So you think that's sharp...

[tosh]

I've seen numerous videos on youtube on so-called sharp blades.

But, they all seem to think if it cuts paper then it's sharp................

The edge of a piece of paper must show an enormous amount of resistence to any edged blade - implying anything that's remotely keen would possibly slice though.

I think it's time for a different approach?

I'd like to see a blade cut through a single piece of cotten thread hung from the air with no tension on the the thread it's self - just hung. The chosen blade is to cut upon contact in a VERY slow slicing motion - no flick of the wrist or simply slashing through the air.

Or we could be even more demanding.

How about cotton wool, again suspended with no tension applied. That would actually make for a VERY good test of sharpness - as any ragged edge no matter how small would simply snag the the fine fibres of the cotten wool and simply push it away as opposed to slicing through.

I've actually tried it tonight using a brand new razor blade - no luck, it simply pushed the cotten thread  away - but then, using a strong magnifying glass it's possible to see the ragged edge even on a brand new razor blade!!

Does anyone think it's possible - could an edge of steel be taken to such a microscopic level of sharpness??

Anybody care to rise to the challenge??

[Grant Lamontagne]

I'm no physicist, but I really don't think that's possible.  The only way you could possibly make it work is to use a downward angled cut with a very sharp knife moving very fast, but then that would get back to the tension issue.

This strikes me as one of those Kevon Costner Bodyguard type things, where he demonstrates how sharp the sword is bu dropping a sheer scarf on the blade and it cuts it.  Simply does not happen.  Anything that could be cut that easily simply would not be strong enough to survive in an open environment on the planet- and remember, thread may not seem like much, but for it's size it's quite sturdy- it has to be, as it's what holds out clothes together.

Paper is also surprisingly resistant, and the test isn't whether or not it can cut paper, but how cleanly.  Paper is fibrous- it's tiny strands of wood fiber going in all different directions stacked on top of eachother then heated and bonded together, similar to plywood.  When you tear paper you see lots of fibers, but when cut with a nice sharp blade, the edge of the paper should be very clean, and that is where the challenge lies.

Def

[Syph007]

For me its not just if it can cut paper, but if i can move the blade in tight curves while cleanly slicing.   Like cutting wavy spiral cuts.  At least that's what I always do to prove a SAK is sharp.  Most people also test against paper since odds are its easily at hand.

[tosh]

I'm no physicist, but I really don't think that's possible.  The only way you could possibly make it work is to use a downward angled cut with a very sharp knife moving very fast, but then that would get back to the tension issue.

This strikes me as one of those Kevon Costner Bodyguard type things, where he demonstrates how sharp the sword is bu dropping a sheer scarf on the blade and it cuts it.  Simply does not happen.  Anything that could be cut that easily simply would not be strong enough to survive in an open environment on the planet- and remember, thread may not seem like much, but for it's size it's quite sturdy- it has to be, as it's what holds out clothes together.

Paper is also surprisingly resistant, and the test isn't whether or not it can cut paper, but how cleanly.  Paper is fibrous- it's tiny strands of wood fiber going in all different directions stacked on top of eachother then heated and bonded together, similar to plywood.  When you tear paper you see lots of fibers, but when cut with a nice sharp blade, the edge of the paper should be very clean, and that is where the challenge lies.

Def

That is pretty much my own assumption, Def.

But I'm beginning to suspect the major problem is the bevel angle and wether the build up of steel to the secondary bevel actually creates a drag thus impeding the performance of the cutting edge. I'm thinking along the lines of a filleting knife but even thinner but gradually easing towards a stonger spine.

Basically, I'm  going to assume the secret is to do away with the secondary bevel, but to simply hone down the blade to an almost polished state...ouch   

[tosh]

Just found this:-

http://www.physicsforums.com/showthread.php?t=161956

http://en.wikipedia.org/wiki/Damascus_steel

Makes for interestin' reading.

[theonew]

I haven't played around with thread but this is one of my fine blonde hairs slivered twice while hanging solely under its own weight.



I consider that to be pretty sharp.

[Mike]

I'm no physicist, but I really don't think that's possible.  The only way you could possibly make it work is to use a downward angled cut with a very sharp knife moving very fast, but then that would get back to the tension issue.

