How sharp is sharp?

[Jan]

I've used the SJ250 (250mm) after using the very little worn SG250 (245mm) and you can already see that the SJ250 would leave the middle of the cutting edge untouched. With higher pressure it will "clean" all the edge.
So on first thought using the SG200 first IMO would be to big different. On second thought it could be a nice effect and worth trying.

WolfY, thanks for sharing your interesting results. 

I am very surprised, that you were able to observe untouched cutting edge in the middle of the bevel sharpened using SG-250 with 245 mm diameter.

When you have set a bevel angle of 15.0^o for SG250 (245mm) and used the same position of the USB for the SJ250 (250mm) than you should sharpen a secondary bevel of circa 15.4^o. It is hardly visible difference.

Possible explanation for the untouched part of the cutting edge could be slightly larger hollow depth for the smaller SG250 (245mm) grindstone then for the larger SG250 (245mm) grindstone. But this difference is really microscopic.

As you mentioned, pressure differences during sharpening could also influence the result.

Jan

[grepper]

Which BESS sharpness tester are you guys currently using?

[Ken S]

My favorite is the PT-50. I have used all three models, and like all three. I chose to purchase the PT-50, which probably speaks louder than word.

I like the increased accuracy and it seems easier to use (just my opinion, I could be happy with any of the three)

Ken

[yargnoj]

Well, this was a long read.

Anyways, I too use the BESS method. Though not an ISO, ASME, ASTM, etc. standard, it is a fantastic tool. I have been collecting data with it and my whetstones (having newly acquired a T-7 have not had experience enough to collect data yet for the Tormek). I am working on a graph of "BESS mass [g]" vs "sharpening medium [µm]". The BESS system is great for this because you really only need the independent variable to be standardized of sorts. Granted, the sharpening/honing grit is only as good as the operator.

In an effort to tie into the original question of "how sharp is sharp", I move to offer that the answer is relative to the task. In my perfectionist mind, nothing is sharp unless you get cut just by looking at it (see most any Japanese sharpness story). I have trouble stopping when I take on a set of knives to sharpen, I would call it a problem if I was not proud of my work. How could I not produce an edge that I am not proud of? However, this once again leaves us with the relative sharpness of an edge. To me, sharp happens when I run out of honing paste that is smaller than what I am currently using; as you can imagine, this is not practical. That is why I am so dedicated to producing this graphical representation of "sharpness vs µm".

I would love to help answer any questions with this regard. My current items to question are as follows:

What µm medium is the minimum requirement to break the BESS "sharp cutlery" requirement?

What grit is the apex most prominent?

How does this method compare across steel types?

how does this method compare across blade types/thicknesses?

Sharpness of knife upkeep (steeling vs stropping)?

As an aside, I would like to take magnified photographs of each sharpness at multiple blade points to try to qualify the "look" of a blade with sharpness. Perhaps a diagnostic visual test. Perhaps an arbitrary method, but say... 2 visible blade imperfections per inch qualifies the blade to the "not sharp" category.

I also want to test this on my personal knives with longevity and durability. There is no standard for when someone needs to resharpen their knives, just hone often enough and you are fine for 6-12 months. I don't like this. Perhaps I could have my neighbors contribute to the study and let me take BESS scale measurements and record them?

OORRRRR, I may just be taking this too seriously and sharp happens when you can cut a tomato. 

Jon 

[Herman Trivilino]

Interesting experiment, Yargnoj. Another variable to consider is the edge angle. At smaller angles the steel gets thinner faster as you approach the edge. At some point you will run into a limit due to the fact that the steel is too thin to hold an edge. You may be able to measure a sharper edge, but as soon as that edge is used to cut something it crumbles and thus gets very dull very fast. A knife so sharpened would be a disappointment to use.

[yargnoj]

Ah yes!
Perhaps there is a correlation here as well. I wonder if it would be linear or not? In my mind, BESS sharpness would asymptotically approach an angle of incidence where the edge first becomes 'sharp'. Perhaps this would be 90°. I would expect this curve to follow a quadratic function, thus making it difficult to choose the best angle to sharpen at. The observation of infinitesimally small edge, though being sharp, and its lack of durability would half to play an important factor. Perhaps if there was a systematic approach to dulling a knife consistently, at a small rate; a before and after "snapshot" would be useful.

