How to categorize my sharpness?

[knifeydsd]

First off, could I recommend a sticky somewhere for what I'm going to ask? Cause I know this is a bad question with answers everywhere, but not sure where to look. I tried the search feature, but it didn't work, so, here I go. Yes, got my blade back from Ben. Very nice, but I have no idea how to quantify. He reprofiled to 19 degrees (does anyone know what a stock endura 3 had as a angle degree?). Anyway, you can't feel the edge with your finger- it probably is piercing the first layer of skin, so it feels smooth anyway, what tests can I do to show how sharp it is? I have no idea how one is to cut through a sheet of paper hanging from the ceiling, or through a plastic bottle- are there any primers that you guys have? I just want to do his work justice before I talk about it, cuz I know enough to know I know nothing.
Oh, and finally: what tests are good for a mirror edge vs. a microserrated one? Cuz I tried to show it to a chef friend, who says it isn't that sharp, but he was trying to do a pull cut on cardboard- is he right, wrong, or what? Anyway, thanks for any help.

 

[Cliff Stamp]

I tried to show it to a chef friend, who says it isn't that sharp, but he was trying to do a pull cut on cardboard- is he right, wrong, or what?

He is right in that sharpness is specific to the media and how it is cut. A couple of easy tests for sharpness are to push cut newsprint and slice a fairly thick piece of rope. The former shows push cutting sharpness while the latter shows slicing aggression. Rope cutting is also very sensitive to the geometry of the knife, in particular the edge angle so you can't compare the sharpness of two dissimilar knives with thick ropes.

-Cliff

 

[sodak]

Try the newsprint. Measure how far away from where you are holding it you can push cut it. I'd like to know, Ben is quite well known for his edges.

 

[knifeydsd]

I read about the newsprint thing, but was unsure exactly what was meant- i.e., when pushed down it cuts 4 inches? Or when you drop the paper onto the knife it cuts 4 inches? Or how far down the handle you hold it when you cut?


------------------------------------____________
----------------------------------- ___________/


Alright, so: if the dotted line is the handle, the solid line the blade, then
Do you mean:


10---9----8-----7------6-----5---4-----3---2-1-____________
10---9----8-----7------6-----5---4-----3---2-1-___________/

What is the highest number I can hold on the knife while the blade cuts through paper?

And final question:

-----------------------------------------_____________________
-----------------------------------------__6__5__4__3___2__1_/
Would it matter if I cut with the tip (number 1), or nearest the handle (number 6)?
Because I'm trying to challenge my leatherman's crappy stainless to my endura; and they both cut newsprint; but the leatherman doesn't pop hair off my leg so... help? could anyone post step by step pictures of how to do these tests? I barely passed high school geometry for a reason... no talent for visualizing geometrical stuff in my head.
Anyway, much appreciation for helping out a novice of knives.

 

[Cliff Stamp]

I read about the newsprint thing, but was unsure exactly what was meant ...

Hold a piece of newsprint between your thumb and index finger. The further you can cut from the point, the sharper the blade. Cut directly down into the paper with no slicing motion and the blade perfectly square to the paper.

-Cliff

 

[gunmike1]

Ben said he sharpened my Spyderco R2 at 17 degrees, but trigonometry tells me it was about 12 degree per side. The edge pro settings don't take the angle of the blade on the platform into account. My enduras were right around 15 degrees per side from the factory, so the 19 degree setting sounds about right in comparison to my R2.

Oh yeah, and after Ben Sharpened my R2 it could push cut newsprint over 4" from the point of hold (method as described by Cliff), and tree top arm hairs. The factory edge could push cut newsprint about 3.25" from the point of hold, which was the sharpest I had seen until Ben sharpened my R2 (it also tree topped arm hairs). The factory edge did have more slicing aggression, though. And if you care about how an edge looked, the polished edge was pretty. Personally the only thing I really care about is how it cuts, and after Ben sharpened it it was pretty in that respect too, as it could cut like hell.

 

[kel_aa]

If I recall correctly Cliff says sharpness? is linear to edge angle. So 15/12 = 1.25. And so 3.25" x 1.25 = 4.06". But off course the newspaper relationship is definately not linear. How much of the increase is possbile from the angle decrease as opposed to Mr Ben Dale's sharpening?

 

[Dog of War]

If I recall correctly Cliff says sharpness? is linear to edge angle. So 15/12 = 1.25. And so 3.25" x 1.25 = 4.06". But off course the newspaper relationship is definately not linear. How much of the increase is possbile from the angle decrease as opposed to Mr Ben Dale's sharpening?

