Step-by-Step Lawnmower Blade Sharpening Instructions and Tips - - The Friendliest Tractor Forum and Best Place for Tractor Information
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post #1 of 1 Old 03-30-2012, 12:40 PM Thread Starter
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Arrow Step-by-Step Lawnmower Blade Sharpening Instructions and Tips

Well, I'm sure with spring well on the way already, many of you have already, or will have to sharpen your lawnmower blades, so I decided to put together a little write up with some simple methods, and tips, on how to sharpen your lawnmower blade. Sure, to the many of us that have sharpened our own lawnmower blades all our lives, it is definitely not a difficult task, but for those of you that are perhaps sharpening your own lawnmower blade for the first time, or are looking for a few tips, and tricks, on how to go about it, read on!

Having a sharp lawnmower blade is very important in getting an even, satisfactorily cut when you are mowing the lawn. Depending on how large your lawn is, you will have to sharpen your blade one, two, or even three times in one season.

#1 Safety comes first, pull the spark plug wire to make sure the mower does not accidentally start. Professionals would recommend that you never work on your mower with the spark plug installed, because the engine can start unexpectedly. This is especially true when removing the blade, as you can easily turn the engine. As an extra precaution, you could also remove the spark plug completely to remove all chances of the lawnmower starting.

#2 The next step, of course, is removing the blade, which can be quite difficult. A good way to start is to squirt some Kroil, WD-40, or penetrating oil on the blade bolt and nut, and let it stand for a few minutes.

Block the blade as shown in the first picture below so that it does not turn while you are removing the blade nut. An easy way to do this is to use a clamp a block of wood on the inside of the deck with a C-clamp.

#3 Once the blade is removed, use a scraper, wire brush, or steel wool to remove any grass build up from it. This will enable you to balance the blade properly when finished sharpening.

#4 The primary goal in sharpening your blade is to maintain the correct angle. 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 - 45 degrees, as is shown in the diagram below, is typical. 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 angle will not provide the same quality of cut, even though it might wear more slowly.

Whether you are sharpening your lawnmower blade using a file, handheld grinder, or benchtop grinder, many of the same principles do apply. However, I have always used either a handheld grinder or my benchtop, so that's the angle from which I am sharing these tips.

The first step, as you sharpen, is to take a few passes directly over the cutting edge of the blade to remove the nicks in the blades (shown in the 3rd photo at the bottom of the page). After the majority of the nicks are no longer visible, continue grinding the blade at the correct angle that is shown in the photo below. 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 cause the metal to lose its temper. 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 major nicks should be replaced, but a few can be tolerated.

#5 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. One simple, easy way, to check the balance of your blade is to hang it on a screw or nail on the wall (shown in the last photo at the bottom of this page). Make sure that the blade is level, not one side hanging lower than the other. There are many other methods of doing this, such as a blade balancing cone that can be purchased rather inexpensively. 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 high RPM under the deck. The potential liability or injury is not worth the cost of a replacement blade.

Once you have finished balancing the blade and checking it for straightness, clean any burrs or jagged edges with a metal file.

#6 This point is optional, but I thought I'd add it anyway for those of you that will be draining your motor oil at the same time. Because many walk-behind lawnmowers have the oil drain plug on the bottom now, it's a good time to drain the engine oil before you replace the blade.

#7 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 it. Also, remember to replace the blade with the blade angle turned up towards the mower deck, and the cutting edge towards the ground. Tighten the blade nut until it is snug against the blade, then tighten it another turn. If your lawnmower has a lock washer, be sure the washer is pressed flat against the blade.

Happy mowing!
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Last edited by tiretrx; 04-01-2012 at 06:08 PM.
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