My Tool of Choice
I get a lot of use from my radial arm saw, including using it as a grinder to sharpen my mower blade.
Step 2: Using a Radial Arm Saw As a Grinder
Remove the blade and its guard. Turn the motor 180 degrees and lock it in position. Put a coarse to medium grit grinding wheel on the arbor.
Step 3: Make a Guide to Support the Blade
A simple upright guide from scraps works very well. The piece clamped to the table is simply glued onto the vertical piece with the "V" cut into it.
Step 4: Align the Guide With the Grinding Wheel
The bottom of the "V" needs to align on center with the grinding wheel. Each side of the "V" is cut at an angle 33 degrees off of the horizontal.
Step 5: Position the Motor and the Guide
The flat part of the blade in the area of its mid-point will ride on the guide. Move the guide so its distance from the grinding wheel allows the blade to slide over the guide without riding up onto a curved section of the blade, and yet the full length of the sharpened portion of the blade can come into contact with the grinding wheel as the blade slides over the guide.
Step 6: Clean the Blade Before Grinding
Scrape the crusted old grass from the sides of the blade so it rides smoothly on the guide.
Step 7: Grinding--part 1
Position the mower blade on the guide and the wheel as shown to sharpen the upper side of the blade. For the best control, grasp the end of the blade with your finger tips to guide it and keep it on the wheel. Only light pressure is needed. Hold the blade firmly down on the guide with the other hand. I like to count the number of passes I make so I can make the same number of passes on both ends. This keeps the blade from getting out of balance. Three or four passes will sharpen most blades, unless they are very dull or badly nicked.
If you suspect your blade is out of balance because someone ground more on one side than the other, you can buy a little cone tool that sets on the tip of a spindle to see how it balances. Although I have one of those, I have not found it completely helpful. I like to put a small bolt through the blade's mounting hole and slip a collar onto the bolt so the bolt fits the hole closely. I secure the bolt with washers and a nut. Then I chuck the bolt into a variable speed drill. If the blade is out of balance, it should gyrate as the drill spins the blade. It does not need to spin very fast. Bring the spinning blade near to an old board with paint on it so the end of the blade begins to thumb the board a little. Stop the drill and see which end of the blade has paint on it. Grind a bit of steel from the end of the blade and repeat the drill test until both ends have about the same amount of paint on them.
When using a radial arm saw as I have described, the sparks will fly away from you.
Step 8: Grinding--part 2
When the upper part of the blade looks good on both ends, turn it over and grind a little on the underside. A couple of passes should do the job.
How often should you sharpen the blade? A lot depends on how much mowing you do each week and if your mower picks up small rocks while mowing. If your lawn is not large and the mower does not kick up small rocks, once a season should be enough. If you keep your blade fairly sharp, it does not take long at all to touch it up a little when it begins to dull.