Tapered Blades

[Jesse Smith]

I was watching a youtube of a guy forging a knife and complaining that stock removal guys don't taper the blade like he does, and that's why they break at the tip or finger guard, because they aren't flexible enough. Where as he gently tapers the thickness of the blade from the tang down to the tip.

Yet, one of the selling points I see on knives is often that the blade remains full thickness almost to the point. So, I'm confused. Does the thickness make a stronger blade or is flexible better? This is referring to small bowies, or 3-5" edc blades, boot knives, etc. since I'm know some blade types obviously wouldn't preform without being very flexible to begin with.

 

[Overmountain]

The strength or flexibility of a blade has more to do with the heat treatment than the size or thickness of a blade. If he is forging, he should be normalizing before he heat treats his blade. This process makes his steel structurally the same as any stock removal blade. Brittleness, which is caused by the heat treatment and a tempering cycle which leaves the blade in too hard a state is what would cause a blade to snap vs bend.
Just because someone has a hammer and a forge doesn't mean they have been studying metallurgy or even proper blacksmith/bladesmith work...

In point of fact, many stock removal guys grind a distal taper (the gradual tapering and thinning of a blade from ricasso to tip) when they grind a blade. And both forged and stock removal blades can carry full thickness to the front edge of the spine...

Ultimately there is little difference in forged vs ground blades. I do both to make my knives. There are a few little things that are associated with/common with each style, but ultimately we are all baking the same cake, just using different boxes of cake mix...

 

[idaho]

I was watching a youtube of a guy forging a knife and complaining that stock removal guys don't taper the blade like he does, and that's why they break at the tip or finger guard, because they aren't flexible enough. Where as he gently tapers the thickness of the blade from the tang down to the tip.

Well, people like to add some magic to their work

Yet, one of the selling points I see on knives is often that the blade remains full thickness almost to the point. So, I'm confused. Does the thickness make a stronger blade or is flexible better? This is referring to small bowies, or 3-5" edc blades, boot knives, etc. since I'm know some blade types obviously wouldn't preform without being very flexible to begin with.

But to the point.

Gradual distal taper is good for "proper knives".
You can build a 1/4 inch convex grind short drop point monster, that would be impassible to break by hand. But that is not a knife, that is a toy

"Proper knife" is thinner, sharper and with some tip. That is where gradual distal taper is useful.
Thinner steel "bends more" than thicker one without breaking, and with taper whole knife acts like a fishing rod - tip is bending more, hilt is bending much less. It gives better feedback to the user, and allows to unload stress to whole blade, not just to the weakspots.
BUT, distal tapered knife is NOT stronger, it is just more optimized.

Take full thickness blade with thin sharp tip - If you bend it, whole elongation MUST take place at the tip and it will break, while with tapered - the elongation will happen at more of blade length, and it will not break.
But you can allways build full thickness blade with thick tip - and then it will NOT bend and it will NOT break.

 

[Overmountain]

I agree with your statements about blade shape and qualities attributed to those shapes to a degree but I think the OP and the video he watched was saying that stock removal guys as a blanket statement do not taper their blades. Just a flat out false statement...

And there are times when a full 1/4" spine on a knife is necessary. Some tactial knives have a full thickness blade for strength so that the knife can be used as a lever, or worse; a step... (I still think that is a proper knife... I carry one on my entry vest like that...and certainly not a toy )Still, that blade will bend or snap depending on the HT of the steel, not just the thickness...

 

[james terrio]

I'm not familiar with that video, but it sounds to me like it falls at least partly under the category of "myth, half-understood 'facts', and outright BS."

All else being equal (same steel, same HT, same profile, etc.), thicker is stronger. There's no getting around that. Blades with distal taper are not automatically stronger than full-thickness ones - they just cut and handle better.

Flexibility is almost entirely dependent on geometry - whether the steel is annealed or fully hardened and tempered. A nice thin filet blade of stainless steel at 64Rc will flex very nicely without breaking - as will the same blade design made of carbon steel at 56Rc.

The finest knives ever in history are being made right now, by both forgers and stock removal makers.

 

[Bo T]

All things being equal a thin blade will flex further before failure than a thick blade. There are a number of postings showing the knives of 'stock removal' guys. A lot of them will show a gradual taper to the tip. So the statement by the blacksmith should be, "Some stock removal guys don't taper their blades, and some smiths don't taper their blades either.

 

[Karl B. Andersen]

In one aspect - this is incorrect.
Flex is geometry. Failure is heat treatment.

Two blades of identical steel and geometries - one properly heat treated and one in a non-heat treated state of pearlite - will flex exactly the same until the un-heat treated knife experiences failure and takes a set.
Whereas the heat treated knife will return to straight.
I've seen it demoed again and again.

The strength or flexibility of a blade has more to do with the heat treatment than the size or thickness of a blade.

 

[idaho]

In one aspect - this is incorrect.
Flex is geometry. Failure is heat treatment.

Two blades of identical steel and geometries - one properly heat treated and one in a non-heat treated state of pearlite - will flex exactly the same until the un-heat treated knife experiences failure and takes a set.
Whereas the heat treated knife will return to straight.
I've seen it demoed again and again.

I think that a graph would be extremely useful.
Something like this... I know it is for martensite, but it is good example.

HOWEVER I bet we can produce superfine perlite with tensile properties BETTER than for tempered martensite.

 

[Karl B. Andersen]

The important part of your graph is the first sentence - "......fracture of quench and tempered carbon steels."
Until the steel fails, they flex the same.
I've witnessed it demonstrated over and over.

The whole point is, flexibility of steel is dependent upon geometry - not heat treatment.
Failure is a different topic.

I think that a graph would be extremely useful.
Something like this... I know it is for martensite, but it is good example.

