Verhoeven Experiments on Knife Sharpening--Wow!

[Cliff Stamp]

I notice a great improvement with an unloaded leather strop on some of my edges, but he seems to think that isn't the case.

I have tested sharpness after CrO and after leather and then paper and see improvements in sharpness, but they are small, on the order of 10%. He is judging primarily by edge width and the variance along the edge is significant and thus you need in general a difference much greater that that to be noticed. In short, he is making a very coarse measurement. You can judge by feel much smaller differences that he can measure by edge width by visually comparing two sections. You could measure this to a high degree but you would want mean and variance estimates along the edge, which would require computer analysis or a lot of eyeball time. Otherwise you are left with a coarse estimate like 0.2-0.4 microns, note the limts are double one another. Just think about what you can tell as a change in sharpness.

He states that trailing strokes on waterstone produces larger burr formation presumably because debris is carried to the edge. On the other hand, CrO loaded leather clearly removes significant amounts of material, yet does NOT produce a significant burr formation, even though the stoke (or wheel direction) is always trailing.

Debris can be carried in the swarf of waterstones, in the loaded leather it sticks into and becomes part of the abrasive.

I also start to wonder which edge is perceived to cut better.

This depends on what you are cutting and how, push/pull and how much force. On slices there is a critical level of force required to start the sawing action, if you go under this the aggression is low, however once you exceed it, there is a *massive* change in cutting ability. I assume it is because at low loads the pressure on the teeth doesn't exceed the rupture pressure of the material and it just flows over them. He uses the same defination of sharpness as Lee and J.J. that a straight razor is optimally sharp. I don't like this defination because the cutting ability isn't optimal with that finish on all media with all methods. I fail to see the logic in calling an edge finish sharper when it cuts worse.

When I diamond stropped my plastic film knife its free hanging plastic film cutting ability went down.

How are you cutting it? Is there an improvement in push/pull cutting sharpness on the diamond, you can measure this quantitatively by just cutting newsprint and using the length away from the hold point to measure sharpness.

-Cliff

 

[HoB]

Debris can be carried in the swarf of waterstones, in the loaded leather it sticks into and becomes part of the abrasive.
-Cliff

I have thought of that but I don't believe it. A) I don't really believe his debris argument at all, just look at the massive burrs that the wheel that is rotating into the edge is creating. B) Even if the swarf is carrying debris, you still need to have it somehow bond to the edge, or you simply could wipe the burr of by holding it under running water or by simply running your finger down the edge. But the motion on the waterstones isn't fast enough and the water would prevent that and I would be surprise if even the dry wheel is running fast enough to have that occure. Verhoevens other argument of deformation for burr formation, which Jeff pointed out, makes a whole lot more sense to me. The leather wheel or strop would then reduce burr formation because it distributes the load at the edge.

 

[Cliff Stamp]

Even if the swarf is carrying debris, you still need to have it somehow bond to the edge, or you simply could wipe the burr of by holding it under running water or by simply running your finger down the edge.

The burr isn't formed of the debris, it is made from contact with the debris which doesn't cut the steel just deforms it, it just acts like you are not sharpening the blade but actually using it to cut something. This is also used to explain why the edge irregularities are so large compared to the abrasive marks.

In edge trailing the edge is layed on the abrasive and just rides over it, getting abraded from underneath. On edge into honing, this abrasion is still present but there is also the action of the abrasives which are higher than the edge now ploughing into it directly. It would be hard to imagine the resulting honing would be idential in behavior.

Consider the effect on the burr which always forms on the side opposite to the abrasive. Take a blade which has been finished honed on one side and thus has a prominent burr, if you place it edge down and trail it can be pulled back out straight, in fact this is the exact action the deformation will produce. However if you reverse the action, the deformation will act to push it further and cut it off.

Note there is always deformation when using an abrasive, when an abrasive particle meets steel, the steel will start to compress as will the abrasive under the influence of the shared action/reaction force. However the compressive strength of abrasives like silicon carbide is 3900 MPa compared to ~2500 MPa for 60 HRC alloy tool steels, thus the steel will be readily cut by the abrasive because it will yield long before the abrasive.

However there will still be deformation around the cut, as the bonds have to be broken which will lead to plastic and likely fracture damage, how much depends on the steel, really weak steels will deform readily and form large burrs, this is common knowledge, especially for people who hone at low angles, Verhoeven just verified this under magnification directly, some of the more brittle steels can also fracture under heavy pressure and waterstones are known to produce better edges. Less abrasive hones and dirty hones also induce more burrs as does excessive pressure.

