I'm Bored...

I'm glad you spoke of that, Jerry. Have you any other intelligent test you can share?

You've alluded to a test using beef-bone, before. Can you tell us something more about it? It seems like one might be able to find several corrollaries to real-word knife uses with that.

Impact stresses are notoriously difficult to calculate or measure but they are high, way above anything created by bending. I questioned the bending test in its present form on another forum an at least one ABS master smith agreed with me (on most points ;>)) but untill something better is developed I guess we'll see that.

Standard engineering test are not necessarily well suited to blade world and do not give the right picture of of any material or blade.

Hmmm... maybe we should have a committee... no heaven forbid... a development group to look at the matter of standardizing a set of tests suited to the needs of knife blades.

Am. master smith convinced me with good argumentation that some of the tests do have fairly good predictive value but on heat treatment and strength (strength includes a LOT of things) the bending test leaves many things open and there are actually some cheats around it.

TLM

There are several ways to work around the bending test. It very much favors a flat ground blade for one thing. A hollow ground blade resists bending a lot, and will usually break before it bends very far, but that's normally well above the pressure needed to bend most flat ground blades. I'm not denegrating using the test to measure brittleness, but it's not always a clear measure of a blade.

Mike Snody made a composite blade of titanium and stellite (the edge was stellite), and was able to bend it 90 degrees and back a couple times. Stellite won't bend 90 degrees without breaking, but it did.

Lots we don't know....

How a blade handles bending depends on its moment of inertia and thickness. Both are quite well defined geometrical quantities. I= moment of inertia defines the stiffness and thickness/I defines the strength.

A corollary is that flat ground blades are stronger than hollow ground.

TLM

"A corollary is that flat ground blades are stronger than hollow ground."

That is simply untrue, as is your first statement to a large extent. It might look good on paper but in practice it doesn't work like that. Blades are not measured as two dimensional cross-sections, and all - including flat ground blades - are of variable geometry. How the blade handles bending, at any given point on the blade may fit your equation, but a blade is three dimensional and thus the cross section is ever changing. How the "blade" handles bending is a function based on the integral of the cross-sections in any given area of the blade, and like a chain it (the blade) is only as strong/flexible/etc. as its weakest point.

If you think of a hollow ground blade as an I-beam, the strength is derived from the vertical member, which in the case of the hollow ground blade is usually the spine. It resists bending because of it's geometry, not its mass or cross-sectional area.

Further, a functional blade is a composite of its parts, each of which is designed to perform certain functions. Part of it, the edge, cuts and must be thin (small frontal area). Part of it is for penetration, the point, and must also be thin in cross-section (also small frontal area). In a flat ground blade these thin parts are achieved in two dimensions, as is the edge on a hollow-ground blade. The point on a hollow-ground however can be given a small frontal area using a cross-section that is shaped like a three or four-pointed star, giving it a much different strength to weight ratio.

To achieve a thin point on a flat-ground blade you must use a distal taper, so a blade which may be 1/4" thick at the guard, may be well under 1/8" thick throughout forward third of the blade. That same piece of steel hollow ground may retain the full 1/4" thickness at the spine throughout 90% of its length.

Knives are also defined by handling attributes like overall weight and balance. The distal taper is again used to move the center of mass rearward in the flat-ground blade. The hollow achieves that in the hollow-ground blade, without thinning the spine or the overall "strength" of the blade in its forward most section. In other words, the cross-sectional area of the hollow-ground blade is more constant along its entire length.

There are a number of other aspects of the blade in which flat and hollow grinds achieve their purposes, each in their own way, but that book that someone asked for on another thread is not quite ready to be written. The important thing to consider though is that blades are roughly analogous to airfoils. Even the best computer generated designs and simulations require proof in testing, and are vastly more complex than is immediately obvious, even for a well educated scientist. Were it simple, I am sure at least one German or Japanese cutlery company would have designed the perfect blade long ago.

I'm no longer bored....

