Lawn mower blade.... any point in going beyond a simple grind?

[JackBauer24]

Every couple of months I pull my lawn mower blade off and run it across the belt sander. Would there be any benifit to actually making it sharp? or would it just be gone too fast and or weaken it too much?

 

[hatchetjack]

A bit off topic but I couldn't help remember the scene from Billy Bob Thornton's "Sling Blade"

Doyle: What in the hell are you doing with that lawn mower blade?
Karl: I aim to kill you with it. Mmm.

 

[db]

I’ve put a good edge on my lawn mower blades and it’s gone after one cutting. There are many who say the best thing to do is put a small 1/16 flat 90 degree bevel on the edge. The idea is the 90 degree corner is enough to do the cutting on a lawn mower blade and it will last longer than the ground to an edge bevel. I’ve done it and it does cut, not sure it lasts any longer though.

 

[Cliff Stamp]

No it doesn't weaken it, but lawnmower blades blunt by rocks usually and not grass. So there is little benefit either. That being said, you can make a blade shave off a belt sander if done right.

-Cliff

 

[JackBauer24]

I’ve put a good edge on my lawn mower blades and it’s gone after one cutting. There are many who say the best thing to do is put a small 1/16 flat 90 degree bevel on the edge. The idea is the 90 degree corner is enough to do the cutting on a lawn mower blade and it will last longer than the ground to an edge bevel. I’ve done it and it does cut, not sure it lasts any longer though.

I only think I understand that

 

[Rick in KY]

In a recent conversation with a lawn mower serviceman, I was told never to sharpen a mulching lawn mower blade.

Apparently, a mulching blade is easy to unbalance and cause all kinds of problems with the drive shaft dojamahicky.

 

[WhitleyStu]

I have read it is better for the grass if it is cut rather than ripped at the top of the grass blade. I try to keep my three blades on my 60" MMM deck some what sharp, but mowing 3 acres takes a toll on the edge. The second paragraph here tell what it does...

 

[Broos]

You can balance them while sharpening them pretty easily - buy the little cone shaped balance stand to check them (or you can rig something up yourself pretty easily).

 

[the possum]

I was thinking about this earlier today, since a co worker asked me to sharpen his mower blades. They were duller than,... well, I don't know what, and had gouges in 'em a quarter inch deep. On the bottom, they were stamped "Grass Side" behind the edges. So I told him, "Here's your problem. You had 'em on backwards. See here where it says Grass Side? The other side must be the Rock Side."

Because I work long hours, my lawn is usually pretty tall by the time I get to cut it. And, since I have to do so with only an old Lawn Boy push mower, I can tell a huge difference between sharp blades and when they get a little dull. I usually touch up the edges with a file every time I mow, since I can just flip it over & give the blades a couple strokes as needed. When they're sharp, I can literally run through thick foot tall grass (usually in my dog's favorite pooping areas ) without bogging down the motor. I'm limited only by how fast the blades spin, since otherwise I can pass over the ground before the blade has time to come around. When the blades are dull, I have to go slower to keep from bogging down the engine, which means a half hour job easily turns into an hour long task. Folks with powerful riding mowers may never really notice the difference, especially since their blades are almost always very dull anyhow.

There are many who say the best thing to do is put a small 1/16 flat 90 degree bevel on the edge. The idea is the 90 degree corner is enough to do the cutting on a lawn mower blade and it will last longer than the ground to an edge bevel.

Sure. After all, back in the day when wheat was harvested by hand, folks would scrape their scythes against a rock or try to chop up iron bars to make sure the edge wasn't too sharp. Even today in South America, folks who use a machete for harvesting prefer to use the flat spine rather than the edge. Oh, wait a minute. Maybe I've got that wrong. Maybe that makes no sense at all. Why not just round the edge over completely? Then it will never get any worse!

Even if your sharp edge is gone by the time you're done, it should still be sharper than a 1/16" flat, and it will have helped you immensely until then. Yeah, you have to keep touching 'em up. What would you expect of steel that's only in the 40's to low 50's Rc that makes 800 chops per minute on sticks, rocks, and dirt/dust laden grass? Those sharp 90 degree corners will wear off your flat edge pretty quick, too.

