So what is the measure of sharpness to go by?

[Sikael]

Considering that the diameter of an atom -- it varies, since a Hydrogen or Lithium atom is obviously smaller/lighter than an Iridium or Uranium atom -- is between 0.1 to 0.5nm (1 × 10[SUP]-[/SUP][SUP]10[/SUP] m to 5 × 10[SUP]-[/SUP][SUP]10[/SUP] m), and the diameter of an iron atom is 0.25 nanometers, while that of a grain is usually 10 to 20

m, I'd say an apex of 30.47nm qualifies as 'sharp'... if that photo's numbers are correct, that would mean the apex pictured might only be the width of @60-300 atomic diameters (!?), with fewer actual atoms and a lot more empty space. That measurement -- 0.25nm -- quantifies the outermost electron orbit; the nucleus has a much smaller diameter, 10(-12) cm, one ten-thousandth of the diameter of an atom itself. So... a 30.47nm-wide apex seems unbelievably 'sharp'.

 

[singularity35]

Considering that the diameter of an atom -- it varies, since a Hydrogen or Lithium atom is obviously smaller/lighter than an Iridium or Uranium atom -- is between 0.1 to 0.5nm (1 × 10[SUP]-[/SUP][SUP]10[/SUP] m to 5 × 10[SUP]-[/SUP][SUP]10[/SUP] m), I'd say an apex of 30.47nm qualifies as 'sharp'... if that photo's numbers are correct, that would mean the apex pictured might only be the width of @60-300 atomic diameters (!?), with fewer actual atoms and a lot more empty space. That measurement -- 0.1nm -- quantifies the outermost electron orbit; the nucleus has a much smaller diameter, 10(-12) cm, one ten-thousandth of the diameter of an atom itself. So... a 30.47nm-wide apex seems unbelievably 'sharp'.

Strictly speaking, it can't be an atom unless the blade is made from that element. It's has to be a molecule.

 

[Sikael]

Strictly speaking, it can't be an atom unless the blade is made from that element. It's has to be a molecule.

Obviously... I can't disagree with that.

LATE EDIT: I'm not sure what was meant by 'it can't be an atom' because it 'has to be a molecule'. Elements take a molecular form, but the atoms can take different molecular forms. Oxygen is typically made up of 2 oxygen molecules, but a molecular bond can be broken by ionization, as in the case of ozone. If the meaning was that you'd see molecules first, then atoms, then I agree completely.

I was trying to illustrate was how small the 30 nanometer measurement on the photo really is, by using the smallest physical components before hitting the subatomic scale -- we are supposedly looking at a width which could contain a maximum of 120 iron atom-diameters... if the measurement is correct, and probably less. Does that look right? We should be seeing the actual molecular latticework of the steel matrix, and a lot of empty space. Maybe it's not nanometers indicated.

 

[Surfingringo]

four fingers on spine, touch the blade with the pad of the thumb. The idea is to see how easily the blade will bite into the first layer of skin. I can tell a great deal about an edge like this...not only sharpness but I can tell with a pretty high degree of accuracy how coarse/fine the finish is. Another quick test i like it to touch the edge lightly to the back of my hair like a brush. If the edge instantly bites into the hair then it is quite sharp.

 

[Sikael]

It's mostly iron, around 1% carbon, with the molecular structure of steel. Unless it's stainless or a tool steel, etc.

 

[Sikael]

I stumbled onto a forum devoted entirely to straight-razors, and as you can probably imagine, they take this sort of thing pretty seriously. They expect a razor to pass the 'HHT', Hanging Hair Test, just letting a three-inch or so hair hang loose, and see if the blade will catch the strand an inch or so below the point where you pinch it between two fingers. It's the same idea as trying to 'tree-top' arm hair, seeing if it bites in midway up the length of the hair. None of my knives have passed the HHT, but I'm learning to live with the shame.

 

[singularity35]

I stumbled onto a forum devoted entirely to straight-razors, and as you can probably imagine, they take this sort of thing pretty seriously. They expect a razor to pass the 'HHT', Hanging Hair Test, just letting a three-inch or so hair hang loose, and see if the blade will catch the strand an inch or so below the point where you pinch it between two fingers. It's the same idea as trying to 'tree-top' arm hair, seeing if it bites in midway up the length of the hair. None of my knives have passed the HHT, but I'm learning to live with the shame.

