# How acute an angle will (insert steel name) take at the very edge



## Nemo (Oct 20, 2016)

G'day all,

Relative J knife newbie here but I have become fascinated with knives and edges over the last 6 months. Learning heaps from the forums. I have been sharpening with Edgepro which despite some limitations was a great tool that give me the confidence to buy and use great knives and also taught me a lot about edges (a bridge to something bigger, maybe). I have just ordered some stones and am planning to learn free-handing.

In a previous post on a different topic, a senior member advised that my 14dps sharpening of SG2 is probably too acute, and that it would cope better with 35-40 degrees inclusive at the very edge.

This got me thinking about other knife steels.

So my questions to the experienced sharpeners (knife-makers also feel free to chime in!) out there are:

1) What properties of a steel determine the angle (at the very edge) that the steel will cope with? Is it hardness, carbide size, a mixture of the two or some other property?

2) Assuming a competent if heat treatment (and does it vary with a very good HT?), what angle (at the very edge) will specific steels take?
Steels that I'm initially be interested in hearing about include (feel free to insert your favourite knife steel in your answer):
Carbon: Shirogami, Aogami, AS, 52100, specific Swedish steels (e.g.: Misono, Shig), soft Euro carbon.
'Stainless': Soft Euro/Krupps, AUS8, AUS10, VG10, AEBL (is it very different between good and bad HT due to K2 carbides?)
Tool: HD2, Carbonext, SKD-11 (is this similar to SLD?), SKD-12.
PM: SRS-15, HAP 40, Niolox, RWL34, ZDP189.

I hope this is not too much of a can of worms. Please say so if my question doesn't even make sense (or if it's so much more complex that I've set out). Thanks for sharing you knowledge.


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## Steampunk (Oct 20, 2016)

Edge geometry is a variable not just on steel and heat treatment quality, but also rockwell hardness, thinness behind the edge, and - most importantly - application. 

It is kind of a can of worms, but as a sound rule of thumb, harder, finer grained, lower-alloyed steels used for less abusive applications can be take much more acute angles without the edge immediately crumbling or rolling. Softer, coarser grained, or highly alloyed (Lots of hard carbides) steels, and or those used for abusive applications require more obtuse angles to prevent the edge from crumbling or rolling. 

Let's take an example... Say you've got 1095 steel that has been heat treated well and is very fine grained. You can harden it pretty high (Say, 63hrc, although I've heard of people taking it way higher.), and you can make a single bevel knife out of it with an inclusive angle of maybe 5-degrees that will hold up just fine for light applications. Make that steel softer, and 5-degrees could potentially roll on you when cutting a piece of copy paper, or shaving a hair. Use it hard, and that 5-degree angle could chip out at virtually any hardness level. At the same time, make that same 1095 steel - say 55hrc, and give it a 40-50 degree inclusive double-bevel edge with plenty of metal behind it, and you could have a tool you could chop through a cinder block with without experiencing catestrophic edge failure. How you sharpen your knife depends upon a whole lot of factors beyond just being made with a certain steel.

To further complicate the matter, the edge angles _I_ personally put on my knives could be considered conservative or extreme for another user with the exact same knife using it for the exact same things, let alone different knives at different hardnesses for different applications.

Some steels do genuinely have limitations as to how acute you can practically make them that goes beyond usage or heat treatment. Steels with lots of hard carbides for example can't support super acute edges without experiencing carbide fallout, so it is best to sharpen them more obtusely. You can sharpen CPM-S110V at 10-degrees per side, but that doesn't mean that the edge can demonstrate the same extreme longevity it can at 20-dps. However, this is an extreme example. 

Somewhat irregardless of steel type, I personally find that edges more acute than 25-degrees inclusive on double bevels tend to roll pretty aggressively on knives hardened below ~60hrc for my culinary applications and in my hands. Above 60hrc, I can typically have a 20-25 degree inclusive edge that's pretty stable; maybe even 15-inclusive if it's a light use knife in a very hard, very fine grained simple steel. On knives in the low-mid 50's, 30-degrees inclusive is pretty extreme geometry, and 40 might be more appropriate for most applications to prevent rolling. However, this is a gross generalization. I typically start out with a 25-30 degree inclusive edge edge on a double-bevel knife, and experiment with it to see if it should go up or down in my specific usage. Sometimes I need to raise the angle, or add a high angle microbevel, and for others I can go a bit more extreme with it.

There are others with more knowledge and experience than I, but hopefully this helps...

- Steampunk


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## Nemo (Oct 20, 2016)

Thanks steampunk- I wondered if it might be more complex. This gives me an idea of the kind of things that require consideration.


