Edge Retention/Rolling Test Stand

[grepper]

Mr. Bud uttered, "Now I don't think I know anymore how to sharpen an edge that dull. May have to get out the sandpaper like EOU did." 

That's funny.  I remember when you first started posting here you were producing 250 edges!  Now you have to "learn" how to do what you already knew how to do!

I know what you mean!  I have no idea how to sharpen a blade to 300!  Without even trying all my edges turn out @ 130-150.  

I'm thinking what I need to do is sharpen, dull with sandpaper and then do a light sharpening with 150 grit abrasive to reapply tooth.  Who knows, we may find that for a general purpose blade,  a 350-400 really toothy edge is optimal.  Anything goes at this point.  Geeze!  This is a lot of work, and that's not fair.  Nobody ever told me I'd have to work.  

[EOU]

"When a sufficient load is applied to a metal or other structural material, it will cause the material to change shape. This change in shape is called deformation. A temporary shape change that is self-reversing after the force is removed, so that the object returns to its original shape, is called elastic deformation. In other words, elastic deformation is a change in shape of a material at low stress that is recoverable after the stress is removed. This type of deformation involves stretching of the bonds, but the atoms do not slip past each other. When the stress is sufficient to permanently deform the metal, it is called plastic deformation."
 
"Yield strength can be explained, in engineering and materials science, as the stress at which a material begins to plastically deform. Prior to the yield point, the material will deform elastically and will return to its original shape when the applied stress is removed."
 
We thought that we would begin this post with a couple of definitions for reference later but first we would like to thank KG, Grepper and Bud for their most recent and insightful posts. We're sure that the rest of the followers of this thread appreciate their input as well as all must be tired of listening to only us droning on.
 
So what have we accomplished so far with our tests? We have pushed the apex of a sharpened knife edge to one side using a fixed amount of force and then measured how much the result of that "push" has affected the cutting ability of the edge. We have discovered then that our "push"  exceeded the yield strength of the edge apex and resulted in a plastic deformation, or "permanent rolling", of the edge. We suspect three things now as a result of our tests and we cannot emphasize enough the term "suspect" strenuously enough because our test results are far from mature. The results are certainly definitive enough though to give us good reason to investigate further.
 
We suspect:
 
A. That very sharp edges are are more prone to rolling than less sharp edges and significantly so. This suspicion is not difficult to get one's arms around from a logical standpoint. Thinner things bend more easily than thicker things of the same material.
B. That edge apexes assume new mechanical attributes that are different from the metal they emanate from. We can suspect that the sharpening process is responsible for this phenomena but it might easily be something inherent to the steel itself. Ductility is a prime example. Once again, not difficult to see how this might be true in that edge apexes can be rolled (bent) nearly 180° without fracturing. The constituent metal they usually emanate from cannot.
C. That hardness ratings of the knife blade material do not translate to edge apex strength in a direct or semi-linear fashion. This is likely the least mature aspect of our testing. We suspect that hardness is definitely a contributor to apex stability but perhaps not as directly translatable as some may think. We think that metallurgical science lends support to this supposition in that burr formation is regarded as the result of plastic deformation. The mechanical characteristics of metals that have been plastically deformed are changed. Does that mean that all of the hardness qualities have been lost through the deformation process? It doesn't seem so in our tests. It remains to be seen what percentage of hardness, if any in the final analysis, is lost. Deformation may not turn out to be the culprit at all. Foil properties are a real consideration here. It could be foil and it could be deformation - or both or something else entirely.
 
We appreciate your taking this ride with us because "a ride" is exactly what this is. We are not performing a year or two worth of careful research and then publishing a white paper of our findings. We are asking you to ride along as we conduct experiments and talk about the results. We'll likely go down some rabbit holes during this process and then have to dig ourselves back out but that is how discovery works. Experiments need to be repeated and data confirmed. Right now we're just trying to figure out where the fences are. Once we and our outside testers establish the parameters then, perhaps, we can start driving stakes in the ground. Until then, we can use all the help and suggestions we can get.

Here's an experiment that all PT50 or ID75A owners can conduct. Power your instruments up and place your finger on the force plate and press down. You'll get a feel for exactly how little 150 grams, the force level we conduct these measurements at, actually is.  

EDIT POST - Case in point. After posting this morning we decided to retest one element of our $5.00 knife test. While the results from the end of this knife that we had dulled to 292 seemed perfectly within bounds the factory ground edge did not even though we had confirmed the measurement. We were suspicious though of  exactly what kind of edge the $5.00 factory had left us with so this morning we sharpened it using our techniques and equipment. Yesterdays test on this section of blade began at BESS 168 and this mornings BESS 164.  Yesterday, after one set the factory produced edge rolled to 592 and our in-house sharpened edge produced this morning rolled to 435 and we confirmed the measurement at 442 . Those numbers make more sense. Just one more thing that we have to be careful about. We're going to have be careful about trusting the sharpening efforts of low end knife manufacturers because, obviously, it can have a significant bearing on our test results. We're going to edit yesterday's post and place an asterisk next to yesterday's numbers.
 

[grepper]

   

So, with starting sharpness of ~165, edge resistance to rolling appears to increase with an increase in steel hardness, but then levels off at HRC 55-61 range. 

Maybe that is the "sweet spot" of price/performance and why most of the  blades we see lie in that hardness range.

It would be interesting test these three blades at starting sharpness 300, 350 and 400 just to see if/when the harder HRC 60-61 shows an advantage.

