Rehashing "flex"

To get some hard numbers to apply to this I just watched a DVD of Tim Zowada’s demo on this topic from two years ago at Ashokan (thank you Chris). I think most present would have thought the principles laid down here as nonsense before that demo but after actually seeing it applied right before their eyes, they accepted what was the undeniable truth about how these things really work.

Tim made two identical bars of O1 that were 1” x 1/8” and around 8” long. One he left annealed and the other he heat treated to 60.8 HRC. Both were clamped on a base and weights were hung off the ends. When they both deflected exactly the same amount, you could see the eyes opening wider in the crowd as the whole world began to change and become more clear.

Tim then put both pieces in a device he made with a strain gauge for precisely measuring force applied and an indicator to show any permanent bending. The Annealed one took a set at 50 lbs. of pressure (easily done with your bare hands) and was totally bent beyond use at 120 lbs.

The piece hardened to 60.8 HRC required 350 lbs to take a set and broke only after 425 lbs. So the question we need to ask is what do we want as far as strength and toughness in use? Do we want a 1/8” thick knife that will require 350 lbs to bend and 425 lbs to break or do we want one that will bend between 50 and 120 lbs? Both knives will behave exactly the same in flexing below 50 lbs. but the through hardened one will handle around 7 times more force before showing any effects, if flexing knives flips your switches.
 
Kevin R. Cashen said:
Indeed in matters of stiffness geometry is the bottom line. Increasing the ammount of material to resist tensile forces on one side and compressive forces on the other is how you make things stiff. And it does't have to be pure mass, one can play games with distances from the center line where the two forces are divided, as in "I" beam type shapes, the mass is not the key as much as the distance from the centerline that the materil extends thus requiring more force to overcome it. This comes into play with rapiers. Real rapiers are not these flippy flimsy whip like foils that we so often see, they are rigid and as stiff as they can be made and this has nothing to do with their heat treatment. Good old rapiers had heavy fullering or even sometimes star shaped cross sections to give rigidity in order to drive them through their targets.
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Kevin R. Cashen said:
I thinkI would say strength is the ability to resist plastic deformation and toughness is the ability to "tolerate" deformation. 1018 is ductile and very soft so it can tolerate getting smacked with a hammer and taking a bend without breaking, it is tough. While hardened 5160 is strong and can resist the bending under the blow but once the bend is inevitable it will not tolerate it very well.
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Kevin, you did the great job to explain what the moment of inertia is as well as ductility.:thumbup:
 
bambright said:
Kevin, you did the great job to explain what the moment of inertia is as well as ductility.:thumbup:
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Ah, moment of inertia in the non "spinning ballerina" sense. I almost used that term earlier, but I figured people would think I'd gone crazy. I've also heard it referred to as sectional modulus.

Regardless, I'm surprised how good Kevin's understanding is, that isn't really even a metallurgical concept, but more a mechanics of materials thing. Kevin is a bright dude.
 
bambright said:
Kevin, you did the great job to explain what the moment of inertia is as well as ductility.:thumbup:
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Thank you very much! That means a lot since it have been criticized for putting this all under modulus of elasticity instead of getting very specific about aspects such as the moment of inertia, but I ask myself what that would accomplish. Would I just lose the folks I am trying to reach by getting buried in the minutiae instead of sticking with the big picture. To those who want to split hairs about where the numbers actually fall, I would ask if the slope of the proportional range ever changes with heat treatment if I fail to mention the more specific areas? The general concept is sound and works just fine on communicating things in lay terms. Now if I were making errors or omissions that would lead people to believe that the proportional line can instead be a curve with special quenching techniques, then I need to be called on my lack of clarity. I have always found the greatest difficulty in carrying on the conversation with both the people new to the subject as well as those quite familiar with it and engaging both. With that being my approach you then can understand that I consider it a great compliment that you found my approach illustrative of the finer details as well. :)
 
Nathan the Machinist said:
...Regardless, I'm surprised how good Kevin's understanding is, that isn't really even a metallurgical concept, ...
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I am painfully aware of that, metallurgy comes easy to me but these kinds of material physics discussions are a bit more of a challenge and require something akin to a tightrope walk for me, but the basic principles always seem to work.