I haven't found that edge angle strictly correlates with sharpness as measured by push cutting newsprint. I have two knives that can be sharpened on a ceramic CrockSticks sharpener giving edges that push cut newsprint just over 4" from point of hold, both with a 20 deg/side microbevel. Since this angle seems more optimal for these steels in terms of edge retention as well, I assume there are edge stability issues actually affecting the sharpening process. Also I imagine that at the very start of a push cut into newsprint there's virtually no wedging effect, so whether the apex is 20 degrees or 40 degrees doesn't really matter, just the thinness of the edge at the very apex.

 

[Cliff Stamp]

If I recall correctly Cliff says sharpness? is linear to edge angle. ?

Sharpness is independent of edge angle as I would define it as the condition of the very edge. Cutting ability on binding media tends to be proportional to edge angle as a first approximation. Since newsprint isn't very binding in single sheets you would expect only a slight dependence on edge angle. If however you tried to press the blade through a book then you would find the edge angle was very critical.

-Cliff

 

[kel_aa]

Thanks for the correction. Does anyone have any ideas on the ease of obtaining any level of sharpness at various angles? I understand that with lower angles the horizontal component of the forces on the edge will be higher as you sharpen and hence more likely to be subject to deformation rather than abrasion. For some of the blades whose primary grind allows an aggressive edge angle, what are your experinces at making use of them as opposed being more conservative (10/12 vs 15/20), with respect to obtaining any level of sharpness?

 

[Cliff Stamp]

Does anyone have any ideas on the ease of obtaining any level of sharpness at various angles?

It is actually easier to get a higher sharpness, if you define the term to mean the amount of imperfection at the edge, at a higher angle. Verhoeven showed in detail how increasing the edge angle would reduce the burr width at a given finish. However this tends to not be what most people would say because they are really measuring cutting ability when they determine sharpness.

If you give most people a blade with an angle of 10 degrees vs one of 20 degrees, even if the 10 degree blade has a large burr, most will still call it sharper because it cuts way better for most tasks. If the edge thickness is also really reduced then this difference can be massive, many times to one and thus for most people sharpening at lower angles is far easier because the inherent cutting ability is much higher.

However, if you consider sharpening from the perspective of just the last 0.1 mm or so and view the rest as just shaping, then it is harder to achieve a high sharpness at acute angles. Especially if you are working with high carbide steels because they will fall apart at low angles. This of course is only in regards to a high push cutting sharpness. For coarse edges, the carbide instability of high carbide steels doesn't limit the sharpness to the same extent.

-Cliff

 

[a223cat]

Then Cliff, If I understand you, their is a point at which S30V's angle is too small to be usable? Any idea what angle that might be on a decently heat treated S30V blade? Then I should not try to get that blade under 10* or so?

 

[me2]

It is actually easier to get a higher sharpness, if you define the term to mean the amount of imperfection at the edge, at a higher angle. Verhoeven showed in detail how increasing the edge angle would reduce the burr width at a given finish. However this tends to not be what most people would say because they are really measuring cutting ability when they determine sharpness.
-Cliff

This must be whats happening to my Kershaw Vapor. I sharpened it on the 320/leather belts on the sander and have been touching it up on the medium Sharpmaker rods at a much higher angle. The sander is about 15 degrees, the touch ups are at 22-25 degrees. However, the touched up edge will whittle hair as well or better than the stropped edge off the sander.

 

[Cliff Stamp]

Then Cliff, If I understand you, their is a point at which S30V's angle is too small to be usable?

The high carbide steels are all limited in the ability to take and hold a high polish at acute edges. This refers to the angle of the very edge so when issues are noted the edge is just microbeveled at a more obtuse angle, the entire edge does not need to be reground. Depending on the level of carbide then the width of the microbevel has to be adjusted but you are still speaking of a small fraction of a mm.

I looked at S30V in the small Sebenza in acute edges and the performance was very poor in regards to edge stability and even at a microbevel of 20 degrees it fractured readily unless I thickened the microbevel to above 0.15 mm wide. However that blade also had some very low performance in other areas which has lead me to conclude that it is defective because I have seen better performance in other areas. I have been meaning to look at the South Fork in detail (custom S30V, Wilson) with high polishes and investigate that matter specifically.

S30V according to Landes perspective is a Type II class steel in regards to edge stability so the edge responds best to high polishes in the 12-20 degree range. This is a wide range as he only uses three classes for every every steel from the ultra fine to excessively coarse. Based on work to date I would put S30V in the middle range which agrees with gud4u's microbevel comparisons.