I usually study the kind of damage the blade has been getting, looking at the deepest nicks. I thin down the edge creating a back bevel behind these nicks, and sharpen a steeper secondary bevel to maybe 40-60 degrees at the edge itself, depening on whether it needs to be reground or how many sticks are layin' in the yard. I walk through the lawn first, and pick up anything that I wouldn't want to chop with a good knife, and all the sticks I reasonably can down to pencil diameter. (I burn leaves & sticks in the ditch, being sure to let it smolder after dark so the cops can come tell me to put it out. )

The factory edges are usually chisel ground from the top, but I usually work the bottom side a bit too. On the blades I did today, if I tried removing all the damage from the top side, I would've had to grind away twice as much steel. The blades will be completely worn out long before you have to worry about the edge getting too high towards the centerline, and frankly I'm not sure you have to worry about it at all.

So, I don't try to maintain my mower blades at shaving sharpness, but I can definitely tell the difference by keeping them sharp enough that I wouldn't want to rub my finger down the edge. Especially since it only takes me like 2 minutes to do so, and saves me lots more time in the long run. Whether you want to bother is up to you and your situation.

 

[Bungwrench]

This is good information. I need to sharpen my mower blade. I bought it used and I have had it for 4 years now and never sharpened it.

 

[StretchNM]

I sharpen mine on a 6" grinder (after cleaning with water and a nylon scrubpad) until it's *very* sharp. I mean sharp enough to slice your finger. But then it may be a month or so before I rotate a sharp blade on, and then sharpen the old one. Of course the blade edge doesn't look like a knife because it has all these little knicks and chips that I don;t bother to grind all the way out.

I really don;t know if it needs to be that sharp, but whoever said it was better to cleanly cut the grass is right, I believe. With a dull blade, the tops of the grass blades are literally shredded... you can even see it with the naked eye. This gives the lawn a brown cast and can also avail the grass to disease. I learned this from Jerry Brown's lawncare guide.

All I really know is that, when I put a clean, very sharp blade in the mower, I can literally (truly) *hear* the blade cutting the grass. I know it sounds phony, but it's true. I mean, above and beyond the sound of the mower, there is a "wishing" slicing sound that I only hear after putting on a sharp blade. After about 3 or 4 mowings, when the blade edges are very dull, I can hear these tiny tiny little screams...shrieks, if you will. Then I know it's time to grind.

 

[db]

Well Possum make fun and criticize my post all you want. It does work and cuts the grass just as well as a chisel sharpened blade. I’ve tried it myself and it works. And yes a round corner doesn’t cut but a nice sharp 90 degree corner does and it cuts very well on a lawn mower. It is also very easy to touch up with a file and I get much less chipping compared to before. Give it a try you may just be surprised. You ever hear of Gator Blades? They are a high-end blade and guess how they come sharpened?
...
Btw how wide is your 40 to 60 degree secondary edge?

 

[Jared Stenoien]

Well I'd sharpen the blade on my lawnmower, if the engineers had actually thought about that part when they designed the mower. The way that you drain the oil on my lawnmower is tipping it, and not by much at all, I've gone up a steep slope and had the engine catch on fire before.

 

[conan1]

I use a dremel tool to sharpen my lawnmower blades. Does a good job and doesn't take too much metal off. Also use a cone balancer just to make sure. Balancing will increase the life of your mandrel bearings for sure.

 

[sputnick]

db: I thought it was pretty funny, no hostility intended. My blades are ground like picture (a) so thats how i sharpen them. I am assuming you are telling us that edge (d) will cut as well as edge (a). Initially i thought you were proposing that edge (b) was the optimal lawncutting edge, which somehow seems less than efficient.

 

[Allan Molstad]

Balancing will increase the life of your mandrel bearings for sure.

I found that it's $100.00 to replace the mandrel bearings ....

 

[the possum]

Well Possum make fun and criticize my post all you want.

Calm yourself, dude.

It does work and cuts the grass just as well as a chisel sharpened blade.

It may cut as well as a dull chisel sharpened blade. I don't see any way it could cut like a blade that can easily slice paper. But tell ya what. In the spirit of fairness and learning, next time my blades get a little dull I'll swipe a flat on the edges and see if I can tell the difference. If there is, my thick lawn and wimpy push mower will tell me pretty quick.

And yes a round corner doesn’t cut but a nice sharp 90 degree corner does and it cuts very well on a lawn mower. It is also very easy to touch up with a file and I get much less chipping compared to before.

