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 Post subject: Sharpening angle vs. steel
PostPosted: Sat Jun 15, 2013 7:25 pm 

Joined: Sun May 05, 2013 10:52 pm
Posts: 12
This may be a difficult question to answer and I'm sure it differs significantly depending on the sharpener, use of the knife, product being cut, etc. But anyways as I continue to sharpen I'm challenged by finding out just how far I can take my angles without running into an edge folding or chipping. I can achieve a very useable extremely sharp edge fine, but I always wonder what if I go steeper how sharp will it be then? What success has everyone had with sharpening angles with different steels? Maybe like a unified breakdown by all the steels ( vg-10, white 1, white 2, AS, etc) and the farthest you have had success pushing the angle.


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 Post subject: Re: Sharpening angle vs. steel
PostPosted: Sat Jun 15, 2013 8:08 pm 

Joined: Wed May 16, 2012 1:49 am
Posts: 317
Location: Amstelveen, The Netherlands
It will greatly depend on the Heat Treatment, so general statements about steel types have not that much sense, I'm afraid.


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 Post subject: Re: Sharpening angle vs. steel
PostPosted: Sat Jun 15, 2013 8:31 pm 

Joined: Sun May 05, 2013 10:52 pm
Posts: 12
Maybe the better question is to categorize it by HC Rockwell then. Just curious that's all. I keep my carbon knives at 12-10 degrees. I tend to go more than 10 degrees with my white #1 goko since its slightly more brittle than my AS that I keep right on 10 degrees.


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 Post subject: Re: Sharpening angle vs. steel
PostPosted: Sat Jun 15, 2013 10:57 pm 

Joined: Wed May 16, 2012 1:49 am
Posts: 317
Location: Amstelveen, The Netherlands
It's all about trial and error. When the blade is sharpened correctly, with an thin relief bevel, the primary edge is not that important and can be changed with a few strokes.
You may use a single microbevel if you went too far with thinning, but that's not very common.


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 Post subject: Re: Sharpening angle vs. steel
PostPosted: Sun Jun 16, 2013 3:56 am 

Joined: Fri Mar 22, 2013 12:44 am
Posts: 161
Location: Northern Virginia
Wallidt,

When I get a new knife, here's what I do.

I thin and then sharpen them to about 5 degrees per side using a digital meter for reference, then use the knife as normal checking the edge condition often. As I see damage occurring, I resharpen adding a degree or so until the damage occurrence is reduced to an acceptable level. Acceptable is micro chips small enough that they come out more or less without adding sharpening time on a 1.2K stone or Aoto.

I use the knives to cut veg and boneless meats at home, and once a week, I prep enough veg to make about 200 servings of soup. I use a not very great supermarket bamboo board.

Here is where my knives ended up.

240 Richmond AS Laser 7.2 degrees a side.
210 Richmond AS Laser 6.8 degrees a side.
240 Heromoto AS 7.0 per side.
210 Goko White #1 7 degrees a side.


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 Post subject: Re: Sharpening angle vs. steel
PostPosted: Sun Jun 16, 2013 6:04 am 

Joined: Sun May 05, 2013 10:52 pm
Posts: 12
Branwell,

Thanks that helps a lot. Unfortunately, my knives meet much higher demands than at home. I'm on the line 6 days a week and plow through hundreds of pounds of produce and proteins throughout the week so unfortunately I don't think 7.0 degrees for my goko is appropriate for the workload I put it through. I love my knives and I treat them accordingly but I only have time to sharpen about one a week...more than one at a time and I loose efficiency with holding an angle.


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 Post subject: Re: Sharpening angle vs. steel
PostPosted: Sun Jun 16, 2013 10:55 am 

Joined: Tue May 29, 2012 12:29 am
Posts: 926
Unless you are using a CATRA edge protractor I highly doubt anyone is getting edge angles of 6-7 degrees per side. That's basically the angle knives are ground at.

To give reference, 1-2 penny height from the spine will average 10-12 degrees per side if freehand sharpening and with very good angle control.


