Strength: 3/16 flat ground vs. 1/8 Scandi/saber

[RX-79G]

For a "survival" or field blade, which are so popular at the moment, these two grinds are very popular. The Scandi has many adherents, but is generally used with thinner stock to be useful. Full flat grinds also cut and carve great, and are often seen with wider spines. Which one is more reliable from a strength standpoint?

This is actually a complicated question because "strength" in engineering isn't necessarily what we think of as strong in a knife. A blade that deformed only slightly after a hard blow would be considered strong, even those an engineer would say that the blade "yielded" and was taken past whatever its maximum strength was.

Anyway, I thought it was interesting that a wider blade might actually be weaker than a thinner one because of where the metal is. This cross section illustrates the problem. These 2D shapes demonstrate area, and therefore blade volume and weight:

The green tick marks stand for 1/16", so the Scandi blade is 1/8" wide, and the full flat is 3/16". To calculate the area of a triangle, it's length x height divided by 2. So if the full flat is 3 ticks wide by 10 high, that's 3x10/2=15. For the Scandi, the top half of the blade is 2 ticks by 5 high, and the triangular part is also 2 by 5, which makes its area 2x5 + 2x5/2 = 15.

So these two very different blades contain exactly the same amount of steel.

If you wanted to make a very rigid structure, a triangle is very stiff. For a given area the outer points are further from each other than a rectangle, which braces the shape better. Like the three guy wires of a radio antenna. The most stiffness for the amount of material.

A Scandi or saber grind packs more material into a thinner blade. That makes the blade more flexible, which is less strong from that point of view. But it also means that the blade can flex more without bending or cracking, even though there's just as much steel there.

In straight chopping, both blades are the same height with the same amount of metal in line with the force. The full flat is thinner closer to the edge - you can see that 2/3 of the FF is thinner than the Scandi is in the middle. If the impact causes torsion, the FF has less material to prevent the area above the edge from twisting.


If either blade is bent sideways enough to permanently deform, the shape of the Scandi should allow it to bend further before anything cracks. The wider FF will resist bending more, but when it does go it is more likely to simply break.


I like both grinds, but I'm starting to suspect that, for two blades of equal height, the Scandi will survive more than a full flat that is 50% wider.

 

[trevitrace]

Strong taper on that ffg.

 

[RX-79G]

It's only an illustration. All the information remains the same as long as you're comparing two blades of the same height.

 

[shinyedges]

I was think exactly what trevitrace was, who says the ffg has that strong of taper?

Or that both have to be identical in steel mass. I think for your example what you worked out regarding strength is dandy. That said the slope on a ffg can make it have more support or less support than a scandi depending on the thickness and preference of the maker.

In the end I think this is a case of "could be" but is also a case of "could not be". Basically it is possible but not guaranteed that either grind can be made to have more supporting steel than the other.

Hope that makes sense.


Also what about the slope of the scandi thinning out to have less metal and support than the ffg even in your example?

I guess you're pointing out that at that one point the scandi has more metal than the ffg supporting it, but you miss that below that point the scandi begins to taper more drastically and thus starts to have less material for support than the ffg. So is the scandi actually "stronger" because it has slightly more support in one location? What about when the taper occurs and it has less support at the edge? Truly is the scandi stronger than the ffg? I don't think so.

 

[rwn2000]

It's only an illustration. All the information remains the same as long as you're comparing two blades of the same height.

your example blades are only 10/16" or 5/8" tall, if I am following your diagram correctly. This is about normal for a scandi ground puukko, Mora, etc, but pretty short for a survival style knife made from 3/16" stock. I would be curious to see the results for a 1 1/4" tall blade and the same scandi angle. I think you will only get the same results if the saber grind is 1/2 the height.

randy

 

[RX-79G]

I was think exactly what trevitrace was, who says the ffg has that strong of taper?

Or that both have to be identical in steel mass. I think for your example what you worked out regarding strength is dandy. That said the slope on a ffg can make it have more support or less support than a scandi depending on the thickness and preference of the maker.

In the end I think this is a case of "could be" but is also a case of "could not be". Basically it is possible but not guaranteed that either grind can be made to have more supporting steel than the other.

Hope that makes sense.


Also what about the slope of the scandi thinning out to have less metal and support than the ffg even in your example?

