A visual guide to Edge geometry vs Blade geometry

[JRinFL]

It can depend on how the material responds to being cut.

All knives eventually taper to the full thickness of the stock. FFG cross section is a wedge, HHG is a curved sided wedge, like some axes. Flat sided with parallel sides might be the most efficient, but then everything would look even more like a cheap steak knife than they do now.

Not true. Some step up to full stock thickness at the plunge grind. You see it a lot on older traditionals, especially multi-blade models. Often the tang is the only part left full thickness. Oddly enough, those thin blades cut with much less effort than modern "tacticool" knives, but they don't make good crowbars.

Gah, burned by making another generalization. You are correct and I could have seen that by looking at some of the many traditionals I have sitting right here. Most of my knives taper to the full thickness, but not all.

You are also correct in that those old timers got a lot correct in their blade grinds. Knives made to actually cut with ease.

[sal]

Very good thread David. Thanx. I must say, "It all depends".

I'll get back into it when I have more time.

sal

[elena86]

First of all : thank you David for this thread. We are on the same page… once again. The answer is clear IMO : high hollow grind plus thin blade stock. One of the reasons I like the ARK so much. There is another option: slightly convexed full flat thin grind. The name of the game is : Opinel. Name any spyderco folder and my Opinel nr.8 will outslice it. And yes, thickness behind the edge IS important. I really hope Spyderco will make a shift regarding the grinding and the thick blade stocks. Talking of thickness bte, I love my Delicas but most are ground very thick behind the edge. Why ??? Warranty issues ?! Really ?! I bet most are not complaining about their Civivi blades. People will be tempted to do stupid things(like prying and twisting) when you give them a thick blade yet ground thin bte hence the warranty “issues”. They should use a chisel or a prybar instead. People should learn to use the right tool for the given task.Or just change the warranty policy but not deprive us of enjoying a thin bte ground blade. I don’t even use my plain blades to cut open thick cardboard boxes. I have other tools for that and sometimes( but rarely ) I use my serrated hawkbills( the Tasman or even the Ladyhawk).

[Wartstein]

Pretty obvious, said often already, but I still want to make clear:

When wishing for more thin(ner) blade stock, of course it is clear that this just one of several factors that (can) enhance the cutting/slicing performance of a folder.

Grind angle, thickness behind the edge, blade height, tapering, "sharpness", and so on are important factors too.

Thin(ner) stock can just provide a better "starting point" for having/making a slicier knife (as Davids pics somewhat show) than thicker stock.

So after really using and abusing knives like ffg Delica or Salt and Chap, and never having any problems with the thinner stock concerning "strength", but just enjoying their "sliciness", I feel that in a small folder there is really no need to start with thicker stock (despite thicker stock can be slicey too of course, but has a disadvantage right from the start)

Again, I´d like to know who has ever not just "somehow felt", but actually experienced that the ffg Delica or the Chap had "too weak" blades in any even just remotely sensible folder task.

That said :Of course thicker stock has other virtues: More comfortable to put a finger on the spine, larger lock interface for comp. lock or linerlock (but then there are 3 mm and even 2.5 mm comp.lock Spydies out there), or even that in certain tasks one actually want a more "wedge-like" blade.

[Wartstein]

... People will be tempted to do stupid things(like prying and twisting) when you give them a thick blade yet ground thin bte hence the warranty “issues”. They should use a chisel or a prybar instead. People should learn to use the right tool for the given task.Or just change the warranty policy but not deprive us of enjoying a thin bte ground blade. I don’t even use my folders to cut open thick cardboard boxes. I have other tools for that.

Well, I actually think my main example Chaparral is quite the opposite out of the box:

Thin blade stock, but for that rather thick behind the edge (never measured it, but did thin out mine a bit, just cause I felt it would perform even better then).

I am with you generally on "right tool for the given task"!

