how far is too far for normal cutting : edge geometry

[Cliff Stamp]

I was repeating the work I had done with the 1095 paring knife with a larger blade in O1, which also had a similar hollow grind. I had used it extensively already just as EDC and set out recently to run it through a full standard of uses to benchmark it fairly quantitatively. I sharpened it just as with the paring knife, flat to the stone, starting from 220 SiC all the way up to a fine chinese waterstone which is finer than a 4000 grit synthetic japanese waterstone.

I used it in the kitchen on vegetables, meats, breads, etc., no problem, cut very well as expected. I then tried it out on fabrics, paper, cardboard, plastics, again no problem. I then took out a piece of hickory flooring and carved up a piece, about 200 slices, to my surprise the edge had folded over. Damage was barely visible, but I could feel it burred to one side. Now the 1095 knife easily did this with no damage, and in fact cut much harder woods outside.

It was odd I thought that O1 at 63.5 HRC was so much weaker than 1095 at 66 HRC, then I suddenly realized that even though both knives were sharpene by the same method since the O1 blade was much wider it was taking a more acute angle. So I do the math and determine the edge is sharpened at 2.3 degrees per side, the 1095 blade is at 4, so it is almost twice as obtuse.

So that is the lower limit, two degrees is fine for foods and most materials but you need more obtuse angles to carve hardwoods, how much more obtuse I'll find out over the next few days on some various types of wood.

-Cliff

 

[Mitchell Knives]

Wow, that's an extremely low angle. I generally run my knives in the 13 - 17 degree range.

 

[sodak]

That's pretty thin! I like between 10 and 15 deg total, but still have damage (sometimes) depending on the steel type and thickness behind the edge. But once you get them thin, they sure sharpen up fast, and stay that way for almost forever. It's definitely worth the risk once you get a feel for the knife.

My Dovo straight razor works out to about 12 - 14 deg total, but that incredibly deep hollow grind makes it quite fragile. I gave it a barely perceptible dent in the edge with the calipers, but it sharpened out fairly quickly.

 

[Habeas Corpus]

So that is the lower limit, two degrees is fine for foods and most materials but you need more obtuse angles to carve hardwoods, how much more obtuse I'll find out over the next few days on some various types of wood.

Interesting results. What I'd like to know is what the lower limit is for other types of steel. I noted that my Calypso Jr. in ZDP-189 has a somewhat lower angle than the gray Calypso in VG-10 and works really well at that low angle.

But which angles can one reasonably use with, for instance, S30V, or other common types of stainless steels?

Hans

 

[Cliff Stamp]

But which angles can one reasonably use with, for instance, S30V, or other common types of stainless steels?

Stainless can't come close to properly hardened tool steels, you have to go much more obtuse. I'll be doing more work of this type as I get the time.

I repeated the wood cutting with the angle doubled to 4.5 degrees per side, another couple of hundred slices and the damage was barely noticable.

There were a couple of rough spots made during the tougher cuts where the blade twisted, so about 6 degrees per side should be fine.

Sodak hit the critical point which is ease of sharpening, once you start working with blades of this nature you also start changing your methods as well.

You start using less and less force and more control, as a consequence the safety goes up.

-Cliff

 

[Habeas Corpus]

Stainless can't come close to properly hardened tool steels, you have to go much more obtuse. I'll be doing more work of this type as I get the time.

I agree, stainless doesn't usually come close to decent tool steels. I think a straightforward steel like 1095 works better than most "modern" stainless steels at low angles even if it hasn't been hardened to over 60 RC. 1095 seems me to be much finer grained than, say, S30V, and doesn't have any monster carbides that destroy the edge at low angles as soon as there's any sort of lateral pressure. With S30V I'm rather inclined to stick to something like 25 to 30 degrees total at the moment.

Of all the steels I've used over the years only ZDP-189 seems me to have any chance of coming anywhere close to tool steels. In some ways it behaves more like a tool steel than any other stainless steel I've used so far, for instance when it comes to sharpening. I think I really like that steel.