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## Steampunk (Oct 20, 2016)

Another thing to consider, is that edge geometry isn't necessarily a factor of actual _sharpness_ (One can whittle hair with even a 40 or 50 degree inclusive micro-bevel.), but it does play a role in cutting performance as it controls how much metal is directly behind the apex and how quickly it thickens. Basically, from the edge apex up, one can view sharpening a knife as an exercise in _thinning_ on multiple geometric planes. Some people treat it as such, starting at the blade road, then the 'edge bevel' without forming a burr, and then using a micro bevel to actually form the apex. It's also kind of a similar concept as convex sharpening, where one is sharpening along a single - and much larger - curved bevel. 

Food for thought... Been pondering this subject recently.

- Steampunk


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## Nemo (Oct 20, 2016)

I'm guessing that the geometry behind the edge also affects the stability of the edge at any given angle?


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## Steampunk (Oct 20, 2016)

Yes, it is one of the contributing factors... In and that the geometry contributes to the _thickness_ behind the edge. 

A 10-dps edge is a somewhat different story on a steel which is .020" or more behind the edge, vs one which is .005" behind the edge, with metallurgy, grain size, usage, and hardness also factoring. Look at a scandi grind vs. a traditional double bevel; not many thin knives you can successfully baton through a log (Extreme), or carve hardwood with a 7-dps edge! Axes are also surprisingly acute when sharpened correctly, but hardness, steel type, and geometry behind the edge (As it pertains to thickness) helps much with durability.

- Steampunk


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## Benuser (Oct 20, 2016)

Some steel behind the edge will surely contribute to edge geometry. But there's another effect as well. If the blade has got thicker behind the edge, the user will use more force to get it through the food, and the edge will suffer from hard board contact. Very obvious with neglected blades when only the very edge has been sharpened and no thinning took place.


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## Nemo (Oct 21, 2016)

So is it a case of 'pick an angle' thst you think is reasonable (based on steel type, blade geometry and intended use) Then flatten it if edge retention is poor. Or gradually steepen if edge retention is ok (until it becomes a problem)?


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## Benuser (Oct 21, 2016)

Don't try to find the steepest angle. No need for it. Performance is much more determined by thinness behind the edge.


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## Nemo (Oct 21, 2016)

So I should really be thinning the blade until performance is good then putting a narrow (because of the thinning) but relitavely obtuse bevel or a micro-bevel on it? When do I stop thinning? When the bevel is half a mm or so wide? Does it depend on the knife? Whar angle do I thin at? Presumably to Shinogi line (or just above) if there is one? If there isn't one?


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## Benuser (Oct 21, 2016)

Start by easing the shoulders. You may go further, and build a relief bevel at the lowest angle you're comfortable with. Go on until you reach the very edge -- look at the scratch pattern or use the marker trick. Even further thinning with stones is possible but you will scratch a great part of the blade.


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## Nemo (Oct 21, 2016)

Presumably thin with a coarse stone? Do I need to refine the relief with a medium at all?


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## Benuser (Oct 21, 2016)

Yes, I'd refine it a bit, for both performance and easier maintenance. So next time you start with the coarse stone you better see where you're abrading. But again, start with the shoulders, to get an idea.


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## Nemo (Oct 21, 2016)

This is kind of what I have been doing with the EP, although the minimum angle is about 7 degrees. It forms a kind of shinogi line.


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## Nemo (Oct 21, 2016)

Benuser said:


> Start by easing the shoulders. You may go further, and build a relief bevel at the lowest angle you're comfortable with. Go on until you reach the very edge -- look at the scratch pattern or use the marker trick. Even further thinning with stones is possible but you will scratch a great part of the blade.



Meaning stop grinding the relief just as/ before it forms a burr, then cut a new primary bevel? If you want to do further thinning without scratching the blade, do you use a belt sander?


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## Benuser (Oct 21, 2016)

Nemo said:


> This is kind of what I have been doing with the EP, although the minimum angle is about 7 degrees. It forms a kind of shinogi line.



Send a picture, please. I'm a bit worried about that quasi-shinogi-line. A choil shot if possible.


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## Benuser (Oct 21, 2016)

Nemo said:


> Meaning stop grinding the relief just as/ before it forms a burr, then cut a new primary bevel? If you want to do further thinning without scratching the blade, do you use a belt sander?


Yes, indeed, both.


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## Nemo (Oct 21, 2016)

Sorry for delay. Trying to work out how to put images in. I'll have to sit down and try to work out how to upload images to forum.


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## Benuser (Oct 21, 2016)

www.dropbox.com/sc/1pmjayvwa5h4v9j/AABo1M0qPUTEOKtDmhv0W47wa


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## Nemo (Oct 21, 2016)

Ah, did they get through?


