HOW TO: Sharpen rotary mower blades

There is no technical mystery to sharpening a lawnmower blade. It just takes common sense and a bit of know-how. If you have a good bench grinder or a professional blade grinder, you can do a professional job.

  1. Remove the blade

    Removing the blade can sometimes be the hardest part of the job. A good way to start is to squirt some penetrating oil on the blade bolt and nut and let it stand for a few minutes. While you are waiting, pull the spark plug wire to make sure the mower does not accidentally start. I have talked to some people who had stitches in their hands or even a few missing fingers because the engine fired unexpectedly.

    Next, block the blade so that it does not turn while you are removing the blade nut. A blade holder will make this easy. However, if you do not have one, a block of wood under the deck and a C-clamp will do the job.

    Once the blade is removed, use a scraper to remove excess grass build up around the center hole of the blade. This will ensure a good, tight fit and assists in keeping the mounting bolt from working loose.

  2. Sharpen the blade

    Now that you have removed the blade, it's time for sharpening. The primary goal is to consistently maintain the correct angle on the blade. Manufacturers perform hours of testing to determine the angle that will give the user the best cut with the longest span of time between sharpening. It's important to keep the angle as it was intended. Around 40 degrees is typical, but this can vary, so check with the blade manufacturer to obtain the exact figure.

    A narrower angle, such as that of a pocketknife, will cut well initially, but will dull quickly and nick easily. On the other hand, a blade with a less severe (more blunt) angle will not provide the same quality of cut, even though it might wear more slowly.

    Blades come from the manufacturer with a milled edge. Milled edges are the best, but machines that provide a milled edge are expensive. You still can do a good job with a professional blade grinder. A sharpener with a grinding wheel is not preferred, because it will give you a hollow grind.

    As you sharpen, move the blade back and forth across the grinder, maintaining the proper angle until you get the edge you need. Do not force the blade into the grinder. Forcing the blade to grind faster heats the blade and will bein cause the metal to lose its temper (hardness of the blade). Some mechanics will keep a bucket of water handy and will dip the blade in it to cool. If the blade turns a straw color while grinding, it's too hot and the temper is likely gone.

  3. Check for balance and straightness

    It is not necessary to grind a blade until all nicks are out. Grind until you have a sharp edge on the blade in the area where there are no nicks. A blade with numerous nicks should be replaced, but a few can be tolerated.

    Try to grind both edges of the blade evenly, removing the same amount of metal from both ends. This is important when you check the balance. An inexpensive cone-shaped blade balancer can do an excellent job. Wall-mounted blade balances are also available. These help you see if the blade is straight.

    An out-of-balance or bent blade can cause severe vibration and damage to your equipment. You can balance a blade by grinding just a little more metal off the heavy end of the blade. However, never try to straighten a severely bent blade. Straightening it could cause a weakened or cracked blade. A cracked blade could break apart when turning at the high RPMs under the deck. The potential liability or injury is not worth the cost of a replacement blade.

  4. Remount the blade

    Once you have finished balancing the blade and checking it for straightness, clean any burrs or jagged edges with a metal file. Now it's time to put the blade back on the mower deck. Remember you now have a very sharp blade. Use extreme caution when installing.

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Donald C. Wineland is a product consultant for Stens Power Equipment Parts, a division of Ariens Company. www.stens.com; 800-457-7444.

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