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Why the 40° edge will not score as well as the 20°
#21
Good point Jan and with little doubt the bulk of the  article the excerpt was lifted from seems to be pointed at straight razor users. However - we're thrown a curve ball with this portion of the excerpt -  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.  That doesn't sound much like a reference to straight razor sharpening.

It makes little difference, we don't hold the author of the excerpt forth as the be all and end all of edge creation. It just seemed to us as if he had thoughts on the matter and then proceeded to advance those thoughts in a reasonably clear fashion. It just wasn't clear enough to avoid our being left  with a few questions. 
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#22
(08-29-2019, 11:04 PM)grepper Wrote: 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.

Appreciate your re-wording the concept I am trying to explain. Rather than the somewhat confusing "keenness/sharpness" construct, it may really be easier to comprehend by seeing the edge apex width as sharpness, that combined with the bevel angle together dictate how the edge will score on the sharpness tester. But both are talking about the same things.

Edge-on SEM could tell us that the apex width in properly sharpened edges is the same disregard of the edge angle, but nevertheless the more acute edge will score better on the sharpness tester.  As you say, that is not saying that the apex is not as sharp, but that it does not test as sharp.

For example, from my experience, the 20 dps edge that is as sharp as a razor at its apex, scores on the BESS sharpness tester near 110 BESS at the best. Still at its very apex this 20 dps edge performs similarly to the razor in the hanging hair test, splits and whittles.
http://knifeGrinders.com.au
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#23
Yup.  We are on the same page Mr. KG.  It was only after rereading your post that I understood that you were offering an explanation of why test results differ for edges of the same sharpness but with different bevel angles.  I feel a bit meager of mind because you stated it most clearly.  After all, the subject of your post is, “Why the 40° edge will not score as well as the 20° “, as opposed to Why the 40° edge is not as sharp as the 20°.

If I understand correctly, you are suggesting that test media must be penetrated to some depth before it cleaves and that this depth is enough so that the bevel interaction with test media measurably affects the sharpness reading.   Additionally, the impact of bevel angle is significant, accounting for a ~40 point variation between 15 and 20 degrees of bevel angle for similar edges of the same sharpness.

I like that hypothesis because I can now blame test instrument limitations for what I had previously assigned to my own faulty technique.

Test media is some amazing stuff and I am personally hesitant to assume understanding it's qualities or how it works.  With very sharp edges it severs with almost no force and yet is able to withstand 2,000g+ of force with a dull edge.  I don't understand how it functions well enough to suppose if it supports the bevel hypothesis or not.  

While interesting, I have no idea as to the validity of your hypothesis.  Moreover, verifying it is problematic as the only accurate way I have of measuring sharpness is with the instrument in question.  Yes, there is empirically derived evidence of hair splitting, etc., but in my mind that is only suggestive and not quantifiable.  

This is really some cutting edge stuff here and we are considering hair splitting details.  (Puns intended).  I hesitate to delve into subjects like this because I think it way over complicates practical knife sharpening adding to the mysticism so prevalent in knife forums.  

That said, I know you are very meticulous in your technique and when you present a 40 point sharpness difference, even though for all intents and practical purposes for cutting onions it matters little, it is curious and methinks merits a bit of head scratching.
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#24
(08-30-2019, 10:26 PM)grepper Wrote: ...
I like that hypothesis because I can now blame test instrument limitations for what I had previously assigned to my own faulty technique.
...
LOL That is exactly what makes this hypothesis so plausible, isn't it?
http://knifeGrinders.com.au
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#25
Count me in fellows! From now on, every time I see a number I don't like I'm going to bounce my edge tester off the wall. I've already wrapped it in foam rubber. Wouldn't really want to hurt my best sharpening tool.
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#26
Many a man on this forum put true razor edge on the knives, so can you confirm my observations that the razor sharp edge will score:

- 10 dps edge near 50 BESS;
- 12 dps edge 50-70 BESS;
- 15 dps edge 70-90 BESS; and
- 20 dps edge near 110 BESS.
http://knifeGrinders.com.au
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#27
Thanks for bringing this up again Mr. KG and pestering the point.  While I have done no experimenting to verify it, I think that without giving it much thought at the time I have actually noticed that more acute bevels seem to end up sharper than those less acute.  I can’t say for sure, but I have that gut feeling and it’s an interesting hypothesis.  

Knowing your amazingly exhaustive methods, I suspect that you would not have even asked the question without prior extensive experimentation and data.

Am I correct in my assumption?  For instance, considering you’re finely honed sharpening expertise, do you find difficulty in getting a 20 dps edge sharper than near 110 BESS?
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#28
(09-15-2019, 10:02 PM)grepper Wrote: Thanks for bringing this up again Mr. KG and pestering the point.  While I have done no experimenting to verify it, I think that without giving it much thought at the time I have actually noticed that more acute bevels seem to end up sharper than those less acute.  I can’t say for sure, but I have that gut feeling and it’s an interesting hypothesis.  

Knowing your amazingly exhaustive methods, I suspect that you would not have even asked the question without prior extensive experimentation and data.

Am I correct in my assumption?  For instance, considering you’re finely honed sharpening expertise, do you find difficulty in getting a 20 dps edge sharper than near 110 BESS?

First, SHARPCO started me thinking of this when he asked me whether it is possible at all to hone a 20 dps edge to 50 BESS. He was frustrated he could not.
About the same time there was a thread by Sirano on this forum that he could not get any decent BESS, sharpening a blade with steep edge, some dagger.
Back then I hardly ever sharpened knives at 20 dps. But I often sharpened folders at 15 dps, and could tell straight away that they never score as well as the 12 dps, let alone 10 dps edge, despite showing the same keenness in the HHT test.

I simply kept registering all these occasions in my memory till could see the above pattern.
Naturally, I also kept thinking of an explanation for this.

I am looking for confirmation from others of the "best score by edge angle".
But probably I better ask at the Tormek forum, as there is over a thousand using our software for grinding angle on Tormek, and among them there should be many who also have the BESS sharpness tester.
http://knifeGrinders.com.au
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#29
I haven’t done any systematic tests, but have observed the same phenomenon. I once asked about it on this forum, and was essentially told I was wrong, but a little thought experiment I used was to imagine two edges, both one-molecule thick, one at 179 degrees, and one at 1 degree (and both unsusceptible to deformation). I can’t imagine the 179-degree edge cutting the media without considerably more force than the 1-degree, so it comes down to within what range of angles do the differences become irrelevant. Thanks to your efforts here, I now have something quantitative, which I very much appreciate. I usually sharpen at 15 dps, and no matter how hard I try, can’t get below 70~80 BESS, though I can easily whittle hair when I try. Now that I have your numbers, I’ll check against them when I sharpen at other angles. Much appreciate your work here!
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