This strikes me as one of those Kevon Costner Bodyguard type things, where he demonstrates how sharp the sword is bu dropping a sheer scarf on the blade and it cuts it.  Simply does not happen.  Anything that could be cut that easily simply would not be strong enough to survive in an open environment on the planet- and remember, thread may not seem like much, but for it's size it's quite sturdy- it has to be, as it's what holds out clothes together.

Paper is also surprisingly resistant, and the test isn't whether or not it can cut paper, but how cleanly.  Paper is fibrous- it's tiny strands of wood fiber going in all different directions stacked on top of eachother then heated and bonded together, similar to plywood.  When you tear paper you see lots of fibers, but when cut with a nice sharp blade, the edge of the paper should be very clean, and that is where the challenge lies.

Def

That is pretty much my own assumption, Def.

But I'm beginning to suspect the major problem is the bevel angle and wether the build up of steel to the secondary bevel actually creates a drag thus impeding the performance of the cutting edge. I'm thinking along the lines of a filleting knife but even thinner but gradually easing towards a stonger spine.

Basically, I'm  going to assume the secret is to do away with the secondary bevel, but to simply hone down the blade to an almost polished state...ouch   

Knocking that secondary bevel down a bit will keep your knife a nice slicer. That's the premise behind spyderco recommending use of the 30 degree angles every once in awhile. By knocking that shoulder down, it keeps the angle of the edge from becoming too obtuse. Now, you do need a certain amount of steel behind the edge to provide strength and keep the edge from rolling. This is best achieved by trial and error as it depends on the steel and heat treatment.

A very good book on sharpening is The Razor Edge Book of Sharpening by John Juranitch. Bear in mind that it's geared towards the sale of his products, but the principles are sound and easy to understand.

Knife edge grind reference link:
http://zknives.com/knives/articles/knifeedgetypes.shtml

Mike

[Grant Lamontagne]

I think they mostly just started cutting paper because their arms were bald.

Def

[Carlos]

That's no excuse, while they still have their legs available:

[Carlos]

But I agree that cutting general paper is a weak test. However cutting very soft paper is surprisingly a nice test, and not many blades can do it flawless. For instance, try to make one of those paper-cut tests on toilet paper instead: incredible hard.

[tosh]

But I agree that cutting general paper is a weak test. However cutting very soft paper is surprisingly a nice test, and not many blades can do it flawless. For instance, try to make one of those paper-cut tests on toilet paper instead: incredible hard.

I had actually considered the toilet paper test  - however, having once attempted to cut the stuff using a stanley blade - experiance now tells me it's almost impossible.

[OddFuture]

What about a test like that.

[tosh]

What about a test like that.

Hmm........Is that water on the Blade ?

Not entirely convinced that's authentic. Either the tissue was moistened or the blades were lubricated.

I could be wrong, but you can clearly see somethings running off the blade.

[OddFuture]

What about a test like that.

Hmm........Is that water on the Blade ?

Not entirely convinced that's authentic. Either the tissue was moistened or the blades were lubricated.

I could be wrong, but you can clearly see somethings running off the blade.

Yes the paper towel was wet, so when you go to cut it. It want's to tear instead of cut, but if you get a clean cut then it's sharp.

[tosh]

What about a test like that.

Hmm........Is that water on the Blade ?

Not entirely convinced that's authentic. Either the tissue was moistened or the blades were lubricated.

I could be wrong, but you can clearly see somethings running off the blade.

Yes the paper towel was wet, so when you go to cut it. It want's to tear instead of cut, but if you get a clean cut then it's sharp.

No denying those blades are sharp..ouch. much sharper than any I own.

But, surely water softens the fibres of the tissue.

Lets see those blades slice through cotten wool or as originally specified a single piece of  hanging thread.

[Carlos]

I had actually considered the toilet paper test  - however, having once attempted to cut the stuff using a stanley blade - experiance now tells me it's almost impossible.

I have been able to cut toilet paper -- dry, wet I've never tested -- but the cutting movement has to be slow and I have only two blades capable of that: an actual razor blade (dovo) and a kitchen knife (kai shun damascus santoku).

[Carlos]

Actually I've retried that just now and the paper ends up a little teared.
I don't have any video facilities but I leave a picture:



The bottom cuts were made by the kitchen knife, the upper cuts by the razor blade. The upper cuts are more clean, but they also have teared the paper a bit.

