Does it have something to do with that the knife is thicker at the spine than at the bevel so it tapers a little from "top" to bottom (sharp side)?

Hi Joergen,

Yes, I believe that's the problem. As Ken said, with something with a nice wide bevel, like a chisel, you can put the angle master directly against the bevel and should get an accurate reading. But, for something like a knife with a small bevel, if you follow the advice of resting the angle master up on the side of knife (the primary grind), there will be an error introduced because you are not measuring against the center line of the knife.

Three degrees sounds about right for that error. I recently worked the math with an Opinel No. 10 and found that I had to use a correction of 3.15 degrees for it.

Here are the calculations I used in case you or anyone else is interested in using them or wants to check my math:

Spine width: 2.14 mm

Distance from spine to edge: 19.45 mm

We are interested in the right triangle that is formed between the edge, the center line of the knife (running from edge to the middle of the spine) and the primary grind (the hypotenuse of the triangle), so we need to divide the spine width by two before calculating the primary grind angle:

primary grind angle = tan

^{-1}((2.14 / 2) / 19.45) = 3.15 degrees

(Make sure your calculator is in degrees mode if it supports both degrees and radians).

That amount needs to be

*added* to your desired bevel angle to get the angle to set with the angle master. In my case, for a 15 degree bevel, I would have had to use 18.15 on the angle master.

The method of setting the bevel angle using the projection distance and USB distance from stone or case does not have this problem (assuming the knife is centered in the jig). I use it when I care about grinding a specific angle so that I don't have to worry about applying a correction to the angle master setting.

Greg