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 Post subject: Re: Sharpening angle vs. steel
PostPosted: Sun Jun 16, 2013 5:28 pm 

Joined: Fri Mar 22, 2013 12:44 am
Posts: 161
Location: Northern Virginia
Hi Jason,

I did my measurements with a digital angle meter so thought them to be accurate. Your post based on the penny measurement suggests I am off by quite a bit. Not liking being off by quite a bit :D and having some spare time, I thought to confirm the digital meter and the penny trick with some math. Heres what I got.

The math for angles is a bit of a pain if one is not into math, which I am not, so I used an online calculator to calculate the angles. The one I used is here.

http://www.endmemo.com/geometry/triangle.php

I started with the penny trick which says 1 - 2 pennies under the spine should give 10 to 12 degrees.

So I measured 2 pennies to give it a best case scenario and got 2.75mm of thickness.
I measured the depth of the blade mid way down the blade and got 45mm.
I measured the thickness of the spine mid way down the blade and got 1.25mm.

So the calculation peramiters are.

Two lines of 45mm separated by two pennies at the spine edge, 2.75mm, plus half the blade thickness at the spine, .62mm bringing the total separation to 3.375mm.

The calculator says the angle is : 4.2982 degrees.

This did not match your angle, so I though maybe if the pennies where pushed under the spine the full width of the pennies. What would that give?

I measured the diameter of the pennies and got 19mm. If I subtract 19mm from the 45mm blade width I get 26mm.

Inputting the new length of 26mm into the calculator I get 7.4427 degrees which still doesn't match the penny trick by more than 20%.

So I moved on to confirm the digital meter. I measured the knife as above and took digital readings of the angles. They matches the math pretty perfectly, so I am thinking my use of the meter and the math is likely okay.

So whats the deal with the penny trick which is wrong by more then 20% given a best case scenario? For it to be accurate, the knife would need to be thicker or shallower, or both.

Anyways, it was a good exercise and I learned a thing or two doing it. Many thanks :P


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 Post subject: Re: Sharpening angle vs. steel
PostPosted: Sun Jun 16, 2013 5:32 pm 

Joined: Fri Mar 22, 2013 12:44 am
Posts: 161
Location: Northern Virginia
Wallidt,

My girlfriend has 4 different cooking jobs and gets to cut up soooo much stuff all day. I am totally jealous.


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 Post subject: Re: Sharpening angle vs. steel
PostPosted: Sun Jun 16, 2013 7:32 pm 
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Joined: Tue Apr 23, 2013 5:29 pm
Posts: 123
Location: San Francisco
The good thing about math and sharpening angles is that nobody is going to check you for accuracy -- repeatability, perhaps, but this ain't a moon shot. If the edge is holding up, then it is a good angle, no matter if it is 7.2 deg., or 8.3 deg., that is the angle that works for you.

While arc sines are greek to many, the good news is that for the angles we are generally talking about, the angle is pretty close to

((height over stone to center of spine) / (width of blade) ) * 60 degrees

so, with angles per side, it is roughly:

1 in 4 ~ 15 deg
1 in 5 ~ 12 deg
1 in 6 ~ 10 deg
1 in 8 ~ 7.5 deg
1 in 10 ~ 6 deg
1 in 12 ~ 5 deg

It's a pretty good approximation, compared with the "real" value
1 in 4 => 14.5 deg
1 in 6 => 9.6 deg
1 in 12 => 4.8 deg

If I measure a penny I get 1.5 mm thick

If you have a 30 mm wide blade, then that is 1 in 20 per penny, or about 3 deg for each additional penny, plus one for the half-width of the spine (half of 3 mm)

30 mm knife, two pennies under spine = 2*3 + 3 ~ 9 deg per side (compare asin(4.5/30) ~ 8.6 deg)


If you have a 45 mm wide blade, then that is 1 in 30 per penny, or about 2 deg for each additional penny, plus one for the half-width of the spine

45 mm knife, two pennies under spine =2*2 + 2 ~ 6 deg per side (compare asin(4.5/45) ~ 5.7 deg)

Yeah, you can get more accurate, but the sixty-degree approximation is pretty good and you can do it on most any calculator, or in your head when your hands wet and the knife is already in the stone.


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