Hans

 

[Cliff Stamp]

I think a straightforward steel like 1095 works better than most "modern" stainless steels at low angles even if it hasn't been hardened to over 60 RC. 1095 seems me to be much finer grained than, say, S30V, and doesn't have any monster carbides that destroy the edge at low angles as soon as there's any sort of lateral pressure.

Yes exactly, even if the edge isn't strong enough, it will just roll a little on the fine grained steels where it just collapses/breaks on the coarse steels. You can actually take advantage of this if you leave the edges really coarse. Take a S30V blade and grind it really fine, <10 degrees per side and leave it with a 100 grit AO finish. The edge breaks apart, but it creates a really rough and jagged edge which will act like a fine serration pattern and cut for a very long time. This however will wear the blade out quickly.

Of all the steels I've used over the years only ZDP-189 seems me to have any chance of coming anywhere close to tool steels.

I have seen S90V as quoted as having a very fine grain structure as vanadium carbides are very fine. If you get a chance you might want to try that or S30V heat treated by Phil Wilson who gets them very hard and runs the high temperatures necessary to dissolve the coarse primary carbides. One of these days I'll get a small custom from him in S90V at 63/64 HRC and see how it runs that acute and thin.

-Cliff

 

[thombrogan]

Even a proper heat-treat won't let you get durable angles of less than 20 degrees with S30V. It's just a slightly tougher eggshell.

 

[yuzuha]

I've taken M2 down to around 7 degrees, but D2 and most stainless steels don't seem to hold up at anywhere near that angle.

 

[db]

I'd agree this would be much more interesting and informative if done with common steels at common hardnesses. I sure wouldn't want to put a 2 degree edge or even a 6 degree edge on my SAK. I think my Sak edges are around 15 degrees but I'd guess they could go lower and still be reasonably useful but I don't have any idea. Same with Ats at rc 60, S30v, D2, 1095 at factory made hardnesses, and others. It's kind of cool to see whatcustomised heat treatment and grinds can do, but most will neverhave or find a knife like that.

 

[Habeas Corpus]

I think my Sak edges are around 15 degrees but I'd guess they could go lower and still be reasonably useful but I don't have any idea. Same with Ats at rc 60, S30v, D2, 1095 at factory made hardnesses, and others. It's kind of cool to see whatcustomised heat treatment and grinds can do, but most will neverhave or find a knife like that.

I'm not so sure. For instance, I think 1095 can easily take quite a low angle even if it isn't at full hardness. The edge may roll in some situations alright, but you'll never see the edge just falling apart like in some steels with large carbides. AFAIK the grain structure is what's really important here - the finer the better. Such steels may not be quite as wear resistant as some others, but they do take a very nice edge, provided of course they're at least reasonably hard. I think a good example of that type of steel is Sandvik 12C27 with a decent heat treatment.

Hans

 

[Habeas Corpus]

Even a proper heat-treat won't let you get durable angles of less than 20 degrees with S30V. It's just a slightly tougher eggshell.

I've got no experience with S30V with a custom heat treatment, but I'm on the whole not that happy with that steel. I think it's somewhat overrated. It doesn't work at low angles, and at more obtuse angles it's difficult to get it *really* sharp for push cutting. (Or maybe that's just my lack of proper sharpening technique ... )

I think VG-10 is somewhat better in this respect. I'd quite like to know more about that steel, especially its grain structure. I've taken VG-10 to angles of about 20 degrees, and the edges seem to be pretty durable.

Hans

 

[sodak]

Some of this depends upon overall blade geometry as well, not just edge angle. I had an Opinel that I thinned the edge out to about 20 deg total with a high carbon blade (not stainless), and simple tasks like cutting fuzz sticks took large chunks out of the edge. There was some lateral pressure to the process, so I obviously (greatly) exceeded what this steel could take. The edge was better left obtuse, and the thinness of the stock would still make it a good slicer. Conversely, I have some thicker blades such as a CS Master Hunter and SR Howling Rat that I've thinned out to 15 deg total, and they show no damage performing the exact same tasks. They also have more steel behind the edge, even with a thinner angle.