But, stupid or not (rather the former ) : For me the fascination of an EDC folder sometimes is to push it a bit into the direction of an "all around tool" - not that I´d actually pry with my folders (or just lightly), but twisting the blade in for example harder wood is something I do occasionally

Doing so with my Chap is exactly one of the reasons why I came to advocate for thinner stock (which, I know, is something different than your thin bte!).
Really, if I had the money I´d think about gifting a Chap to ten experienced forum members and ask them to really test the thin stock "hard".. I believe none of those would find it had just a "dainty gents knife blade"...

[olywa]

I think the perfect Spyderco response to this thread would be a Centofante 4 run in S90V or K390.

[Wartstein]

I think the perfect Spyderco response to this thread would be a Centofante 4 run in S90V or K390.

... and perhaps in higher sabre hollow grind or in ffg... ?

/ though with the high performance Delica a great "response" might be in the works already...

[Evil D]

Pretty obvious, said often already, but I still want to make clear:

When wishing for more thin(ner) blade stock, of course it is clear that this just one of several factors that (can) enhance the cutting/slicing performance of a folder.

Grind angle, thickness behind the edge, blade height, tapering, "sharpness", and so on are important factors too.

Slicing is the sum of everything you put into a blade. All other things being equal, thicker blade stock is going to make a thicker wedge to push through material. If you managed to make a blade just 1 atom thick, it would still slice better if you managed to make it 0.5 atom thick.

Blade height also has a huge impact on all this, which this pic should illustrate. Both wedges are the same "blade stock thickness" at the top and both are perfectly flat or "full flat grind" but it's not hard to imagine which is going to slice better.




Still, the problem of stock thickness still becomes an issue when material is rigid and doesn't separate around the blade, because eventually even a tall blade runs into the full stock thickness at the top and will start to wedge.

Blade grind is probably next in line. If you made them both hollow grind, it would make slicing easier up until... again...the full stock thickness hits the material you're cutting. When you have a low saber hollow grind you lose much of what a hollow grind can add because it doesn't have much room to transition into the spine, and you basically get a hollow wedge.

Thin blade stock alone won't get you there either if the edge bevel is 90 degrees inclusive and dull. You could have a blade that's only 0.25mm thick but if the edge is blunt then obviously it's not going to cut well (imagine a dull utility blade).

This topic to me is sort of like building a high performance engine. You need to match the parts to each other, they need to compliment each other. You can't just look at a catalog of parts and pick all the ones that seem like the highest performance and throw them all together. This is why back in the day when engines were carbureted, you couldn't just put a 1000cfm carburetor on your Ford 289 and expect it to magically make a bunch of power, it just doesn't work that way. In the same way you can put a super thin hollow grind and razor edge on a blade but it will only help so much if you start with a blade that's 10mm thick.

Of course all of this depends greatly on what you're cutting. I only used the rigid board as an example because it's something we all encounter and can cut to see this for ourselves and is a known rigid material.

[Wartstein]

Pretty obvious, said often already, but I still want to make clear:

When wishing for more thin(ner) blade stock, of course it is clear that this just one of several factors that (can) enhance the cutting/slicing performance of a folder.

Grind angle, thickness behind the edge, blade height, tapering, "sharpness", and so on are important factors too.

Slicing is the sum of everything you put into a blade. All other things being equal, thicker blade stock is going to make a thicker wedge to push through material. If you managed to make a blade just 1 atom thick, it would still slice better if you managed to make it 0.5 atom thick.

....

You framed a lot better what I tried to say.

[endura3]

Great example and observations! Thanks for sharing.

[yablanowitz]

I think the perfect Spyderco response to this thread would be a Centofante 4 run in S90V or K390.

... and perhaps in higher sabre hollow grind or in ffg... ?

/ though with the high performance Delica a great "response" might be in the works already...

Do any of you remember the passaround of a Tom Krein regrind Centofante4? I got in on it and it was pretty impressive.

