cbwx34 wrote:


Well, I am going to return them both to pocket carry over the next week then re-test them both. But it is probably clear that there have been some zealous overstatements that have been made prior to the testing Cliff had an interesting series of posts once about how knife ergonometrics can effect "apparent sharpness feedback" to the user. i think we need to find those comments and re-post them somewhere..

cbwx34 wrote:


that is still a rather sharp knife, it will easily shave arm hair, it will not direct push cut tomateo skin, but will cleanly slice a tomateo using an easy draw cut under the weight of the blade, it will still 90 push cut copy paper if you handle it carefully and keep the finger hold on the copy paper within 1/4 to 3/8" of the blade contact. -Will work on getting a parts list together this weekend. the thing is so simple you can basically figure it out from the videos and the posted pictures. The part of it that necessary to understand more fully is the proper placement and use of the testing media, how to run a 'test series' properly, the nature of the whole concept of "testing sharpness", and a few other details that we need to put in some sort of documentation sheet. The whole thing is still a work in progress. .. Cliff has been very kind to work out an Excel template that crunches the numbers. From a recent email to Cliff, here is how I feel about it so far:

... I am not yet confidant enough in the 'testing jig' to accept it as a tool to measure anything but gross ranges of data. In fact because of what it is, and how it is built, it probably has limits that must be "respected" (if that is the right turn of phrase -grin!-)..

I feel an acceptable level of comfort with it as a rough gauging tool, within 20 grams push cut increments starting from commercial razor blades thru a range to kitchen knives, in that range I feel it is testing well, and the data it is returning is meaningful for that use and can be relied upon. When we are taking about testing subtle variations on very sharp high end blades with subtle variations in sharpening technique, well then, ... humm not so sure.

A simple fit will turn up :

Sebenza has an initial sharpness of 237 (5) the ZT was 283 (6). This difference of about 15% is significant (meaning it isn't random) both from the standard errors being much larger than the difference (it would pass a t-test for significant at 0.05%, and if you look at the data you can clearly see that while they are scattered, the Sebenza scatters around a lower value (and thus higher sharpness).

Now that just means *for this run* it was sharper, it does not mean that it always would be, Dan would need to sharpen a few more times and if the Sebenza was consistently sharper then an inference could be made about the steel. But I would bet that this small difference in sharpness is simply the expected variation, it isn't like you would expect the initial sharpness to be exactly the same each time you sharpened a knife.

The slope for the Sebenza was 1.35 (12) and for the ZT 1.39 (14), both of them showing a strong non-null value. This can be further checked as the F-statistic for both (136 and 92) easily pass the test of significance which show that the data do not remain constant, there is a statistically significant decrease in sharpness during the test. This is also, as Dan noted, easily verifiable in the data itself because you can see that towards the end the data flattens to a value about 100 g higher.

Dan if you want I can generate a template for that type of work which will generate all of these values and the graphs and email it back to you.

If you have the time/inclination you could carry the Sebenza and ZT and then periodically just record the sharpness and see how well the numbers match your perceptions. The ideal way would be for you to make your notes on the physical handling, and ask a friend to measure the forces and only look at the forces when you have decided to stop the experiment. This way you have reduced one of the largest possible sources of bias.

CliffStamp wrote:


This is interesting, and I will put this on the 'to-do'list. I was looking through my stack of testing that I have done with the jig and came across one early on that was done with the ZT and the baseline numbers started as low as 175g so I know that I can get at least that far with it. It would be interesting to see if that could be brought even lower, and if the sebbie could be brought along with it. It would be interesting to see if I could 'find the bottom' (so to speak..), but with S35vn I don't think I have the equipment or the skill here to really find out the answer to that, but it would make an interesting afternoon of sharpening I do know that I have found such "bottoms" with certain knife steels: for example I tried pushing a CRKT M16 every finer and found out that the jig told me I could never (no matter what I did!) get it sharper than 180 or thereabouts. CRKT says the alloy is 8Cr15MoV.

It might be interesting to set a few benchmarks which should be extremely high (or low as it were) :

-standard utility razor blades as-new
-double sided razor blades
-"shave ready" straight razor

CliffStamp wrote:


I have tested utility knives like what you find sold as 'utility knife replacement blades' at Home Depot and True Value hardware under brand names like "Stanley" and "Irwin" but they are just very dull, usually baseline numbers in the 350-400g. The Gillette type double edge replacement blades in the pharmacy are much more like what you would want as a reference, as they are consistent and very sharp across their entire length. I do not have access to 'straight razors' so, sorry to say, can not test them

The following chart is an example of results from a generic double edge razor sold by the Rite-Aid pharmacy chain stores. This data is simply sorted low to high, not put thru a proper data cruncher, but I thought I would present it this way as an easy way for 'laymen' to get the gist of it. (throw out the lows and the highs!)



