I'm Bored...

Crucible Steel lists WC at the lower numbers. I got that from their little orange book.

I think/hope there's room for healthy disagreement here. I suspect the problem that developed was largely one of semantics and perspective. I probably over-reacted to the blanket statement than flat ground is stronger than hollow ground, which I remain unconvinced of, but I apologize for the way I expressed myself on that point.

It occurs to me that unless you are aware of how knives are made, the comparison might be based on two blades of equal mass and different maximum thickness or equal maximum thickness and quite different mass. Further, knowledge of how exactly blades are shaped by the two grinding methods helps in understanding why black and white aren't always black and white in knifemaking. I'd really like to continue the dialog on a respectful footing.

An Internet search yielded data that says WC in a cemented cobalt matrix does indeed have Rockwell C hardness 89-92, so that memory was still intact! I also remember that hardness would be all over the place based on composition, %C, free C, etc. That raises the interesting question, based on Crucible's entry, of whether the tungsten carbide crystals formed in an alloy steel would indeed have a lower hardness due to the presence of all the other competing alloying elements.

The coolest thing about knifemaking is that it never stops raising another question for the knifemaker, whether it be steel selection, HT & metallurgy, design, etc. The fact that there are such a number of great combinations provides a wonderful ground for variety and creativity. Despite the fact that sometimes the debate gets heated and ventures into "less than academic" discussion, these forums provide a marvelous avenue for disseminating all this great information, thereby increasing our knowledge, and thus our options.

I am sure appreciative of all those who make this a great place to learn.....thanks!

It may well be that in an iron matrix where there is competition for the carbon, not all the carbon binding sites on the tungsten are filled and thus the lower mean hardness. That might also explain the range. Even in a cobalt matrix, which often includes some chromium, there would be some lesser amount of competition.

One thing that perplexed me (so far) with the discussion that has taken place would be the definition of "strong". To me, whether a blade is flat ground or hollow ground is akin to the approach of the differences regarding how an oak and a palm weather a storm. Both do it in totally different ways but end up accomplishing the same thing, surviving.

I did and do appreciate the correlation of an I-beam. That is very good example. But I am STILL stuck on the definition of strong. After that, someone always ask, which is better? In my opinion, we must then ask, for what purpose.

Craig

To Don Cowles:

When I registered I gave the information asked, including e-mail address. Name:Tapio Manner, email: tapio.manner@kolumbus.fi, occupation: design engineer, location: Vantaa Finland. Somehow they haven't ended up on the register.

Jerry apparently didn't like the reference to "marketing clown", here it is not a general term but refers to certain profit surfing, pseudo working bungee managers who often seem to have a marketing background (not allways), origin of "clown" is obscure but maybe it refers to that all other people would have been happier if he would have been replaced by a clown. I didn't refer to him but to the hypothetical person giving an idiotic assignment "best knife in the world", I guess I wasn't clear there.

The discussion started on bending of a knife blade and wheather that can be calculated.

Suppose for a moment that I am right. I presented a well known engineering fact (not a natural law but...) that has been in use for more than a century (and worked well) and from my point of view isn't even close to an opinion. I get an answer that, when I am looking at a knife blade bears, no resemblance to it, far relation but very far. And when trying to explain it I get back (from my point) meaningless references to gravity loaded building arches. Apparently we were not talking about the same subject.

I was definitely not trying to be a nosy expert on everything but to explain a minute detail that I have been working with for years, seems to have blown up.

Jerry, as far as I know, is a respected knife maker and I am not trying to say anything else and I have not referred to his ability in any way but does this mean that regardless of subject he is allways right!?

From my point of view he has very nearly called me a liar which wouln't mean anything if we were having opinions but when talking (remember my viewpoint) about accepted 'engineering facts' just sounds plain silly.

I have been following this forum to learn new things and I have. This just makes me wonder if the following rules apply:

1. The resident bladesmith is allways right.
2. If he happens to be wrong apply rule 1.

Lets see...

TLM

Since you directed this to me, I will be happy to respond - and thanks for providing your contact information.

Now that I know you are from Finland, it sheds some light on the possible semantic issues that could have been the root of some misunderstandings here.

