Lawn Mower Blade Sharpening
The blade in a rotary lawn mower doesn't need to be sharp as a carving knife or scalpel but serious dents and nicks will result in a less than perfect lawn (to say the least!). In addition, a seriously unbalanced blade can result in excessive vibration and eventually, possible internal damage as well.
First, drain the gas or remove the gas tank. If you will be filing steel, you get sparks. Sparks are not the greatest thing to have around gasoline vapor. Enough said. Disconnect the spark plug wire and tie it safely away from the spark plug or remove the spark plug entirely. Turn the mower on its side.
CAUTION: Immediately check for oil leaks at the oil filler pipe or elsewhere. If there are any, you will need to work on the mower just propped up by 45 degrees or so. Or, use this as a good excuse to perform an oil change and drain the oil (even if the engine is cold, most of the oil will drain out but it will take a little longer). Just don't forget to refill the crankcase with fresh oil once you have completed work on the blade!
Check that the blade isn't bent. Locate a reference point on one side and note the height of the blade tip at that location. Rotate the blade 180 degrees and check the height of the opposite blade tip. There should be no significant difference - say no more than 1/8" or so. If it is greater, the blade is bent or the crankshaft is bent. Either will require further investigation as running the mower under such conditions will probably result in excessive vibration and can be dangerous.
Lawn Mower Blade Sharpening Guide
Slight nicks and dents can be cleaned up with a file while the blade is still installed on the mower. Unless you have run into a curb, this is probably all that is needed on an occasional basis. Removing this small amount of metal will also not unbalance the blade enough to worry about. Refer to the section: Non-violent blade removal if it needs to come off the mower.
If the damage is severe, consider replacing the blade entirely - they are not that expensive (usually under $10). Otherwise, you can use a file, a bench grinding wheel, or a grinding wheel mounted in an electric drill (there are special attachments for this specific application).
Since the rotating blade also contributes to the proper air flow, you do not want to upset the shape. Grind in such a way that the original blade angle is preserved. It doesn't need (or want) to be razor sharp. A 1/64" edge is fine. Anything finer will quickly be dulled by little bits of stone and dirt in any case. Safety is not the main concern here - if any part of your anatomy contacts the whirling blade, you **will** be in trouble no matter how dull or sharp the blade might be!
Attempt to remove approximately equal amounts of metal from both ends and in roughly similar areas if possible. If there are a few large nicks, it isn't necessary to remove them completely - your lawn (and neighbors) will never know the difference.
Check the balance by positioning the blade at the center hole location on a pencil or other rod - you don't need a fancy blade balancer but can use one if you like. If it tips one way or the other, remove more material from the heavy side a little at a time.
Remove Mower Blades for Sharpening
For just some minor touch up, there is no real need to remove the blade. For major grinding and balancing, removal will be needed. Removal will also be required to inspect for a damaged or sheared blade lock key and to replace the blade or blade related parts if necessary.
In either case: disconnect the spark plug wire and tie it safely away from the spark plug terminal (several inches minimum) or remove the spark plug entirely to prevent accidental starting.
If the nut holding the blade on is just on very tight, use a block of wood to prevent the blade from turning. Use a good quality socket wrench or box-end wrench of the correct size. An adjustable or even open-end wrench may not be enough and will damage the nut and tend to slip off, possibly quite violently. The nut usually unscrews counter-clockwise. However, check this out first! A careful inspection of the threads on the end of the crankshaft will reveal the direction. Or, determine the direction of rotation which will be designed to tighten, not loosen the blade. Most, if not all, single blade mowers rotate the blades clockwise as viewed from above which will therefore use a normal right-hand thread nut.
Pay close attention to the arrangement of the blade and associated hardware so you can replace everything properly. If the (Woodruff) key waas damaged or sheared replace *only* with a similar *soft* key. Else, you'll be replacing the engine next time there is a minor incident. Tighten the blade nut to the recommended torque. If it's too loose, the key may shear fairly quickly from normal start and stop cycles as the blade whacks back and forth.
The blade replacement procedures for riding mowers and lawn tractors generally differs significantly from that of walk-behind mowers. At the very least, it's probably not possible (or at least easy or desirable) to turn them on their side. Some may require removing the deck or it may be possible to work from underneath (with appropriate precautions and accident insurance). Refer to your owner's manual.
(From: Graduate student of school of hard knocks.)
"I'd wish I'd read this a few years ago. I used an open-end wrench and it 'flexed' off of the bolt. Needless to say, my next week wasn't a lot of fun with 10 stitches in my hand."
Use some penetrating oil (e.g., liquid wrench or WD40) on the nut and threads if there are signs of rust or corrosion. Allow it to soak in for a few minutes before attempting to remove the nut.
You will prevail. A hammer or other more violent approaches should not be needed.
Once the nut is loose, unscrew it the rest of the way by hand and remove any washers or mounting plate and note their exact position and orientation. The blade and adapter should come off easily. Some penetrating oil (e.g., WD40) may help if it is difficult to remove.
If your adapter/blade doesn't pop off after removing the nut or bolt, it may be mounted using a taper like the flywheel. This is somewhat unusual on a walk-behind lawn mower but might be present on a larger machine like a lawn tractor. A wheel puller is best for dealing with this situation but first see if it isn't just gummed up or rusted in position - try the WD40.
Inspect the key or locking tab for damage. You may have:
- A rectangular blade adapter with a welded-on soft metal ring with a tab protruding into a slot in the crankshaft.
- A blade adapter that locks to the crankshaft with a rectangular or D-shaped (Woodruff) soft metal key.
If the adapter's tab is broken off or the key is sheared or damaged, then replacement of the entire blade adapter or just the key (depending on your mower's design) will be needed upon reassembly. For now, if you will be sharpening the blade, replace all the hardware in the correct positions (except the blade) and finger tighten the nut so you won't lose anything.
WARNING: Do not install a hard steel key in place of the recommended blade lock key as you will lose the protection that the soft metal provides and the next incident may be the last... See the section:Why soft metal keys must be used.
Once you have reground the blade or obtained a replacement, reassemble in reverse order and then tighten the nut to the proper torque.
(From: Gib Gahan ([email protected]).)