What is possible?

[aread]

What level of knife sharpness is possible today? Not just a good useful, practical edge, but the maximum level of silly sharpness just for the sake of doing it.

I've never really had a sharp knife before I got an Edge Pro Apex. Since I lack whatever it takes to become good at freehand, the best that I could hope for was the edge from a Sharpmaker and I wasn't even good with that either.

Now that I have the EP, I see how much sharper I can get a knife with the proper tools, even without any real skill. Of course this gets me thinking "How much further can I take it? & What does it take to get there?"

I guess one problem is measuring the sharpness. push cuts phone book paper? shaving sharp? cut TP or paper towels? Hair whittling? I've achieved all of those, but it seems that there is a level above this.

Your opinions / experiences?

Thanks,
Allen

 

[gooeytek]

I've seen some people test their kitchen knives by push-cutting a standing, folded piece of receipt paper. Also seen a cherry tomato dropped on a blade resting on its spine, edge up.

 

[Lagrangian]

Here is a list of length-scales related to knife sharpening. If you were to zoom-in to the knife's edge, at the apex, you would see a rounded ridge. To "measure" the sharpness, one can use the the width of the round part of the apex. This is only an approximation in that the complex micro geometry of the knife edge is reduced to a single number. But it is fairly reasonable.

Prof. John Verhoeven measured this sharpness using a scanning electron microscope (SEM) for modern razor blades. He found the sharpness it be around 0.4 microns (see URL below for a reference).

The following list may be interesting, and I've posted it before. But I think it may be worth looking at again. In particular, note the sharpness of modern razor blades and diamond microtome knives. The diamond microtomes are _literally_ a hundred times (100x) sharper than a razor blade.

180 - 7 Microns: Diameter of human hair. https://en.wikipedia.org/wiki/Hair
100 Microns: Approximate thickness of paper (copier paper of weight 24 lbs; 500 sheets is about 2 inches thick).
16 Microns: Thickness of household aluminum foil. http://en.wikipedia.org/wiki/Aluminum_foil
25.4 Microns: = 0.001 inches (1.0 mil). Standard resolution for an imperial caliper.
2.54 Microns: = 0.0001 inches (0.1 mil). Standard resolution for an imperial micrometer.
0.75 - 0.38 Microns: Wavelength of visible light. http://en.wikipedia.org/wiki/Visible_light
0.4 Microns: Sharpness of a modern razor blade. http://www-archive.mse.iastate.edu/...te.edu/static/files/verhoeven/KnifeShExps.pdf
0.2 Microns: Resolution limit of optical microscopes. http://www.microscopyu.com/articles/optics/objectiveproperties.html
0.05 Microns: Sharpness of diamond coated razor blades. http://www.technologyreview.com/computing/25988/
0.005 Microns: Sharpness of a diamond microtome knife. http://www.tedpella.com/diamond_html/diamondk.htm
0.003 Microns: Sharpness of concoidally fractured obsidian. http://en.wikipedia.org/wiki/Obsidian
0.003 microns: Distance between a hard drive head and the spinning platter. http://en.wikipedia.org/wiki/Disk_read-and-write_head
0.00034 Microns: Van Der Waals diameter of a single carbon atom. http://en.wikipedia.org/wiki/Van_Der_Waals_Radius

The needles used in scanning-tunnelling-electron-microscopes (STEM) and atomic-force-microscopes (AFM) are so sharp they literally have a single atom at their tips.
Also, x-ray telescope mirrors are atomically smooth. http://www.mpe.mpg.de/xray/wave/technologies/mirror.php

This is also fairly interesting when combined with Komitadjie's Grand Unified Grit Chart, which is an approximate conversion between sharpening stone grits and microns. The graph below is made by Mr. Wizard who used the data compiled by Komitadjie.
http://www.knifeforums.com/forums/showtopic.php?tid/904090/tp/7/

Sincerely,
--Lagrangian

 

[BluntCut MetalWorks]

I've seen some people test their kitchen knives by push-cutting a standing, folded piece of receipt paper. Also seen a cherry tomato dropped on a blade resting on its spine, edge up.

I saw & replicated those tests too. A hair whittling / tp cutting edge with sub 14* inclusive bevel will get the job done. No extra sharpness, just thin geometry.