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## Benuser (Oct 21, 2016)

It seems more or less Ok, has nothing to do with a shinogi line. It is rather a relief bevel, but as always when done by the EP, with two new shoulders who will cause unnecessary friction. Ease them by hand with only a few strokes on a 2k to convex the entire configuration. 
But the blade has become very thick as far as I can see. Perhaps you should send it to a good sharpener.


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## Nemo (Oct 21, 2016)

Sorry for the poor quality shots. This is a Carbonext Suji that I am using as a testbed. You can see the remnants of the most recent thinning line in the blade shot. I have since tried to re-polish the blade with a wet & dry progression (the camera really makes the scratches seem a lot worse :O), so the thinning line is not so obvious.

On my first attempt at thinning it, there was a thinning line about twice as tall as the one you can see now (about 1/5 the height of the blade?) but this was removed when I tried to clean up the scratches with wet & dry.


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## Nemo (Oct 21, 2016)

Benuser said:


> It seems more or less Ok, has nothing to do with a shinogi line. It is rather a relief bevel, but as always when done by the EP, with two new shoulders who will cause unnecessary friction. Ease them by hand with only a few strokes on a 2k to convex the entire configuration.
> But the blade has become very thick as far as I can see. Perhaps you should send it to a good sharpener.



It's really a blade that I bought to help me learn about knife geometry and sharpening. Sounds like you think it needs a lot of thinning. How far up the edge should it be thinned?


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## Benuser (Oct 21, 2016)

Do you experience wedging and or steering? I 'm asking because that's quite common after some EP sharpening sessions on an asymmetric blade.

http://www.kitchenknifeforums.com/showthread.php/5656-Asymmetry-–-The-REAL-DEAL


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## Benuser (Oct 21, 2016)

Nemo said:


> It's really a blade that I bought to help me learn about knife geometry and sharpening. Sounds like you think it needs a lot of thinning. How far up the edge should it be thinned?




Can't tell from pictures. But perhaps you may measure thickness above the edge, and find out where the 0.5 and 1mm lines are along the blade.


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## Nemo (Oct 21, 2016)

Benuser said:


> Do you experience wedging and or steering? I 'm asking because that's quite common after some EP sharpening sessions on an asymmetric blade.
> 
> http://www.kitchenknifeforums.com/showthread.php/5656-Asymmetry-–-The-REAL-DEAL



No steering although I don't really make cuts that deep with it (I actually don't use it much because I don't often use a suji) and the blade was always very wedgey, even OOTB. If anything it's maybe slightly better after thinning.


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## Benuser (Oct 21, 2016)

Can you test for steering by cutting through copy paper? And to explain why I'm asking for measurement: quite often, blades are much thicker near to the choil. A fat choil pic doesn't always reflect poor performance. 
If I were you, I would ask a sharpener to thin the entire left face, and the last 10mm of the right one. You want to preserve the convexity of the right face for food separation.


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## Nemo (Oct 21, 2016)

Needs to be ~0.5 mm thick 5mm from edge and ~1mm thick 10mm from edge, right?


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## Nemo (Oct 21, 2016)

Benuser said:


> Can you test for steering by cutting through copy paper?



Yes it steers to the right but not severely. Makes sense because I've thinned more on the right side. Having done a bit more reading, thinning should be symmetrically, even on asymmetrical knives, shouldn't it?


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## Benuser (Oct 21, 2016)

No need, but those are good figures. Some slicers are much fatter, others much thinner.


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## Nemo (Oct 21, 2016)

I'll get the callipers out tomorrow


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## Nemo (Oct 21, 2016)

Benuser said:


> It seems more or less Ok, has nothing to do with a shinogi line. It is rather a relief bevel, but as always when done by the EP, with two new shoulders who will cause unnecessary friction. Ease them by hand with only a few strokes on a 2k to convex the entire configuration.
> But the blade has become very thick as far as I can see. Perhaps you should send it to a good sharpener.



How does a shinogi line look on a choil? A more pronounced angle?


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## Benuser (Oct 21, 2016)

Nemo said:


> Yes it steers to the right but not severely. Makes sense because I've thinned more on the right side. Having done a bit more reading, thinning should be symmetrically, even on asymmetrical knives, shouldn't it?



Once steering has been redressed, you better do. But I saw some left side convexity as well, and the edge having been recentered. Normally you have some steering to the left -- clockwise so to say -- you compensate by increasing the left bevel angle. Just to make sure: you're right -handed, aren't you? And it's no blade that has been neutralised for a left-hander?


