How quickly should an edge lose "Super Scary Sharpness"?

Uncle Rukus

Uncle Rukus

Joined
Mar 13, 2007
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133
Lets say you have a knife made of S30V and it's sharpened at 15 degrees with a 20 degree microbevel. The microbevel is taken up to approx 1800 grit on a spyderco UF ceramic and there is no sign of any burr under 100X magnification. How quickly should the knife lose it's hair cleaving, single ply toiletpaper cutting sharpness? I'm just curious because I seem to lose it after doing light cutting such as nylon cord or cardboard (light being maybe 4 of 5 cuts in cardboard). Is this normal or is something wrong with my edges?
 
I've been having that problem for a while with my Native. A lot of people said that sometimes the factory grinding can weaken the steel at the very edge, and after a few hand sharpenings you get to some good steel. I'm still having slight chipping near the tip, but I'm thinking this will probably improve after another few sharpenings.
 
Thats what I was thinking but I have the same problem with my BM 610 and I've probably sharpened it 15 times.
 
I have a BM210 in S30V and I noticed that, when it arrived from the factory, as scary sharp as it was, you could clearly see the grind lines on the bevel. I don;t know how they sharpen but it appeared to me it was done on a belt sander with maybe 600 grit or so.

Now admitedly, that's a pretty rough edge, but is it possible that 1800 is too fine for S30V? I sharpen mine with a Spyrderco Sharpmaker and I don;t do anything past the fine white rods. I think they're about 1200 grit or so. That little knife keeps its edge for quite some time....

I'm just guessing here..... just spouting a little to see what comes back....
 
Most of the time, that scary edge leaves pretty quickly, but you have a serviceably sharp edge for some time. With some better steels (52100, D2, INFI), I've been able to retain the scary sharp edge for some time, but it helps if you are whittling wood rather than cutting cardboard.

Most factory knives, I've noticed, seem to be sharpened on a belt sander then touched up by a buffing wheel. Which is why you see the rough grinds, but the very edge is grabby and polished.
 
Sodak said it already: The super scary sharp edge is gone very quickly. Cliff collected some very nice data on that and showed that blunting is highly non-linear, meaning the very, very fine edge goes quickly, after that the blunting process continues to slow.
 
Thanks for the input guys, I just wanted to make sure it wasn't just my edges that were losing scary sharpness quickly. I did suspect that edge degradation was non-linear, just wanted to see what everyone else thought.
 
I haven't found any steel that stands up to cardboard for very long. After a few feet of cardboard, a decrease in sharpness is noticeable to me. Nylon cord and poly rope is rough on an edge also. Keep in mind when I say noticeable, I mean Knife Knut Knoticble. If I handed that same knife to somebody else, it would still probably be the sharpest knife they ever handled. This forum has been both a blessing and a curse. :)
 
There have been reports for a while that S30V will microchip at the edge, sometimes during sharpening. Sometimes repeated sharpening and making sure the metal at the very edge wasnt fatigued or stressed solved the problem.

I have experienced the loss of the hair popping edge during cardboard cutting tests on my knives. A more polished edge helped on my Kershaw Vapor II. I was amazed that it would readily shave after over 300 cuts, approximately an inch long, on the same 1 inch of blade section. Of coure that was a mirror polished edge from stropping on a compound loaded belt on the 1x30 sander, but it out lasted my patience for that night. By readily shave, I mean it would shave against the skin w/o trouble, but not above the skin, like it would before the test. On another note, I had a Shrade Old Timer peanut that would still shave after 128 cuts through aluminum cans, ~3.5" long cuts, over the length of the blade, ~2 inches. Again, this was by no means an edge that would shave above the skin, but it would shave hair from my arm, and I suspect that I would have had a pile of Moutnain Dew cans at my feet waist high, and a week without sleep to clean them up, if I had kept going until it would no longer shave. It had the same edge as the Kershaw.
 
I don't understand the "hair-popping" edge. I can;t get any of my knives that sharp. All of them, even my D2 blades, will shave hair from my arm, with 154CM and 1095 doing it best (so it seems). Oh and the S30V...shaving sharp.

But I'm still hunting that hair-popping edge. My arm hair is very fine...maybe I need to drink some motor oil, I don;t know. Or maybe strop after sharpening. The thing is, I don;t want a hair-popping edge on my knives so much as I just want to see it happen once or twice.
 