I became a darksider some time after I joined BF. For a while I had a hair saloon that I got hair from because I used a lot of hair testing my hair whittling edges. The hair from my wife's hairbrush wasn't nearly enough because I had to test each time I sharpened my edges.

 

[Sikael]

I became a darksider some time after I joined BF. For a while I had a hair saloon that I got hair from because I used a lot of hair testing my hair whittling edges. The hair from my wife's hairbrush wasn't nearly enough because I had to test each time I sharpened my edges.

LOL. I was thinking that if anyone would understand why you wanted hair to test an edge, it would have to be hair salon owners or barbers. But I'm not so sure... do barbers still offer shaves with straight-razors? I wouldn't be surprised if the quality of disposable razors and the emergence of AIDS in the 80's kind of put an end to the old-fashioned straight-razor shave.

 

[singularity35]

LOL. I was thinking that if anyone would understand why you wanted hair to test an edge, it would have to be hair salon owners or barbers. But I'm not so sure... do barbers still offer shaves with straight-razors? I wouldn't be surprised if the quality of disposable razors and the emergence of AIDS in the 80's kind of put an end to the old-fashioned straight-razor shave.

LOL, not at all. The salon owner at first though I was doing some vodoo or witchcraft thing. I hadda show her that I could actually whittle hair with my knife and show her my YT vids. After that, they started saving me hair in a ziplock bag that I left with them.

 

[Rhinoknives1]

LOL. I was thinking that if anyone would understand why you wanted hair to test an edge, it would have to be hair salon owners or barbers. But I'm not so sure... do barbers still offer shaves with straight-razors? I wouldn't be surprised if the quality of disposable razors and the emergence of AIDS in the 80's kind of put an end to the old-fashioned straight-razor shave.

Barbers use a straight edge razor with disposable blades for shaves here in California at least. They are machine sharpened and my barber will strop them before use.

They work fine.

 

[Surfingringo]

Typically, the word "sharp" is used to describe the ease of cutting. There is nothing magical about a blade; it has a geometry that can be quantified and that geometry determines the perceived "sharpness." What complicates this, is that the geometry should be matched to the cutting task. For example, a straight razor edge is destroyed by a single cut through a piece of paper.

I would suggest that our perception of sharpness can be attributed to two aspects of knife geometry; the width of the apex (I think best described by the word "keenness") and the thickness of the blade behind the apex (which can be measured at various distances from the edge). I sharpening, we typically first set the geometry of the blade behind the apex - eg thinning a knife from the factory grind to a thinner grind. And second, set the geometry of the apex. The relative importance of these two parameters will depend on the material being cut.

Most of this discussion of "sharpness" is really describing what I call "keenness" - the last micron of the blade. Keenness is responsible for the ability to cut paper or grab fingerprints, shave or tree-top hair, etc. The apex may not be uniform, a property sometimes described as "toothy" but this is still a property of the last micron of the edge.

Hi Todd, I'm not sure I agree with this, nor have I ever used "sharp" in that context. I don't remember ever hearing anyone use the word that way either. I believe sharpness is simply the width of the apex. An axe can be sharpened to whittle hairs yet wouldn't be very good at slicing deli meat...would it not be considered sharp? Could just be a simple regional/geographical difference in semantics though.

 

[ToddS]

Hi Todd, I'm not sure I agree with this, nor have I ever used "sharp" in that context. I don't remember ever hearing anyone use the word that way either. I believe sharpness is simply the width of the apex. An axe can be sharpened to whittle hairs yet wouldn't be very good at slicing deli meat...would it not be considered sharp? Could just be a simple regional/geographical difference in semantics though.

If you gently take the edge off a blade with a fine stone the geometry is unchanged but the blade is no longer keen. For a task that does not require shaving ability the blade is still sharp.

The word sharp has no precise definition but I think it is the most suitable word to describe geometry or thickness behind the edge

 

[whp]

If you feel they are sharp...sharp enough to cut the things you want to cut....they are sharp. End of story.

There is no objective standard or measure for "sharp."

Ninety seven posts in this thread, and the thread could have ended quite well with this one.(number 2).

 

[Surfingringo]

If you gently take the edge off a blade with a fine stone the geometry is unchanged but the blade is no longer keen. For a task that does not require shaving ability the blade is still sharp.