I guess you're pointing out that at that one point the scandi has more metal than the ffg supporting it, but you miss that below that point the scandi begins to taper more drastically and thus starts to have less material for support than the ffg. So is the scandi actually "stronger" because it has slightly more support in one location? What about when the taper occurs and it has less support at the edge? Truly is the scandi stronger than the ffg? I don't think so.

I don't know what you mean by the Scandi tapering more. Scandi bevels are almost always more obtuse than a FFG. A common edge angle on a Scandi is 25° inclusive. The primary bevel on a FFG is going to be something less than 11° inclusive, which is more acute, which I assume acute = more taper. When is the Scandi going to be tapering more?


In terms of different variations, if you make the Scandi taller on the same thickness stock, it just gets stronger and heavier because more of the profile is rectangular before the bevels start (since you are still using the same 25° bevel), so the ratio of height to mass changes. As you make an FFG taller on the same stock, it reduces the primary bevel more and the strength near the edge goes down and the mass to height ratio remains constant.

 

[shinyedges]

Take a look at the photos you posted, which has more material supporting the edge?

And speaking of acute, which angle is more acute in your photo examples? The scandi is what I believe has a more acute taper.


Granted the scandi taper starts later than the ffg, true scandi grounds are flat all the way to the apex leaving less steel behind the edge.

The better comparison for strength would have been a saber grind I think.

 

[chiral.grolim]

... "strength" in engineering isn't necessarily what we think of as strong in a knife. A blade that deformed only slightly after a hard blow would be considered strong, even those an engineer would say that the blade "yielded" and was taken past whatever its maximum strength was...

"Strength" on what level? I think that "strength" in engineering is exactly what we should be talking about with regards to knives as well, doing otherwise confuses the matter.

Strength relates directly to material hardness which, in steel knives, is generally measured in Rockwell C though other scales can be used. The rockwell hardness indicates the blade's resistance to flex/strain - how much bending-stress it can endure prior to permanent deformation/fracture. A blade that only deformed slightly after a hard blow would be considered strong by an engineer as well in context of what it is expected to endure. Every blade has its yield point. The stronger blade has a yield point beyond the weaker blade. Strong/weak are relative terms.

The strongest materials are the hardest materials. Carbide is many times stronger that steel and so can be taken to much finer (thinner) geometries to withstand stresses that would fold/squash steel.
However, carbide is not very tough nor is it easy to restore when damaged (at current levels of technology), making it a poor choice for a "bushcraft" woods knife.


Beyond hardness-strength at the same geometry there is also stiffness-strength dictated by the thickness of the material. This is where your diagrams come into play. Stiffness/strength is cubically proportional to thickness.
As you step incrementally back from the apex of each blade, that blade which is thicker is also stronger at that distance. In your diagrams, the scandi-blade has thinner edge-geometry making it weaker but also having higher penetration/cutting-efficiency (mechanical advantage) close to the apex. This continues to be the case until the scandi's primary bevel flares beyond the thinner primary bevel of the FFG blade. But, as you note, the FFG bevel continues to a thicker spine, making it stronger in the spine.

The question you must ask is, "Where do I need my blade to be strong at the expense of cutting efficiency? Where can I allow my blade to be weaker for the advantage of cutting efficiency?" Here is a diagram from one of my reviews:

 

[RX-79G]

We seem to be using "strong" in the same way, since an engineer would talk about strength as what happens before yield, but you are talking about strength accepting minor yielding. This is what a metallurgist would call "toughness" in the matrix, but the idea also applies to the structure. I was taking the same POV.



In terms of "strength behind the edge", this requires you to decide to compare two different edge angles. If you sharpen your Scandi and FFG at the same angle, they have the same edges. And if they have the same edges, the Scandi's 25° primary bevel supports the 25° edge better than the FFGs ,<10° primary supports its 25° edge.

I don't know of any rule that a non-Scandi blade must have a greater edge bevel than a Scandi, and plenty do not. But even if you do arbitrarily choose a 40° inclusive edge for your FFG, the more obtuse edge angle may protect the edge itself (as you would hope an obtuse edge would), but doesn't do anything for the strength of the blade just a millimeter or two above the edge bevel where the large round breakouts we see from chopping occur. Edge damage is repairable with a stone, but a big crack propagating away from the edge is the death of the blade. This type of failure, or a full crack all the way through:

Even if you were comparing a 25° Scandi edge to a 45° FFG edge, if the FFG is reasonably thin behind the edge, it is only going to be thicker than Scandi for about half a millimeter. At 30° the FFG is thicker than the Scandi for so little distance that it would be hard to measure with calipers. I can see how having an obtuse edge is good for preserving that edge, but it doesn't do much for the preservation of the overall structure. And, it is ultimately less sharp.