I even did not mention cross sectional area because knife blades are seldom used in pure tension. I never mentioned that moment of inertia should be constant, the said fact holds at any one section, handling continuously changing sections is not a problem anyway, it is routine.

Flat ground blades are stronger because of the way material is distributed in comparison to hollow ground. Of course I have to compare about similar blades.

Or I can but it this way: if you make two knives of the same material and same HT (and your manufacturing proces works) and you tell me how you are going to load them, I can tell which one is stronger and about by how much. That is fairly elementary stress engineering which has been proven to work on paper and in practice. I don't quite get your reference to I-beam, to me that has no relevance.

I don't disagree at all on most of the handling related properties you are talking about.

I have been a professional design engineer in aerospace, machine design, materials engineering for twenty years. I know that I can calculate the bending strength of a blade if I am told: material properties, blade geometry and the way it is going to be loaded. These are not necessarily simple.

It does not work that I am told: "design the best blade", that is said by the marketing clowns who should stay by their beans anyway and not try to be clever.

I get to hear that comment "it only works on paper" often, mostly by people who don't know much about stress engineering, it often also takes the form "why can't you calculate a simple thing like that".

I have used various edged tools and weapons for as long I can remember. I am not claiming that I know how to make the best knife in the world. But I do know what can be calculated in relation to bending.

TLM

Well, I happen to have been one of those "marketing clowns" and hold a number of patents resulting from what the "design engineers" said couldn't be done.

I don't think you've addressed the I-beam.

You asked the wrong people, part of the job is to know whom to ask who to believe (how do I know wheather your patents actually work), patents are a dime a dozen before they have been proven commercially usefull and technically viable and challenged to prove that they are unique (enough). That a patent has passed the patent examination does not mean a thing.

I beams are optimized for bending in the plane of the web, the flanges are just wide enough and thick enough not to lose something for effective width and not to buckle on the compressive side, The web is just thick enough not to buckle or break in shear or vertical compression. Against torque they handle relatively badly as against bending in the other longitudinal plane. They are somewhat lousy tools for cutting.

Moment of inertia is defined as integral(sqr(y)dA) and y is the distance from the neutral axis in question. In knife blades the meaningfull axis parallel to blade plane and perpendicular to knife long axis, the other axis is seldom handled because all but the narrowest blades tend to suffer from warping instability when bended. If stffness is considered, an infinidecimal area on the cross section contributes as its area multiplied by the square of its distance from the neutral axis (for simplicity the plane of symmetry of the blade), the corollary was that if you take material away from the "skin" as in hollow ground blade you lose stiffness quite fast. Strength is a bit more complicated but as a first approximation the same area contributes as the area multiplied by distance, again the hollow ground blade suffers. But it suffers more in stiffness than in strength.

The catch for the spine is that it is farthest away from the neutral axis and so is the most highly stressed part and the load bearing area is much smaller than in flat blades and it usually breaks more easily because of this.

If you were to bend the knife in the plane of the blade you would have something like an I-beam effect but not nearly as much as you intuitively think because the neutral axis is displaced towards the spine. And if your edge is on the compressive side it usually buckles before it reaches compressive yield or break if it hasn't warped before. If the edge is in tension the axis shift has the edge breaking first. Again here hollow ground blades are at a disadvantage because the axis shift is larger and the edge thinner.

All in all, flat ground blades are stronger than hollow ground.

A totally different thing is handling (in all its aspects), if strength (it should be just sufficient) is not an issue,I have no comments, it goes case by case whatever usage one has on the knife.

TLM

In view of the fact that Abbott Laboratories payed $250,000,000 for the company that was founded on the basis of one of my patents, and all of them have been used in commerce (why else bother), I'd say something must have "worked".