Stropping helps a lot, especially if you use the green CrO from Hand American or Lee Valley or any other reputable supplier. One stropping paste that's *really* worked well for me is this one - http://www.classicshaving.com/catalog/item/522944/564416.htm. I don't know why, but this seems to get everything to that final stage for me, my razors "sing" when I use it, my knives that are very sharp can *easily* tree-top trim with it also. I highly recommend using this with a hanging strop.

One thing I've found that determines blunting is a combination of factors and how you cut. For example, I have some very hard knives in the 60 - 64 HRC range. If I get them wickedly sharp, and go and say, whittle soft wood (pine, aspen), they tend to stay wickedly sharp for a little while. Notice they are hard, and I'm push cutting into a relatively soft media with little abrasive properties. I can turn around with the same knives, and start pull cutting in cardboard or carpet, and lose that wicked sharpness almost immediately. It's almost like I'm ripping the edge off the blade.

Then again, I have a knife which is highly optimized for cutting (read thin), very hard (64), and full of very hard carbides (10V). It gets sharp but not wickedly sharp (shaves but not tree-top trims, although I haven't honestly tried to get tree top with this one) but loses almost none of it's sharpness in either cutting method for a *very* long time. And I mean very. I am being forced to get a scale and rig up a measuring method for sharpness with this knife. It's edge lasts long enough, that after *many* cutting sessions, I can't tell any edge degradation yet.

So I think geometry, heat treat, steel type, and - just as important - what you are cutting, and how you cut it all factor in to the equation.
 
It makes sense especially on a beveld edge, the thickness of the blade on the edge is already a curve, so as the steel gets thicker when the edge dulls more metal must be groudn off broken off or bent in order to further dull the knife.
 
smegs said:
I haven't found any steel that stands up to cardboard for very long. After a few feet of cardboard, a decrease in sharpness is noticeable to me. Nylon cord and poly rope is rough on an edge also. Keep in mind when I say noticeable, I mean Knife Knut Knoticble. If I handed that same knife to somebody else, it would still probably be the sharpest knife they ever handled. This forum has been both a blessing and a curse. :)
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My thoughts exactly. In my experience, VG-10 tends to keep a shaving edge a bit longer than S30V. That said I imagine S30V will keep a working edge a bit longer than VG-10, but I don't have any problems with VG-10 getting that far gone.
 
Steel is certainly a factor. Those with very high edge stability -- Sandvik stainless steels, for example -- will hold a very fine, acute edge much longer than a high carbide, low stability alloy when cutting relatively low-abrasion materials.
 
i dont know what you call scary sharp but my razor when its honed and stropped is scary sharp indeed, and it will lose that sharpness in only one shave so i guess it doesnt take alot. for knives that wont go near your face i´m satisfied with the brown SM-rods on the flat side, but thats just me.
 
The carbides in S30V are larger than your "scary sharp" edge. A super sharp edge is less than a micron, while S30V's carbides easily average 3 microns, with many larger. Also, a highly polished edge doesn't cut nearly as long as a coarse one when slicing, which I assume is the type of cut you are using to cut nylon cord or cardboard.
 
Where did you find this info on the carbide size? I've heard much hearsay on it, but never seen anything factual (including Crucible's data sheet on S30V; unless I missed it in there!). Just being curious, not doubtful.
 
Buck268 said:
Where did you find this info on the carbide size? I've heard much hearsay on it, but never seen anything factual (including Crucible's data sheet on S30V; unless I missed it in there!). Just being curious, not doubtful.
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My info is a combination of high magnification micrographs of S90V contained in its patent, compared with a lower magnification micrograph of S30V from a knife steel presentation sent to me by Crucible, and also measurements of carbide sizes from Roman Landes on Powder metallurgy stainless steels, I don't know if he has specifically measured S30V, but he has measured RWL-34, for one.
 
So if the carbides average 3 microns, with some larger, about how much larger are we talking? I've been going down to 9 microns (I think, DMT x-fine 1200grit) with mine and the egde retention has slowly increased while the chipping seems to be getting better. I don't use this knife very hard at all, so it only gets sharpened when I bored, but still. I stropped it a little today and seems to have helped bring out a better edge. Made a couple fuzz sticks and seems to hold that razor edge better than with just finishing on the XF DMT stone.
 
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