Knife Sharpness Scale

I'm not so interested in how sharp it is. I'm more interested in if it is sharp enough for me, and how long it can stay sharp. That still requires some kind of testing.

After reading and studying steels and sharpening, different edge angles, thickness behind the edge, sharpening grits, it seems that there are very many variables involved. Put aside for a moment thoughts about how sharp and think about how you would test the sharpness in an accurate and repeatable manner, a test that would not be sensitive to the sharpening angle and edge thickness. Then we can go back to talking about how sharp. But coming up with a test method is tough enough to kill the project before it starts. If we do have to consider things such as sharpening angles, grits and edge thickness, then there are so many variables that the testing would be very difficult. Now we are back to shaving arm hair. Or do like I do and slice the thin paper found in advertisements in our Sunday newspaper.
 
OK I will attempt to respond to questions in broad form and to the best of my ability. First, the question about "standard blades" is a really good one and caught my eye immediately so here goes. It is amazing how close most of these blades are with regard to initial sharpness level. I hesitate to name actual product names/brands because I am bound to step on someone's toes that shaves with one of these blades and believes that it is much sharper than the others listed but in fact, it isn't. I have personally tested eight different brands of blades and found only one brand that is, in fact, significantly sharper than the rest. I'm about to tell you what the sharper brand was but first I want to say this.. It doesn't make any difference. The blades that BESS refers to as “standard” are those that are the most common, inexpensive and readily recognizable to users. Among these dozen or so brands there may be only 3 or 4 grams of pressure difference between them. The BESS relies on the sharpness of razor blades as only a reference point so that the scale has a meaningful beginning point. The general sharpness level of any DE razor blade is something nearly anyone can relate to hence the well known terms "razor sharp" and "sharp as a razor". The BESS standard for a zero point is 50 grams of total pressure. That's how much the piston assembly on a BESS certified instrument must weigh, hence a razor blade has a BESS value of "0" meaning no additional weight (pressure) need be added in order to sever the test media. The Feather DE blade is indeed sharper than what BESS would term a standard blade. This is notable for general edge (knife) sharpness testing only in the sense that BESS certified instrumentation can not only detect a difference in the sharpness levels of these DE blades but measure and quantify these differences as well.
With regard to instrumentation: The instrument is a simple as it gets but the test media is quite complex since I know that it was in development for over three years. I can’t comment on the test media makeup because I don’t know. I can say that it is about nine thousandths of an inch in diameter, has the consistency of a polyethylene thread but the sudden shear characteristics of something akin to a glass fiber optic fiber. The system works by spanning the sharpened (knife) edge with test media and then adding weight until the test media severs. The added weight is measured (weighed) and directly compared to the BESS. Each measurement consumes about two cents worth of test media. The instrument is a pure edge sharpness tester without regard for sharpening angle or blade geometry. Comment was made that the instrument/BESS failed to factor in several other variables important to blade performance i.e. edge durability, blade thickness, sharpening angle etc. and nothing posted in this thread could be truer. I think of the instrument and scale as I do my old mercury thermometer. I may wonder whether I need to put on a coat in the morning so I glance out the window at my thermometer and it tells me, quite accurately, what the outside air temperature is. It does not tell me the humidity level or the wind velocity however and those are two contributing factors affecting my comfort level as well. I will leave to my fellow forum members the question of which factor is most significant.
One last question I may have failed to address. If I were forced to commit I guess I would describe the test as a “push cut” although with a test media .009” in diameter and that severs at first incision it’s not much of a “push”..
I really appreciate the questions and apologize if I have left something out or failed to answer coherently. If I have, please bring it to my attention and I will give it another go.
 
It seems useful when comparing a blade to itself. Go to your final stone and then curious how much difference stropping might make? Now just measure before and after.
And next time you sharpen the blade at the same angle with the same technique you could then test a different stropping compound and compare the sharpness you get to the previous experiment.
Still doesn't say if it cuts better what you need to be cut but it's a good starting point.

