Why the 40° edge will not score as well as the 20°

[grepper]

Mr. KG does bring up an interesting point: Considering Why the 40° edge will not score as well as the 20°, I have no idea as to the validity of that, but I wonder how deep a cut must be made in test media before it cleaves?

Does test media actually tear or does it snap more like a thin string of glass when the surface is broken? Media is not very thick so I don't know how much difference it actually makes, but it's interesting nonetheless.

[grepper]

Perhaps I should post from a less tongue-in-cheek perspective.

I’m sure everybody understands what sharp is.  How sharp is the apex of the edge.  Thanks to BESS not only can we accurately measure it, but also communicate it.  If I say the blade measured 100, everyone with an edge tester understands what that means.

We all know about bevel angle and no doubt most folks understand how more/less acute bevels effect cutting.  

Everyone has probably experienced the difference when using a thick heavy blade and a thin blade to cut stuff.  

I’m not talking about burr or wire edge, etc., but just the basics of sharpening; sharpness and blade geometry.  

Other than edge finish, toothy or polished which I think is a separate subject, what else is there?  That all is so clearly described and easily understood by anyone who has used a knife.

Does this need to be more complicated?  Am I missing something here?

[KnifeGrinders]

Knowledge augments through better discrimination.

Discrimination between the keenness (apex width) and sharpness (width behind the apex) allows us to better define criteria of the clean, burrless edge tested on the sharpness tester.

Talking of the polished edges, we now see that:
- a burrless 10 dps edge will be scoring near 50 BESS, while the residual burr mushrooming against the test line will show over 90 BESS;
- a burrless 12 dps edge will be scoring 50-70 BESS, while the residual burr will show over 100 BESS;
- a burrless 15 dps edge will be scoring 70-90 BESS, while the residual burr will show over 120 BESS; and
- a burrless 20 dps edge will be scoring 110-130 BESS, while the residual burr will show over 150 BESS.

We now have an answer to the question haunting advanced sharpeners why we struggle to make the 20 dps edge score the same as the razor on the sharpness tester, despite all the efforts.

[Jan]

Ooops!  Almost forgot to answer your question:  I believe a blunt edge can be either nose or elbow and both can be extremely sausage.

Mr. Grepper, your mind and tongue are as keen as is the razor’s edge. In your terminology the sentence would read: your mind and tongue are as blunt as is the razor’s edge.

Jan

[EOU]

It would seem that this might resonate with some of our members. It's an excerpt from a more extensive piece and authored  by Keith Johnson with Tomo Nagura. 

About sharpness – most people regard edge width to be the main critical concern where sharpness is concerned. To a lesser degree, the topography of the edge’s apex can be a factor also. It can be smoother or toothier, and that can aid or distract from its cutting ability. But at the end of the day, width of the edge is the common denominator that must be factored in first and most heavily. Edge width is controlled by bevel angle and only bevel angle.


So - are we to take from Keith  that the final arbiter of "sharpness" will be determined by  the bevel angle because smaller sharpening angles produce narrower edge apexes or are we to assume that when smaller angles are utilized the sharpness is improved due to improved geometries of the edge? 

We know how Rupert would answer this question - He'd say "Yes" and Rupert is seldom wrong.

[Rupert Lucius]

It would seem that this might resonate with some of our members. It's an excerpt from a more extensive piece and authored  by Keith Johnson with Tomo Nagura. 

About sharpness – most people regard edge width to be the main critical concern where sharpness is concerned. To a lesser degree, the topography of the edge’s apex can be a factor also. It can be smoother or toothier, and that can aid or distract from its cutting ability. But at the end of the day, width of the edge is the common denominator that must be factored in first and most heavily. Edge width is controlled by bevel angle and only bevel angle.


So - are we to take from Keith  that the final arbiter of "sharpness" will be determined by  the bevel angle because smaller sharpening angles produce narrower edge apexes or are we to assume that when smaller angles are utilized the sharpness is improved due to improved geometries of the edge? 

We know how Rupert would answer this question - He'd say "Yes" and Rupert is seldom wrong.

[Rupert Lucius]

As always EOU is spot on.