OK. Let's think about this. The blades get dull when they get too rounded over, right? The edges/corners on your flat edged blade are starting out at an included angle of around 130 degrees! (on the top side; the bottom corner should be closer to 90 degrees.) It only takes a tiny bit of wear before it's rounded to a 1/16" circle. Compare that to an edge that starts out at only 40 degrees, and you have to wear a hell of a lot more metal before things get rounded over to that extent.

Lawn mowers do not defy physics. They simply swing the blades fast. A sharp edged machete cuts grass better, and the same is true of a mower.

You ever hear of Gator Blades? They are a high-end blade and guess how they come sharpened?

I don't care how some manufacturer sharpens their blades. Knife manufacturers can't even get edge geometry right!

Btw how wide is your 40 to 60 degree secondary edge?

On my personal mower, since I try not to hit junk with it, the secondary bevel is maybe 1mm wide or so. The primary edge bevel is more like 20-30 degrees. On other people's blades, if it looks like they hit rocks all the time, I'll leave things beefier.

 

[db]

Calm yourself, dude.

Yeah sure thing dude.

It may cut as well as a dull chisel sharpened blade. I don't see any way it could cut like a blade that can easily slice paper. But tell ya what.
In the spirit of fairness and learning, next time my blades get a little dull I'll swipe a flat on the edges and see if I can tell the difference.

A sharp 90 degree edge doesn’t have any problem cutting paper. In fact I’m not sure of the angle but I think paper cutters are pretty darn close to 90 degrees. But yeah try it on your blade and see if it works for you or not if you want. Even if you don’t it makes no difference to me.

OK. Let's think about this. The blades get dull when they get too rounded over, right? The edges/corners on your flat edged blade are starting out at
an included angle of around 130 degrees!

Huh? I said a very small 1/16 wide, 90 degree bevel, similar as your 40 or 60 degree secondary edge. It is put on a main primary chisel edge.

I don't care how some manufacturer sharpens their blades. Knife manufacturers can't even get edge geometry right!

That is obvious. You probably also know a lot more about lawn mowers than me. I don’t even know how they sharpen their lawn mowers in South America or in your area. However, up here in WI I have only seen chisel ground edges on lawn mower blades and never have seen even a 90-10 V ground edge. Yes, my 90 degree flat edge bevel is on a chisel ground edge. I really don’t know why but I can make a guess as to why the cutting edge is kept down on the low side of the blade. Guess 1 is it probably helps with airflow pulling up the grass blade to be cut. Guess 2 is to keep drag/friction on the blade to a minimum. If this will make a difference in cut quality from a blade with a 90-10, 30-70, or a 50-50 v ground edge I don’t have any idea. I am just assuming the manufactures are all keeping the edge on the bottom for a reason. I’ve never seen nor used a lawn mower with any type of V edge on it. I do know you will get a better cut depending on design, if your mower deck has a 1/8 to ¼ inch forward slant. Meaning when the blade tip is forward it measures say 3 inches off the ground, and after it spins and faces the back it is 3 1/8 to 3 ¼ off the ground. But what ever works, right? What type of grass? How sandy and rocky is your soil? How fast do you cut and how long is your grass? What type of mower do you use? All have a lot to do with how your lawnmower cuts.

 

[db]

OK. Let's think about this. The blades get dull when they get too rounded over, right? The edges/corners on your flat edged blade are starting out at an
included angle of around 130 degrees! (on the top side; the bottom corner should be closer to 90 degrees.) It only takes a tiny bit of wear before it's
rounded to a 1/16" circle. Compare that to an edge that starts out at only 40 degrees, and

Ok I think I see where your confused. The cutting edge isn’t the corner between the 90 degree flat and the primary grind your thinking of a v grind with one side at 90 and the other side of the edge at 40. That’s not correct. The cutting edge is at the bottom of the 90 degree bevel. Think of a chisel with one side of the cutting edge at 0 degrees and the other side with your 40 degree primary and a small secondary at 90. Now is having that secondary at 90 better worse, the same as having it at 60 degrees? I don’t know but touching up the 90 degree flat is easy and quick to do, and I get much less chipping than I did without it.

 

[hardheart]

a 90 degree machined edge should do ok with the force from a 5 horse motor behind it. I used to 'cut' down tall weeds with a length of 3/4 conduit when I'd get bored. Shearing a blade of grass along a machined edge should leave a clean enough tear. I can use a sharp knife or 'dull' scissors to cut paper easily.