The graph of inverse durability vs angle and sharpness vs angle, when charted together, may yield a max or min that could show the best combination of the three variables.

I believe my time spent per knife sharpening has just doubled to accommodate data collection.

[grepper]

"The graph of inverse durability vs angle and sharpness vs angle, when charted together, may yield a max or min that could show the best combination of the three variables."

Jon-

Well..., that would be very cool to see!  I for one definitely encourage you to do that.

Please do it for about RHC 52, RHC 58 and RHC 62.  Measure the sharpness, then chop up 100 carrots with each blade, take another sharpness measurement and do another set of graphs.

That real world testing would be extremely informative and well worth your effort!

[wootz]

... I am working on a graph of "BESS mass [g]" vs "sharpening medium [µm]". The BESS system is great for this because you really only need the independent variable to be standardized of sorts.
Jon

Generally, the resultant edge is finer than the sharpening grain size.
But Jon, I honestly cannot understand how this can be measured for the continuum of sharpening mediae by BESS tester. With sharpening media coarser than JIS #6000  the ragged burr will be severing the test thread rather than the edge, and this reading cannot tell much.
e.g. my repeated testing of an unhoned edge after #220 Tormek grindwheel gives 130 units variance on BESS, you cannot rely on that; while method of de-burring you may use will effect the BESS reading...

[Herman Trivilino]

Generally, the resultant edge is finer than the sharpening grain size.

Wow! That's interesting. Do you know how wide a knife edge would be after sharpening with the SG finely graded? A typical bevel angle is around 15° or 20° making the edge angle around 30° to 40°.

The steel used in knives varies greatly in composition, but I wonder if something can be quantified. I refer to the microscopic structures, whatever they might be called. These would be the grains of steel, for lack of a better term. Do they exist in a form where their size can be measured?

[Jan]

The size of steel grains can range from nanometres to millimetres depending mainly on cooling rate. Rapid cooling generates smaller grains and strengthens a material.

Typical grain size may be ~50 000 atomic diameters which is more than 10 micrometres. Please take it with a big reserve, it is a very rough estimation.

Jan

[wootz]

Generally, the resultant edge is finer than the sharpening grain size.

Wow! That's interesting. Do you know how wide a knife edge would be after sharpening with the SG finely graded?

Easy, if you have a BESS tester.
1 unit on BESS scale roughly = 0.01 micron of the edge width (the correlation is practically straightforward in the lower range of the scale form 0 up to 500).

A JIS #1000 stone has grain size of 16 micron, so the best edge you can get will be somewhere around 10 microns.
Obviously, after #1000 stone some de-burring must be done, and the method of honing you use will effect the BESS value.
E.g. if after the Tormek #800-1000 grindwheel I hone on a paper wheel with 0.5 micron diamond paste, the final BESS reading is about 80, i.e. edge under 1 micron .

As to what can be achieved by honing on the Tormek honing wheel with the Tormek honing paste, if I remember right Ken was planning to test this...

[SharpenADullWitt]

Generally, the resultant edge is finer than the sharpening grain size.

The steel used in knives varies greatly in composition, but I wonder if something can be quantified. I refer to the microscopic structures, whatever they might be called. These would be the grains of steel, for lack of a better term. Do they exist in a form where their size can be measured?

Electron microscope.
But then, as with your evidence post, you will get averages across the knives that you submit to the microscope.  You may find a more consistent batch, but what you choose to use out of that batch, may or may not be the same.

[wootz]

 
... and for those who don't have access to the scanning electron microscope, BESS reading can give some idea of the edge thickness...

[Jan]

Wootz, I would like to ask you what is the argument to support your statement that  "1 unit on BESS scale roughly = 0.01 micron of the edge width"? 

Jan

[wootz]

Wootz, I would like to ask you what is the argument to support your statement that  "1 unit on BESS scale roughly = 0.01 micron of the edge width"? 

Jan

Unpublished data from Mike.
I couldn't stop thinking that one unit on BESS may correspond to 0.01 micron edge. I gathered a number of observations, and wrote to Mike, and he replied with data that support this correlation.
There is little theory behind this, but many facts that make it practically useful.
This straightforward correlation gives BESS clear advantage over other sharpness scales.