Edge angles effect on sharpness

M

marshall3

Joined
Dec 12, 2022
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I had a Emerson cqc7 back in the early 2000s. It was the sharpest knife I had but what I wanted to discuss was do you guys think it was so sharp because it was like 20 degrees inclusive or because it was chisel ground. It think it was the low total edge angle. I see sharpening videos where people perform all sorts of feats of sharpness after sharpening their kitchen knives at what looks like 10 degrees per side. I sharpen at 20 degrees per side and can cut a freehanging hair after stropping but don't think I could cut a blueberry sideways without holding it like I have seen done. Anyhow I was wondering what other people think.
 
Alot more goes into it than just the the degree of the apex. Thickness behind the edge, the type of grind, and the overall thickness of the blade are just as important as how "sharp" the knife is, and what angles it is sharpened to.

I could sharpen my ESEE 4 to 10° per side, and then sharpen my CRK Inkosi to 25° per side. They will both still shave hair, sure.. but even with a 50° inclusive apex, the Inkosi will still cut most materials (rubber tubing, fruits, block cheese, etc.) much better just due to the design, grind, and geometry of the blade.

But I'm just some redneck from Alabama, so ymmv 😆
 
E)
Sharpness is independent of edge angle. The things that affect cutting performance are sharpness, edge angle
and thickness of the edge (BTE)
 
I like to distinguish between sharpness (which is only determined by how small a radius is at the edge apex) from cutting ability by describing the latter as the "keenness" of the blade. One could put a very fine edge on a metal cube but that edge will still be 90° and not very penetrating as a result.

Proximity to the apex increases the magnitude of the specific geometry's influence on cutting performance. That is to say that the importance of the geometry of the blade becomes DRASTICALLY more important as you approach the edge, and small changes just behind the edge, while much less important than the apex itself, are still WAY more influential on cutting ease than, say, the spine thickness (which still has some influence, especially on narrow blades, but very little compared to the region around the edge.)

So edge angle does make a HUGE difference in cutting performance, but so does the region just behind the edge, though to a lesser degree.
 
To me, what's more important is how the edge cuts after it dulls a little bit. That's where narrower geometry and thinness of the grind behind the edge will make the biggest difference. Initial sharpness can be very good with a perfectly executed NEW apex as keen as it can be, regardless of how wide the edge angle is or thickness behind the edge. But after the apex wears, flattens or rounds off a little bit, the edge angle and thickness behind it can completely stop cutting in its tracks if it's very wide or thick. But a thinner grind with relatively acute edge geometry can continue to work pretty well after the apex has worn a bit, thanks to the still-narrow geometry remaining behind it. This has been verified in CATRA testing and is the basis for why some blade mfrs. have changed their default edge specs to something narrower and thinner-ground, which effectively makes the working edge more durable (cuts better, with less applied force, for longer).
 
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Agree with the preceding posters -- thickness behind the edge is critical. That said, it's related to edge angle in a sense. If you have an equal-sized apex on a knife sharpened at 12 dps and one sharpened at 20 dps, and you abrade away a couple microns of steel from the apex, the knife with the 12 dps edge will be much sharper than the 20 dps edge, because ... geometry. Likewise, if you have a very thin knife that's, e.g., 6 or 8 thousandths behind the edge, it will cut soft materials (food) reasonably well even if it's completely dull.
 
Blade Lab said:
Agree with the preceding posters -- thickness behind the edge is critical. That said, it's related to edge angle in a sense. If you have an equal-sized apex on a knife sharpened at 12 dps and one sharpened at 20 dps, and you abrade away a couple microns of steel from the apex, the knife with the 12 dps edge will be much sharper than the 20 dps edge, because ... geometry. Likewise, if you have a very thin knife that's, e.g., 6 or 8 thousandths behind the edge, it will cut soft materials (food) reasonably well even if it's completely dull.
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You beat me to it.

Another way to think about it is that edge angle DETERMINES thickness JUST behind the edge.

I’ve always felt that the thickness “behind the edge” measurement is essentially meaningless unless you also indicate HOW FAR behind the edge you are measuring. Is it 1mm behind the edge? 5mm?? 0.02”??

Or, if you’re “BTE” means you’re measuring thickness where the edge grind meets the primary grind, you also need to include how far that intersection is from the edge apex to fully describe the shape of an edge.

It would be nice if there were an accepted norm for this measurement so that everyone was talking about the same thing…
 
What does the knife need to do ?
Chopping ? 20 degrees + depending on the strength of the steel ..
Slice N dice ? What ? If all your slicing is soft stuff like blue berries , then yeah , take it down to 10 degrees .
I had a factory knife that was crazy bevelled , and I re edged it to 14 degrees , edge rolled at 500 slices of rope . ( That's not bad )
Maybe next time I try for 16 degrees or 18 degrees ?

Thing is , a lot of people push edge angle till the edge rolls ..
If your steel is up for it , then 14 degrees or tighter may not be out of the question . ( If the blade steel supports it )
And tighter bevels cut / slice better .

Again , just depends on the use the knife will be put to .
 
How does steel type influence the optimum edge angle, all other things being equal? Are certain steels more tolerant of thinner edges than others against chipping or rolling, for example?
 
Blue Sky said:
How does steel type influence the optimum edge angle, all other things being equal? Are certain steels more tolerant of thinner edges than others against chipping or rolling, for example?
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Steel type and heat treatment impact the minimum edge angle permissible for a given application, yes.
 
So discussions of angles are incomplete unless the type of steel and their heat treatment is included. Has anyone ever compiled information like this into an easily referenced form? It would be quite useful I think to anyone who regularly uses and sharpens knives of various steel types.
 
One thing to keep in mind is that rolling isn't necessarily better than chipping. Some people seem to think that you can just use a steel or stone to straighten a rolled edge and it's all good, but that's not the case. A rolled edge results in fatigued steel that's just going to roll again the first time you cut with it. Consequently, it may take just as much work to really fix a rolled edge as it does to fix an edge with small chips. In both cases you have to sharpen back to clean steel.
 
Blue Sky said:
So discussions of angles are incomplete unless the type of steel and their heat treatment is included. Has anyone ever compiled information like this into an easily referenced form? It would be quite useful I think to anyone who regularly uses and sharpens knives of various steel types.
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Different heat treatments will result in different values, and different use cases will similarly yield different minimum acceptable angles for the given context. There's really no replacement for experience in this sort of case.

Regarding fatigued steel it will be prone to folding back over or breaking off, but most honing rods have some degree of mild abrasive action to begin with. It is undeniable that side loads do inevitably introduce some degree of fatigue, and once stressed, performance will be reduced until you get back to unaffected steel. The performance difference is simply often not so significant or on so large a portion of the blade as to warrant sharpening the entire blade back for the sake of one rolled spot. But the difference is measurable for sure.
 
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