Totally Unscientific Test: Izula at 40 degrees vs Izula at 20 degrees

[KennyB]

My girlfriend was over this weekend with the Izula that I got for her. I'm not usually one for all that knife testing stuff but I realized that I had two identifical knives, ground at to distinct angles, so I figured why not go and see how they differ performance wise. Mine was the 20 degrees inclusive grind, hers was the 40 degrees inclusive from the factory--both were touched up on my little fine diamond hone until I felt they were just about the same sharpness.

What I was cutting? Some dead, dirty, ratty, weathered old bamboo stalks I found on a buddy's ranch that had been sitting out in the dust and dirt in a pile for who knows how long. The task was simply to slice half-way through the stalk, which was about an inch to inch and a quarter thick. Here is how each knife did...

40 Degree Izula
This one got all the way to half-way through and didn't show any signs of wear on the edge. You could tell that it was duller, but the edge was still in good shape, the apex wasn't dinged or rolled over. After the first few cuts though, it became quite difficult to force the blade through.

20 Degree Izula
This one rolled its edge almost immediately. Seems like right after getting through the hard, weathered outer surface... In any case though, even rolled over, the more acute edge glided through the bamboo with much less effort and even when taking much bigger carvings I was able to slice the stalk in half in almost half the time as with the 40 degree edge.

At the end of the test, each knife was tested on a tree limb to see how sharp it still was. Neither of them were anywhere near arm/leg hair shaving sharp anymore, but both still shaved the bark off the limb and whittled the hardwood underneath.

I think this test kind of shows a couple of interesting things to me. The thing that really surprised me was that the thinner edge still cut much better than the more obtuse one even when it was definitely rolled over. I wonder if one that had been ground at 30 degrees inclusive wouldn't be a good middle ground between the two.

However, it got me thinking, if you had to choose between the two... The thin edge that rolls but still cuts better, or the obtuse edge that holds better but doesn't cut quite as well? I guess this is really the choice we always make when deciding on edge angle, but I never really considered the aspect of what if a thinner edge, even when dulled, cutting better than the obtuse one.

I'm personally not sure what to think about it. On the one hand bamboo is a pretty soft wood so I don't know that the fact it could still cut that should be an indicator, but I could also still cut through bark and other wood pretty easily. What occurred most to me though was how insignificant the rolling on the edge really was, because a handful of passes on the hone set it back straight.

I didn't test any other materials to see if the knife would still cut. I figured if you're out cutting bamboo, you're probably going to be cutting mostly bamboo or other vegetative type materials. Probably should have thought of some twine or something like that though. In any case, going by the good ol' finger test I would bet that the edge on the 40 degree Izula wouldn't have a problem cutting printing paper or opening packages with ease after all was said and done, but the 20 degree Izula on the other hand was just a little too roughed up to do anything but wood-work until I touched it back up on the hone.

So, yeah, anyway... Just sharing, if anyone finds it interesting at all. Questions or comments welcome

 

[Stark Knives]

Nice report! Would be better with pics though.

Erd

 

[Jason B.]

First problem is you have 1095 at a low hardness with a extremely low angle of 20 degrees inclusive. The edge stability at that point is not enough to support the thinness of the edge so rolling sounds about right.

Next, bamboo is very tough and not a soft wood.

Your results sound correct to me, try and keep it above 30 inclusive and smooth the shoulder some. Adding a little convex flow to the thicker izula blade will help with its cutting performance.

 

[P2P]

First problem is you have 1095 at a low hardness with a extremely low angle of 20 degrees inclusive. The edge stability at that point is not enough to support the thinness of the edge so rolling sounds about right.

Next, bamboo is very tough and not a soft wood.

Your results sound correct to me, try and keep it above 30 inclusive and smooth the shoulder some. Adding a little convex flow to the thicker izula blade will help with its cutting performance.

I'm trying to make sure I understand here.

So your saying rebevel to 30 inclusive? Then smooth the shoulders? Is this like a micro-bevel?

 

[bergholz]

I convexed the whole edge on my Izula. Seriously it cuts so much better now. All around really, keeps an edge longer too. It made this knife a must carry for me and I haven't stopped carrying it since. Here's a picture, hope you can tell.

 

[Jason B.]

@P2P, More like a convex, the thicker blade of the izula causes resistance in the cut. Allowing material to flow smoothly over the surface reduces this resistance.


I'm saying to not go so low, keep the angle a little higher on softer steels. If a steel is really hard you can make the cutting bevel very thin, if not, or the blade is very thick then changing geometry such as convexing the edge is often the solution. It thins the metal behind the apex while keeping the apex angle more obtuse.