The important part of your graph is the first sentence - "......fracture of quench and tempered carbon steels."

HOWEVER I bet we can produce superfine perlite with tensile properties BETTER than for tempered martensite.

 

[Karl B. Andersen]

Make/grind/shape/taper two fillet knives from the same steel.
Harden and temper one.
Leave the other annealed.
It will require the EXACT same amount of energy to make them FLEX the same distance.
At some point during the flex curve, the annealed blade will exceed its modulus of elasticity and the steel grain will crush and fracture, BEND, and take a set.
At that same point of the flex curve, the tempered blade will return to perfectly straight.
Geometry determines flex. Heat treatment determines failure.

 

[idaho]

Karl - the graphs prove your point!

They show elastic "flex", then they show plastic failure, and the they show fracture - end of the line.
The elastic part of the graph is the same no matter how tempered, higher tempering temperature - the faster the (plastic) fail,
The only thing is they do include perlite, only differently tempered martinsite.

The whole point is, flexibility of steel is dependent upon geometry - not heat treatment.

That is extremely confident statement. Wanna bet?

 

[Robber58]

I was a mechanical engineer for 30 years and worked with the structural properties of materials daily. I have seen many arguments on the forums about the improved "strength" of tapered blades over full thickness ones. The fact is, Karl is correct. For identical materials, a tapered blade will bend more than a full thickness one under the same load because it is not as stiff. Some people seem to believe that because it does bend more it must be stronger, but that is simply not the case for identical materials. It is true that a tapered blade can offer better handling and balance but it will not provide increased strength.
Randy

 

[Karl B. Andersen]

The whole point is, flexibility of steel is dependent upon geometry - not heat treatment.
That is extremely confident statement. Wanna bet?

I've seen it proven. I don't need to bet.
Heat treatment determines failure of two similar steels/dimensions.

 

[james terrio]

It is true that a tapered blade can offer better handling and balance but it will not provide increased strength.

Thank you, and thank you Karl.

I can't believe this is still up for discussion in 2014. How people manage to convince themselves that less material is somehow stronger is beyond me.

 

[Jesse Smith]

Thanks for all the replies guys. I should clarify that the guy in the youtube was ranting about, paraphrased the best I could remember his words, "most stock removal guys, pay attention, I didn't say all, I said most stock removal guys" so he wasn't talking in absolutes as much as I made it sound like. And he said he wasn't looking down on stock removal guys until they start looking down on him... That all spurred from some comment someone left him on another video i think. Whatever.

Anyway, so what I gather from this thread is a tapered blade gives better feedback to the user, but as one would intuitively think, a thicker blade is actually stronger, all other things equal? Or is there still something too the bending/spreading the load idea?

Lets say I take two identical knives with identical profiles, material, heat treating, etc. and I mount them by the handle, horizontally in a vice. Then I take sandpaper and taper one of the blades so it's more flexible than the other one. I then attach big heavy weights to the very tip of the blades. Should I expect the full thickness, stiff blade to snap the last 1/4" or so off the tip, while the flexible blade bends in an arch?

 

[Brian.Evans]

Karl and James said it above. Geometry determines flex, ht determines whether a blade stressed past the modulus of elasticity breaks or takes a set.

I'll let someone more knowledgeable answer your question for sure, but I believe that given identical ht, the knives in your scenario would respond thus:
The tapered knife would bend since it is thinner, the thick knife would snap. To give another example:
Three knives, all heat treated to 64 rc, one 1/16" thick overall, one 3/16" thick overall, and one tapered distally from 3/16" to 1/16". Obviously this is exaggerated, but the thin one bends a lot until it hits the MoE, then breaks, the tapered one bends a little less before reaching the MoE, then breaks, the full thickness blade bends very little then breaks. Say the blades are 25 rc instead of 64 rc. It will take the same amount of force to move each blade the same distance, but once the steel reaches the MoE, it will take a set instead of breaking.

Despite what many people would like you to believe, steel and heat treating is plain science, not magic. When it was just a forge it was still science, only looked like magic. Steel is steel is steel. Chemistry and physics don't change.

 

[HallHandmade]

Distal taper is a feature found on premium blades, but they are not mutually exclusive. There is nothing wrong, weak, or bad, about a knife with no taper, assuming it is designed and intended.

I'm roughing the below knife tonight for HT tomorrow. It will have some inherent taper due to the swedge and grind geometry, but it is an untapered stock removal knife when you get down to it. I just wanted to give an example that taper in the blade is a superb feature for a good knife, but not a required feature.

 

[idaho]

Karl and James said it above. Geometry determines flex, ht determines whether a blade stressed past the modulus of elasticity breaks or takes a set.

I'll let someone more knowledgeable answer your question for sure, but I believe that given identical ht, the knives in your scenario would respond thus:
The tapered knife would bend since it is thinner, the thick knife would snap.

Yes. But you will need more force to bend thicker knife to break it, unless there are stress concentrating weakspots.

 

[sunshadow]

Yes. But you will need more force to bend thicker knife to break it, unless there are stress concentrating weakspots.

The advantage you will get with a distal taper blade that gives the appearance that it is stronger is that a distal taper blade with a bending force applied at the tip will dissipate the bending force over the length of the blade, if properly done it will give a deflection over the length of the blade, whereas a non tapered blade will resist bending, which will concentrate the bending force between the tip and the fulcrum, the added thickness actually adding to the stress load on the surface opposite the fulcrum which can rapidly exceed the tensile strength of the steel, which is why thick blades are prone to tip breakage in extreme situations. Physics is neither your friend or your enemy, it just is . . .

-Page

 

[Brian.Evans]

Similar to a Colt mainspring. A good spring should taper along its length as to distribute the load across the entire spring.