Is there a way to test the whole edge and not just one point of it. Ideally the whole edge would be sharpened the same way but we know there is some variation.
 
CharlieSoxx said:
... I have personally tested eight different brands of blades and found only one brand that is, in fact, significantly sharper than the rest. I'm about to tell you what the sharper brand was but first I want to say this.. It doesn't make any difference. The blades that BESS refers to as “standard” are those that are the most common, inexpensive and readily recognizable to users. Among these dozen or so brands there may be only 3 or 4 grams of pressure difference between them. The BESS relies on the sharpness of razor blades as only a reference point so that the scale has a meaningful beginning point. ...
Click to expand...

It makes ALL the difference. If you don't use a specific brand of blade every time, your results are not repeatable by you or anyone else. You are not doing a controlled experiment.

You might as well be doing the "arm hair" test.
 
I think I'll stick with stuff like this: https://scienceofsharp.wordpress.com/2015/03/31/the-pasted-strop-part-3/


Sharpness is a measurement of edge thin-ness, corresponding to the distribution of cutting pressure presented at the apex. Cutting performance is a matter of area displacement. The OP mentioned that the cutting medium is 0.009", so the sharpest blade is thinnest at apex (probably 0.1 - 0.3 microns) and thinnest at 0.009" back from the apex. The end.
 
I think I may have failed to make myself clear in a portion of my earlier post so I will attempt to clarify now. With regard to the following " I'm about to tell you what the sharper brand was but first I want to say this.. It doesn't make any difference." This is to say that it makes no difference that there may be a particular brand of razor blade out there, in this case Feather, that is sharper than those more common brands that BESS considers as standard. I hope this serves to clarify and apologize for any lack of clarity in my previous post. To me, the most important message in that section of my post was that there now exists a means of measuring and quantifying sharpness levels as small as those that separate a Feather blade from let's say an... Astra or Bic or Gillette.
 
CharlieSoxx said:
I think I may have failed to make myself clear in a portion of my earlier post so I will attempt to clarify now. With regard to the following " I'm about to tell you what the sharper brand was but first I want to say this.. It doesn't make any difference." This is to say that it makes no difference that there may be a particular brand of razor blade out there, in this case Feather, that is sharper than those more common brands that BESS considers as standard. I hope this serves to clarify and apologize for any lack of clarity in my previous post. To me, the most important message in that section of my post was that there now exists a means of measuring and quantifying sharpness levels as small as those that separate a Feather blade from let's say an... Astra or Bic or Gillette.
Click to expand...

No there doesn't. You don't have a standard of sharpness. Its not a controlled experiment. Not reproducible. Science fair stuff here.

Again, it's like saying the sharpness test will be based on arm hair.
 
Sharpness is an inherently subjective subject. It depends on user preference and technique, and to objectively quantify it implies a reproducible test. Such a test would need to encompass both push and draw cutting a number of media, and it would neglect the most important feature of an extremely sharp knife - its ability to hokd that edge. In one test, obsidian would undoubtably win. However, in its actual ability to perform as a sharp object, it will fail. Any test needs to encompass a great many factors. Ultimately, if all you want is the geometric details, I still think you need three or four variables (depending on how you count), no one of which is sufficient to characterize it fully.

1. Angle to the apex of the edge

2. Width of the apex (equivalently, how fine an abrasive medium the edge was finished with)

3. Width behind the edge and the angle of the bevel behind the cutting edge


In convex or hollow blades, 1 and 3 become increasingly difficult to measure. As grits grow higher and abrasive sizes approach the grain of the steel, materials dominate 2. So as I said, there are too many variables to make one all encompassing scale, and even if one were to exist, it would be meaningless except for in one specific application. From what I understood, perhaps all you need is scale for razor-blade sharpness. However, for DE razor blades of fairly uniform thicknesses, angles and finish, the scale would really become an ordinate ranking of materials.
 