It seems pretty obvious that 150ish sharpness is ephemeral at best and that an initial sharpness of maybe 300 or so will produce a much more durable and survivable edge. 

If that proves out, it begs the question of if there is any advantage with a HRC 60 edge over an HRC 57 edge.

For me, this is the elephant in the room question:  Is there any advantage to using a more expensive harder steel blade, and if so, how to take advantage of it. 

If there is an advantage, cool!  If not, great!  I can look at my drawer full of HRC 55-58 blades with renewed respect.  After all, they are easier to sharpen and never have chipped.

Testing so far indicates that when sharpened to ~150, a reasonably priced HRC 55-57 blade performs as well as a far more expensive HRC 60-61 blade.

[Bud]

Don't disagree Grepper that 55-61 may be a sweet spot for edge retention but only the lower Rockwells in that range are a sweet spot for price. You can buy Rockwell 55 everyday for $35.00 and maybe less but  every 61 knife i can find is at least $80.00 and some, like the Shun, are $180.00. I agree that tests at higher beginning sharpness levels will be very interesting. Like I said, this is just great stuff in my opinion.

[Bubby]

I have to admit that I'm just sitting here shaking my head. This isn't the first time that I've seen something like this but you don't see it very often. You go along forever thinking that this is just the way it is and then you get bounced out of your buggy. Then you look at all the information and think "why didn't I put all this together before?". If you stop and think about it burrs and edges don't act anything like hardened steel and we all know it. If they act like anything they act closer to aluminum foil than hardened steel. I mean, how strong could an edge be if you can bend it with whiskers and straighten it back up with a piece of leather? 150 grams is nothing but apparently its bending the crap out of these high dollar edges.  So what's supposed to be harder bone and meat or steel but every good butcher I've ever seen is never more than a foot away from his steel. Guess what? He's using hard steel to straighten softer steel. If that steel on the edge was resistant to rolling it wouldn't bend 90 degrees it would just break off. I've been through meat processing plants where they use hundreds of knives every day and I've never seen a Shun or anything like it. Outfits like IBP and Cargill. They use a bunch of twenty dollar knives. I think they figured out where their best bang for the buck is a long time ago. Hardened steel cracks like an egg if you stress it enough and I've busted enough bearing races with a drift punch to know that.  I think that all these clues have been right in front of us for as long as we used and sharpened knives and we just never put it all together. I'm not threatened one little bit by this information. When the testing is done I'm going to use this information and start building better edges with it. That's no big deal for me because I don't make a living with knives but I would think it would be a big deal for people who do and want to make a better product.

[grepper]

Yup.  Umm…, that about sums it up, and I must say, beautifully and succinctly put.  I was drawn to embellish, but I would only be gilding the lily.

That said, until proven otherwise I'm still open to the possibility that harder steel may be advantageous when not over sharpened to an excessively thin edge. It could pan out that harder steel is only an advantage when initial sharpness is so dull it it irrelevant, or maybe not.  We will see. But until then I remain open minded.

And even after that, I'll still be open minded because I've grown to not be surprised to discover that everything I think I know is wrong.

[KnifeGrinders]

I've also been pushed to the brink of cognitive dissonance about the edge vs steel of the blade by the last EOU post.
But I am not going to fall into it till we get x100 more data.

[EOU]

And that is exactly how you should be regarding the information gathered to date KG. It's going to take a lot of additional testing and time before we know exactly how this all shakes out. Some premises are likely to be confirmed and some will fall by the wayside. Conflicting data must be sorted out. In all cases, a better understanding of edge structure will lead to more durable edges. 

We're going to be a little out of pocket for a week or so. We will be working on this project and others over the next week or ten days so will likely have little to report during that period. We'll be back on the Exchange shortly thereafter with a new report.

[KnifeGrinders]

Just in time has come a thread on the Blade Forums about "supersteel" knives edge rolling, started by a man who sharpens knives manually on DMT diamond plates to cut cardboard:

https://bladeforums.com/threads/m390-and...g.1562908/

.

[grepper]

Thanks for passing that along Mr. KG.

The OP in that post starts by saying, “I love razor edges.”  I have no idea what a “razor” edge is because the entire thread is devoid of BESS sharpness numbers.  Nonetheless I presume he’s probably getting a pretty sharp, thin edge.  Other posters claim their edges don’t roll, but without sharpness readings to back it up.
 
Another poster states, “Corrugated cardboard shouldn't ordinarily cause many edge rolling issues…”.  Really?
 
Another poster said something more interesting, “Depending on what's meant by stropping with 'emery compound', you might also omit that part of the process. If it's actually 'emery', a.k.a., 'corundum' or aluminum oxide, it may not be handling the vanadium carbides in this high-wear steel very well. It'll erode the matrix steel from around the carbides (vanadium) at the edge, while failiing to cut or shape those carbides effectively, which leaves them standing proud with little support and weakens the edge. You'll effectively end up with an edge that behaves just like a folding burr.”  And then goes on to say, “So something else in the process is disproportionately weakening the edge…”
 
I have no idea if steel removal around carbide particles actually happens or not, but it makes more sense than a lot of the posts.
 
I have yet to see a “razor” edge that does not roll. 
 
There are a lot of opinions and guessing going on there.  All of the posters could benefit from a sharpness tester.  You know, sharpen the blade, test the sharpness, cut some cardboard and then test again.  I suspect that some of those guys who claim their edges don’t roll may well be surprised at the results.