Let's all have a great Memorial Day Weekend.

Rupert

[grepper]

I have noticed that more acute bevels like 15 dps seem to come out sharper than less acute bevels like 20 dps.  (I'm typing on a laptop and can't figure out how to make a degree symbol).  Since I don't chase super sharp and anything 150 or less if fine with me for kitchen knives, I never really checked it out but I have noticed it.  

I've even pondered it occasionally and it always seemed like both 15 and 20 degrees come to the pretty acute edge and it seemed like both should be able to be equally sharp.  Nonetheless, it has sort festered in the back of my mind as a curiosity I never really figured out.

The EOU post quotes a source stating, "Edge width is controlled by bevel angle and only bevel angle", as though bevel angle is the limiting factor of how sharp an edge can be.

The subject of the KG post is, "Why the 40° edge will not score as well as the 20°", and here it seems that "score" is key referring to BESS testing insofar as the edge must actually penetrate the test media to some depth thereby bringing the bevel angle into play as explained in the post.  That is not saying that the apex is not as sharp, but that it does not test as sharp due to how test media severs.

I have always had this nagging suspicion that more acute bevels can be made sharper than more obtuse bevels but except in extreme examples I don't understand it because at the very apex an edge is formed.  It is only when a surface is penetrated that bevel angle becomes significant as it interacts with the material being cleaved.

As I understand it, the whole keen/sharp thing attempts to conflate edge apex width and bevel angle as a unified definition of "sharpness".  In my mind they are separate entities.  Edge apex width is sharpness, and bevel angle is, well, bevel angel.  Together they dictate how a blade will perform in a cutting operation.

All this is actually pretty ambiguous because none of our edges come to a perfect edge.  The edge has width and rounding so it begs question of where it's measured;  1 micron back from the edge, 2 microns, 3 mm, etc.  It's all so vague that when the minutia is considered it's almost impossible to nail down and since there is no standard I just don't go there.
 
So I have some questions:  If we had the equipment to do it, could both a 15 dps edge and a 20 dps edge be 1 atom or molecule or whatever the smallest steel particle is wide?  Is bevel angle a limiting factor in apex width or is this a limit of our ability to test it because bevel angle effects the test?

Of course for all intents and practical purposes this is academic and I doubt I'd notice when slicing an onion, but it is interesting nonetheless.

[grepper]

Thanks for the post Mr. KG.  While I find the keen/sharp way of describing things puzzling, the point of why test results with different bevel angles differ is interesting.  Does bevel angle dictate apex sharpness or with these extremely sharp edges are we seeing how bevel angle effects test media cleaving.

Maybe you have it exactly right when you said, "Of all 3, only the BESS Edge Sharpness Tester tests both the keenness and sharpness".

I hate the terminology, but I understand what you are saying.

[Jan]

It would seem that this might resonate with some of our members. It's an excerpt from a more extensive piece and authored  by Keith Johnson with Tomo Nagura. 

About sharpness – most people regard edge width to be the main critical concern where sharpness is concerned. To a lesser degree, the topography of the edge’s apex can be a factor also. It can be smoother or toothier, and that can aid or distract from its cutting ability. But at the end of the day, width of the edge is the common denominator that must be factored in first and most heavily. Edge width is controlled by bevel angle and only bevel angle.


So - are we to take from Keith  that the final arbiter of "sharpness" will be determined by  the bevel angle because smaller sharpening angles produce narrower edge apexes or are we to assume that when smaller angles are utilized the sharpness is improved due to improved geometries of the edge?

In my understanding the quoted text oversimplifies the situation. There is no doubt that the bevel angle is of major importance, but for complex understanding we have to include steel properties also. The way how the blade was ground is important also.


In my thinking the quoted text was written to describe the specific situation to straight razors, where the included bevel angle is mostly in the interval 17 to 19⁰. During honing this angle is fixed by the spine thickness and blade width.

If it is true that the quoted text was intended to straight razors, which are very shallow cutting tools, than it is not simply transferable to knives, whose behaviour is influenced also by cleaving and displacing the material far behind the edge.

Jan