Sharpening knife edges, what is actually going on?

[dbcad]

Hello folks, my study of physics is 20 years old or so please bear with me.

I enjoy sharp knives and blades in general and am curious about what is actually happening when I sharpen them. I know these steels are a matrix composed of different elements tightly bound by nuclear forces, but I'm curious as to the weak link where the material gives at a small scale. ie. carbides, when you're sharpening the edge. I believe it's impurities that cause a discontinuity in the steel matrix.

In retrospect I probably should have searched for a metallurgy forum, but I figure you folks might be able to help me out. I did pass quite a few 600 level courses in quantum physics and mathematics before I stupidly withdrew from the universty so many years ago...

Thanks for your patience:)

 

[turbo]

OK, the basics of knife sharpening are easy enough. When a knife has been dulled, you have a couple of options. If you have a very finely-figured edge, you may be able to re-align the edge by honing it on a steel. This is the preferred method of "sharpening" chef's knives and butcher's knives when you need only light maintenance to keep the performance tip-top. If the edge is very worn and/or won't hold a decent edge after being honed on a steel, you need to actually re-shape the edge. I recommend that you you buy a decent 6" diamond-impregnated stone (or two, maybe fine and very fine) and re-shape your edges with these stones lubricated with clean water to clear away abraded material, and keep the edge cool. If you have spent $70-80 on a hand-forged French chef's knife, the last thing you want to do is ruin that edge.

Don't worry so much about metallurgy - let the knife-makers take care of that. Your primary concern is taking care of your knives properly and not un-doing all the good work that you paid for. BTW, if you have ever seen Alton Brown's show on knife-sharpening, please disregard it at all costs. Letting some itinerant sharpener with a van draw sparks off the edges of your prized knives with belt-sanders is beyond ignorant. Brown is clueless, and his production team really screwed up this one.

 

[dbcad]

Thanks Turbo-1 for your reply.

I do know how to get the knives sharp and what not to do while working them.

I'm curious as to what's going on at a very small scale (nano) as I'm sharpening them. This is a purely theoretical excersize on my part. Where in the overall structure of the metal does the material choose to leave?

 

[Darken-Sol]

in addition to turbo i'll say this, that feel you get when you close shears and that clean sound is what you're looking for when putting steel to stone. learned from a hillbilly in minnesota. his knives are sharp and stay sharp. mine are sharp but require more upkeep lots ov practice on cheapies is recomended.

 

[0xDEADBEEF]

There is really no magic involved in sharpening. You simply rub away the metal. Parts that are rough and stick out get carried away first.

If you go to the Angstrom scale, you see that the metal is not taken off as a large chunk at once, but there are waves of lattice distortions running trough the material first, then there is some bending and finally it rips off. The distortion waves can run into defects, which stops them temporarily. This is why steel is harder than pure iron.

But that is it, the alloying is really just changing the hardness and flexibility, there is not too much else going on when sharpening afaik.

 

[JaredJames]

You make the edge of the metal as fine as possible.

By doing so, you decrease the area of the edge of the blade which in turn increases the pressure.

A blunt blade has a large surface area on the edge of the knife, a sharpened blade has a small surface area on the edge of the blade.

Pressure = Force / Area

Now in both cases, force can be the same. But the smaller the area value the higher the pressure.

Example (numbers exaggerated):

Let's say you have a knife that is blunt. The area of the blunt edge is 20cm². You apply 100N of force. This gives a pressure of 100/20 = 5N/m².

Now you sharpen it, the area reduces to 5cm². You apply 100N of force again. This time you have a pressure of 100/5 = 20N/m².

Basically, you are removing atoms from the material with each pass.

 

[dbcad]

Thanks for all of the replies folks. They are appreciated. I believe I didn't make my initial query quite specific enough.

This probably involves more metallurgy than pure physics, but I believe any knife steel alloy consists of lattices of iron and the alloying materials held together by strong bonds. Also in any steel alloy there are impurities such as sulphur which create weaker bonds within the lattices. I'm theorizing that the material is being removed primarily at those weaker impurity bonds rather than at the stronger alloying element bonds??

