Edge Geometry + (What?) = great distal taper?

[Brian Avila]

I've seen info on his jig, and I have thought about going down that road. Then you just get into that boat with everyone else who likes or dislikes that idea. I don't think either is better than the other, I think the results is what really matters. In my training and in my job, process and procedure plays and important role in an acceptable outcome that you can derive decisions from, and that is just how my brain works right now I guess. If I had the money and time, I guess I would take many different knives, measure them and classify them and make a conclusion from there. Depending on how you look at it and what you do, everything can be measured.

I have a feeling that there is more thought that goes into developing a good knife, and for a new knife maker it is hard to grasp or even absorb those slight tweaks that helps make a superb knife. Trial and error works.... but there are generalizations for each grind, and everyone will have their own preference.

I started ranting and forgot my point....

 

[james terrio]

Then you just get into that boat with everyone else who likes or dislikes that idea.

Forget who likes it or doesn't! If it works for you, take the ball and run for the end-zone. All that really matters is whether or not you end up with a product you're proud of.

I don't think either is better than the other, I think the results is what really matters. In my training and in my job, process and procedure plays and important role in an acceptable outcome that you can derive decisions from...

I like the way you think. Go for it.

 

[cdent]

After hearing that for years and subsequently having several long and detailed conversations with a mechanical engineer, and breaking several blades on purpose... that's a myth. The lateral strength of the blade depends entirely on it's thickness at the point of the most stress. As far as whether or not a tip is going to snap off, tapering it simply removes steel and only makes it weaker. A non-tapered tip is flat-out stronger than a tapered one, when everything else is equal (I'm talking two blades made from the same bar, ground to the same profile, HT'ed by the same professional at the same time.)....

I couldn't help but get a bit hung up here. I believe, the more metal that is present, the more strength that is possible. It doesn't take into account that a good distal taper engineers a lot of weight out and a lot cutting potential in. All else equal, an equivalent amount of metal in a good 4" hunter blade might be a 2 1/2" piece of sharpened bar stock. I think your engineer buddy would agree that a triangular form resists deformation better than a rectangle, as long as it's a true apples to apples comparison. The distal taper might be a good option to balance weight, strength and cutting preference for a given profile.

Just thoughts, Craig

 

[james terrio]

I think your engineer buddy would agree that a triangular form resists deformation better than a rectangle...

No, he doesn't. We're not talking about equilateral pyramids or braced corners in a house-frame that don't collapse on themselves here, we're talking about solid, long thin sections that don't snap under lateral stress.

All else equal, an equivalent amount of metal in a good 4" hunter blade might be a 2 1/2" piece of sharpened bar stock.

The same amount of steel, forged into a sphere, would be strongest of all. Wouldn't cut worth a hoot, though.

I believe, the more metal that is present, the more strength that is possible.

Exactly my point. The cross-section of the blade is what we're talking about. Thicker is stronger.

The distal taper might be a good option to balance weight, strength and cutting preference for a given profile.

Absolutely. As I said earlier, I almost always prefer a full distal taper for precisely those reasons. "Thin is in and light is right" is my slogan. But when folks claim that grinding a blade or tip thinner makes it stronger... sorry guys, that's nonsense.

 

[stezann]

So I have been doing research on grinds and using a grinder to preform the grinds and I came across the thread about edge geometry, it was a very good read:

http://www.bladeforums.com/forums/showthread.php/838481-Edge-geometry

Lets talk about the specifics of the flat grind:
1) If you have a 4" blade
2) Blade width of ~1.5"
3) Drop point that is just around .5" drop from spine,
4) 5/32" thickness
5) You have a "perfect" flat grind that is somewhere in the number of 3.25 degrees,
6) You have decided not to break the spine and leave 1/8" left on the flat of the knife to the spine

If you grind your knives and just drag your blade across the belt without putting any pressure on the tip, the thickness of the tip located at the 1" mark from the edge of the blade would be ~.1138". (I sat down and did the math and this would depend on perfect angle and the width and thickness of the material)

That would lead me to believe that pressure must be applied around the tip area to create a distal taper. The "correct" thickness of the tip around the distal taper would probably depend of the intended use of the knife, type of steel, shape, etc.

What do you guys think about when you make your knives and are doing the distal taper?