I don't have a stanley cutter, but have an OLFA one and also tried with it. It just pushed the paper, no cut or even ripping was done.

[dks]

I test my knives by push cutting paper (80 whatever units they use, g/cm or something), not shaving paper which is easier and maybe shaving hair if I feel adventurous... This is usually as sharp as knives come when new and this level of sharpness lasts for some time with resonable use (dont cut wire or on ceramic plates.

The problem with these extra sharp edges is their durability. I can spend an extra few hours to get  a scary sharp blade but then that extra sharpness will dissapear after the first few cuts, which makes the whole process of achieving that super-sharpness pointless, for me at least.

Shaving toilet tissue is the new craze that started from the usual forums... I think you are supposed to use one ply for testing, dry not wet....

[tosh]

Shaving toilet tissue is the new craze that started from the usual forums... I think you are supposed to use one ply for testing, dry not wet....
[/quote]



Didn't know that - has anyone yet succeeded??

[Mike]

When I read stuff like this, one thing comes to my mind. OCD

Mike

[dks]

When I read stuff like this, one thing comes to my mind. OCD

Mike

It depends.. picture this: you are sitting on the toilet, it is quiet, you are pushing away and you realise you will be for some time..you are a knife nut, so you can not/do not want to read, so you pull out your knife, get some toilet paper and slice away.....the time passes and you are happy.....
 

EDIT: apparently brand matters too
http://www.bladeforums.com/forums/showthread.php/799423-Toilet-Paper-Sharp

[Mike]

When I read stuff like this, one thing comes to my mind. OCD

Mike

It depends.. picture this: you are sitting on the toilet, it is quiet, you are pushing away and you realise you will be for some time..you are a knife nut, so you can not/do not want to read, so you pull out your knife, get some toilet paper and slice away.....the time passes and you are happy.....
 

EDIT: apparently brand matters too
http://www.bladeforums.com/forums/showthread.php/799423-Toilet-Paper-Sharp

 

[Sazabi]

When I read stuff like this, one thing comes to my mind. OCD

Mike

It depends.. picture this: you are sitting on the toilet, it is quiet, you are pushing away and you realise you will be for some time..you are a knife nut, so you can not/do not want to read, so you pull out your knife, get some toilet paper and slice away.....the time passes and you are happy.....
 

EDIT: apparently brand matters too
http://www.bladeforums.com/forums/showthread.php/799423-Toilet-Paper-Sharp

 

If it's bad for your butt, it's probably worse for your knife, regardless of how sharp it is...

[jzmtl]

I remember seeing a video on youtube of a german company making some sort of diamond blade, and was impressed by its sharpness, though I can't find that video now.

When you get pass paper cutting sharpness it's mostly a question of bragging rights, since you'll lose that sharpness the first time you cut anything (other than hair or toilet paper). I don't bother with that level of sharpness anymore, not worth the time. A few swipes on sharpmaker white rod then couple more on strop and it's good.

[AimlessWanderer]

I only strive for "usably sharp" too. I don't bother following up on finer stones or stropping normally either. As soon as it's sharp on the stone I'm using ... I stop sharpening

 

[tosh]

I only strive for "usably sharp" too. I don't bother following up on finer stones or stropping normally either. As soon as it's sharp on the stone I'm using ... I stop sharpening

 

I'm in the same camp with you 50ft -

However, I'm basically curious as to what is the absolute edge steel can take. there must be a peak before it then begins to break down.

Just how sharp would that peak be?.......Sharp enough to slice through a single thread ??

We know that a honed blade can shave hairs, slice paper etc etc - and it's a relatively easy goal to achieve.

I'm no metallurgist, but would I be correct in assuming  "normal"  steel blades simply do not have the grain structure to allow the honing of a super fine edge.

Would laminated steel be the better choice?