The CKD is indeed a community -perhaps even a family- where we *all* learn from one another, and always in a spirit of friendly cooperation. We try not to be confrontational, even when our opinions are strong.

One of the many things I have learned here is that there are no hard-and-fast rules in knifemaking. For example, published heat treating information from steel manufacturers might not apply to sections as thin as those used for knife blades.

It is the spirit of the dialog that is important here; not who is right and who is wrong. A gentle approach is more effective than a manifesto.

It is also helpful to establish your bona fides by maintaining a presence in the forum for some period prior to holding forth on your particular area of expertise. Most of us listen more willingly to people we know, and whose credentials and experience we respect. Thanks for coming back.

By the way, you can update your personal information by clicking on your Profile and filling in the fields.

A R&D design engineer works mostly on shades of gray, very few things are black and white. I don't have to be right, after all these mistakes and errors collected during my working years it somehow has little meaning.

The reason why I originally commented is the ABS bending test. It is often referred to and bending is used as THE test on strength. That test does (as I read the requirements) not take into consideration the cross sectional geometry (all things included) of the blade and because of that it is not very well suited to general testing of blades. I was trying to tell that some things can be calculated and predicted without actually testing and saving a lot of work that way or maybe used to design a slightly better blade for a specific purpose. That message got lost somewhere along the way...

My credentials as a knife maker do not amount to much but I have done design on very mixed subjects with very mixed lot of materials and couldn't help learning something.

Anyone having had the basic "strength of materials" course could do these bending calculations. No "theory of elasticity" or FEM needed.

TLM

OK, let's try again. Maybe the problem is in the language. Maybe it's because most knife blades in Finland are on average fairly thin in comparison to those we use here. Here's what I had trouble with.

"...flat ground blades are stronger than hollow ground"

Is this statement true if the blades are of the same length and total mass? I am not disputing the engineering principle you've stated; I am challenging its application to the question.

OK. (You are correct there, most "puukko" are thinner than 3 mm (1/8"))

For the comparison to be meaningfull the blades should be fairly similar. I dont think we'll lose too much generality if we take the following example, assuming both blades to be of equal width and equal thickness at the back and the bevelled area goes all the way to the back. Taking the width as 38.1 mm (1.5") and thickness at the back as 5 mm (1/5"). The flat ground blades cross section is a thin isosceles triangle and the hollow ground sides go 0.5 mm 'below' the surface at midpoint. At these dimensions the HG blade is some 25 % weaker in bending at this section, if the back were thickened it would have to be 12% thicker than FG blade (5.6mm) before the strengths would be equal. The thickness of the back affects the bending strength as invers square relationship and the width linearly.

If the bevel would not go all the way to back and there were some parallel sides, I estimate that 1/3 of the width parallel would be enough to hide this difference in practice.

It also follows that if we make a thicker blade without making the hollow 'deeper' the effect is less.

If one does not use a knife as prybar the difference may not be meaningfull but it exists.

This is what I was meaning but is it what you were asking?

TLM

Almost, but not quite. Consider the issue of the point. The flat ground blade must be tapered towards to the point to make it a point and not a wedge, so it is impossible to maintain the same spine width for some of the blade's length for that reason alone. A hollow ground blade can maintain it's spine thickness much closer to the point, especially if the grind is angled a little as it approaches the point. Many hollow ground blades are not ground all the way to the spine, so the steel thickness near the spine can be greater (also related to the point issue). However, most hollow ground blades would go more than 0.5mm below the imaginary surface at the center, some would go much below that, so there is probably more strength loss there than in your example..

The reason I mentioned mass and holding that constant in your equation is that a blade must be reasonably light or the knife will be blade heavy. Unless the blade is for chopping wood, a knife should be reasonably balanced so the handle and the blade are not too different in weight. Obviously that is somewhat more easily achieved with a hollow grind, but as you observed the removal of steel diminishes the bending strength. Some of that is offset by maintaining the thicker spine and the area just below the spine. Some further rigidity (I contend) comes from the geometry of the hollow - the arch. To achieve that same weight (and weight distribution) without resorting to much thinner steel, the flat ground blade must be tapered towards the point, so the distal taper is not only needed to shape the point, more of it is needed to remove and redistribute weight from the entire blade, moving the balance point rearward towards the handle. Basically this has the effect of using thinner steel, but only in the forward portion of the blade.

