Mods Please Lock

I'm seeing a lot of comments where the poster has constructed a model of the system in which to apply their reasoning. And not a lot of it make sense.

I'm going to go with either "this is a complex question which requires a complex answer" or simply "the way blades work is through the application of moderate pressure over a very tiny area (the edge) which causes massive levels of force to occur at the edge which we perceive as 'cutting' when the blade is applied to a substrate. It's less of a chemistry question and much more physics.

Think about it this way- even with super fine 0.05 micron (50 nanometer) alumina abrasive used to obtain that 'perfect edge' the reality is there are still very small scratches present everywhere. From this wikipedia entry you can see that the covalent radius of an iron atom is 140 pm (picometers) or 280 pm for the diameter. Meaning, a perfect 50 nm wide scratch in your blade (which to our eyes appears as a flawless mirror polish) could still house ~180 iron atoms in a line side-by-side across the scratch. So even our craziest abrasives cannot produce anywhere near an "atomic" or "molecular" edge.

Furthermore, the discussion on the "touching of atoms" is flawed since there has been no agreed upon understanding of what an atom is or what constitutes them touching. The electron cloud surrounds the nucleus and both are part of the atom as a whole. Not to mention the weirdness of quantum mechanics: while the electron cloud density is highest near the nucleus, that density also tapers off into infinity as you move away from the atom. So when do they truly begin to "touch" or how much perturbation to the electron cloud constitutes "touching"? It goes even deeper but ultimately, it doesn't matter for the blades we use.

Overall, while the chemistry of the blade steel is very important for determining its properties and potential, the discussion of cutting with "normal knives" has more to do with elementary physics (force over area) than any kind of "stiction" or intermolecular forces that may occur between the knife edge and the substrate being cut. [Note: 'stiction' may be one reasons why convex edges cut better than flat bevel edges, but I'll leave that for a different discussion].

Anyways, I'm here for future questions if this thread doesn't get locked.

/working on a PhD in Materials Inorganic Chemistry
 
BladeChemist said:
I'm seeing a lot of comments where the poster has constructed a model of the system in which to apply their reasoning. And not a lot of it make sense.

I'm going to go with either "this is a complex question which requires a complex answer" or simply "the way blades work is through the application of moderate pressure over a very tiny area (the edge) which causes massive levels of force to occur at the edge which we perceive as 'cutting' when the blade is applied to a substrate. It's less of a chemistry question and much more physics.

Think about it this way- even with super fine 0.05 micron (50 nanometer) alumina abrasive used to obtain that 'perfect edge' the reality is there are still very small scratches present everywhere. From this wikipedia entry you can see that the covalent radius of an iron atom is 140 pm (picometers) or 280 pm for the diameter. Meaning, a perfect 50 nm wide scratch in your blade (which to our eyes appears as a flawless mirror polish) could still house ~180 iron atoms in a line side-by-side across the scratch. So even our craziest abrasives cannot produce anywhere near an "atomic" or "molecular" edge.

Furthermore, the discussion on the "touching of atoms" is flawed since there has been no agreed upon understanding of what an atom is or what constitutes them touching. The electron cloud surrounds the nucleus and both are part of the atom as a whole. Not to mention the weirdness of quantum mechanics: while the electron cloud density is highest near the nucleus, that density also tapers off into infinity as you move away from the atom. So when do they truly begin to "touch" or how much perturbation to the electron cloud constitutes "touching"? It goes even deeper but ultimately, it doesn't matter for the blades we use.

Overall, while the chemistry of the blade steel is very important for determining its properties and potential, the discussion of cutting with "normal knives" has more to do with elementary physics (force over area) than any kind of "stiction" or intermolecular forces that may occur between the knife edge and the substrate being cut. [Note: 'stiction' may be one reasons why convex edges cut better than flat bevel edges, but I'll leave that for a different discussion].

Anyways, I'm here for future questions if this thread doesn't get locked.

/working on a PhD in Materials Inorganic Chemistry
Click to expand...

I've been watching this thread, and wondering how long it would take for that important point (highlighted above) to be made. Even the real, bona fide 'experts' in physics still have to theorize about how these things happen. Until there's actually a means to see for real, with our own eyes, how these 'interactions' between atoms/molecules happen, there is no 'factual' answer to the question. It's all speculation (more respectably known as 'scientific theory'), although some 'explanations' might make it more believable, at least. It's those more 'believable' (but still unprovable) explanations that usually end up in textbooks, to be inferred as 'fact'. Everything needs to be taken with a grain of salt, so far as I'm concerned.

The older I get, when I watch or read the news or some magazine article (in a 'scientific' journal, or the like), the more I notice so-called 'facts' being articulated in descriptions that include terms like 'possibly', 'likely', 'could be', 'might be', 'presumed to be', etc. A real, hard, bona fide FACT would need no such qualification to describe or illustrate the point. Simply, a true fact is obviously and undeniably just that.

Still an interesting topic for creative discussion, though. :)
 
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knarfeng said:
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Interactions between solids is PhD level stuff.
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If you really want to pursue this, get a PhD in Physical Chemistry. It's that complex.
Click to expand...

BladeChemist said:
...
...
Anyways, I'm here for future questions if this thread doesn't get locked.