Nathan the Machinist said:
...
but more a mechanics of materials thing. Kevin is a bright dude.
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But I wouldn't get too carried away, as even a broken clock can be right twice a day, and even blind squirrels can occasionally find acorns, the law of averages can be a wonderful thing as well:D A little luck doesn't hurt either;)
 
So,... when it all boils down!... any given piece of steel is only what it is, and no more or no less...
 
... a 1/4 inch round piece of 01, is the same thing,... anyway you slice it!

... what really matters is what you want to do with it,... or what you want it to do! LOL :D
 
All right, so in light of this thread I think we ought to be touching on other knifemaking parameters such as the 'edge flex test'. What, if anything, is being proven by flexing the edge of a knife over a brass rod?
 
Matthew Gregory said:
All right, so in light of this thread I think we ought to be touching on other knifemaking parameters such as the 'edge flex test'. What, if anything, is being proven by flexing the edge of a knife over a brass rod?
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Easy!

It proves the edge will flex over a brass rod,... under the given circumstances.
 
... it's poetic!

... like sliding "Y" up and down the scale,

... not that much different than playing a slide trombone. :)

... it's all about curves and scales.
 
Kevin ,in different words, within [below] the elastic limit the bending is controlled only by 'E' [modulus of elasticity] .Stiffness is a function ,as far as shape , is controlled by 'polar moment of inertia' . Modern wood frame construction uses wooden 'I' beams which are much stiffer than a joist made of standard 2x10 of the same weight. The 'I' beam has better Polar moment of inertia than the 2x10 !
 
Tai Goo said:
Easy,... by ear.

Play from the spine, skip the brain and let it go straight to the ear...

... and your hands will catch on with practice.
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Cool! Okay, guess what song I'm playing on the brassrodbone now...:D
 
mete said:
...can't think why anyone would want a flexible boner...
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Neither can I, but I get e-mail all the time from people wanting to sell me stuff to fix it:D:foot: sorry mete I kind of had to since I am surprised nobody else took such easy bait:p

Tai Goo said:
Whatever I'm smoking probably isn't any better than what Kevin's drinking!

.. but I drink too! HA HA LOL :D
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Actually I have had two cups of Earl Grey, and one mug of pale ale after splitting a load of wood (I would rather cut steel with a hatchet than split red elm:grumpy:). Perhaps I will have some scotch or a good Irish whisky tonight. Yesterday it was the bourbon and a decent inexpensive chianti with my spaghetti dinner. I enjoy some sort of alcohol just about every day, but I cannot remember the last time I was inebriated, I just enjoy the flavor of very good spirits, for that reason I don’t bother too much with the cheap stuff. All the alcoholics I have known in my life drank the cheapest stuff they could find because they didn’t care about flavor all they wanted was to get a stupid as they could on a buck. I say life it too short to drink lousy booze! I can’t speak for Tai but I only try to smoke the best I can get as well, I have never smoked cigarettes for that reason, but enjoy a good cigar from Nicaragua or the Dominican Republic, occasionally a Honduran as well, I have some Cubans but find them entirely overrated. But my favorites are ones I can steal from Matt when I see him:D. However it is winter now so I only light a cigar perhaps twice a month but enjoy a bowl of tobacco in one of my calabashes at least once a day.