 

[aread]

Lagrangian,

THANK YOU! That's some great information! There's quite a few hours of interesting reading there.

On the list of various thicknesses, where does a well sharpened EDC knive blade come in? My old knives were worse than the 180 micron, but more recent results are better than that, but likely not in the sub-micron range.

Thanks,
Allen

 

[Obsessed with Edges]

Lagrangian,

THANK YOU! That's some great information! There's quite a few hours of interesting reading there.

On the list of various thicknesses, where does a well sharpened EDC knive blade come in? My old knives were worse than the 180 micron, but more recent results are better than that, but likely not in the sub-micron range.

Thanks,
Allen

It's generally assumed a 'shaving edge' on an EDC blade will be somewhere on par with a razor blade (~0.5 micron, give or take), or maybe up to ~1 micron or so, in terms of the edge thickness. As mentioned before, the thinner the geometry behind the edge is, the easier it becomes to accomplish that sort of sharpness. That's demonstrated in the thin geometry of a straight razor, at ~15° - 17° inclusive (7.5° - 8.5° per side), which is roughly about 1/2 the edge angle of a more typical EDC knife blade (30° - 40° inclusive). On an EDC blade, any edge much finer than 'shaving sharp' won't typically last anyway, assuming the blade gets used for anything more than shaving. I tend to believe that ~1 micron is a pretty good target to shoot for, and depending on the steel & heat treat, can be relatively durable and easy to maintain as well. An edge like this may or may not shave (mostly depending on the hair itself), but it'll do pretty much everything else expected of a knife edge, and do it quite well.

 

[Lagrangian]

Hi aread,

Glad you like the list of length-scales.
Also, I just wanted to say I agree with Obsessed with Edges.

Sincerely,
--Lagrangian

 

[Lagrangian]

btw, it's been suggested that "sharpness" and "cutting performance" should be defined separately. They're related of course. I don't know what the best way to do this is, but here's what I've heard suggested:

(1) Sharpness is only about geometry the knife edge. It is a measure of how close the knife is to a perfect geometrical wedge, and the shape of the wedge.

(2) Cutting Performance is only about how easily the knife cuts through material. This will depend on the material being cut, and the geometry of the knife behind the bevel (ie: the size of the "shoulder" or if the knife has a convex edge, etc.).

Not clear how to define "how easy the knife cuts" but there are suggestions based on the force and energy required, as well as physiological considerations (how easy it "feels" for us, as humans with muscles to cut).

Some definitions are based on cutting tests, like the Hanging Hair Test. See this link: http://www.coticule.be/hanging-hair-test.html

Other tests are slicing paper in various ways, push-cutting toilette paper, slicing a phone book, etc.

Personally, I've gotten to hair whittling, but not beyond that.

Sincerely,
--Lagrangian

 

[Bo T]

I'd suggest that the grain size of the steel and the lack of large carbides would influence how sharp an edge could become. If I understood Ed Fowler correctly a grain size of 10 (fine) was considered good a couple of decades ago while he currently makes blades with a grain size of 14 (very fine). The finer the grain, the sharper the edge can become. Ed uses 52100 steel in his knives.

 

[Lagrangian]

Hi Bo T,

I'm curious... what does "grain size of 10" mean? Can that be converted to a micron range?

Sincerely,
--Lagrangian

 

[Obsessed with Edges]

I'd suggest that the grain size of the steel and the lack of large carbides would influence how sharp an edge could become. If I understood Ed Fowler correctly a grain size of 10 (fine) was considered good a couple of decades ago while he currently makes blades with a grain size of 14 (very fine). The finer the grain, the sharper the edge can become. Ed uses 52100 steel in his knives.

Hi Bo T,

I'm curious... what does "grain size of 10" mean? Can that be converted to a micron range?

Sincerely,
--Lagrangian

Curious about this too. In Googling for possible explanations, I found this excerpt in a Google Books search result (linked below). Don't know if it's what Bo or Ed Fowler is referring to, but maybe....