I enjoyed oxDEADBEAT's reply as he tried to describe what was happening to the lattice at close to the nano scale. Distortion then ripping makes sense. But where does it want to rip? Posting my thread here has helped me refine my thoughts a little more.

You folks have quite a friendly place full of folks that like to explain and assist I appreciate that and thank you. When I post another question I'll do my best to be more precise.

Charlie

 

[dbcad]

oxDEADBEEF, this means that the ripping is more likely to occur at the defects?

 

[Andy Resnick]

Hello folks, my study of physics is 20 years old or so please bear with me.

I enjoy sharp knives and blades in general and am curious about what is actually happening when I sharpen them.

JF Sackman wrote a charming article about ensuring that razor blades remain sharp:

http://iopscience.iop.org/0305-4624/9/5/305;jsessionid=5F39410DB46CDF3CD4595EDA38411135.c2

I forget why I got this article, but it was very enjoyable to read.

 

[EWH]

Sharpening on a steel straightens the folded-over edge and does not remove much material. Grinding on flat blocks gives a flat-sided "V" edge. Belt grinders (with the appropriate slack and fine abrasives) give the longest-lasting edge, with a curved side and a section like a pointed "U" which is stronger than the "V" edge. All these methods are prone to error and damage to the blade if the angle is not exact. Sharpening by hand requires a great deal of practice to do optimally.

The grinding methods will remove the softer parts of the steel somewhat more quickly than the harder parts, but usually the abrasive is so much harder than any component of the steel that the differential wear is slight, and only evident at the microscale. The structure of the steel may give rise to micro-serrations, this is evident in true Damascus steels, for instance. Most such micro-serrations are due to the grains of the abrasive, and do not persist in the same way that one gets with an oriented cementite inclusion microstructure.

(I'm not an expert - recalling old reading, perhaps incorrectly.)

 

[turbo]

Many years back there was a debate in which knife-makers were touting the advantages of a slightly convex curved edge profile. Some were calling it the apple-seed edge. I built a highly hardened knife using that edge, and it is pretty useful, perhaps because I like that little knife and only use it appropriately.

I think this is the profile that you call a pointed "U". I created that edge with no resort to belt grinders or over-heating. You can do a lot of with flat diamond stones.

 

[DickL]

Ron Hock has written a book, 'The Perfect Edge', which mostly explains how to sharpen woodworking tools (chisels, plane blades, etc.). It also provides a good explanation of what is happening and how the type and metallurgic treatment of the steel effects the sharpening, including some micro graphs of edges with various treatments.

The crystallization of the metal has a significant effect on the sharpening. The grains (carbon, carbide, etc.) effect both how the edge wears and how it sharpens. For example the grain size of stainless steels tends to be greater than for some tool steels and therefore can not achieve as sharp an edge. As an edge wears, or is sharpened, the grains break off. Small grains leave the edge sharp, large grains leave a rough and duller edge.

 

[dbcad]

Thanks again for the replies folks:) Another small nugget of understanding came to me:)

I have to let you kind folks know I'm pretty good at making an edge razor sharp. With some of the newer powdered metal materials I have on hand posess a very fine grain structure and keep the edge very, very well!

My geuss is that the grains are the very hard carbides. The Bonds that hold the carbide "nugget" itself together are very strong, it doesn't want to come apart. The bonds between all of the very little carbide "nuggets" are the failing points in the material.

With a finer grained overall structure, there are more of the weaker bonds to share the force load.

Not quite complete, but fleshing out my understanding a little:) The bonds holding the carbides together can be different also.

Charlie

 

[LostConjugate]

The surface of all things is like the earth. Smooth from a distance but huge mountains and valleys when viewed from up close. See Electron Tunneling Microscope images. When you sharpen a knife you are smashing mountains from one material into the mountains of another. A whole lot of damage is done but the weaker material ends up with smaller mountains left over first, that being the knife.

 

[dbcad]

I will look at the electron tunneling images if I can find them. Haven't tried yet. Thanks Lostconjugate, nice imagery:) You seem to be a bunch of very nice folks here, despite my antiquated and very rusty training:)