I remain unconvinced on your arguments, in view of the fundamental principles of an arch (and the I-beam of course). The arched structure resists bending. In fact, Sir Richard Burton (not the actor) observed in the late 1800's that the hollow ground blade was substantially more resistant to bending, based on an impressive amount of testing of blades available at that time. The Japanese sword (flat ground) is notorious for bending and taking a permanent set if used in a poorly directed cut. That is one reason novices are never allowed to handle a valuable sword in Japan.

It might be that you are basing your argument on hollow grinds that are more shallow than those of which I'm speaking (and do). Most factory-produced hollow ground blades are very shallow and use fairly thin steel for economy. Most are weak. Many factory produced flat ground blades are heavy choppers and might be judged strong based on the sheer mass involved. Most custom made blades, whether flat or hollow ground are quite different from these.

BTW, you are not reducing the "skin" of the blade with a hollow grind. In fact you are increasing it. Further, it is difficult in most knives to laterally stress any part of the blade independent of the spine. I think you need also to think about the distal taper in the flat ground blade and consider that "strength" must be measured along the entire lenght of the blade, including the point, to assert a comparison of strength.

I'd really like to continue this dialog, but would appreciate your dropping the need to denegrate my and others' professions and personal accomplishments, about which you know nothing, as a necessary part of your technical arguments. It's unnecessary. You'll find that most professional knifemakers today come from a wide range of professions and as a whole are a fairly bright bunch of people, well able to discuss the technology of their work as well as the craftmanship. Most of us do an extensive amount of testing, and are deeply involved in improving our technoloy.

I am denigrating no one, and as stated ,a patent does not mean anything. If yours has been successful on all counts, good for you. It was you who stated that a blade's bending strength cannot be calculated. It can, easily. If you dont believe it, it's your loss. It was you who stated that I was wrong. I haven't written anything about knifemakers doing this or that but generally their understanding of the effect of geometry on strength and stiffness is not very good.

Your I-beam theory is relevant in the sense that it is a beam in bending, not much else can be applied to a knife blade.

I repeat: if a given a cross section and bending moment I can calculate the stress. I can compare two cross sections with fairly good accuracy. If given the variation of cross section I can calculate the weakest section, variation is not a problem. These - and several other interesting things- can be done by any stress engineer. It is not even a problem, I don't know why it is not common knowledge. How do you think modern aerospace structures hold (sometimes;>)) together, they are calculated! Next time you fly, you put your life on some one having got his figures right, why can't you believe that the same type of engineering can applied to knife blades? A knife blade in its most diffult loading condition is very simple in comparison to an ARIANE5 or the satellite it carries.

There is big difference in analysis and synthesis or design.

I have been using some every day terms because I have no idea wheather you or other readers understand correctly the proper ones. This causes problems.

Strength is depending on material properties and geometry, geometry is the easy one material causes much greater spread and uncertainty. Trying to make general statements like "flat grounds are stronger than..." is dangerous because I have no problem making a stronger blade of some other ground type. But if you compare blades that have most other things similar it holds. There are exceptions to exceptions and I have not taken into account second degree interactions like: effect of geometry on HT, stress concentrations caused by torque, yielding redistribution etc.


Nothing is as important as gardening and even that is not very important.

The standard method of doing the impossible is taking a well known experts opinion that something cannot be done and then doing it.

TLM

TLM- We don't normally have a lot of room in this community for supercilious, overbearing experts who jump in out of the blue as the final authority. Jerry is too much of a gentleman to call you on it, but I'm not. I would have sent this to you by private email, but your profile is empty.

Right on, Don!

Bob Sigmon

Yes Don, most trolls have an empty profile.

There is room for considered dissent here. I felt sorry for him at first, but now he shows his true colors. Too bad.

Many years ago I worked a year doing research on tungsten carbides cemented with cobalt for lathe bits, tire studs, etc. I distinctly remember hardnesses being about Rockwell 90, but it was stated here they're 72-77 Rc. Memory fades with time! Am I remembering the wrong scale or is the 70's number WC in steel,and does it matter?

BTW, it was an interesting discussion until it got personal....