This must be whats happening to my Kershaw Vapor. I sharpened it on the 320/leather belts on the sander and have been touching it up on the medium Sharpmaker rods at a much higher angle. The sander is about 15 degrees, the touch ups are at 22-25 degrees. However, the touched up edge will whittle hair as well or better than the stropped edge off the sander.

I had a similar issue with the Vapor, the problem here isn't the carbide structure as AUS-6 is a very fine carbide steel but Kershaw is significantly underhardening it which makes it difficult to sharpen to acute angles as the burr formation is increased with a lower hardness. AUS-6 is actually on the same tie line as 440C which means it has a similar maximum hardness. It is however just used as a cheap steel in "beater" class knives so rarely is optimally hardened.

The common underhardening of steels in cutlery leads to some pretty severe misconceptions in general. One of the main ones is looking at steels with about 0.6% carbon as being fairly soft whereas in reality that much carbon allows full hardness of 66 HRC. Of course other alloying elements in the austenite will increase its stability and lower as-quenched hardness so you need oil/cold to maximize hardness in the high alloy steels.

-Cliff

 

[hardheart]

what depth are you looking for in the newsprint test? I sometimes can get a cut to start, but wedges at the height of the relief. Does that just mean the edge/microbevel are clean and the angle is too great? I just decrease the distance until I can cut the full height of the primary. If that ends up <1", I do some work on it.

 

[Cliff Stamp]

Generally I have not found wedging to be an issue because newsprint opens readily when cut, even with thicker papers like cardboard, the most difficult part I have observed is starting the cut. Due to the nature of the force mechanics, when the cut is started, a blunter blade should be able to continue. This is why for example you can often start on a slice but continue on a push.

-Cliff

 

[a223cat]

Cliff, it was a Sebenza (small) that I was referring to on angles to best sharpen it. Are you thinking just your sample was not hardened properly, or do you think it might be a CR wide problem?

I dislike the stock angles now and want to go more acute but dont know what I can get away with safely and still get a workable stable edge. any best guesses?
and have you tried any Strider s30Vblades?

 

[sodak]

In reference to the newsprint, what Cliff said. I can usually hand sharpen to the point where I can hold the newspaper in a pinch grip between my thumb and forefinger and push cut newspaper 2 inches away from my grip. If I'm holding the paper vertical, the push cut is straight down to the ground, not angled away from my grip. That's cheatin'.... If I spend a lot of time on it, I can go out to 3 inches, 4 is currently beyond my skill even with my Edgepro. That's amazing...

 

[Dog of War]

what depth are you looking for in the newsprint test? I sometimes can get a cut to start, but wedges at the height of the relief. Does that just mean the edge/microbevel are clean and the angle is too great? I just decrease the distance until I can cut the full height of the primary. If that ends up <1", I do some work on it.

I've noticed this as well when you're dealing with edges in maybe the ~2.8" push cutting range or better. There's another interesting behavior at this level of sharpness as well, the knife actually starting a push cut and cutting maybe 1/16-18" deep without your being aware of it unless you're watching carefully (not a good explanation I realize, maybe you've observed this as well and can describe it better?)

There are other peculiarities I've been noticing with newsprint as well, my guess is they mostly relate to different tensions in the paper that show up depending on actual size and even weight of the paper, how it's held, and how it flexes when cutting.

Generally I have not found wedging to be an issue because newsprint opens readily when cut, even with thicker papers like cardboard, the most difficult part I have observed is starting the cut. Due to the nature of the force mechanics, when the cut is started, a blunter blade should be able to continue. This is why for example you can often start on a slice but continue on a push.

Cliff, maybe hardheart and I can persuade you to look into this some more sometime, IMO there is something going on .... perhaps not wedging in the usual sense but rather just enough extra resistance being met while the cut is still quite shallow due to the paper not splaying away from the cut.

 

[Dog of War]

If I spend a lot of time on it, I can go out to 3 inches, 4 is currently beyond my skill even with my Edgepro. That's amazing...

For me the "leap" from getting maybe 2.5" sharpness into the 3"+ range and even 4" with a couple of knives was all in using an incredibly light touch; I use ceramic rod sharpeners but I suspect the same principle applies regardless of method, and probably easier with an Edgepro because of the larger contact area.

I would describe the difference as almost shocking it's so subtle, just 3-4 feather-light passes is often all it takes. At first I thought this was so minimal that something wasn't right, so little extra metal would be removed after all .... and I really wondered if such an edge was any good in terms of retention, but you do see the benefits of high initial sharpness like that even after a fair amount of use.