There are a number of interesting questions about the pictures, note that the maximum edge irregularity is massively larger than the abrasion lines as noted. It would be interesting to have treated the edges to see the grain and carbide structure. Is this a factor, or is it just an effect of random distribution of abrasives or the debris hypothesis which Lee also shares. Note the really low angle edges show stress cracks running 90 to the edge.

Also note the contrast with J.J. findings which showed a dry hone was superior, Verhoeven showed the opposite. I would also be really interested to see how the actual edges cut and stayed sharp. As well what about varying the intersection of the grind lines, a maker friend of mine is convinced from his experiements that you get the best edges if these cross over each other at 90 vs running parallel, it is pretty easy to imagine why with even simple visual experiments.

-Cliff

 

[STR]

I was hoping after getting there toward the end of the study that he would touch on blue jean stropping and show us any effect if any like with the non loaded leather. I know a lot of guys that strop their knives on their jeans including myself. I can't say I've ever seen or felt any significant improvement doing this but it would be nice to see how it fairs compared to un loaded leather. I suspect it would be the same but can't say for certain.

In line with the same way of thinking there are some that use cardboard to finish strop instead of leather. I have noticed that this sets the edge up nicely and do it a lot using the back card from a legal pad. I'd love to see comparisons of this to both plain ole leather and loaded leather.

STR

 

[HoB]

The burr isn't formed of the debris, it is made from contact with the debris which doesn't cut the steel just deforms it, it just acts like you are not sharpening the blade but actually using it to cut something. -Cliff

Well, that is what I think as well, so we are in agreement on that. But that is not what Verhoeven says in this article. I talkes about the debris carried to the edge in the trailing stroke. He talks about the deformation as a seperate event.

On edge into honing, this abrasion is still present but there is also the action of the abrasives which are higher than the edge now ploughing into it directly. It would be hard to imagine the resulting honing would be idential in behavior.
-Cliff

That is mainly the reason why I follow Murray Carters example and hone with a trailing edge. That and that I simply got better results.

 

[Cliff Stamp]

I can see his arguement with the high speed belt sanders as the steel is actually sparking so I can see the possibility of deposition, but hand honing on a waterstone there is no way you can reach essentially welding temperatures especially it is all water cooled.

This is pretty easy to test, you could subject the wheel to a series of high pressure jets so the edge essentially always hit a fresh abrasive with no swarf and see if that makes a difference. I know a guy who hones under running water like that.

The abrasives meeting the edge directly should give it a better finish. You can see this when filing soft machetes or cheap stainless where strokes from shoulder to edge can produce a burr so wide you can see it, it actually bends and stays there like an additional edge.

It depends though on how you hone, the Japanese style one section sharpening is more effective on burr elimination than the common all in one pass style, but I find that Japanese way very difficult to freehand and maintain consistent angles.

Note as well that what Verhoeven calls burrs are not what most people call a burred edge which is a deforformation so large you can readily see it even under 10-15x mag magnification, so they are above 10 microns. Verhoevens burrs are often two orders of magnitude down.

I would wonder what they look like after cutting, just light use and extended, does the burr immediately go out of alignment or crack off, what actually does the cutting. I have seen arguements that for coarse edges the side scratches are a significant factor in the aggression, it isn't just the very edge itself.

-Cliff

 

[DGG]

Need some help with this paper.

Here is my question -

If I have a knife with an edge that is .001 inches thick, and I put a 1 pound weight on the spine of the knife, am I pushing down at 1,000 pounds per sq. inch on the edge? How exactly is the force measured or calculated? Does sawing with the knife with the 1 pound weight increase the force on the edge or does is just stretch out the material being cut so it becomes easier to make the cut?

 

[HoB]

For your pressure calculation you are missing the edge length. Since most edges are curved (belly) you have to figure out what the actual contact length is. If you assume that your contact length is a 1/4", the pressure is 4000 psi (1pound/0.001x0.25). Obviously on deformable material the pressure is lower than on non-deformable material, on a piece of steel the contact length is probably no more than 0.01" and you are at 100000 psi.

If you make a pull-cut, the microserrations of the edge catch the material and you are excerting two forces (pushing down and pulling) which both which both help to overcome the cohesion forces of the material being cut. So yes, it adds a force on the edge but in a different direction.

But you are the "other" DGG (I assume his son), again, right?

 

[DGG]

Wow! Thanks for the reply.

Those are some truly awesome psi amounts (like resting a car on the edge of your knife blade). I can understand how the edge breaks down. It is almost like leverage where the longer the lever the easier it is to lift something, but in this case the smaller the blade edge the greater the cutting force exerted.

