How to Measure Blade Angle?

Lenny

Lenny

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Oct 15, 1998
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I did something slightly stupid with my Sebenza.
I wanted to put a lower edge angle on it with my Edge Pro.
I set the angle at about 19 degrees (indicated on the vertical rod that the sharpening stone rod attaches to)
I proceded to sharpen to a mirror edge.
Unfortunately, I laid the whole edge of the knife on the sharpening table (instead of just the flat surface) so I have
no idea what the actual edge angle is.
It was however, low enough to remove a tiny bit of the thumb stud. (DOH!!!)
Anyway, it's back at CRK for a new thumb stud.
BTW, the edge is stupid sharp and has a beautiful polish on it.
Should be a piece of cake to touch it up on the 40 degree position of my Spyderco Sharpmaker now.
Anyway, could somebody please explain to me, in simple terms, how to determine the actual edge angle?
Thank you.
 
You could mark the primary bevel with a sharpie. Use the finest stone you have (you don't want to grind away metal) and play with the angles until you find the right one that removes the sharpie along the primary bevel.

Or you make a diy goniometer like demonstrated in this video:
 
I have no idea how accurate are those degree (angle) markings on your sharpening system.
First: put your knife back on sharpener at approximately same position and measure angle of the stone with angle cube.
Second: according to dimensions of your blade calculate the angle of primary bevel of your blade for one side (not inclusive angle)
Third: calculation
'Number on angle cube' minus 'angle of your primary bevel' = your sharpening angle
 
[I edited this to correct an earlier error.]

The angle may not be that bad, depending on how far you ground down the thumb stud.

On my large Sebgenza, the distance from the edge to the middle of the thumb stud is 0.9645 inches. (I used a caliper, so the decimals can go out a ways.)

From the middle of the spine to the outside point of the thumb stud is 0.3407 inches.

So with trig, you have the sine equals the side opposite divided by the hypotenuse, or 0.3407/0.9645 = 0.3532.

The arcsine of 0.3532 is 20.7 degrees, or 41.4 degrees inclusive. It would be less, depending on how far down you ground the thumb stud. But even if you ground the thumb stud down to 0.24 inches from 0.28 inches, the angle would be close to 39 degrees inclusive. The angle would be different for a small Sebenza, but the math works out the same.
 
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Twindog said:
[I edited this to correct an earlier error.]

The angle may not be that bad, depending on how far you ground down the thumb stud.

On my large Sebgenza, the distance from the edge to the middle of the thumb stud is 0.9645 inches. (I used a caliper, so the decimals can go out a ways.)

From the middle of the spine to the outside point of the thumb stud is 0.3407 inches.

So with trig, you have the sine equals the side opposite divided by the hypotenuse, or 0.3407/0.9645 = 0.3532.

The arcsine of 0.3532 is 20.7 degrees, or 41.4 degrees inclusive. It would be less, depending on how far down you ground the thumb stud. But even if you ground the thumb stud down to 0.24 inches from 0.28 inches, the angle would be close to 39 degrees inclusive. The angle would be different for a small Sebenza, but the math works out the same.
Click to expand...
Thanks Twindog.
I only ground down a tiny bit of the thumb stud.
It's still full height, with an angled grind on the side facing the edge.
I'm sorry, but I've forgotten all my Trig from my younger days.
What would be great is if somebody could draw a pic of a knife with all the pertinent
measurements and angles marked. I'm bad at visualizing things.
I'm not asking for much, am I? :)
Anyway, my small Sebenza is at CRK so I'll have to wait till it comes back to measure anything.
Happy Holidays all!
 
Lenny said:
Thanks Twindog.
I only ground down a tiny bit of the thumb stud.
It's still full height, with an angled grind on the side facing the edge.
I'm sorry, but I've forgotten all my Trig from my younger days.
What would be great is if somebody could draw a pic of a knife with all the pertinent
measurements and angles marked. I'm bad at visualizing things.
I'm not asking for much, am I? :)
Anyway, my small Sebenza is at CRK so I'll have to wait till it comes back to measure anything.
Happy Holidays all!
Click to expand...
Sure. You just need to make a right triangle.

2v2eLJP4UxAWtWs.jpg


You want to find the sine of the edge angle, which is the angle of just one side -- degrees per side.

Sine = side opposite divided by the hypotenuse (the side opposite the right angle).

The side opposite is the length between the center of the spine to the outside of the thumb stud.

So 0.3407 inches (half the spine width plus the length of the stud) is side opposite.

The hypotenuse is the length between the edge apex and the outside edge of the stud, or 0.9645 inches.

0.3407 divided by 0.9645 = 0.3532 (the sine)

To get the angle of the sine, you use a calculated to hit the arcsine button. The arcsine is 20.68 degrees. So the edge angle on one side is 20.7 degrees, or 41.4 degrees inclusive.
 
You rock Twindog, Thanks!
I'm remembering Trig now. :)
Just so everybody realizes, this method only works for my situation where
I ground the top of the thumb stud off.
This is the only instance where the angle in your picture is actually the edge angle.
 
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just make sure you are doing the edge angle rather than the blade angle—unless you are grinding flat for 0 edge which it doesn’t seem like you are doing.
 
The Razor Edge Book of Sharpening mentions a simple method but one I have used for years. Its not absolutely perfect but it works.


Basically I took two feeler gauges pieces. Place it over the edge as below. Run it down until it catches. Place it on a protractor and you get an idea of angle.

Here is a store bought axe I applied the above logic to.

A25-AEA9-C-0220-4-D3-A-8379-EEDD658-AA95-F.jpg


482-E76-B7-70-E2-4-E39-8467-894-D16-A238-F5.jpg


CF0-F86-FF-4-E19-454-D-BD2-B-B463-AAE80140.jpg



Compared to my heavily used Manix
12-A21-AD4-8-B35-4-BDD-8622-DAB6823276-E9.jpg
 
When I sharpen my kitchen knives on magnetic table I usually add 2 degrees to my sharpening angle. If I want to sharpen the knife at 16 degrees per side I set my stone at 18 degrees.
Why?
Because you need to take in account the angle of primary bevel of the knife (on one side).
-measure the thickness of the blade and divide it by 2. This is yout T.
-measure the width of your knife (spine to edge). This is your W.
The angle of the primary bevel of your knife (on one side is):
Angle=arcus tangens(T/W)

The same equation goes also for Edge Pro. The knife rests on the table of the Edge Pro with the surface of primary bevel.
 
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