Most of my stainless steels haven't done well at all with thin edges, even with thicker stock blades.

 

[Cliff Stamp]

It's kind of cool to see whatcustomised heat treatment and grinds can do, but most will never have or find a knife like that.

The above are optimal performance benchmarks. In regards to obtaining one, any custom maker can easily duplicate the heat treatment, this is why the full details were given along with the necessary information to duplicate the performance on other steels. They can simply be obtained by taking any factor blade and having a custom knifemaker reharden it.

In regards to exploring this limit on more common steels, note it is actually only possible to go as low as the above on particular grinds on very thin stock, in particular it takes a deep hollow grind to go significantly under 15 degrees and not have the sharpening become *really* inefficient due to the bevel becoming so wide sharpening times *slows*.

For example if I sharpened the above blade at 10 degrees I would be honing a strip of metal about 0.1 mm wide, very efficient even on hard to machine steels. In contrast if I honed a typical puukko at the same angle I would be honing a stip of metal 100 times wider which means I need to remove 100 times as much metal, really inefficient.

In general there are three types of materials in regards to setting edge angles for cutting (not chopping), by this I mean the cross section necessary to make a few cuts without immediate damage, fatigue and wear of course require more than the minimum :

-soft : paper, cardboard, ropes
-medium : thick plastic (bottom of a pop bottle) and hard woods (hickory)
-very hard : bones, thin metals (cans)

For soft materials there is no limit on edge angle, for thick plastics and hard woods the edge starts at about 5 degrees per side on optimal steels, for bones and metals, the edge angle is about 10 degrees per side on optimal steels. This assumes optimal techniques.

It would be interesting to take some more common blades and do the necessary modifications, I'd need a wet wheel grinder to do this, I may pick up one later on. It would be nice to have a nice hollow grind on a SAK. Krein also does the necessary mods for anyone interested.

[s30v]

...it's difficult to get it *really* sharp for push cutting.

The machinability is really low, the only way to sharpen such steels is to use a micro-bevel on a heavily undercut primary edge. You also need really clean and aggressive hones. The problem with a lot of the blades is that they are way under hardened and you end up with a weak steel which has a low machinability, this is the worst combination of properties for a knife.

Some of this depends upon overall blade geometry as well, not just edge angle.

Yeah, without the necessary edge thickness the very edge can remain intact but the whole bevel can deform, as what happened on the Extreme Judgement during batoning. The edge was fine, but the primary grind rippled, while the edge was obtuse enough it was too thin.

A lot of it depends on method, specifically how much torque and lateral loads the edge experiences. The more that these types of stresses are applied to the blade the more edge thickness needed in order to prevent it from buckling.

Most of my stainless steels haven't done well at all with thin edges, even with thicker stock blades.

They are simply not intended for cutting tools despite the propoganda to convince people otherwise.

Nice work in general with the reprofiling, informative, I hope you keep it up, the pictures and details are interesting.

-Cliff

 

[sodak]

Thanks, it's been fun and informative. I must say, the more that I read and test for myself, the more focused I am on what I want to buy next (and what I don't want to buy). So you could say that this is saving me money! Yeah, that's it!

 

[me2]

At widely accepted blade geometries, stainless steels seem to work fine. One stainless and one carbon steel in the same shape, the stainless held its edge longer. Of course this is with a pukko(sp) with relatively thick grinds, compared to what we're talking about here. From what I've seen, the more complicated stainless steels have higher heat treating temperatures, which lead to coarser grains which wont hold the thin edges that are being discussed. 12c27 is a relatively simple stainless, and one of my favorites. I wonder how it would hold up to this kind of geometry. I dont know the heat treating specs, but the temp is probably relatively low, below 2000 deg F maybe. Any one know the top hardness and HT specs?