I like the concept of having a quantitative measure of edge sharpness, but I'm leaning toward the side of "it's too complicated" - certainly for it to be boiled down to one number. Thus, I have questions.

1. How do you control exogenous forces from ruining the result you get? For example, adding weight to the apparatus seems nontrivial. If the additional mass were dropped from even a small height, the resulting instantaneous force pressing down onto the media would be greater than the static force of that same total amount of mass. Similarly, if you choose not to drop the mass, it would be impossible to place it without unintentionally exerting additional force on the apparatus.

2. How is Newton's Third Law dealt with in the media? Cutting a limp rope is a lot more difficult than cutting a taut one. The applied force simply deflects it in one case, vs. cutting it in the second. That is related to what material is pushing back on the media from side opposite the edge, and how the media is mounted, at least.

3. Is it temperature dependent? "Hot knife through butter", and all that.
 
Blade steel makes a huge difference as well. CPM vs nonpowder. It depends how you define sharp. Even if you compile all this information, it's only for one type of material you're cutting. What happens when you cut meat or wood? The carbide structure in the steel will perform differently for each case. I suggest taking a high resolution camera and taking pictures of the steel up close and you will notice that all your blade samples will look different unless you have a CPM type steel. How about if you use a Titanium blade? or a ceramic blade?

Also, what's the Rockwell of these test blades?
 
Ok I think I saw some questions that I can help with having been there done that with the instrument. Weight may be added either in fractions of grams per second or in tens of grams per second and is done so dependent on the edge to be measured. It is a very intuitive process and I can promise does not represent an issue. I can't speak to Newton's third law but I can say that the test media when loaded is neither limp nor taut. Test media is loaded so that the slack is removed and that is all. Once again and in my experience it is an issue that simply never arises during the process. There is some measurable temperature dependence in the process and that is all test media related. I only know this because it has been stated both at meetings and in the manual. If memory serves it can be as much as 10 grams of pressure but to me when I'm sharpening to say 360 on the BESS it is transparent to me, from a practical stand point, whether the edge is actually 355 or 365. I don't think I get that much variation however because I'm always sharpening and testing in a temperature controlled environment. Secondly, I use the device most of the time in a relative fashion as opposed to an absolute fashion. The only time I think in absolute numbers is when I'm finished. The rest of the time I concentrate on using this stone to take the blade down by X, then by X with the next stage, and only then do I pay attention to my final destination for the knife in question but even then I'm tracking down in relative numbers...30 grams better, 20 grams better etc. Hope this all helps.
I do have a question for participants in this thread however that you may be able to help me with. No matter the knife, I sharpen to a certain sharpness level based on what the knife if designed to do. For me that's 400 with my hunting knives, 350 for my chef's knife, 275 for my Santoku and 55 for my straight edge razor. This all is regardless of the type of steel, sharpening angle, blade geometry etc.. With some of these knives I remove some or most of the wire edge and in the case of my straight razors I'm shaving with the wire edge. I can't say that I have a single complaint but I've been reading about all these other factors that should be considered. Am I living in blissful ignorance? Please be gentle with the refernce to ignorance. I didn't mean to open any doors here.
 
Thanks for your answers - they all sounded reasonable to me and it's helpful to hear you talk about the range of values you are targeting and how that relates to the reproducibility in the measurements (I think you said +/- 10 grams earlier?)
 
Just measure the thickness or width of the apex. That should really be empirical and reproducible. You'd precision instruments for that but what the heck, aren't we all OCD here? :p
 
Thank you for the thank you and Yes that is correct. I believe the resolution is 5 grams because I know it is better than ten grams. I don't know who benchmade is but no the instrument i use is not even close to CATRA's device. My device cost about 1 per cent of the CATRA unit. I do believe the CATRA unit might be better suited however for some of the folks who have posted here with regard to a more total test. I'm going to hang it up for tonight. You guys have a good evening.
 
You must log in or register to reply here.
Back
Top