Sharpening the end of a lawnmower blade?

[Blues]

I must admit, btw, that Benjamin, @FortyTwoBlades has me beat when it comes to how well his scythe tackles its work...

Not sure which video it was where he was basically push cutting with his scythe. Impressive.

 

[ToddS]

As a first attempt, I ground the leading corner of the ends of the blade, and put an edge on it. I left the radial edge alone, although it was in need of sharpening. Anecdotally, the battery powered mower was able to cut about 4 inches off the damp grass, something that I would normally expect to bog this mower down and require raising the deck to the highest setting.

 

[sickpuppy1]

That’s because nobody who cares about their yard waits to cut 4” off the grass! Shouldn’t cut more than 2”. I don’t care if it did rain. I also hope you rebalance that blade after butchering it. They can and often do break if too far off balance. People get badly injured every year by doing stuff like that. The reason it’s cutting tall grass better is your hurting the lift of the blade by reshaping it. I don’t want to hear of that happening to you.

 

[Mr.Wizard]

sickpuppy1 So what I wrote in post #5 is correct? Why does "hurting the lift" result in better cutting of tall grass? Are you implying that the grass is still bent over and not all of it is being cut? If the blade is kept in balance so it does not break are there any other problems?

 

[sickpuppy1]

Your post is correct in that the blade is design to lift grass and 90% of the the actual cutting is done by the outer 1/2” of the blade. That is under normal conditions . With his reshaped it does lift normally and it’s not standing up right. Where the grass is lifted it may be cutting grass at 3 inches tall, it now may be 4 inches tall when done. These are hypothetical heights of course. By balance, I was referring to the actual mower blade getting out of balance just like a car tire at speed. Mower blade are solid steel and can break at high speed. l actually don’t give a rats A about how his cuts, I just don’t want him to get hurt. I used to have a Honda mower that uses two blade. The top blade was about a quarter to half inch in front of the lower blade so it cut first and then the lower blade hit. Left tiny pieces of grass which are supposed to decay faster. Their mulching solution.

 

[Mr.Wizard]

sickpuppy1 I wonder if it would be sufficient to check static balance over the mounting hole, or if the flex and axial forces (lift) would require dynamic balancing. I am really glad I never sharpened mower blades while being ignorant of this risk.

 

[sickpuppy1]

I actually use a dremel with a sanding disc to sharpen the outer inch and only need to do it once a season. My mower is closing in on ten years so I’ll just get a new blade actually. But I would think dynamic balancing would be fine. It it’s taken to a lawnmower shop that’s all they do. When I do get a new blade it will be an oem from the mfg. Reasoning is, as a equipment mfg the are governed and have safety specs to follow. The generic ones have no such rules to follow. So oem may cost 12 bucks vs 5 or 6. but the piece of mind is worth it considering how often they are needed

 

[ToddS]

I use one of these cheap balancers
https://www.amazon.com/s?k=Oregon-Mower-Blade-Balancer-42-100
It's quite easy to balance when you are grinding from the ends.

However, I'm skeptical that a 20 inch blade at 2500rpm is going to explode if its a few grams light at one end.
This particular blade is a high-lift bagging blade with the fins on the trailing edge, behind where I ground, so I don't see how this modification would have any effect on lift or suction.

At the same time, I'm struggling to understand where the lift comes from when the mower is used in mulching mode. It seems to me that unlike with bagging, there is no airflow in or out, only swirling.

 

[Mr.Wizard]

I am picturing a tip vortex. If it is swirling like this that is still upward flow at the outer edge of the blade deck where the cutting is taking place, right?

 

[Diemaker]

What mower do you have?

The best mower I ever had was a gen 1 Ryobi battery mower. It was only a mulcher, nothing else, and mulched so well you wouldn't want anything else. The underside of the mower deck looked identical to the vortice in the photo above, so I would say there is still a lot of lift with a mulching blade. I inspected my new blade today and noticed the wear is even from the tip in 4" or so until it bends up, no extra wear at the tip. The tips on it sweep inward around 20 degrees so if I bevel them it will reduce the cut diameter quite a bit. They are high lift Ego blades at $35 each. Nothin special but the price.

Mr.Wizard, for the mower blade vortices you need to invert your diagrams since it is reverse of a lifting wing. The mower blade tip vortices would push the grass down at the outside and lift the grass inside the cut diameter, in theory. There is also the endplate effect from the mower deck.

 

[ToddS]

I have an older Ryobi 40V. The bagging blade I modified has the lift wing behind the corner I removed and only sweeps in a few degrees, so I didn't reduce the diameter by any appreciable amount.

https://www.homedepot.ca/product/ryobi-20-inch-bagging-blade-for-40v-brushless-lawn-mower/1001087155


I assume that the intention is for air to flow upward at the outer part of the blade, near the fin, and downward near the center of the blade where there is no cutting.

 

[Diemaker]

I think the air is up and out into the bag when bagging. Downward near the center when mulching when all other egress is blocked.

 

[sickpuppy1]

Mr wizard, while a airplane wing is used to create lift due to the curved top part of the wing, a lawnmower blade is shaped to create suction.Air going over a wing lifts the plane up while a blade sucks the air and grass up and creates enough suction to circulate and expel the clippings.

 

[Mr.Wizard]

I feel silly for not seeing that the vortex would be backward.

 

[sickpuppy1]

Thats not silly, My minor interest in it came from the flying nun tv show. "when lift plus thrust is greater than load plus drag, you can fly!"
Direct quote from Sally Fields. Now THAT'S silly!!