Help interpreting goniometer results

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evert67

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Oct 22, 2011
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Hi all,

I've got a goniometer that aids in determining the angle at which a knife has been sharpened. It works quite well for knives with a flat grind, as for example my Swamprat Ratmandu:

IMG_2274c.jpg


The edge has got an angle of about 20 degrees per side.

It also works quite well for thin knives with a convex grind, However, I cannot interpret the results the goniometer gives for a thick knife with a convex grind, my Fallkniven A1:

IMG_2277c.jpg


Can you help me interpret what my goniometer tries to tell me?

Thanks!
 
thats a neat gadget. i have never seen one before but it looks pretty simple to use if i can understand how it works from the picture.

is there any mention of blade thickness in the instructions? it could be the thickness of the blade making it give you the odd reading.
 
Seems that the convex is between 8-10 per side (the bright long band) with a microbevel at 20. The microbevel isn't continuous, that's why it 'jumps' from 10 directly to 20.

That's how I see the second photo.
 
Where did you get the goniometer, and what instructions came with it? I would guess that the manufacturer would be able to help. The thick knife probably has much more of a curve as well as a longer surface on the basic "taper" which would spread the laser beam all over the place. A short flat taper would probably give the most succinct readings.
 
You start by aligning the dots nearest the zero degree mark, once aligned a equal distance from zero you can take a angle measurement. Edges with curvature give you the full range of the edge geometry so the tip and tail of the laser is the apex and shoulder of the sharpened bevel. In the picture below the convex bevel has a apex angle of 16 degrees per side with a 10 degree shoulder. The distance between shows that I have a 6 degree roll in the geometry of this bevel. Don't worry about the faint trails beyond the points of measurement that's just light distortion. In full convex blades the laser may even go from zero to the apex angle without a break in the beam, FYI.

Picture1874.jpg
 
a convex edge does not have any set angle but a continuous radius to the edge so really there is no "angle" to be measured.
the point where the laser reflects down to give you the reading could be a few thousands of an inch or maybe +1/16" from the very edge. i think that is why you get the different readings between the 2 knives. a wide blade will have a wider/longer radius and will give you a different reading.
 
Thanx guys, for your responses. For the people interested in the device: I got it from Catra in the UK: http://www.catra.org/pages/products/kniveslevel1/lkep.htm

I guess I’ll have to live with the fact that my knife is too thick for exact results with the goniometer. Depending on how I hold the knife exactly, the light pattern is quite different: just a tiny bit of a degree already makes a world of difference. And there must be some unevenness in the knife, because the patch of light on the right (around 15 degrees) is consistently larger than the patch of light on the left (around 9 degrees).

Probably the large patches of light are just reflections from anywhere on the blade. I was puzzled for some time, thinking: “a convex edge varying from 10 degrees till 28 degrees, that cannot be right”. But now I come to think of it, the 28 degrees (in the photograph 22 and 31 degrees, but twice 28 when I slightly twist the knife) must be the dullest angle of the edge. And since it is convex, that must be the final angle of the edge of the blade. Does this make sense to you?
 
i think you can get a ballpark idea of the angle on a convex edge but not like you would with a v edge. at the website there is nothing mentioned of measuring a convex edge that i could find.
 
I've read the instructions that come with the device, it reads with a accuracy within 2 degrees and can read convex edges as I described above.

I've also noticed that if the bevel is coarse and has more distortion at the apex you will see more of a half moon shape reading. Your guess would be as good as mine in that situation.
 
28 degrees final edge for convex might be if the person sharpening it put a bit more 'roll'. Seems to make sense to me.

To think about it, the laser has to be paralel down and the width of the beam must cover the whole bevel thickness for measurement to make sense .. something like | | ...

Am I making any sense?
 
Chris "Anagarika";10482567 said:
28 degrees final edge for convex might be if the person sharpening it put a bit more 'roll'. Seems to make sense to me.

To think about it, the laser has to be paralel down and the width of the beam must cover the whole bevel thickness for measurement to make sense .. something like | | ...

Am I making any sense?
Click to expand...

The laser dot is not round but rectangle, this allows enough laser beam to be split by the bevel.
 
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