 

[RX-79G]

I added some new parts to the diagram to answer Randy's question and illustrate the edge issue:

If you keep the same thickness stock (which is how knife buyers look at knife shopping), and make the blades taller, the Scandi just gets material added to the spine. The FFG elongates and becomes more acute, which essentially "thins" the blade more, making it more delicate. One thing that never gets much talk is about the FFG primary grind angle. I'm sure knifemakers think about it, which is what a big FFG like a Cold Steel Trailmaster has such a thick spine, or way a Survive knife has a very high saber grind rather than a FFG.

 

[craytab]

Strong taper on that ffg.

This. So simple.

 

[chiral.grolim]

We seem to be using "strong" in the same way, since an engineer would talk about strength as what happens before yield, but you are talking about strength accepting minor yielding. This is what a metallurgist would call "toughness" in the matrix, but the idea also applies to the structure. I was taking the same POV.

Elastic deformation falls within the range of what would be called "strength", the yield point has not been reached. A strong material that can endure a relatively high amount of stress prior to reaching some specified level of strain, but strain (bending) does occur - that is not "toughness". It is only once deformation becomes permanent that you have reached the yield point and enter the realm of "toughness" i.e. the propensity for the material to deform plastically prior to brittle failure. A material that deforms relatively little prior to fracture under an applied stress is "less tough" than one that deforms a lot.

And if they have the same edges, the Scandi's 25° primary bevel supports the 25° edge better than the FFGs ,<10° primary supports its 25° edge.

What do you mean, "better"? The edges, as you said, are the same right up to the transition to the primary bevel of the FFG blade. If you mean that the 12.5-dps bevel is thicker/stiffer behind the edge than the 5-dps bevel, that is so. That does not make it "better", just stiffer, and it comes at the cost of cutting efficiency (mechanical advantage). Whether or not that matters depends on your use.

... if you do arbitrarily choose a 40° inclusive edge for your FFG, the more obtuse edge angle may protect the edge itself (as you would hope an obtuse edge would), but doesn't do anything for the strength of the blade just a millimeter or two above the edge bevel where the large round breakouts we see from chopping occur

Quick note: "obtuse" means >90, "acute" means <90, a "more obtuse edge angle" can only apply to one that is >90 and neither is here. The 40-edge is "less acute", it is not "more obtuse".

Second, I do not normally see those large round break-outs from chopping at all, but when they happen it is due to an excessively thin bevel or the material being used regardless of the edge-angle. Thankfully, most steel tools used for hard-use are thicker than 0.020" behind the edge which, coupled with a 5-dps primary bevel, is more than enough to resist most of the normal stresses involved in chopping. The failure of one knife at such geometry compares nicely against the success of another knife at the same or thinner geometry (e.g. that BG Gerber vs... well, a US-made Gerber! Or an ESEE or Survive! or Becker or .... pick one).

Edge-thickness ~0.020 15-20 dps should be all that is needed for a small-medium knife, increase the thickness to 0.030 for a larger knife, to 0.040 for a 10" knife as needed, accepting that with each increase there is a substantial loss in relative cutting ability.

Even if you were comparing a 25° Scandi edge to a 45° FFG edge, if the FFG is reasonably thin behind the edge, it is only going to be thicker than Scandi for about half a millimeter. At 30° the FFG is thicker than the Scandi for so little distance that it would be hard to measure with calipers. I can see how having an obtuse edge is good for preserving that edge, but it doesn't do much for the preservation of the overall structure. And, it is ultimately less sharp.

Again, not "obtuse" at all. And, very important, a 30-inclusive edge is not at all "less sharp" than a 25-inclusive edge. "Sharp" is defined by apex diameter, i.e. how thin the knife can be made within the first micron or so of edge. Whether sharpening at 25 or 30 or 40 or 90, it is the apex-measurement that determines "sharpness".

Now, why have a 25-inclusive bevel that rides all the way up to 1/8" thick? Is it really "preserving" the "overall structure" of the blade, or is it actually overkill and significantly impeding the cutting performance of a tool which is, at its foremost, designed for cutting? Here is a diagram comparing edge-angles and their relative strength/cutting efficiency:

When it comes to knife blades, the rule is thin wins so long as the blade is thick enough to endure the stresses of use. The smart maker designs a knife that is as thin as possible for the task, thickening the blade where it is actually needed to preserve its integrity. That Gerber definitely needed a thicker primary bevel for whatever it was used for. A lot of other blades could stand to lose a significant amount of weight in the primary - much thicker than necessary.