Note: low numbers= sharper, high numbers= duller

Dan,

That makes sense to me, on the light thread I normally cut a Super Platinum razor blade is 30 (4) grams, if a knife even gets to within half of that at 60 grams it is extremely sharp, i.e., it is long since push cutting a tomato, newsprint, etc. . Most knives that people consider sharp such as the production Spyderco's will in general be about 2.5-3.5 times as high. It has been awhile since I hand polished to a very high grit, but getting past 60 is not easy so I would be curious if you could bring knives down to match the finish of the razor and if so how long and what grit was required.

CliffStamp wrote:


Sorry I had a interesting week to say the least so I have a bit of catching up to do on this thread.

Just curious do you have data for the linear changes of performance with hardness? I would assume some variation between different steels as well which would muddy the picture. If this was the case and microstructure was more indicative of performance or retention I would be willing to ascertain that hardness may have a congruent relationship with microstructure and retention. Although maybe not. So to say it is a good indicator of retention seems like it is to be determined still.

Even with a linear relationship there would be no question what knife I would choose if I was presented with two knives, one had 3x better retention!

I agree with the bias statement, but without properly reported methods and lack of generated data it is hard to avoid. Consider a phase II drug trail where x many patients are given a new drug and are observed for adverse reactions. Study participants will talk to each and generate a similar type of bias. It may even statistical significance. My point is that bias occur and are sometimes unavoidable but then again thats what statistically analysis is for right?! haha that was definitely a joke!

Back to the knife at hand. I agree that it's a burr problem but I just don't think it was made from sharpening. I know I formed an apex/burr and I felt. No, I did not feel a burr after the ceramic and I did not mean I felt a burr after it stop slicing, haha that would be ridiculous. What I meant by that was that I know that I apexed the edge and formed a burr on the lower grits.

Also for clarification, I have high standards and when I say it stopped slicing after 10 slices it still would but I actually had to move the blade as compared to just blowing right through it. I may have been a little histrionic! ha However when it was sharpened up the the 0.6micron ceramics and stropped down to 0.25 I expected a lot.

Anyway, interesting thread, Thanks guys! This is why I posted it here instead of the CRK forums on BF where I would have been eaten alive! (Disclaimer I am a CRK fan just not a fan boy!)

Rlb wrote:


Yes, as one example see "The Effects of Carbon Content, Matrix Hardness and Microstructure on the Wear of Steel Grinding Balls During Wet Copper Ore Grinding". Here 1090 carbon steel is examined for wear resistance when hardened from 64 to 31 HRC. The changes in wear (material lost) are linear incrementally. In regards to strength, hardness is a direct measure of compression strength, and the relation between hardness and tensile strength is linear and can be seen in any simple table where one is used to estimate the other.



This is the critical point, hardness is only a measurement of the materials resistance to being dented, if the hardness changes and the microstructure is the same class then you can make inferences, however blades at the same hardness can have vastly different properties if the microstructure is different (i.e. bainite vs martensite being an obvious striking example of difference in toughness at the same hardness).

As an example critical to edge holding, if a blade has a high level of retained austenite, even if it is hard as in 60 HRC, the edge will still be highly subject to localized rolling and chipping and denting in these patches of austenite as it is very soft. Similar the grain of the steel can have a large impact if it runs from spine to tip (blade and edge are very tough) vs spine to edge (blade and edge is now very fragile).




Medical research is insanely difficult just for that reason, all of the bodies are different and all of the people will describe the exact same condition differently. It is why you have to just get so much of it and pool a big bunch of studies especially when you are dealing with people who can simply lie to you about what they did or ate or past history etc., and some who just do trials for the money. It makes looking a knives for sharpness, etc. trivial in comparison as you only have to deal with minor biases such as data forcing (i.e. we see what we want to see).



To be frank, if you are careful enough in how you measure sharpness, and you can do this by feel on paper, you should be able to detect a difference almost immediately as blunting is actually highly nonlinear so the change initially is extremely fast compared to later. As a rough rule of thumb, in order to see similar changes in blunting you have to keep doubling the material cut. This isn't an exact rule but it gives you an idea.

So when you are cutting paper you would check at 1, 2, 4, 8, 16, 32, 64, 128, 256, etc. . You can quickly see how it will appear that any blade will plateau very fast, this behaviour alone is responsible for some of the worst misconceptions about steel performance. You can see this in Dan's data, see how it changes sharply at first and then starts to slow down, this gets very dramatic later on.

It also produces curious effects such as if you are willing for example work with a blade at 5% of optimal sharpness vs 10% then AISI 420 will easily outperform S30V as it will take longer for AISI 420 to get to 5% vs S30V to get to 10% as they are in the very slow plateau region at that point.

This is one of the reasons why people who don't sharpen knives often, but let them go extremely dull don't tend to consider edge retention at all as they won't even notice any difference because once you get in the plateau of very low sharpness it takes forever for a blade to change any more regardless of the steel.