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## Benuser (Oct 21, 2016)

Nemo said:


> How does a shinogi line look on a choil? A more pronounced angle?


Very exaggerated, concave. Think made with a huge waterwheel.


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## Nemo (Oct 21, 2016)

Benuser said:


> Once steering has been redressed, you better do. But I saw some left side convexity as well, and the edge having been recentered. Normally you have some steering to the left -- clockwise so to say -- you compensate by increasing the left bevel angle. Just to make sure: you're right -handed, aren't you? And it's no blade that has been neutralised for a left-hander?



I realise that I did the initial paper test wrong (I only held the paper on one side of the knife). When I held the paper on both sides of the knife, there was very little steering. Maybe a cm to the left (I thought I remembered it being away from the side of the thinning!) over the 21cm cut. Yes, I am a right hander and this is a right handed knife. Nominally 60/40.


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## Nemo (Oct 21, 2016)

Benuser said:


> Very exaggerated, concave. Think made with a huge waterwheel.



I obviously used the wrong word when I said "shinogi line". Clearly I should have left it at "relief line". They have a somewhat similar appearance from the side of the blade but it sounds like the cross-sectional geometry (which is much more important to the knife's function) is very different.

I am learning a lot from you, Benuser. Thanks again.


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## Benuser (Oct 21, 2016)

Some clockwise steering is common. Less friction on the left side due to the off-centered edge. Take the left angle higher and don't convex that thiny bevel.


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## Benuser (Oct 21, 2016)

My pleasure, Nemo


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## Nemo (Oct 21, 2016)

Benuser said:


> No need, but those are good figures. Some slicers are much fatter, others much thinner.



Indeed, the choil is a little fatter at 5mm than the rest of the blade: 5mm from the edge: Choil: 0.74mm, then gradually tapers from .68 to .58mm.

10mm from the edge: pretty constant at 1.1-1.2mm.

I sliced quite a few more pieces of copy paper today- I think that any steering is probably less than the variation in my (probably poor) slicing technique.


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## Benuser (Oct 22, 2016)

Have you measured above the bevel?


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## Nemo (Oct 22, 2016)

Benuser said:


> Have you measured above the bevel?



I hadn't but I have now .

0.33 at choil. Within 1-2 mm, it settles down to 0.22-0.23 which it maintains throughout the length of the edge.


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## Nemo (Oct 22, 2016)

That's at a 15 dps edge angle.


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## Benuser (Oct 23, 2016)

Seems quite OK to me. With slicers, all will depend on where you use if for and above all, what you like. Some want them stiff, and it isn't easy to have them both stiff and very thin. Thickness is of no importance with raw meat. With a cooked pork roast though you want your slicer to be very thin, and have to accept some flexibility.


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## Benuser (Oct 23, 2016)

Only now I realise it's a 300mm suji, so it needs to be a bit stouter.


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## Nemo (Oct 23, 2016)

Benuser said:


> Seems quite OK to me. With slicers, all will depend on where you use if for and above all, what you like. Some want them stiff, and it isn't easy to have them both stiff and very thin. Thickness is of no importance with raw meat. With a cooked pork roast though you want your slicer to be very thin, and have to accept some flexibility.



This slicer does a bit of both. It doesn't seem very whippy but I can cut reasonably thin (?5mm) slices of roast withut problems. My knife skills are OK but definitely not super so I guess a little stiffness is a good idea. OTOH I'm a home cook, so not as much time pressure.


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## Nemo (Oct 24, 2016)

Benuser said:


> Only now I realise it's a 300mm suji, so it needs to be a bit stouter.



Is it reasonable to continue to use it as is? Or should I thin out the right side of very heel a bit? Am I right in guessing the next thinning should focus a little more on the left side?


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## Benuser (Oct 24, 2016)

No need for thinning the heel area. I like the heel to be a bit more robust, but I'm used to old Sabs. But do verify whether the heel is protruding, though. Very common with sharpeners who aren't aware of it's relative thickness.

No as well for extra thinning the left side. As it slightly steers clockwise keep thinning on the right side, reduce the angle of the very edge, and increase the left one.


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## Nemo (Oct 24, 2016)

Thanks Benuser.


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## DanHumphrey (Oct 25, 2016)

Every time I read a thread like this it makes me afraid to put my knives on my stones at all! :O


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## Benuser (Oct 26, 2016)

It may seem a bit complicated, but that's because we're trying to restore more or less the original asymmetric configuration that has undergone some EdgePRO sharpening. If you follow the blade's geometry from the very first sharpening it's all much simpler.

http://www.kitchenknifeforums.com/showthread.php/5656-Asymmetry-–-The-REAL-DEAL


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