What is so great about tapered tang?

[tomh]

I feel your pain Ed, well at least the sitting around part. I hate that too! See ya soon!

 

[Will Leavitt]

Wouldn't fullers act as an I beam in the blade cross section?

 

[Lucky Bob]

Speaking as a mechanical engineering student, removing material from a solid object (filleting outside corners are an exception, but that's nitpicking) will never strengthen said object. What it will do is increase its specific strength, ie strength per weight.

When you flex an object, the parts that are subject to the most stress and therefore require the most strength are the ones being compressed or stretched the most, which are the parts farthest from the plane that is neither stretched nor compressed, usually the outside. That's why an I-beam has a higher specific strength when you apply a load along the vertical of the I, but not when it's perpendicular to that. When it's along the vertical, you have full width of material at the inside and outside of the curve, but when you do it the other way you have just the corners trying to resist a ton of stress. In knifemaking terms, that means that fullers lighten the blade but actually weaken it, just insignificantly against loads applied in the spine-edge direction.

Also, along the same lines, I'm pretty sure that, ignoring any lightening holes and resulting stress concentrators, a tapered bar will be weaker than an untapered one, just because there's less material to spread the load.

Unfortunately I don't have the relevant textbook handy to cite, but if you all would like, I'll see if any of my friends/classmates have a copy I can borrow.

 

[Rick Marchand]

Lucky Bob

Are you measuring "strength" as the ability to resist stress (ex. a prybar) or the ability to displace/transfer stress/energy (ex leafspring). I'm no engineering student... I don't know all the lingo... so bear with me.

Perhaps toughness or resilience is a better word for what I'm speaking of. I have performed a flex test on tapered blades vs. non-tapered and the tapered pull ahead by a longshot. Like the limbs of a Bow the distal taper allows the blade to flex rather than focus stress to the weakest point on the curve. Try to draw an untapered bow and you are going to snap it.

Isn't an I-beam the most rigid in the direction straight down through the "I". I mean thats how they are used... from truckbeds to spanning gaps in construction. if a fuller is running down the center of a blade, wouldn't the knife be rigid along the edge to spine axis?

 

[Edro20]

I thought that a tapered tang meant that as a knifemaker you had passed a test. That being the shower of sparks that burns your hands and arms while you grind it.

 

[sunshadow]

Hey Ed, have a quick and complete recovery

-Page

 

[J.Marsillo]

Exactly Ed..... great point. Not many folks can digest the fact that removing material can strengthen a structure if done properly. I believe fullers(blood grooves) were used to enhance rigidity as well as lighten blades... am I right? There are many myths about that.

Rick

After seeing a forged fullered sword do some rediculous bending I'm sold on that... I think the maker was Pavel Mach (sp?)

 

[Justin King]

A tapered tang is simply the most cost- and weight-effective design, if done right. It makes the blade thick and ridgid where you want it to be and eliminates excess material where it is not needed, creating and ideal flex curve when the blade is put under lateral stress. It would be very difficult to replicate this effect by drilling and the customer will not be able to see it.

Fullers do not make a blade more ridgid. I have experimented a good deal with fullering sword blades and I have not yet been able to make a blade more ridgid by grinding metal off. Some cross sections will suffer more in this regard than others when a fuller is ground in, but none will become stiffer. It makes the blade more flexible but allows one to remove weight without affecting the functional dimensions or edge geometry of the blade. On some designs it allows a good trade-off between weight and flexibility but one might note that most dedicated thusting swords did not have fullers in the last third of the blade towards the point. Many had none at all.

 

[Lucky Bob]

Are you measuring "strength" as the ability to resist stress (ex. a prybar) or the ability to displace/transfer stress/energy (ex leafspring). I'm no engineering student... I don't know all the lingo... so bear with me.

Perhaps toughness or resilience is a better word for what I'm speaking of. I have performed a flex test on tapered blades vs. non-tapered and the tapered pull ahead by a longshot. Like the limbs of a Bow the distal taper allows the blade to flex rather than focus stress to the weakest point on the curve. Try to draw an untapered bow and you are going to snap it.

Isn't an I-beam the most rigid in the direction straight down through the "I". I mean thats how they are used... from truckbeds to spanning gaps in construction. if a fuller is running down the center of a blade, wouldn't the knife be rigid along the edge to spine axis?

