How To Correctly Sharpen Your Lawnmower Blade
Posted: Monday, November 07, 2005
by outdoor power
Outdoor Power, Powersports & Marine Engines
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. 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.
Once the blade is removed, use a scraper to remove excess grass build up. This will enable you to balance the blade properly when finished 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 - 45 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 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 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 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. You can also insert a screwdriver in the center hole and check to see if the blade remains horizontal. If the blade does remain horizontal then the blade is in balance. If one end falls lower than the other then you will need to grind a little more off the lower end. Keep doing this until the blade remains horizontal
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. 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.
A few words about safety Small engines and the equipment they drive can and are very dangerous to work on. Always use your good judgment when working on power equipment. Never wear loose clothing and never rest any part of the machine on your legs or any other part of your body. Read and understand the owner's manual that came with your equipment. Always remove the spark plug from your engine when doing any type of maintenance. It's not enough to just remove the plug wire, take the extra step and remove the plug from the engine.
This and other articles are written and archived by Outdoor Power Parts and can be viewed at http://www.outdoorpowerparts.com and http://www.oppsme.net/support
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More commentsthank you
This article was very helpful and Thanks for the Q&A which confirmed my 5-7 yrs since last time I sharpened a blade. Cheers
I use a fine wire thru the bolt hole to balance the blade. is this OK?Yes that is fine.
Which way do you turn the nut to remove it. Is a standard tread or revierse thread???Most mowers have the standard thread direction, righty tighty.. lefty loosy..however, some do use the reverse. Its pretty easy to tell though, start with turning to the left if it feels like its getting tighter then go the other way. you shouldn't have to turn it more than a half turn to know which direction.
Great Article! Have a question, do you have to use a torque wrench to tighten the bolt when you are done, or can you just tighten it down the best you can? My manual recommended it, but they are almost half the price of the lawn mower!
Torque wrenches are good to have, but it's just lawn mower. I never use one.
Thanks for the article. Do you have to have a bench grinder, or can you just use metal rasps/files to get the edge you need? Thanks!I've always used a file myself. It takes a bit longer, but not much if you have a good file, and you'll also have more control over the entire operation, and be able to obtain a keener edge. A good set of files is far superior to a bench grinder.
The blade configuration of the new blade i purchased is different than the original blade does make a difference?As long as the dimensions are the same, no. There are different types of blades such as mulching, gator, high lift, low lift, etc and they do look different in how they are shaped.It's important to use a blade style similar to the original one. Most mower decks are designed for a certain type of blade, and using the wrong blade can screw up the air flow pattern that the mower manufacturer spent a lot of engineering time and effort trying to get right. You can't just convert a "standard" mower into a mulching mower just by switching blades.
Put "neversieze" , or equivalent on the blade attaching bolt, it will come off a lot easier next time. Available at your autoparts store. Great forum and instructions, thanks.
I didn't unplug the plug. now i have nine fingers,
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