Doing small tests such as the OP's to the knives we carry every day will show you the strong and weak points on everything from the cutting edge to the handle ergonomics.

 

[Obsessed with Edges]

(...)I think this test kind of shows a couple of interesting things to me. The thing that really surprised me was that the thinner edge still cut much better than the more obtuse one even when it was definitely rolled over. I wonder if one that had been ground at 30 degrees inclusive wouldn't be a good middle ground between the two.

I wanted to shout to the rafters when reading this, about the significance of the blade & edge geometry. I've noticed this too. If the material being cut is very thick at all, most of the difficulty in cutting it will come from the friction against the sides & shoulders of the blade & bevel. I see this every time I slice a simple ol' apple. I've used some of my sharpest-edged knives (Opinel, sodbuster), and also a simple and very dull kitchen paring knife, albeit with a very thin blade. Never ceases to amaze me, the simple & thin paring knife, with it's flat and thin profile, is the easiest cutter of all. My dad actually uses a simple butter knife to push-cut through an apple, and even that works well enough. That blade is equally thin, and the edge as blunt as can be. The thin blade geometry makes all the difference. Even a simple box cutter, with it's thin utility blade, keeps on cutting cardboard long after the edge itself has lost it's shaving sharpness. Not quite so smooth, but it still works.

However, it got me thinking, if you had to choose between the two... The thin edge that rolls but still cuts better, or the obtuse edge that holds better but doesn't cut quite as well? I guess this is really the choice we always make when deciding on edge angle, but I never really considered the aspect of what if a thinner edge, even when dulled, cutting better than the obtuse one. (...)

My preference will always lean towards thinner. Maybe not quite so thin as 20 inclusive, with relatively softish 1095 (maybe 28-30 degrees would suit me here). But I find it very easy to quickly touch up a thinner edge that's dinged or rolled slightly, and keep on working. With very thin edges, I'll also be much more picky about how I actually use such a blade. Most tasks that would easily ding or roll an edge on a knife, might better be done with a more appropriate tool anyways.

 

[Robs92XJ]

I've definetly noticed that my scandi knives will still cut wood extremely well even if there are rolls or chips in the edge. These knives are around 20 degrees inclusive. It's not even that great of a difference from when they were freshly sharpened. Really it only affects paper cutting and shaving ability. For me that negates most of the advantage to the "tougher" knife grinds with thicker edges, for outdoors work.

 

[KennyB]

First problem is you have 1095 at a low hardness with a extremely low angle of 20 degrees inclusive. The edge stability at that point is not enough to support the thinness of the edge so rolling sounds about right.

Next, bamboo is very tough and not a soft wood.

Your results sound correct to me, try and keep it above 30 inclusive and smooth the shoulder some. Adding a little convex flow to the thicker izula blade will help with its cutting performance.

Yeah, I think I definitely underestimated bamboo. I tried cutting up a bunch of different stuff today... A good 16" by 6" piece of cardboard, sliced it up across the corrugations to little bits. Then I went and cut up a bunch of pieces of deadfall around some of my trees. Not sure what all type of trees they are, but there were a couple... Pine, some apple trees, an oak tree, and I think a sycamore but I'm not sure on tree types. No real testing method, just stripping the bark and twigs off and slicing up the wood underneath quite a bit.

Anyway, it handled all that without rolling or really even dulling very much. I think the bamboo was quite a tall order for it. The difference was quite massive really.

What types of materials would compare to bamboo?

 

[me2]

You said after the bamboo that neither edge would shave. Was this after only one cut? Would they shave before the cut(s)? For plain cutting ability, a Victorinox paring knife sets a high standard. By cutting ability, I mean how difficult is it to push the knife through the material. This is completely geometry governed, unless the edge is taking damage like rolling or denting. Obviously when the edge turns side ways significanly, like a dent, or pieces come off, cutting gets much more difficult. Did you by chance try steeling the rolled edge to see what happened?

You may want to try a 15 degree per side microbevel, and if it holds, just keep sharpening there until the cutting ability drops off unacceptably, then reset the 10 degree back bevel, if need be.

 

[hardheart]

The thing to keep in mind is that the only part of a knife that cuts is the edge apex. Every other bit of metal beyond that only hinders cutting ability when contacting the material. The blade needs to be thicker than a micron because we don't have materials able to withstand use at that thickness - but every bit you add to thicken the blade for increased strength then requires increased force to cut. As knifenut said, improving the material of the blade allows you to go thinner, and you should go as thin as you can for the best cutting. The thinner the blade, the duller the edge can be while still cutting for a given force. Getting steel sharp is not that difficult, and it will always dull in cutting, so optimizing the design to allow the blade to cut even after those first few microns are worn away is important to cutting life.