Still, the strength of the blade has to be measured along its entire length. It matters little if the blade is very robust for most of its length if the forward part is fragile. Lateral stress might be applied anywhere along the blade and near the point is where that is most likely to happen.

I make very long, very thin, very light blades using 3/16" (~4.7mm) or thinner steel. This sword is made with 0.175" CPM-3V steel. The blade is 22" long. The full thickness of the steel is maintained to within 3" of the blade's point, and the steel is 1/8" thick within 1/2" of the point. Very little taper. A 3/16" band of full thickness steel runs along the spine to 8" from the point where the swedge (top edge) begins. The total weight of this sword is 22 oz. While it might seem that the sword would be extremely flexible, it is not. Certainly, it flexes but at about 15-20 degrees deflection, it becomes much more difficult to bend. The resistance is certainly not linear. There is no tendancy to vibrate or sting your hand if the blade makes a bad cut. Such a blade has never broken, even under severe test conditions, such as cutting very hard wood or bone. It would be extremely difficult to make this sword with a flat grind. Arguably, a flat ground blade having the same weight, balance, and speed characteristcs might be much weaker.



Thoughts?

(It occurs to me that we are like two men standing on opposite sides of an object, each describing what we see and not understanding why we see different things.)

Hmmm... let's see, said the blind man to deaf.

Broadening the spine is beneficial fast. On a solid cross section, like a blade, the hollow does not have any "form stiffness" like it does on a plate.

The cross section and its distribution defines the stiffness and strength of a blade and their distributions. Totally independent of these, there is loading distribution of which bending is most important, shear and torque are very seldom critical. If you use the knife for prying the max bending occurs at the fulcrum, usually close to the point. If you pound the knife into a tree and then load it from the handle the max occurs just inside the trees surface, again usually close to the point. On a sword things are different because on top of these you also get the dynamic loading. Here you just have to match the assumed use and the following loading (distribution) with cross section (distribution).

Your sword example is a good one because historically sword blades often have a hollow in the middle, there are theories why and I support the lightening one. Let's word the question: what is the solid cross section with minimum area and maximum bending strength.

Because bending strength increases in relation to square of thickness (a small inversion in my first answer) and mass only relative to thickness it is very obvious that the thicker the better, if we can retain an otherwise reasonable section. My first guess would be as follows: take a round bar and attach a hollow ground edge to it on some side. Tapering the point is again a question of use but of course if you want a strong point the thicker the better. It might look funny but it would be quite stiff and strong and take hits well (relative to overall weight). Anyway a rounded spine shape is better over a sharp one if yielding is not allowed.

If we allow yielding the whole question becomes a lot more complicated because it redistributes the stress on a cross section radically. My guess would be that it changes the balance towards sharper spines (like on some rapier blades).

All steels I have met so far are linearly elastic at small enough strains (normal) and bending unlinearities (curved beams) are not apparent at the thickness/radius ratios you get bending a sword blade under yielding, My guess is that the way you are loading it actually changes in such a way that it feels nonlinear, atleast that has been what I have observed with fiber reinforced composites where large deformations are common.

I haven't tried cutting anything with a sword (have fenced with epee for 35 years) but from some other experiments I gather that it depends on having the natural vibration nodes at right part of the handle and if possible using the right part of the blade for cutting (another node).

This is getting interesting

TLM

I think this has always been interesting. It's really quite amazing that here in the 21st century we really know very little about how blades work. Our visual model when we think "knife" is a refined wedge shape. In my experience that is not an efficient cutting shape since it exposes the blade to lots of friction as it moves through the material being cut. We tend to think of strength in terms of the same structure, but need to allow for the 3-dimensional nature of the blade, and that necessarily means that at some point along its length (not width) all values must move toward zero. The geometry of the edge has both a cutting component and a structural component. In a hollow ground blade there is a lot more latitude to manipulate both components at the edge, The same is true of the spine. Weight, unless you are using the blade to chop wood, etc, is a negative element.