/working on a PhD in Materials Inorganic Chemistry
Click to expand...

That would make you the necessary P-chem Meister of which I spoke. Of all of us, you have the background to have an actual answer, though I don't think that answer can be entirely in words.

knarfeng said:
Because electrons alone are not what is holding the material together. You are actually dealing with molecular bonds between atoms. So you need to look at bond strengths. Metal cuts plastic because the metal is a crystal structure, while plastics are covalent bonds. The bond strengths in a crystal structure are greater than those of the covalently bonded plastic.
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Granted that for practical purposes, the molecular interactions are a non-issue. (They happen, but knowing how they happen is not necessary to cut a piece of paper). But, at some point in the cutting process, the bonds holding the atoms of a material together have to be sheared by the material of the blade, else the material is not cut. I talked of molecular bond energies and of the strength of covalent bonds vs crystalline. At my level of understanding, I can look at the bond energies and tell you that one is greater than another and that therefore, one will shear the other. I'd be interested in any understanding you can bring to how that actually happens. (Now bear in mind that I've not used such concepts as molecular bonds vs. atomic orbitals since college, a goodly number of years ago as i am about old enough to be the OP's grandfather. So, be gentle in your answer.)
 
My dear father has a PhD in physical chemistry from a well-regarded university, he taught it for 40 years at another fine university, and he has always loved knives. Aside from the basic force over area equals pressure and resulting benefit of a narrow edge, he has no more idea how a cut progresses at the atomic level than the ordinary knife enthusiast.

My point is that even a whole lot of science education fails to fully explain something as simple as a knife cutting something in two. Our sciences have a long way to go still, and a lot can be learned by experience and observation in the laboratory of everyday life.
 
so take it for what it's worth.....

The relative size issue is the relevant one in separating the molecular discussion from that of the pure mechanical. Molecular, atomic and sub atomic sizes are so small and the edge so huge that the cut is simply a matter of the physics of extreme pressure separating the molecular, crystalline, cellular or even just surface tension bonding of various materials. If the material's bonds are weak, it's easy, if strong; it's difficult. If stronger than the crystalline bonds in the blade; near impossible.
 
jgloveracob said:
Polished vs. Toothy
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This bears mentioning. Many times folks will immediately turn on you if they see a lack of experience or even if they sense less than their own. They will fall back on humor or even ridicule. Don't let it get to you as normally it's only a sign of a lack of understanding of the subject or the lack of something valid to say. You'll see all sides in this thread. I notice the note with your edit, again don't join that fray. Remain above it. Your observation was excellent. You applied what you learned to a real world practical application and I, for one, applaud your effort. The application of what you learn in school is, IMHO, far more important even than what you actually learn.
 
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jgloveracob said:
Sorry guys, Been having a bad day today. I'm calm now. But back to the subject. I'm trying to say that the electrons on the edge are repelling the electrons on the material resulting in a cut. If this isn't the case please tell me in a serious manner why it isn't correct.
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great comeback kid :thumbup:
much appreciated
 
It's the OP's thread. He's shown some real maturity (more than many, even as a 15 year old) in how he's dealt with (some) of the less-than-useful 'feedback' he's received here. It's his choice. Nobody else's.
 
It's OBVIOUS that the OP doesn't know he can "lock" the thread or he would have done it and not put the request in the bloody title!

..also, a MODERATOR can LOCK any thead he wishes to so your comment about this being solely in the hands of the OP is dead wrong.




And, while I'm at it, since the OP is showing such "maturity" why hasn't he changed the friggin' title and removed the "assholes" part in his edited comment box?! :confused:


P.S. I can't be bothered to read this entire thread but leaving things the way they are only invites "trolling"!
 
You know, Ditch Digger, it's funny. None of the powers that be here think the OP is a jerk because of this post. Sure, everybody had a good chuckle, but that's different.

You, however, are cleanly on everybody's radar, and would be well advised to just STFU. Now.
 
Samael said:
You know, Ditch Digger, it's funny. None of the powers that be here think the OP is a jerk because of this post. Sure, everybody had a good chuckle, but that's different.

You, however, are cleanly on everybody's radar, and would be well advised to just STFU. Now.
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I was just about to say the same.
 
That's my claim to fame, I guess. I'm always the first guy to say "eww, what's that smell?" Hey, somebody's got to do it.
 
Now that was not a very smart link for him to post.
Oh well.

Stick to the discussion posted by the OP.
 
Samael said:
You know, Ditch Digger, it's funny. None of the powers that be here think the OP is a jerk because of this post. Sure, everybody had a good chuckle, but that's different.

You, however, are cleanly on everybody's radar, and would be well advised to just STFU. Now.
Click to expand...

:thumbup::thumbup::thumbup::thumbup:

So jgloveracob misspoke or incorrectly phrased his comments. The usual junkyard dogs were here to start barking right away, so he was properly slapped.

I thought he showed a sense of humility and a bit of a sense of humor to come back here and let everyone know he was honestly trying to understand or learn something.

That makes him alright to me, regardless of age. I don't see many around here that know how to retreat from their controversial posting without hurt feelings and name calling.

Good on ya' jgloveracob
.
You too, CTS.

Robert
 
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