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nt1
06-19-2008, 10:43 AM
So the tip of the blade does the cutting but why are all blades of grass cut during one pass of the mower?

If say you have a 21" blade and only 4" 's are sharp on each end, why are all the grass blades cut in one pass with the mower?

I realize the mower creates a vacuum pulling the grass up and on a mulching blade/deck the grass is recut many times. Other than that I have no idea how the grass is cut.

JimQ
06-19-2008, 01:21 PM
Not quite sure now to answer this one...

Assume the following - A mower with a 61" Deck (21" blade) mowing grass at 5 mph.

Blade tip speed of approx 18,500FPM (3300 Blade RPM)

5 MPH = 88 in/sec
3300 RPM = 55 Rev/sec

88/55= 1.6

That means the mower travels forward 1.6" for every 1 blade revolution.

Remember, there are 2 blade tips on a blade so divide that number in half.

1.6"/2 = .8"

So, the mower moves forward less than 1" for each cutting pass of the blade.

If you're looking for a more basic answer, imagine a very fast machete.

Q

k911lowe
06-19-2008, 01:24 PM
Not quite sure now to answer this one...

Assume the following - A mower with a 61" Deck (21" blade) mowing grass at 5 mph.

Blade tip speed of approx 18,500FPM (3300 Blade RPM)

5 MPH = 88 in/sec
3300 RPM = 55 Rev/sec

88/55= 1.6

That means the mower travels forward 1.6" for every 1 blade revolution.

Remember, there are 2 blade tips on a blade so divide that number in half.

1.6"/2 = .8"

So, the mower moves forward less than 1" for each cutting pass of the blade.

If you're looking for a more basic answer, imagine a very fast machete.

Q

a good answer,why the question?are you having trouble with your cut?

Happy Frog
06-19-2008, 02:30 PM
Not quite sure now to answer this one...

Assume the following - A mower with a 61" Deck (21" blade) mowing grass at 5 mph.

Blade tip speed of approx 18,500FPM (3300 Blade RPM)

5 MPH = 88 in/sec
3300 RPM = 55 Rev/sec

88/55= 1.6

That means the mower travels forward 1.6" for every 1 blade revolution.

Remember, there are 2 blade tips on a blade so divide that number in half.

1.6"/2 = .8"

So, the mower moves forward less than 1" for each cutting pass of the blade.

If you're looking for a more basic answer, imagine a very fast machete.

Q

You shouldn't divide the 1.6" by 2. The mower stills travels 1.6" forward per each blade revolution but the grass is cut two times.

JimQ
06-19-2008, 02:53 PM
You shouldn't divide the 1.6" by 2. The mower stills travels 1.6" forward per each blade revolution but the grass is cut two times.

;) I think we're both saying the same thing here...


The mower has moved forward .8" each time the grass gets cut.


Q

Richard Martin
06-19-2008, 03:03 PM
Sound like it's time to split some hairs here.

The .8" is only the first opportunity for the blade to cut grass. The number of times that a blade can potentially cut grass varies depending on where in the arc of the blade rotation the grass is and how far the blade is sharpened down it's length. This is also why some mowers tend to miss blades of grass between the mower blades since you have the least opportunites to cut the grass at that spot in the arc.

nt1
06-19-2008, 10:43 PM
Ok, I get what you are saying about speed of travel but my question relates more to what Richard Martin is saying.

How about another question. Why are blades not sharpened the entire length of the blade?

As far as the fast "machete" comparison, a machete is still sharpened the length of its blade or at least the part that cuts.

Grits
06-19-2008, 10:48 PM
The first few inches of the blade is the main portion that does the cutting. I would think on a mulching blade that more of the blade cuts.

windsory
06-19-2008, 10:52 PM
Ok, I get what you are saying about speed of travel but my question relates more to what Richard Martin is saying.

How about another question. Why are blades not sharpened the entire length of the blade?

As far as the fast "machete" comparison, a machete is still sharpened the length of its blade or at least the part that cuts.