 

[me2]

One more question. Do blades with very thin geometries at 56-57 RC roll when trying to sharpen them? I've got a chef's knife w/ a full flat grind on 2"x3/32" stock that I cant sharpen on my sharpmaker, the edge just rolls from one side to the other. I normally use my waterstone, which is 2"-2.5" wide. My next O1 blade may have to be near 63-64, since the one I just finished seems to be a little soft. Its full flat from 1/8" stock, ~57, with a very thin edge, and I had the same trouble sharpening it on the sharpmaker. The belt sander works for both with a 320x belt and a leather belt w/ white rouge.

 

[Blop]

I do think so. That´s why i think only steel which takes high hardness 60+ is right for very fine edges. If the steel is too soft, it will deform easier. If you change to broader angles this will be reduced. You might change to a little convexed edges. They are not as aggressive as straight V but the edge will be stronger against rolling.

That´s what i thought about setting low angles on a SAK. I wouldn´t do so.

At sharpening the stones should fit. Hard stones (hard Arkansas f.e.) tend to bend the edge over on soft steel.

 

[db]

Also, the Sharpmaker hones are pretty narrow your probably putting alot more pressure on the edge than you think. One good thing about convexing is you can thin and smooth the blade grind wile still leaving enough steel to support the edge. It's not a super thin hollow grind, but it does cut like crazy.
Cliff I'm sure most makers can do it, getting them to do it is another thing. Even if they can be talked into doing it I wonder if the preformance difference from a thinned out convex grind vs the thin hollow is worth the cost. Or, even noticable in general use.

 

[Cliff Stamp]

I'm sure most makers can do it, getting them to do it is another thing.

Many makers run such grinds naturally, Krein for example, it isn't much of a stretch for him to grind as thin as the above, he will even mod production blades and does a really nice job.

Even if they can be talked into doing it I wonder if the preformance difference from a thinned out convex grind vs the thin hollow is worth the cost. Or, even noticable in general use.

Yes, yes. A full flat/convex grind is much thicker, it isn't a small fraction. A hollow grind like the above can be less than 0.020" thick 1/4" back from the edge, in comparison, the main blade on a Rucksack, which is a thin blade with a full flat grind is more than twice as thick at the same point.

There is even a larger difference when it comes to sharpening, especially for full convex ground blades simply due to the difference in metal removal required. As noted in the above, even for moderate angles the difference can be 100:1.

Yeah, those effects are easily noticed.

One more question. Do blades with very thin geometries at 56-57 RC roll when trying to sharpen them?

As steels get weaker they will roll easier, the problem usually becomes an issue when the machinability is also low. Burrs can be a problem on the Sharpmaker due to the narrow stones, Clark has described in detail how to deal with the problem and his method works very well.

At widely accepted blade geometries, stainless steels seem to work fine.

Mainly because people have not compared them to properly hardened tool steels and worked with more acute geometries. Joe Talmadge for example noticed an improvement of hundreds of percent when he adjusted the edge profiles of common folders, many to one. Obviously then this starts becoming the acceptable benchmark for performance.

One stainless and one carbon steel in the same shape, the stainless held its edge longer.

Sure, not every carbon steel regardless of how it is heat treated will outperform every stainless steel regardless of how it is hardened. You can easily take a Case kitchen knife in a plain carbon steel at ~50 HRC and compare it to ATS-34 at 60 HRC and the carbon steel won't outperform it. Of course it should be obvious this is a lopsided and biased comparison. Reharden the case and see what happens. In terms of toughness, it is even more lopsided, the tougher tool steels are about ten times tougher than the common high carbon stainless steels.

From what I've seen, the more complicated stainless steels have higher heat treating temperatures, which lead to coarser grains which wont hold the thin edges that are being discussed.

It isn't the soak temps, M2 for example has a very high soak temp and is a very fine grained steel. The CPM's are also supposed to be fine grained due to the vanadium which will act to prevent grain growth during the soak. Where the problems come from is hard to determine due to lack of information. If the manufacturers/makers would give the exact heat treating recipes it would be easier to compare and high mag grain shots would remove all the guess work.


-Cliff