For those who didn't, the blade had been reground from hollow saber to flat saber, the grind line had been raised substantially and the thickness behind the edge had been taken down to 0.010".

[Evil D]

I think the perfect Spyderco response to this thread would be a Centofante 4 run in S90V or K390.

... and perhaps in higher sabre hollow grind or in ffg... ?

/ though with the high performance Delica a great "response" might be in the works already...

Do any of you remember the passaround of a Tom Krein regrind Centofante4? I got in on it and it was pretty impressive.

For those who didn't, the blade had been reground from hollow saber to flat saber, the grind line had been raised substantially and the thickness behind the edge had been taken down to 0.010".

Sounds like it would be. I'm actually carrying my Centofante 4 today and I can see it would need to be taken down quite thin just to get rid of the hollow grind.


Another thing that may just need thrown out there, is that " saber grinds" are not inherently bad, it (again) depends on how high they're ground and how tall the blade is and how thick it is (once again, the sum of all parts). I would really like to see more HIGH saber grinds, like the Shaman, though with thinner blade stock and a thinner grind below the shoulder.

If you really think about it, what you really end up with is just a full hollow grind or flat grind but with flat stock above it. The flat blade above the shoulder should really only effect your cut in a turning cut and that's more due to blade height. Below the saber grind line is just whatever the blade grind is, and that's the thickest part of the blade. The issue is that many of these are done on short height blades and sometimes also thick blade stock, so they don't have much space to transition from edge to saber grind shoulder, and that's how you get a wedge. I think if more were done like the Shaman, particularly hollow grinds done that high on that tall of a blade, it would be a game changer.

[sal]

A lot of good useful information here. In my opinion;

If you are planning on passing through the material being cut, the ideal grind is the same as the Katana/ Wakizashi. Hamaguri grind (Appleseed) with the spine clipped so as not to create friction. No grind line or spine corner to create friction. This concept was developed over many years in Japan. Early tests were done on
human bodies.

If you are only going to go a few mm deep, like in skinning, then a hollow makes the most sense. Thin where you need it, and thicker support at the spine.

If you are going to cut boxes and go perpendicular, then a very thin blade like a box cutter works best because the structure of the box will pinch the blade.

Food will be similar and that's why kitchen knives are made thin as some food. like boxes, will pinch the blade in the cut.

However, boxes should be cut on the bias because now the "pinch" is is gone as the material will spread above and below the blade rather than side to side so the structure of the box won't be creating a pinch.

Good thread David.

As far as Spyderco making thinner blades, thinner behind the edge, or more hollow grinds, we're always working on that. We do have a "general" market to serve which expects different things from a knife than you. The High hollow Delica test will tell us a lot. If the "Afi's" want that type of knife, we can create a line for that.

sal

[bdblue]

I've always known that thin stock works better with rigid materials. As much as I liked cutting up boxes with my old Manix 2 I know that the standard boxcutter works better. Also something as simple as slicing an apple shows the problems with thick stock. But OTOH I've read the threads about "thickness behind the edge" and fully believed it all, never putting 2 and 2 together with respect to cutting rigid materials. I think thickness behind the edge still matters a little bit- the angle from the edge to the back of the primary grind is still a steeper angle than the angle from the edge to the thickest part of the blade stock.

[Evil D]

I've always known that thin stock works better with rigid materials. As much as I liked cutting up boxes with my old Manix 2 I know that the standard boxcutter works better. Also something as simple as slicing an apple shows the problems with thick stock. But OTOH I've read the threads about "thickness behind the edge" and fully believed it all, never putting 2 and 2 together with respect to cutting rigid materials. I think thickness behind the edge still matters a little bit- the angle from the edge to the back of the primary grind is still a steeper angle than the angle from the edge to the thickest part of the blade stock.

Oh it still matters a lot. Imagine how that Nilakka would cut if it also has a really wide bevel at just a typical 40 degrees inclusive angle like we might see on any other Spyderco that has been sharpened and used for a few years and maybe reprofiled a couple times to fix some edge damage, it would be far far worse.