I guess my wording was a little unclear, yes, that is the direction I-beams are most rigid in. However, a solid block of the same material of the same outside dimensions would be even stronger, it'd just weigh a whole bunch more (and more than it added strength.) However, if you try to bend an I-beam the OTHER direction, it will have lower specific strength than a solid block of the same material.

As for the bow analogy, I think I see what you mean. So you're saying it will flex farther without breaking, not that it will take more force to break it? I can see how that would happen, since the thinner a sample is, the less distance there is from the neutral (ie no compression or tension) plane to the outside surface, and therefore the outside surfaces are linearly compressed or stretched less for a given amount of bend.

However, this falls back into the same kind of argument as those who prefer to partially harden their blades for performance reasons. Given that the spring constant is effectively invariable among steel alloys and particularly among the microscopic structures of a given alloy, hard vs. soft (as far as force and stretching/compressing are concerned) is just a matter of how early it will deviate from the ratio that is the spring constant, with a tendency towards later deviation then being brittle. So yes, the unhardened portion of the blade will bend a lot before failing, but I personally would rather the entire thing be hardened and take 2-3 times as much force to even take a set, even if it will fail shortly after doing so. Similarly, although you may be able to flex it farther, I'm willing to bet that untapered bow/stock will take a whole lot more force before it catastrophically fails, even though it will be at a lesser bend.

EDIT: I just wanted to piggyback off Mr. King's comment and point out the difference between stiffness and strength. Strength, depending on the context, is how much force it takes to either give the object a permanent bend or to break it, while flexibility/stiffness is how much force it takes to deflect it a certain amount. The two, although related, are not identical and can be affected independently, as in my last example.

 

[Rick Marchand]

Thanks L-Bob.



Rick

 

[artmichalek]

It makes the blade thick and ridgid where you want it to be and eliminates excess material where it is not needed, creating and ideal flex curve when the blade is put under lateral stress. It would be very difficult to replicate this effect by drilling and the customer will not be able to see it.

This is based on the assumption that in normal use, enough lateral force is being applied to the tang to make it flex noticeably. I can see tapering the blade to get an ideal flex curve, but the tang is in my hand and it's got thick scales pinned to it.

 

[artmichalek]

However, this falls back into the same kind of argument as those who prefer to partially harden their blades for performance reasons. ... So yes, the unhardened portion of the blade will bend a lot before failing, but I personally would rather the entire thing be hardened and take 2-3 times as much force to even take a set, even if it will fail shortly after doing so.

I think that far too much emphasis is being placed on lateral bending tests. First of all, when you bend a blade laterally, it pretty much all bends to the same radius throughout the height. So it doesn't matter that the minimum bend radius is about half the thickness in the soft part of the blade and six times the thickness in the hard part. The hard steel will fracture first.

The point of differential hardening is to add impact resistance to the blade. The uneven heating and cooling results in a residual stress profile that puts the hard, brittle edge in compression and the soft, ductile center in tension.

 

[Lucky Bob]

The point of differential hardening is to add impact resistance to the blade. The uneven heating and cooling results in a residual stress profile that puts the hard, brittle edge in compression and the soft, ductile center in tension.

Huh, hadn't thought about that angle. While the theory is sound, wouldn't tempering remove those residual stresses, or are they of a magnitude that it would take a full anneal? Also, isn't the point of purposeful residual stresses to have them be opposite to the stress you're trying to resist? So for an edge-impact, you'd want to have the edge in tension, no?

(Wow, that's almost entirely questions, good job me!)

 

[Rick Marchand]

I think that far too much emphasis is being placed on lateral bending tests. First of all, when you bend a blade laterally, it pretty much all bends to the same radius throughout the height. So it doesn't matter that the minimum bend radius is about half the thickness in the soft part of the blade and six times the thickness in the hard part. The hard steel will fracture first.

Ideally, when a differentially hardened blade is flexed to failure, the edge will crack while the softer potion remains intact (though possibly bent) The break is not catastrophic and with a little counter bending you still have a usable knife. This is very important for those who depend on a knife to get them home, safely.

A 90 deg flex test is also mandatory for ABS testing and you fail if your knife finishes the test in 2 pieces. In this case, its more to guage the maker's grasp of metalurgy and blade geometry.


There is a give and take for everything we do as makers. You just have to know what you want, perfomance-wise.... and find a happy medium.