Matthew Gregory said:
All right, so in light of this thread I think we ought to be touching on other knifemaking parameters such as the 'edge flex test'. What, if anything, is being proven by flexing the edge of a knife over a brass rod?
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The edge flex thing is like the file test, adequate if you are not all that particular. It will get you in a wide range but will not tell you anything specific. I would say that the file test can be more accurate since it at least measures a form of hardness (abrasion or scratch hardness but not penetration), while the brass rod flex test tells you a whole lot about how thin you ground the blade and not as much about heat treatment as people would like to think. People who hold it up as the definitive test are simply blowing hot gas. The only way the test can tell you about heat treatment is if you push it to the point of yielding and then you have a kink in your edge- kind of like the nice fellows in the middle ages who would tie rocks to your daughter, toss her in pond and then tell you the great news that she was not a witch after all! Hey that’s great, but it is rather moot point now isn’t it? And of course if you have a good hardness on your edge it will flex like crazy if it is thin. Are we looking for a certain amount of deflection every time? Are we looking for a certain amount of force required every time? If it is a set amount of deflection forget about however you heat treated it and just grind it thinner to meet your mark. If you want to really lay on it and not deform it at all simply grind it thicker. I personally think there has been a lot of slight of hand and misdirection involved in many of these popular tests that make the presenters knives look great, either in their own mind or in the public. Flexing over a rod will get you inside a huge ball park if that is all you need, but it is not a real definitive or even accurate test overall.
 
Kevin R. Cashen said:
Neither can I, but I get e-mail all the time from people wanting to sell me stuff to fix it:D:foot: sorry mete I kind of had to since I am surprised nobody else took such easy bait:p



Actually I have had two cups of Earl Grey, and one mug of pale ale after splitting a load of wood (I would rather cut steel with a hatchet than split red elm:grumpy:). Perhaps I will have some scotch or a good Irish whisky tonight. Yesterday it was the bourbon and a decent inexpensive chianti with my spaghetti dinner. I enjoy some sort of alcohol just about every day, but I cannot remember the last time I was inebriated, I just enjoy the flavor of very good spirits, for that reason I don’t bother too much with the cheap stuff. All the alcoholics I have known in my life drank the cheapest stuff they could find because they didn’t care about flavor all they wanted was to get a stupid as they could on a buck. I say life it too short to drink lousy booze! I can’t speak for Tai but I only try to smoke the best I can get as well, I have never smoked cigarettes for that reason, but enjoy a good cigar from Nicaragua or the Dominican Republic, occasionally a Honduran as well, I have some Cubans but find them entirely overrated. But my favorites are ones I can steal from Matt when I see him:D. However it is winter now so I only light a cigar perhaps twice a month but enjoy a bowl of tobacco in one of my calabashes at least once a day.



The edge flex thing is like the file test, adequate if you are not all that particular. It will get you in a wide range but will not tell you anything specific. I would say that the file test can be more accurate since it at least measures a form of hardness (abrasion or scratch hardness but not penetration), while the brass rod flex test tells you a whole lot about how thin you ground the blade and not as much about heat treatment as people would like to think. People who hold it up as the definitive test are simply blowing hot gas. The only way the test can tell you about heat treatment is if you push it to the point of yielding and then you have a kink in your edge- kind of like the nice fellows in the middle ages who would tie rocks to your daughter, toss her in pond and then tell you the great news that she was not a witch after all! Hey that’s great, but it is rather moot point now isn’t it? And of course if you have a good hardness on your edge it will flex like crazy if it is thin. Are we looking for a certain amount of deflection every time? Are we looking for a certain amount of force required every time? If it is a set amount of deflection forget about however you heat treated it and just grind it thinner to meet your mark. If you want to really lay on it and not deform it at all simply grind it thicker. I personally think there has been a lot of slight of hand and misdirection involved in many of these popular tests that make the presenters knives look great, either in their own mind or in the public. Flexing over a rod will get you inside a huge ball park if that is all you need, but it is not a real definitive or even accurate test overall.
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I totally agree! :)

and ... smoke 'em if you got 'em...
 
Not that you shouldn't use them, but I can't think of any quick little tests that will tell you "everything" you need to know. Some of them are just plain silly and maybe a waste of time.

At any rate, you need to know what you are looking for and have some good theory behind it.

But,... Use whatever you can, whatever you have...
 
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