150 gf sharpness

[grepper]

Almost all of the knives I sharpen are your basic kitchen and pocket knives.  No fine grain really hard super steel, just your basic blades.  As far as I can tell, most of these are in the RHC 56-59 range.
 
The interesting thing is that, almost always, they end up right about 150 gf sharpness.  It’s so predictable I hardly need to bother with a sharpness test.  I don’t do anything special, just throw some belt on the Kally, sharpen, deburr and that’s what happens. 
 
Even though they are all about the same hardness, it happens with different knives with steels of various names sharpened in the 15° - 20° bevel range.  Abrasive coarseness does not seem to be a factor as I see the same results using a 600 grit sic belt or a 120 grit Cubitron belt.  As long as the blade is well debured, they all seem to end up right about that sharp.
 
I have a friend that also sharpens using a Kally and he has noted the same thing.  While I have discussed sharpening with him, whatever exact method he uses is of his own invention.  I only mention it because I found the coincidence curious.
 
Mr. Mark recently posted, “Measuring initial sharpness of all three blades after the 140 Atoma showed I needed to be more careful and thorough about removing the burr. I was getting up to 230, and down to 150 on different parts of the blades. I tried two alternating passes per side +1-2*, and the apexes were clean and sharp. I could see the hint of microbevel with 10x, but they were way more uniform in the 150 range.”
 
There’s that magic 150 gf sharpness number again.  Sharpen, deburr, and… surprise!  ~150 gf.  Mr. Mark mentioned the blade was a 4" Wharncliffe.  I don’t know the exact model, but a quick Google search indicates RHC 57-59 on most of them.
 
Obviously with such a diminutive data set it would be foolish to even attempt to form any conclusions based upon it.  Nonetheless, I’ve seen that happen so many times with my own sharpening and seeing other folks with basically the same results has me wondering.
 
Foolish speculation has me wondering if steel blades within the RHC 56 – 59 sharpness range, sharpened without any special effort, tend to be about 150 gf sharpness and generally not much sharper.
 
If there is anything to that, and I’m not saying that there is, I wonder if it has something to do with the metallurgy of the blade. 
 
This could obviously be nothing more than utter blather, but I found it curious enough to bother to type all this stuff and post it.

[SteveG]

My theory (more speculation than theory, really, as I don't have either the knowledge or data to support it) is that it has more to do with the geometry than anything else. I can get a plane blade at 25 degrees inclusive angle to BESS 100 consistently, but haven't been able to get a knife at 20 dps below 150. I doubt it's the steel, as I've sharpened both carbon steel and high RC (62) plane blades, and low RC (57~58) and high RC (62~65) knives. I wish I had enough steel and time to do some real tests. My guess is that both the apex angle and the thickness of the blade just behind the edge are the biggest factors, assuming no burr remains.

[Ken S]

Steve,

Following your theory, (using the Tormek bevel angle and edge angle terminology):

Your plane blade has a single bevel, with a bevel angle of 25°. With only one bevel, the bevel angle equals the edge angle. With your knife, each of your bevel angles equals 20°. With two bevel angles, the edge angle of your knife equals 40º.

I think your BESS tester is testing the edge angle, without caring whether the edge angle is from a single or double bevel angles. Since 40º is > 25°, my common sense science thinks the higher BESS reading with the 40º than the more acute 25° edge angle is logical.

Following that logic, I would expect a knife with two bevel angles of 15° each (edge angle 30°) to read lower BESS than the same knife with double bevels, each ground 20º (edge angle 40°). 

I would also expect a high carbon plane blade with a single bevel (and thus) edge angle of 25° to give a lower BESS reading than a premium plane blade of A2 steel ground to a bevel (and edge) angle of 30º. Granted, I would expect both the 40º edge angle knife and the 30º A2 plane blade to retain their sharpness longer.

Ken

[grepper]

I have a single bevel, Stanley wood chisel that my dad bought years ago, no doubt from a hardware store, so it is nothing special as far as the steel goes.  While probably inexpensive, it has hung in there for years, the handle is still tight and it is in overall good condition.
 
     
 
I sharpened it @ 25° and it ended up 100 gf sharpness.  Nice and sharp.
 
I was surprised that it ended up @ 100 gf sharpness considering it is a 25° single bevel.  I would have thought that a more acute angle like a 15° dps knife would be easier to sharpen to 100 gf, but that was not the case.  The chisel sharpened to 100 gf without jumping through any hoops at all.
 
I have no idea why the thing is so easy to get so sharp but that’s what happened.