I hope I'm making it clear that the Nilakka is not at all a bad cutting knife, quite the contrary that knife is a laser beam when cutting most materials.

[yablanowitz]

Hollow grind blades, especially hollow saber grind blades, tend to do the same thing. People seem to think "hollow grind = thin behind the edge = better cutter", none of which is automatically true. Hollow grinds *can be* thinner behind the edge, but mostly they aren't. And even if it is thinner there, a low saber grind usually erases that advantage on a cut more than a half-inch deep.

Yep, and even with a true hollow grind, even if it's a full height hollow grind, eventually the full blade stock thickness needs to move through rigid material and that thickness will still matter. For shorter height stuff there is definitely an advantage at least until you hit full stock thickness.

Just so we are on the same page, could you tell me what you meant there? It implies that a hollow grind can be either true or false, while geometry says it is either concave or it isn't. You may have a different definition of "hollow grind" than I do.

[Evil D]

Hollow grind blades, especially hollow saber grind blades, tend to do the same thing. People seem to think "hollow grind = thin behind the edge = better cutter", none of which is automatically true. Hollow grinds *can be* thinner behind the edge, but mostly they aren't. And even if it is thinner there, a low saber grind usually erases that advantage on a cut more than a half-inch deep.

Yep, and even with a true hollow grind, even if it's a full height hollow grind, eventually the full blade stock thickness needs to move through rigid material and that thickness will still matter. For shorter height stuff there is definitely an advantage at least until you hit full stock thickness.

Just so we are on the same page, could you tell me what you meant there? It implies that a hollow grind can be either true or false, while geometry says it is either concave or it isn't. You may have a different definition of "hollow grind" than I do.

Meaning that the blade is literally thinner behind the edge bevel than the top of the edge bevel itself is. I would argue that many blades that are called hollow grind would be more accurately called concave grinds, while a true hollow grind, though still concave by definition, should actually become thinner in the blade than the edge bevel itself. I believe CRK used to this but I'm not sure if they still do.

[SaltyCaribbeanDfly]

Amazing thread!!!

[gunmike1]

Hollow grind blades, especially hollow saber grind blades, tend to do the same thing. People seem to think "hollow grind = thin behind the edge = better cutter", none of which is automatically true. Hollow grinds *can be* thinner behind the edge, but mostly they aren't. And even if it is thinner there, a low saber grind usually erases that advantage on a cut more than a half-inch deep.

Yep, and even with a true hollow grind, even if it's a full height hollow grind, eventually the full blade stock thickness needs to move through rigid material and that thickness will still matter. For shorter height stuff there is definitely an advantage at least until you hit full stock thickness.

Just so we are on the same page, could you tell me what you meant there? It implies that a hollow grind can be either true or false, while geometry says it is either concave or it isn't. You may have a different definition of "hollow grind" than I do.

Meaning that the blade is literally thinner behind the edge bevel than the top of the edge bevel itself is. I would argue that many blades that are called hollow grind would be more accurately called concave grinds, while a true hollow grind, though still concave by definition, should actually become thinner in the blade than the edge bevel itself. I believe CRK used to this but I'm not sure if they still do.

My Sebenza 31s are both thinner above the edge than the top of the edge bevel, so they are still grinding them that way. It is nice to know as you sharpen the edge will get thinner. If only they came ground thinner from the factory it would be nice.

My best slicing pocket knife is a custom slipjoint in AEB-L. It has ~.060” thick stock and was ground to a near zero grind. It is an amazing slicer and cuts better than many factory pocket knives when it is dull. As many have said before, geometry cuts.

[Evil D]

I made it this far before it started to feel like I was carving wood, I had to angle the cut or there was no way I was making it all the way through this box.






If I ever find someone skilled enough to trust who can do a large radius hollow regrind, this will probably be the first one I have done just to see how it changes.