The word sharp has no precise definition but I think it is the most suitable word to describe geometry or thickness behind the edge

Does the word "keen" have a more precise definition than "sharp"? In my vocabulary it does not. As far as the most suitable word for geometry or thickness behind the edge, what about geometry or...thickness behind the edge. I just don't see the need for assuming that sharpness doesn't mean what it does only to replace it with a word that is no more "precise". Again, you may have been taught different meanings to these words than I was so we may not be able to get anywhere with this discussion, but those are my thoughts. You are welcome to tell me yours again but I believe we are going to find ourselves in disagreement on this subject...and that is fine.

 

[Sikael]

Ninety seven posts in this thread, and the thread could have ended quite well with this one.(number 2).

I like that post too. Marcinek is a smart guy who says many awesome and horrifying things. But since this is a forum, it doesn't hurt to let others have a kick at the ball... or the dead horse, depending on your POV.

 

[Converge]

When you close a liner/lock back/frame lock knife and open it again, seeing a thin slice of skin the shape of your finger or thumb, looking at your finger you notice a flat surface where it should be rounded, yet magically there is no blood and you felt nothing!

Joke aside, I actually use my thumb to see how sharp an edge is. I know it's looked down upon but that is just how I do it. You just very, very lightly rub the slightest edge of the thumb on a portion of the blade barely moving. If I can feel or see the edge start to take into that minuscule amount of skin without any pressure, I know it's sharp! I've been gravitating aware from that though and just using paper cuts as a test. Sometimes I still use my thumb though. I've never once cut myself from doing that, however it does leave tiny scratches in the tip of the thumb where the skin is hardest, doesn't hurt but if you do it enough your thumb may catch on fabric if your skin is dry. I don't suggest this, just being honest! Don't know why I picked that up way back in my youth, probably from only being around dull knives!

I still feel that you can really tell the sharpness by just the slightest, slightest touch. On a really sharp knife it doesn't take much at all. It's probably not a good idea, if you by chance have some type of twitch when you're doing it.... You could definitely cut yourself!

 

[Sonnydaze]

As a physicist, I find that some of this stuff is funny.

 

[Surfingringo]

When you close a liner/lock back/frame lock knife and open it again, seeing a thin slice of skin the shape of your finger or thumb, looking at your finger you notice a flat surface where it should be rounded, yet magically there is no blood and you felt nothing!

Joke aside, I actually use my thumb to see how sharp an edge is. I know it's looked down upon but that is just how I do it. You just very, very lightly rub the slightest edge of the thumb on a portion of the blade barely moving. If I can feel or see the edge start to take into that minuscule amount of skin without any pressure, I know it's sharp! I've been gravitating aware from that though and just using paper cuts as a test. Sometimes I still use my thumb though. I've never once cut myself from doing that, however it does leave tiny scratches in the tip of the thumb where the skin is hardest, doesn't hurt but if you do it enough your thumb may catch on fabric if your skin is dry. I don't suggest this, just being honest! Don't know why I picked that up way back in my youth, probably from only being around dull knives!

I still feel that you can really tell the sharpness by just the slightest, slightest touch. On a really sharp knife it doesn't take much at all. It's probably not a good idea, if you by chance have some type of twitch when you're doing it.... You could definitely cut yourself!

This is exactly what I do. I'm not aware of it being "looked down on". Very simple and effective way to test an edge. The only exception is when my hands are wet...the edge doesn't "grab" the same way and it is much easier to use the three finger test.

 

[singularity35]

Hi Todd, I'm not sure I agree with this, nor have I ever used "sharp" in that context. I don't remember ever hearing anyone use the word that way either. I believe sharpness is simply the width of the apex. An axe can be sharpened to whittle hairs yet wouldn't be very good at slicing deli meat...would it not be considered sharp? Could just be a simple regional/geographical difference in semantics though.

Agreed, I'd use the term "cutting performance" for what ToddS is saying. Sharpness is just one aspect of cutting performance, IMHO.

 

[bodog]

As a physicist, I find that some of this stuff is funny.

I'm curious. I tend to disagree with marcinek and agree with proton. I think marcinek is just being an ass to stir crap up like he normally does. I believe the statement is true that given the right tools and technology an edge can be taken as sharp as the steel molecules on the edge allow, in theory at least.

I'd like to hear what a physicist says about it.