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BESS value vs apex angle - Cyrano - 08-25-2018

I've not yet achieved BESS values below 200 for edges with obtuse apex angles, e.g., 30 dps or greater.

Versus acute edges, I'm guessing obtuse edges require greater precision at all stages in the sharpening protocol, for similar reasons as why obtuse edges have poorer edge retention ... but that's just speculation on my part.

What are the best BESS scores folks have achieved for highly obtuse apex angles?

RE: BESS value vs apex angle - KnifeGrinders - 08-26-2018

(08-25-2018, 10:57 AM)Cyrano Wrote: I've not yet achieved BESS values below 200 for edges with obtuse apex angles, e.g., 30 dps or greater.

Versus acute edges, I'm guessing obtuse edges require greater precision at all stages in the sharpening protocol, for similar reasons as why obtuse edges have poorer edge retention ... but that's just speculation on my part.

What are the best BESS scores folks have achieved for highly obtuse apex angles?

a shot in the eye - we discussed this with Youngbin (SHARPCO), as we've noticed it is way easier to get a razor sharp edge under 100 BESS on a knife sharpened at 10 dps as compared to 20 dps, and also attribute this to a greater precision required in grinding and honing because it is easier to round an obtuse edge.

As to the 30 dps blades - out of curiosity, it is not about knives, is it?

RE: BESS value vs apex angle - Cyrano - 08-26-2018

(08-26-2018, 04:23 PM)KnifeGrinders Wrote: ... As to the 30 dps blades - out of curiosity, it is not about knives, is it?

Yes, I'm asking about knives. I have several with very obtuse angles, like this dagger:

[Image: i-VmRPT6q-L.jpg]

RE: BESS value vs apex angle - KnifeGrinders - 08-26-2018

Muela hunting daggers in my collection have factory sharp edges hollow ground at near 25 dps.

The dagger on your image must be a fighting dagger, used as a stabbing weapon.
When I have a request to sharpen a fighting dagger, we agree with the customer on re-profiling the edge bevel to 20 dps.
Another approach is to keep the dagger existing bevel profile and edge angle, and only hone the edge, but don't expect to get it very sharp this way.

RE: BESS value vs apex angle - SteveG - 10-27-2018

I was about to start a new thread, but this title fits my question exactly, and it might be relevant to the original question posted here.

I normally use the manufacturers bevel angles when sharpening (being unwilling to change them until I have more experience), and something I’ve noticed had made me very curious. Most folding knives seem to come at 20dps, and I’m pretty used to how those perform with my sharpening. But some come with more acute angles, and I’m finding that those seem to score better on BESS testing, and are easier to get sharp (less time, less fussing/refining). IIRC, Jan made mention that there’s a higher probability of getting a smaller apex radius with these lower angles, which intuitively makes some sense to me, but I wouldn’t be able to explain to anyone why that would be true. So I’m looking for input; I’d just like to understand why my 15dps knife is so much easier to get sharp than my 20dps knife. For reference, I’m talking about 60~80 BESS.

RE: BESS value vs apex angle - KnifeGrinders - 10-28-2018

Because it is easier to round the apex of a more obtuse edge, methinks

RE: BESS value vs apex angle - SteveG - 10-28-2018

Could it be that simple... One of the things that made me start really thinking about this was that I just a couple of days ago got a new Sypderco Caribbean (which I’m really liking, BTW). It has what looks like a typical factory grind; i.e. belt-sanded at some relatively coarse grit, then with a tiny bevel that was probably done on a stropping belt. It was BESS 135 when I received it, and 16dps. After some use I decided to strop it, and after just a few strokes on CrOx/leather it was able to whittle hair, which was unexpected, and something I’ve not seen from 20dps edges. So I started reflecting on my experiences with my current collection, and failing memory aside, I think there’s a pattern. I’ll be doing some experimenting as time allows, but eeven if the experiments show I’m correct, I have no conceptual understanding of why it’s correct. Since I both freehand and use a guided system, I’m not too inclined to accept rounding unless a soft substrate is involved, but I’ll test specifically for that at some point.

RE: BESS value vs apex angle - KnifeGrinders - 10-31-2018

The more I think of it, the more you seem right, Steve - it can't be that simple as rounding.
All my honing methods are controlled-angle, yet I clearly see the same pattern.

I think it surprises us only because of the wrong vision of the edge apex we have. Thinking of apex as something zeroed as a corner of triangle, it will sure surprise that the sharpness depends on the edge angle.

But in reality the apex has a finite width, a minuscule but width, which is dragged against the abrasive grains.
When the edge is put perpendicular to the honing surface, the whole width of the apex gets abraded. Suppose we hone at 45 degrees - then only half of the width is abraded, and so on, the lower the angle, the less is abraded area. 
With the edge at 90 degrees to the honing surface the apex dulls, while at 0 degrees nothing happens; the closer to 0 the better goes refining, while the closer to 90, the more is dulling.
Especially in the microscopic scale, the apex honing runs in a subtle balance between refining and dulling through abrasion.

What do you think? 

Continuing along the same path of reasoning, what practical conclusion comes? So how can we get the 10 dps sharpness on a 20 dps edge?
By decreasing the size of the honing grains, I suppose. The sharpness we get on a 10 dps edge by honing on chromium oxide, on a 20 dps edge will require 0.1 micron diamonds/CBN and even lighter pressure. But when using the same chromium oxide on 10 dps edge and 20 dps edge, the 20 dps will never get the same sharp, because its apex gets comparatively more abrasion.
Makes sense?

RE: BESS value vs apex angle - SteveG - 10-31-2018

Well, I was starting to lean the other way, thinking that you are right about the rounding, and that my conjectures were just a flight of fantasy. But since you're seeing it, I'm now even more curious.

A sharp 90-degree-inclusive edge cuts easily (I've sliced myself on the planed corners of boards often enough), though not deeply. I have a plane blade that's sharpened at 90 (it's a scraper blade), and it gets nice, clean shavings, albeit thin. It never feels that sharp to me, but it cuts like it's sharp, so I think that the "media" used to check sharpness (my fingers, in this case) don't allow much penetration at that edge angle, hence the perceived difference. Might be interesting to test that blade on the BESS tester.

I believe that for any given abrasive/grit, the edge will be toothier as the edge-angle decreases, which would lead to it feeling sharper, and possibly passing some sharpness tests easier, but that wouldn't necessarily impact the BESS score. With the precision and repeatability of your sharpening methods, the only difference I would expect to see there is that you might have to go to a finer grit for the low angle bevels, but that seems opposite to your conclusions, so there must be more to the story.

To your comments about the whole width being abraded at 90 degrees, I've spent some time trying to visualize the differences, and coming up short. When I have one of those boards I'm planing, if I take another shaving I still have a sharp corner, just a thinner board. I have no idea what the apex radius is in that case, but I don't think it changes much. I would expect the same to be true to some extent with abrasives and steel, and my understanding is too weak to speculate on how/where they would differ. One area where I can visualize it being an issue is that I use mostly edge-leading strokes when sharpening, so perhaps the behaviour of the slurry is to dull more obtuse angles more easily, but that doesn't explain your observations, since (I think) your sharpening is all edge-trailing.

I doubt there's any practical consequence to all of this conjecture, but thanks to your message I'll be dreaming about 90-degree edges for the next few nights. And I'll put my scraper blade on the BESS tester next time I sharpen it, which will probably be this weekend.