(from Google Books search result -->: http://books.google.com/books?id=Kw...wGu8IGwCA&ved=0CEQQ6AEwAg#v=onepage&q&f=false )

It references a standard called 'ASTM E112', so Googling that might add some insight. If I understand the above excerpt correctly, it breaks down like this, from 0 to 14 on the scale:

0 = 0.5 grains/sq. in.
1 = 1 grains/sq. in.
2 = 2 grains/sq. in.
3 = 4 grains/sq. in.
4 = 8 grains/sq. in.
5 = 16 grains/sq. in.
6 = 32 grains/sq. in.
7 = 64 grains/sq. in.
8 = 128 grains/sq. in.
9 = 256 grains/sq. in.
10 = 512 grains/sq. in.
11 = 1024 grains/sq. in.
12 = 2048 grains/sq. in.
13 = 4096 grains/sq. in.
14 = 8192 grains/sq. in.

The individual grain size at '14' would be the inverse of 8192, I believe. This would be 0.000122 inch, which converts to ~3.1 microns. Assuming of course, that I've actually figured this correctly (and quite seriously, I'm not sure of that).

I'm also assuming there may be possible grain sizes smaller than the '14' grade, but this particular scale doesn't seem to go beyond that.

 

[Bo T]

David,

You were pretty darn close! From Verhoeven, ASTM - size 10 grain @ 11 micron, (actually very fine) and size 14 grain @ 2.8 micron, (ultrafine). So I suppose that a better interpretation might be (if we assume the potential of the steel to fracture on the grain boundries) that the ultrafine steel might be easier to sharpen to a very fine edge than fine grained steel given the other variables are the same.

 

[Obsessed with Edges]

David,

You were pretty darn close! From Verhoeven, ASTM - size 10 grain @ 11 micron, (actually very fine) and size 14 grain @ 2.8 micron, (ultrafine). So I suppose that a better interpretation might be (if we assume the potential of the steel to fracture on the grain boundries) that the ultrafine steel might be easier to sharpen to a very fine edge than fine grained steel given the other variables are the same.

Hey, a very hearty 'THANKS!!' to you, Bo. :thumbup:

The more I thought about my 'calculations' earlier, the more my head started to hurt. I was really starting to wonder if I was on the right track, and was beginning to talk myself out of my original conclusions.

I have a copy of Verhoeven's doc on knife sharpening techniques. I'll have to take another look at it.

Edit:
Here's an excerpt from a Verhoeven document, showing the relationship between ASTM Grain Size Number and average grain diameter in microns:

('ASTM' = American Society for Testing and Materials)

 

[Ed Fowler]

David: You put a lot of work into those posts. I thank you.
When we started working with Rex the theoretical limit of grain size in 52100 was an ASTM of10. We had achieved that, thanks to Rex and some other metallurgists who helped us we now have an ASTM grain size of 15 and finer. Actually it is finer than that, but their microscope can only measure a 15. The important aspect to remember is that an absolutely uniform grain size of 15 would be fragile like glass. It is the and finer matrix that provides toughness and more.

 

[Bigfattyt]

Fun thread.

 

[OHALLUM]

Fun thread.

Iv' got a serious headache from all this. Very interesting, but I do not need a knife this sharp for an EDC. If I did, I would be afraid to pull it out and show it to people.

Not to make light to all who dug DEEEP for this information, as it is mind boggling what can be found if you know where to look. My hat is off to all you who researched this and posted your knowledge and the information you found. It is good to know that these "Knife Nuts" on here are well educated and can prove it when necessary.

I stand in awe.

Omar

 

[Obsessed with Edges]

David: You put a lot of work into those posts. I thank you.
When we started working with Rex the theoretical limit of grain size in 52100 was an ASTM of10. We had achieved that, thanks to Rex and some other metallurgists who helped us we now have an ASTM grain size of 15 and finer. Actually it is finer than that, but their microscope can only measure a 15. The important aspect to remember is that an absolutely uniform grain size of 15 would be fragile like glass. It is the and finer matrix that provides toughness and more.

Thank you, Ed. It's great to hear your perspective on all of this. I feel like I'm learning a ton here, and the questions posted in threads like these are what inspire me to go looking for answers. Great fun in the quest to learn something new.

Thanks again. :thumbup:

 

[Lagrangian]

Hi Obsessed with Edges,

Many thanks for figuring out those ASTM grain sizes!

Sincerely,
--Lagrangian