For most knife-use, failure occurs within the edge-bevel not the primary.

 

[RX-79G]

Elastic deformation is "flex", plastic deformation is "bend" or "deform". We had both referred to a "strong" blade as accepting damage without major failure, which is plastic deformation. Tough steels deform minimally.

Elastic deformation is hardness + shape. A blade that can flex further without deforming is not strong from the engineering standpoint because it did changed shape. To a knife user, a knife that withstood bending without damage is strong in a useful way. And a knife that deforms without breaking is tough, which is more valuable to a knife user than one that breaks as soon as it arrives at one of the yield points.

This isn't really "fair", because fair would be to only talk about knife strength in terms of the amount of force required to get past yield. The reality is that we talk about testing and using blades in terms of degrees of bend or how hard we chopped, which isn't meaningful unless the knives being compared have the same mass and CGs.


This is one of the many places where a comparison of an industrial slicing machine and knife break down, because we use knives in a such an uncontrolled manner.

Is a Scandi grind poor for cutting? It seems like lots of people prefer them for carving, which puts a heavy load on both the edge and blade body. Slicing veggies is different, but if we're talking about pushing a knife hard in the field, I don't know where the intersection of common use and inefficiency is.

Is it really a leap to think that the apex measurement of a 25° edge is likely to be smaller than that of a 40° edge? All edges may convex slightly by the imprecision of hand sharpening, but they never concave. So the 25° will be 25° or more, and the 40° will be 40° or more, but neither are going to be less.

Thank you for the reminder on "obtuse".

 

[chiral.grolim]

Elastic deformation is "flex", plastic deformation is "bend" or "deform". We had both referred to a "strong" blade as accepting damage without major failure, which is plastic deformation. Tough steels deform minimally.

Elastic deformation is hardness + shape. A blade that can flex further without deforming is not strong from the engineering standpoint because it did changed shape. To a knife user, a knife that withstood bending without damage is strong in a useful way. And a knife that deforms without breaking is tough, which is more valuable to a knife user than one that breaks as soon as it arrives at one of the yield points.

This isn't really "fair", because fair would be to only talk about knife strength in terms of the amount of force required to get past yield. The reality is that we talk about testing and using blades in terms of degrees of bend or how hard we chopped, which isn't meaningful unless the knives being compared have the same mass and CGs.

Why is changing shape an issue? It is not except in reference to ANOTHER object that changes shape less - again, relative terms. It isn't about how much it changes shape (strain), it's about how much stress is required to induce that change.

Harden two blades of the same length to 60 Rc and then apply the same stress along the length and have one bend (strain) only ~45' and the other bend full 90' with neither "taking" i.e. reaching the yield point which, as it so happens, is same for both blades because both were HT'd the same. How? Make one blade thicker than the other. Thickness is cubically proportional to stiffness i.e. the ability to resist flexing under stress. The thick one can be said to be "stronger" but they have the same UTS, really it is just "thicker". BUT if they are the same thickness then the one that bends less (strain) under the same stress is "stronger".

It isn't about "mass and CGs", only thickness. And remember, "angle" is simply a means of describing the space between lines/planes, i.e. thickness.

Is a Scandi grind poor for cutting? It seems like lots of people prefer them for carving, which puts a heavy load on both the edge and blade body. Slicing veggies is different, but if we're talking about pushing a knife hard in the field, I don't know where the intersection of common use and inefficiency is.

Is it really a leap to think that the apex measurement of a 25° edge is likely to be smaller than that of a 40° edge? All edges may convex slightly by the imprecision of hand sharpening, but they never concave. So the 25° will be 25° or more, and the 40° will be 40° or more, but neither are going to be less.

YES, it is most DEFINITELY a "leap" to think that the apex diameter will be different. Apex diameter is not so much a function of sharpening angle as a function of edge-refinement (e.g. sharpening mechanism & grit) and the strength UTS of the material being sharpened.