[AimlessWanderer]

Laminated steel is just a steel sandwich, and you're reliant on the "filling" to provide the cutting edge. If that steel on it's own won't provide the answer, sticking two bits of something else on either side of it isn't going to help 

[tosh]

Laminated steel is just a steel sandwich, and you're reliant on the "filling" to provide the cutting edge. If that steel on it's own won't provide the answer, sticking two bits of something else on either side of it isn't going to help 

I actually considered that but was unsure as to the steel properties in laminating. I've always assumed laminated steel is simply a skin of softer steel meaded onto a more tensile steel - is laminated and damascus the same principle? Forgive my ignorance.
If we are to assume hard steels are too brittle, would that then favour soft steel ? - more of a meading the steel  as opposed to grinding it.
I appreciate the edge on a softer steel blade would wear faster, but would it ultimately give a sharper edge?

Soft steel, Eh' - 

[AimlessWanderer]

Laminated steel is just a steel sandwich, and you're reliant on the "filling" to provide the cutting edge. If that steel on it's own won't provide the answer, sticking two bits of something else on either side of it isn't going to help 

I actually considered that but was unsure as to the steel properties in laminating. I've always assumed laminated steel is simply a skin of softer steel meaded onto a more tensile steel - is laminated and damascus the same principle? Forgive my ignorance.
If we are to assume hard steels are too brittle, would that then favour soft steel ? - more of a meading the steel  as opposed to grinding it.
I appreciate the edge on a softer steel blade would wear faster, but would it ultimately give a sharper edge?

Soft steel, Eh' - 

OK, two types of hardening ...

For those who may criticise this post I'll preced this by saying it isn't STRICTLY true ... but true enough to give someone who hasn't an in depth knowledge of steels a grasp of what's happening. First type is the conventional hardening that many will be familiar with. Heat something up nice and hot then quench it (rapid cooling) makes it hard, and heating up and slow cooling makes it soft ... why?

When the steel is heated the grain structure breaks down into smaller/finer grain size, and as the steel cools slowly the grain size increases. In order to harden it you need to stop the grain size growing again, so you quench it which brings the steel to "normal" temperature without giving the chance to reform to a larger grain. The easiest way to understand the difference, is imagine belly flopping 10ft into a ball pool ... now imagine doing the same thing on a sandy beach. Although the sand is free flowing, the grains are tight knit ... and that belly flop is going to hurt in the morning. Generally this makes the steel TOO hard and it becomes brittle as the grains are not "knitted" together. The way to overcome this is to temper the steel which is a partial softening process which allows a controlled amount of grain growth to occur returning some toughness (different to hardness) to the steel. Hard steel gives smaller grain which in theory means it will hold a better edge.

The other type of hardening occurs in certain stainless materials and exotic alloys. It's called precipitation age hardening and works BACKWARDS to what we've just discussed. Heat it up nice and hot then quench it, and it SOFTENS the material. Heat up up and let it cool gradually and it hardens ... why?

In these alloys you are trying to work with more than one metal at the same time. There is for sake of arguement your base metal, but within the alloy is an ingredient which has a lower melting point. The softening process is called solution annealing and it gets it's name not from being treated IN a solution, but because the metal BECOMES a solution.

Imagine the metal is a cold cup of tea and the alloying solution we are trying to manipulate is a spoon full of sugar. Heat the tea up and the sugar begins to dissolve and become part of the tea - it is no longer one entity sat within the other, it is now a combined entity - a solution. In the metal, that low melting alloy is weaving about in and amongst the grains of the parent metal in very small form. Quenching the material ensures the grains stay that way and the material is pliable for want of a better term.  To get the hardness up is is precipitation age hardened, and like the name implies to a certain extent this will happen due to time alone. Leave it long enough and it will harden itself, but with the correct heating it can be sped up and controlled.

What happens here is that as the metal is heated the "sugar" in the "tea" begins to recrystalise. It's just like the more common precipitation - rain, where water vapour condenses to form droplets, and all the grain structure of the lower melting alloy begins to grow and "lock in" the grain structure of the surrounding material. The focus here is less about the grain size of the parent metal, but the "glue" that is binding all the particles together

Beyond this there are too many variable to even get close to the hypothetical question you are asking, and I certainly couldn't even begin to understand the physics involved at such a level to ask how sharp is physically possible. Whatever it is, I'm pretty sure it won't be practical 

[jzmtl]

A properly stropped straight razor is probably as sharp as you can get, consider you are not even cutting with the edge but rather the burr, which can be damaged by even paper.

That or a lightsaber.