And then there's the mystical blood groove. What is it? For lightening? Sure, that's more than likely. But is that it alone?. Why do many later swords possess very complex cross-sectional geometries? If blood grooves were just for lightening, why not simply make the blade thinner or just grind a wedge on the spine, since in the material bending-strength equation, that would be a much easier means of reducing weight and retain the same strength as you'd get with a blood groove.

I believe the longitudinal walls, like arches and I-beams and blood grooves, were for stiffening. Given the machinery of the time, and even with modern machinery (of which I own little BTW) a blood groove is a very expensive design element. Blades, in their finest form and especially in their long dimension, are very close in geometry to "plates" where vertical elements are indeed used for stiffening.

It may in part be that with "bending strength increases in relation to square of thickness", the added spine thickness in a hollow ground blade offsets the lower mean thickness of the rest of the blade.

Interesting for sure...

On swords, dirks, daggers etc. I think that smiths have during all times had a need to show off their skill especially to a rich customer. On some sword forums there have been well argued opinions for some features to be just flashy not functional at all.

All broader sword blades are used for hitting and cutting and the edge has to have a lot of transverse compressive strength that severely limits the practical edge geometries, a 'blood groove' in the middle is a good compromise.

On wood working knives I have (and shall use) used a very slightly convex edge that lessens friction and breaks the shaving better, come to think I have used the same knife from moose down to some moles.

On the original subject: if weight is important a HG or grooved blade gives you more possibilities and is even stronger (in bending) than the various flat alternatives but if the max thickness is limited it's the other way round.

Come to think of the many survival knives which are made of quite thick flat stock you could make a lot stronger (in bending) knife with thinner blade proper and a spine on the back on one edged or down the middle on twin edged blade and at less mass.

Ok, lets keep learning

TLM

The problem with the idea that complex geometries were exclusively for show is that they were quite normal on military weapons. Virtually all cavalry sabers produced in the West during the 19th century were remarkably complex. These were mostly produced under contract to the lowest bidder, and not for show. The ubiquitous nature of the blood groove is really difficult to explain as a cosmetic issue only, though it might be convenient for some who don't wish to reproduce them to argue so.

You really need to address the distal taper issue with flat ground blades. How is that stronger than the more uniform cross-section in a hollow ground blade?

Weight is always an important consideration, IMO.

On the edge formation issue, I find the hollow grind, where the apex of the grind is intentionally high to give extra material near the edge, allows me to produce a very wide (>1/8") convex edge that is excellent for heavy cutting or fine with very low friction. The sides of the blade just don't enter into consideration. This also provides local compressive strength without diminishing the blade's cutting efficiency. The edge is generally parabolic, providing the least amount of surface to contact the material being cut. It is very difficult to prevent the flat ground blade from remaining in contact with the material throughout traversing it.

Distal taper seems to be a regional thing , puukko does not really have any but then they are short bladed (<5"), on many types the taper is on the last 20 mm (on the back) the edge curves on last 40 mm, varies of course. Sharper points tend to have longer tapers where this becomes an issue. There are so many possible variations that I don't think that it can be generally commented either way. One thing that affects it really is yielding, if it is allowed or possible in the blade materials state, I think the flat one might have a small edge (pun), more complicated cross sectional geometries tend to have some secondary effects causing problems. With very shallow hollow and flat the difference isn't that much if they are about equal thicknesses, as said I prefer the convex one.

Hmm.. if my memory serves, the cavalry sabers I have seen here in museums are flat sided. Some of them are actually plates in the sense that in bending they start to feel the transverse poison's ratio. Also the way I understand they were used, more complicated geometries might not be useful. References?

Is there actually a thinner portion in the middle of the blade or are you using a fairly shallow hollow? If the portion next to edge is convex it does not really matter all that much what's above it, hollow or flat, at least not on wood, have tested enough variations there. Totally flat is very difficult to control and hollow is impossible. If the bevel on the cross section changes signs of curvature it really is somewhat matter semantics if it is called hollow or grooved.

Weight is important in larger knives and swords, again on small knives it does not matter (within reason).

I have many times noticed that there is a fairly large difference in the knives and there usage between most of scandinavia and north america. Large knives are rare (exept in lappland) and historically they were apparently weapons only, hunters and woodsmen also used a small knife and a small axe or hatchet.

TLM