The tip of the blade is traveling the fastest because it is spinning around a larger radius than the parts that are closer to the spindle so it will give the best cut, thats why the entire blade is not sharpened. The blade is constantly spinning, that's why an entire pass is cut while only 4 inches of the blades are actually sharpened. Picture a cup with wet paint on the brim. If you put the cup brim down on a piece of paper and slide it across, you will get a line of paint the width of the diameter of the cup. The brim represents the outer sharpened edges of the blade while the unsharpened edges of the blade are represented by the empty middle of the cup.

Hopefully this all makes sense. :waving:

gorknoids
06-19-2008, 10:58 PM
They're not sharpened their whole length because the mid-blade speed isn't high enough to produce a cut. Just like I can't throw a golf ball 300 yards.

mx495
06-19-2008, 11:26 PM
I can get the same blade for my 60" mower w/about 4" of the blade sharpened and another with about 6" sharpened. The dealer told me the one with the shorter amount sharpened was better for wet grass. The blades were identical otherwise. Any merit to this, or just a bunch of bull? Doesn't seem logical to me, but I may be overlooking some form of physics at work or something.LOL

hockeypro1411
06-19-2008, 11:52 PM
its because the mower blade spins. (i know right?) as it travels around it cuts the grass in an arc shape. the reason it cuts the entire diameter of the deck is because at one point every blade of grass in one pass was at the edge of an arc (wether its the front or either side). if you took a single blade mower (a 21 inch mower for example) and set it in tall grass while running and lifted it straight out, theoretically there would be a circle in the middle thats uncut. its because it did not get "hit by the blade arc". think of it as the blade clearing a path while it spins at the top of its arc (when its paralell to you) and then making it wider as it spins the rest of the way around (blade perpendicular to you). i have no idea if this makes any sense, or if it even remotely answers the question, but it was a long day so there ya go.

hockeypro1411
06-20-2008, 12:06 AM
http://i141.photobucket.com/albums/r54/hockeypro1411/untitled-5.jpg

im bored so i made a crappy picture that kind of explains how it takes a little more out of the "arc" as it spins. once again, im bored. oh theres no mower or deck... theyre invisible so you can see the blade. imagine that its being pushed towards the "about to be cut" part. ...im a loser

nt1
06-20-2008, 12:24 AM
The tip of the blade is traveling the fastest because it is spinning around a larger radius than the parts that are closer to the spindle so it will give the best cut, thats why the entire blade is not sharpened. The blade is constantly spinning, that's why an entire pass is cut while only 4 inches of the blades are actually sharpened. Picture a cup with wet paint on the brim. If you put the cup brim down on a piece of paper and slide it across, you will get a line of paint the width of the diameter of the cup. The brim represents the outer sharpened edges of the blade while the unsharpened edges of the blade are represented by the empty middle of the cup.

Hopefully this all makes sense. :waving:

Ok, now I get it. The cup was a good analogy for me. Just had to picture the process going on. Explained well in other posts I was just not "seeing" it.
Thanks.

Runner
06-20-2008, 12:51 AM
Actually, the first blade stands it up and lifts the blade straight, while the second blade cuts the grass off clean.:laugh:

whoopassonthebluegrass
06-20-2008, 01:43 AM
im bored so i made a crappy picture that kind of explains how it takes a little more out of the "arc" as it spins. once again, im bored...im a loser

LOL!! :laugh::laugh::laugh::laugh::laugh::laugh:

We all are, I think!

Richard Martin
06-20-2008, 05:28 AM
Ok, I get what you are saying about speed of travel but my question relates more to what Richard Martin is saying.

How about another question. Why are blades not sharpened the entire length of the blade?

Some blades are sharpened almost all of the way to the center.

While it is true that the point of the blade that cuts the grass is moving slower and slower as you get closer to the middle of the blade almost all points along the sharpened length are still moving fast enough to cut grass especially when the blade is really sharp.

On everyday grass it doesn't really matter how far you sharpen the blade. Where it really matters is on really thick, tough grass and long stringy grass and weeds.

I often see complaints that mowers won't cut dandelion heads and seed heads like plantain and Bahia. Usually these complaints are centered around a blade that is only sharpened for a few inches. Sharpen that same blade down close to the middle and it will cut that stuff down.

Nickos
06-20-2008, 07:42 PM
phallic lol