Measuring angle of the edge

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Dave Behrens

Dave Behrens

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I've been mulling this over for a while, and think I know the answer but am curious to how you other makers view this.

When measuring/grinding the cutting edge(or secondary bevel), do measure the degree of the angle relative to the primary bevel, or/as if from the "flat"?

I understand that is measured from the "Flat", the cutting edge will be a shallower(or steeper depending on your view) angle overall, just curious how other approach this situation.


View attachment 270615Here is an illistration, might better describe what Im asking.
 
If I am sharpening, I will gauge the angle in relation to the flat sharpening surface to the spine of the knife (with the edge in contact of the surface).

So I guess that is the same as the angle in relation to the flat? I think.
 
I measure and envision ALL angles from the Centerline of the cross section. Flats change, bevels change... that centerline, although imaginary, is constant.
 
I agree with Rick that bevels change over long periods of time while the center line of all blades stays constant.

Setting the secondary angle at the edge, from the bevel or from the flat [center line] of a blade; makes little difference as long as you are consistent; always using the same point of reference. The knife doesn't know you are setting the angle from either.

Now when it comes to deciding what secondary angle to grind at the edge the three things I consider are the spine thickness, the height of the blade and the angle at which the bevels were ground.
A blade geometry that sports a 1/4 inch spine with a 1 1/2" height and with a 2 1/2 degree bevel grind will take a different secondary angle at the edge than a blade with a 5/32 inch spine 2' height with a 5 degree bevel grind.
So whether you measure from the centerline or the surface of the bevel makes little difference as long as the geometry of the blade is complimentary to its individual parts.
 
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The above is correct. The edge angle is twice the angle formed by the centerline of the blade and the surface of the stone/grinder, with the edge being the apex. It does not change if the bevel is rounded, hollow, or straight. Those tings will determine how it cuts and lasts, but the angle is a fixed number.

The confusion often comes when instructions don't say which angle they are talking about. If yo had a micro-protractor/goniometer you could take a reading of the blade edge and have the edge angle. That is pretty hard to do without a laboratory. The angle between the blade ( centerline to be exact, but the difference is small in a knife's thickness) and the sharpening stone is pretty easy to read or even to guess with experience.The sharpening angle is the actual angle formed by the centerline and the surface plane of the stone. The total angle, or edge angle is often called the included angle to clarify which one you are referring to.

I hope that didn't get too confusing, but that is why one person says he sharpens his hunters at 30° and another says he uses 15° ....they both are the same, just one is including both sides.
 
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Yep, you got it. In your sketch, the flat is a parallel line, the bevel is somewhere around 7°, and the edge is 15°.....in reference to the centerline.
 
bladsmth said:
I hope that didn't get too confusing, but that is why one person says he sharpens his hunters at 30° and another says he uses 5° ....they both are the same, just one is including both sides.
Click to expand...

Stacy, I hope you meant 15 and not 5 or you lost me.
 
bladsmth said:
Typo - fixed - thanks.
Click to expand...
You know, I saw that and thought "What are the odds that I caught Stacy in another boo-boo?"(I think I nabbed you on a typo last month) Then thought twice about pointing it out for fear of accidentally dividing by zero or sumpin'.:p
 
Rick Marchand said:
I measure and envision ALL angles from the Centerline of the cross section. Flats change, bevels change... that centerline, although imaginary, is constant.
Click to expand...

+1 consistently constant
 
bladsmth said:
Rick, I make lots of misteaks.
Click to expand...

Fred.Rowe said:
Make mine a rib I.
Click to expand...

Somehow, this 1-2 punch has me in stitches. Thanks for the laugh.

On to serious matters; I have a related question that I believe I have the answer to, but would love confirmation. If the primary bevel on a full flat grind is, say 6 degrees inclusive, and I lay it down on the primary bevel to sharpen, do I need to add 3 degrees to my sharpening angle to reach my target. I believe I do. How on that?
 
You subtract it, Erik. If laying flat, you already have a 3deg angle from center. If you wanted to finish with a secondary bevel of 22deg inclusive, you would have to raise the spine 8deg.
 
Thank you Rick! I just modeled your description with my hands, and I see exactly what you are saying. I should have been more specific, because not everyone sharpens the way I do. I use a rod guided sharpener. If I lay the same knife on the deck and the rod (sharpening angle) is set at 15 degrees, is it not true that I am turning the angle away from the rod angle and making it more acute? In that example I believe I increase the rod angle by 3 degrees to end up at 15 per side.
How on that - II?
 
For practical purposes, ignore the bevel angle. It is what it is. That is why you use the centerline for exact measurements. If you want to know the exact bevel angle on a flat grind, just measure the thickness at the top of the bevel, and the height from the edge to the top of the bevel. Plug these into a trig calculator or program and you will get the apex angle...which is the bevel angle. For example, if the bevels are 3/4" high on a 1/8" thick blade, the blade's bevel angle is about 10° ( ground at 5° per side). Double the thickness and you double the bevel angle, BTW, which is why slicing blades need to be thin, and chopping blades thick. The edge angle is always higher than this by some amount, or the edge will chip and fail quickly. Usually the edge is about 10-15° per side ( 20-30° included).
 
My wife tells me I'm impractical, shes usually right; so for functional purposes know what the bevel angle is. The more acute the bevel angle the more acute the secondary angle at the edge can be. Thinking in this manner gets a smith to understand which angles "make seance" when applied to a certain bevel angle.

Take a small folding knife blade, that measures 3/8th high by 3/32 thick at the spine ; a blade with this geometry can sport a bevel grind that measures 2 degrees per side. This blade can be sharpened using an angle of 3 degrees or 6 degrees inclusive at the edge measured from the center line. This grind makes for one scary sharp folder blade
 
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