 

[ghost squire]

When I read that file I said "So what?"
I hardly polish my knives smooth, in fact, I only keep one of my knives hair poppin' sharp. I sharpen my Spyderco Para on a DMT coarse diamond hone, then do 5-6 deburring strokes on the Sharpmaker medium rods. The knife never touches the fine rods or a strop (even though I own Sharpmaker fine and UF rods and a strop). The great toothy edge on my Para really cuts great, and is more durable of an edge than when I used to polish the hell out of it. I used to do the whole sharpen till it is a mirror, chase hairs off your arm sharp, but that is only useful for a knife you carry around and show off. I was extremely dissapointed when I tried to cut anything harder than paper. Now that I keep a practical coarse edge on it, it actually does aggressive cutting. I am going to try a DMT fine hone, but will probally no longer use my Ultra fine rods or strop anymore on my user knives.

Is it possible that with a very hard edge (such as 63-65 RC) you would be able to keep that hair popping polished sharpness for a significant period of time?

 

[Cliff Stamp]

Is it possible that with a very hard edge (such as 63-65 RC) you would be able to keep that hair popping polished sharpness for a significant period of time?

Yes, a high hardness is essential to resist the deformation.

-Cliff

 

[DGG]

Yes, a high hardness is essential to resist the deformation.

-Cliff

What about the chipping out that is talked about in high hardness knives? Is that a problem? I just picked up a Spyderco with ZDP129 (or whatever it is) which supposedly has a high hardness Rockwell number.

 

[STR]

I have not heard of much in the way of chipping problems with the ZDP189 steel. The one that came up a few years back was 440V also known as S60V and then they went to S30V which was supposed to be better. I would say it is better in that regard but you still see far too many reports of edge chipping on blades of S30V to be something to ignore. With S60V the people using it dropped the hardness on it and then the problem everyone complained about was the edge rolling. But rolling was the lesser of the two evils for that steel and they went with that procedure until S30V pretty much took its place. I suspect that S30V is going to be phased out and replaced much the same way S60V was with this new steel Spyderco is talking about currently. 125 something or other. I'll have to look it up.

STR

 

[Cliff Stamp]

What about the chipping out that is talked about in high hardness knives? Is that a problem?

It can be in some knives, it depends on the steel. ZDP-189 doesn't seem to have that problem though. I have been fairly rough with the Calypso Jr. I have.

-Cliff

 

[HoB]

Some people reported trouble using the gray sharpmaker rods on the ZDP-189 which may be related to its high hardness, but in practial use I have not seen any chipping with ZDP. Its pretty pronounced in traditional japanese kitchen cutlery though, which have cores hardned to about 64Rc.

 

[Cliff Stamp]

On the corners or the flats of the medium rods? On the Japanese Cutlery are the cores ZDP-189, or are you talking about the white/blue steels?

-Cliff

 

[HoB]

On the corners or the flats of the medium rods? On the Japanese Cutlery are the cores ZDP-189, or are you talking about the white/blue steels?

-Cliff

When the Calys with ZDP-189 came out, quite a few people reported that they saw very rough and unsatisfactorily rough edges with micro chipping using the gray rods of the sharpmaker. I assume they were using both the corners and the flats and quite a heavy hand. I also assume that the chipping came mainly from the corners, but I don't know for sure. I have experienced the same on white and blue steels and recommended to some of the people with the complaints to use only the white rods and to lighten up on the pressure, and they replied that their problems disappeared.

From my own experience I can say that the "brittleness" of the ZDP-189 can not be compared to the white and blue core steels of japanese kitchen cutlery, which in my experience can not really be sharpened on the Sharpmaker. All I wanted to say is that against something as hard and rough as a gray Sharpmaker rod, ZDP might show some chipping aswell.

 

[Cliff Stamp]

I have sharpened it on the mediums with no problems, I generally use very light force, just enough to get it to bite, I never use the corners, they just burr too readily and deform the edge. I never get decent edge holding with those edges.

I can feel a real difference when sharpening the ZDP-189 Calypso Jr. and small Sebenza with idential primary edge grinds on the Sebenza. The Sebenza is very fluid and smoothly cut with the stones, the Calypso Jr. on the other hand feels much rougher on the stones.

However in regards to ultimate sharpness and edge retention there isn't a problem. Though I expected it to be brittle due to how it felt, I need to recalibration obviously. Has Spyderco reported any Q-Fog with the ZDP-189. I recall some CATRA which showed superior performance to S30V, similar to S90V.

-Cliff

 

[ghost squire]

Its pretty pronounced in traditional japanese kitchen cutlery though, which have cores hardned to about 64Rc.

The traditional Japanese kitchen knives are full hard I believe, I don't think they temper them in the slightest to remove stresses. They could probably benefit from some very mild tempering.

 

[HoB]

Yes, that is what I read as well.