Rick

 

[Tinker Simpleton]

I'm a mechanical engineer too, and I'm having a hard time understanding the concept of a tapered tang being "stronger" than a straight tang with a sideways bending load. The ability of a tapered tang vs. a straight tang (given identical material) to resist bending is entirely dependent on the moment of inertia of the cross section of the tang. So, over the entire length of the tang, a straight tang will be stronger.

However, one must also look at the stress applied over the length of the tang. Where is the maximum stress? In my mind, it is just forward of your hand. Where is the minimum stress? In my mind, at the very butt end of the tang. So, at the area where the handle meets the ricasso, the blade should be at its strongest (or widest and thickest). Theoretically, the cross section of the tang at the butt could have a "zero" area, or taper to zero . So, from a design/stress standpoint, a tapered tang is ideal, in that the taper reduces the cross section as the stress is also reduced.

Also, I beams are I beams because solid cross sections are not practical. Again, the cross section of a beam is the sole determiner of its strength, and a solid rectangle is stronger than an I beam. But, maximum strength cannot always be the sole factor in determining proper material usage. Weight savings and material cost savings also factor in, so as a designer, you look for the cheapest, lightest material that is able to withstand the design load.

More importantly for me, a tapered tang, as mentioned, adds immeasurably to the aesthetics of a knife design. I have access to all kinds of CNC equipment but I will never allow myself to mill out a blade profile because I believe that the freehand method of producing a knife allows the artistic, organic side of my creativity to show through. The engineer in me worries about and takes care of making sure holes align, etc., but the creativity involved has been a pleasant surprise as I've begun making knives.

 

[artmichalek]

Huh, hadn't thought about that angle. While the theory is sound, wouldn't tempering remove those residual stresses, or are they of a magnitude that it would take a full anneal? Also, isn't the point of purposeful residual stresses to have them be opposite to the stress you're trying to resist? So for an edge-impact, you'd want to have the edge in tension, no?

(Wow, that's almost entirely questions, good job me!)

Yes, you need a full anneal to get rid of residual stresses. The reason for wanting the hard edge under residual compression is that it's brittle. Tension on the edge causes fractures to propagate.

 

[Bruce Culberson]

Interesting read guys! Nice to hear from the engineers too - thanks Bob & Tinker!

I need to get a 2x72" - the 1/3 hp Craftman doesn't have the balls to taper tangs (or at least I don't have that much patience!)

 

[Rick Marchand]

Great post, Tinker
The perfect marriage of engineering and art.

It reminds me of a quote... and I can't find it on the web, so I'll wing it...

Optimist: "That glass is half full."
Pessimist: "That glass is half empty."
Engineer: "That glass is twice as large as it needs to be."

 

[Lucky Bob]

Ideally, when a differentially hardened blade is flexed to failure, the edge will crack while the softer potion remains intact (though possibly bent) The break is not catastrophic and with a little counter bending you still have a usable knife. This is very important for those who depend on a knife to get them home, safely.

A 90 deg flex test is also mandatory for ABS testing and you fail if your knife finishes the test in 2 pieces. In this case, its more to guage the maker's grasp of metalurgy and blade geometry.

As for the first part, I'm aware of the theory. My point is that, in terms of force required, a through-hardened blade will take more force plastically than a soft blade (or the soft part of a differentially hardened blade) will take before failing. Yes, a soft blade might take a 90 degree bend without breaking, but the same force applied to a hardened blade will just make it flex without any permanent deformation, let alone failure.

As for the ABS, I second Cashen's take (which you seem to, as well) that the test is not so much that these are the properties that every knife should have as much as can you jump through the hoops we set.

Yes, you need a full anneal to get rid of residual stresses. The reason for wanting the hard edge under residual compression is that it's brittle. Tension on the edge causes fractures to propagate.

Fair enough!


And Tinker, no need to quote you, I just wanted to say "right on!"

 

[artmichalek]

As for the first part, I'm aware of the theory. My point is that, in terms of force required, a through-hardened blade will take more force plastically than a soft blade (or the soft part of a differentially hardened blade) will take before failing.

But in practice, blades are not broken by a steady ramp of increasing force. They're broken by some bonehead jamming the blade into something big and cranking on the handle. The bonehead will overload either the full hard or part hard blade in the process, but only one of them will fracture. A blade that can take some plastic bend will hold up to stupidity a little better.