[SteveG]

That all sounds about right to me, based on what I've seen so far (i.e., the more acute the angle, the lower the BESS reading), until yesterday, when my theory was apparently shot down by facts. I bought a Spyderco Endura, and measured it right out of the box, with the following four results: 77, 80, 151, 103. It's my understanding that Spyderco ships their blades with a 30-degree (15 dps) grind, which looked about right with visual inspection. I have no idea what to make of the readings; i.e., did a burr lead to the low values? The high value? Or is it just a finer edge in some areas of the blade...

I was thinking that the more obtuse the edge, the more resistance it would present when penetrating the BESS media, but there seems to be more depth to my ignorance than I had hoped.

Ken, I can my A2 blades as sharp as my O1s (takes longer, though) at the same angles, though the A2s don't seem to like the lower angles as much. Also, for the record, I realized after I posted my last message that my plane blades are often at 30 degrees inclusive, not 25 like I stated (I grind everything at 25 on a Tormek, but then I often microbevel at 30 on bench stones). When I go to the bench stones, sometimes I freehand, which leaves the angle at 25, and sometimes I use a jig, which I set at 30 for most blades.

Grepper, I'm not at all surprised you're seeing 100 gf - I'm not a great sharpener and I'm getting it. And my understanding is that the old Stanleys had good steel. Old tools were often/usually sharpened on oilstones, and are much easier to sharpen than the modern steels.

[grepper]

Mr. SteveG uttered, "I was thinking that the more obtuse the edge, the more resistance it would present when penetrating the BESS media, but there seems to be more depth to my ignorance than I had hoped."

It is my understanding that BESS test media splits when the outer surface is cut, thereby reducing or eliminating the affect of bevel angle on the test.
 
Perhaps Mr. EOU will chime in here.

[SteveG]

Interesting thought, I wouldn't have considered that unless the media was under tension.

I just took a piece of the media, and shaved a bit of it from a section so that it's in two strands, and then tugged on the ends, and it didn't come apart. Not sure how valid of a test that is, but it leads me to believe that some significant penetration is required. Just how much would be very interesting to know, so I'm anxious to see a response from EOU.

[grepper]

I probably didn’t explain that well.  It’s not that if the surface is nicked the media just falls in half, but rather, if I my understanding is correct, the maximum force required to penetrate the surface of the test media will not increase as the media is severed regardless of the bevel angle because the initial maximum force required to penetrate the surface will completely sever the media and does not increase thereafter.  That initial force is the sharpness reading.   
 
I’m sure Mr. EOU will respond.

[SteveG]

Ah, I see what you mean - I think I can visualize that, in that once the surface is penetrated it wouldn't take any more force to sever the remainder (in fact, the momentum from the initial breakthrough might reduce the pressure required). Thanks for clarifying. That notwithstanding, I feel like the initial separation still requires some displacement of material, even if it's only a matter of microns, and at the microscopic level the wedging effect of the edge would require more pressure as the angle becomes more obtuse. If, for example, the edge is half a micron, and it had to penetrate 10 microns to initiate the separation, there would be significantly more surface area in contact with the media, and a significantly more material being pushed further "sideways," at a 40 degree edge vs a 25 degree edge. I think that would increase both the friction and the surface area in contact with the media, both of which would increase the required pressure. But, I'm basing all of this on my visualizations - I'd love for someone to tell me I'm wrong, and why. I only really learn when I'm wrong.

[EOU]

Well, here we are and thank you for the questions. Nothing but good things can come of questions that explore more exactly how BESS measurements are calculated. We are not at liberty to describe, fully, the cleaving qualities of BESS test media but can say this much without reservation; BESS test media is engineered to minimize, if not negate, the effects of blade geometry on the final measurement result. This begins with a test media that is only .009 thousandths of an inch in diameter. If no additional steps were taken to enhance the sever point of the media then common sense would tell us that once the media is severed to the mid-point (radius) that the media would fail due to the decreased surface area presented to the stress source (edge). Now we are talking about an effective sever depth of only .0045 inch, an exceedingly small impingement area. Bess test media is designed to fail well before the edge has reached a depth equal to the radius. With that, we have to stop in accordance with our BESS Universal agreements. 

Using modern test media, we are unable to detect any measureable effect of blade geometry on a measurement result.  This does not mean that some set of circumstances might not exist where blade geometry does effect the measurement and that is where the observations/conversations of folks like Steve G. and Grepper and Ken become very valuable. This is not a matter of ego but one of discovery and if an asterisk should one day appear next to the BESS scale then I think that Bernie and the board will bear up. To tell you the truth, we are very intrigued by the sharpness levels reported in single bevel edges. We do not doubt for a second that these edges are easily sharpened to that level because it is an edge commonly used in medical applications and we have seen reports of very similar successes before. Japanese woodworkers shave meter long wood strips so thin that they are translucent with this edge. We would like to understand why this is possible. We have little experience with this edge and intend to correct that deficiency in the coming months. 

Once again, thank you to all of the participants in this thread.