Please remember that the absolute finest edges, i.e. smallest apex diameters, on knives are made on ceramic carbide hard-metals like WC-Co, and we are talking about 10 - 100X finer than ANY steel can actually be taken and expected to hold-up in use. This is because the hardest steels are still >2X weaker than "soft" (high Co) carbide hard-metals - the steel has a hard time taking and holding a shape that fine (we're talking about apex-diameters <0.1um). The limit of steels is generally ~0.1um and it won't hold that.

IMPORTANT: since steel has a difficult time achieving such fine apex-diameters, is it reasonable to assume that thinning the support metal behind that apex will help it achieve a finer edge? No. Indeed, the evidence indicates that the opposite is the case, as logic would predict. To achieve the finest apex, you need MORE support material, not less. In nearly ever case, if you sharpen two blades of identical material to the same level of refinement, the one with the thicker grind will achieve the sharper apex. Also, it will be better able to hold that shape against stress. Neat, huh?

 

[RX-79G]

I think we are speaking the same language, but talking past each other despite that.

I really just wanted to demonstrate that spine thickness or stock thickness isn't a very accurate way of predicting the strength of a knife blade, as a 1/8" blade of one kind of grind contains more steel than a 3/16" blade of another popular grind.

The rest of the discussion is more about how people can't feel "engineering". We don't know how hard we chop in Joules or how much we may be twisting the handle. Our feedback loop is about transmitting shock, vibration and feeling the object we are handling change shape under stress. So while a particular blade may require more force to break than another would take to bend very slightly, in use the one that broke didn't feel radically stronger, since it broke without any warning at all.


I'm not a proponent of either type of blade, though I do find some of the objections raised about Scandi grinds to not match my experience actually using such knives in the real world. They get very sharp and the grind doesn't seem to interfere with any of the activities I use a knife in the woods for - carving wood and cleaning game. I also have, use and make FFGs, and quite like them. I'm just becoming aware that discarding half the metal on a bar of steel may have some downsides.


As far as the sharpening thing goes, your last statement would appear to suggest that edges get sharper and sharper as they become less acute (or even obtuse!). I doubt a 120° edge would actually be sharper than a 25° one, but I understand what you're saying. In terms of the thread topic, it would seem to be me that a light convex or microbevel buys everything needed in a Scandi grind, without undoing the perceived benefits of that grind. I know you say the grind gets in the way, but that really doesn't seem to be a real world problem with field use for a lot of us.

 

[chiral.grolim]

I think we are speaking the same language.

We are definitely on the same page :thumbup:

I'm not particularly a fan of low-saber scandi grinds because I usually don't need that much material behind the apex of such short blades when a narrow edge-bevel or even a micro-bevel gives me the durability I need up-front, leaving a thinner blade behind for deep penetration or slicing. But folk commonly describe scandi blades as "good for carving" which makes sense since they have similar geometry to a wood chisel - very narrow apex that leads into a stout bevel since the angle remains the same. I wouldn't want to rely on only my 0.005" BET slicers or wood carving I'd likely fracture a bevel since they are <5-dps in that thin primary. But for slicing jobs, that 1/8" stock 25-inclusive scandi is a chore to use Different tools for different tasks.

The experiments on apex-diameter refinement end at 90-inclusive because beyond that it becomes difficult to determine where the actual apex is All apices unavoidably round over convex, and remember that the apex is defined by its diameter, so you can probably understand the problem. The peaks of abrasive particles are often obtuse like that, but they don't require much penetration-depth. Gilette is producing razor-blades that are 90-inclusive just behind the apex, but they taper-down to a thinner angle via multi-faceted grinding (essentially convex).

I agree, a 3/16" blade is not necessarily more robust than a 1/8" blade since what happens between the spine and the edge, i.e. amidst the bevels, is where the thickness counts. That's why I include a super-imposed image with my blade-comparison schematics. In an upcoming review, there will be a Mora Robust included in one such schematic.

 

[bdmicarta]

The green tick marks stand for 1/16", so the Scandi blade is 1/8" wide, and the full flat is 3/16". To calculate the area of a triangle, it's length x height divided by 2. So if the full flat is 3 ticks wide by 10 high, that's 3x10/2=15. For the Scandi, the top half of the blade is 2 ticks by 5 high, and the triangular part is also 2 by 5, which makes its area 2x5 + 2x5/2 = 15.

So these two very different blades contain exactly the same amount of steel.

Nobody seems to have answered the original question. I know how to calculate section properties of various shapes and it turns out that in this simple example the thicker triangular section has 12% more strength in sideways ("weak axis") bending than the thinner section with rectangular plus triangular parts.