When thinking about perfect plane bevels, especially for chisel grinds in japanese style kitchen knives, but it applies to every blade, given the premises, i always wondered how to achieve it without having a full distal taper and an edge that is perfect straight from ricasso to point. Every other design is a compromise as you can't put the whole bevel flat on the stone. The compromises are the craftman dance with geometry I think that distal taper could be one of the detail of a knife and it is to be evaluated among the whole design and purpose of use. As far as the estetic i may say that channels in the blade also improve rigidity and wheight balance but not all the knives design accept it as an improvement of style.

 

[LRB]

After hearing that for years and subsequently having several long and detailed conversations with a mechanical engineer, and breaking several blades on purpose... that's a myth. The lateral strength of the blade depends entirely on it's thickness at the point of the most stress. As far as whether or not a tip is going to snap off, tapering it simply removes steel and only makes it weaker. A non-tapered tip is flat-out stronger than a tapered one, when everything else is equal (I'm talking two blades made from the same bar, ground to the same profile, HT'ed by the same professional at the same time.)

Transferring energy and stress-risers do come into play when there is a sudden and dramatic change in any dimension, which is why we round tang shoulders and why gentle curved and/or angled plunges are stronger than abrupt, square ones.


The correct thickness will depend on whether or not you want to pry or dig with it. For example, Rick Marchand makes some of his knives so their edges are thicker at the tip than at the flat of the blade, specifically because he wants strength there. I'll ask him to explain further.

It is NOT a myth. That is why most leaf springs in guns are tapered. The taper spreads the load. The result may not be as dramatic in a blade, but it is there.

 

[stezann]

It is NOT a myth. That is why most leaf springs in guns are tapered. The taper spreads the load. The result may not be as dramatic in a blade, but it is there.

Without much physics in my quote... i think it may come into play the concept of flexybility more than strenght... and for the blade too....if you taper the blade so it is thinner in the more stressed area it is more likely to bend there instead of snapping, so we can predeterminate where a load is adsorbed by our blade, avoiding for example breaking near the ricasso all of a sudden. (sorry for my english ). They build fishing rods in this way.
I think it is probably what you said with different words from what i can understand

 

[Rick Marchand]

I think James is misinterpreting LRB's "transfered stress" statement. I don't think he is alluding to a notion that a distally tapered blade is stronger but rather that it is more resilient at any given hardness. As the cross-section thins out, it is able to flex more without deformation.

As for Jame's reference to my blades being thick-tipped... he is correct. Keep in mind, that they are not thicker at the tip... just not as thinned out. It is very apparent if I give the edge a consistant v-bevel from ricasso to tip.(see the pic below) You can see how much stock is behind the edge. This, of course is still in the rough. All of the bevels would be blended in. You definately wouldn't win any fish filetting contests with this kind of geometry, but if moderate prying and drilling was something you expected to encounter, the advantage is clear. This is nothing new... most makers do it instictively. I just exaggerate it a bit in my bush knives. It has a place.

I do make many blades with distal tapers. Most of them have the taper forged in. A well made blade with a distal taper is like a finely tuned sportscar.... sleek, fast, great handling, acceleration and suspension... so many advantages over the common sedan... the epitomy of a high-performance vehicle. Though... not so good if you're intending to take the wife, kids, dog and a pop-up trailer camping for the weekend.

Rick

 

[rocketmann]

I have to agree with the statements concerning flexability vs strength. Flexability can equal strength in a lot of cases when talking knives. I know that when I first started out (not to infer that I am a great bladesmith now) that I was so hung up on measurements and angles and tapers that I got what I call paralysis by analysis.. I finally decided that even if I could come up with the perfect numbers, the chance of me being able to pull it off was nearly zero. So I decided to just let it flow and see how it feels. I'm a lot happier now.. BTW did I mention that I suck at math....

 

[Rick Marchand]

Flexability can equal strength in a lot of cases when talking knives.

Hey Louis, can you expand in this a little... I think there might be an issue with sematics, here.

 

[rocketmann]

IMHO I feel that a blade is "strong" when it can take a side load within reason,with out breaking or taking a "set". This is a combination of a distal taper and good heat treating. A rectangular bar of steel ( or any other shape for that matter) that is too hard will snap off when side loaded. If you soften the bar so much that it will bend it can take a "set". When you add in the distal taper in conjunction with the proper heat treating, the blade will react like a spring so to speak. It will flex equally along it's length and then "spring" back to it's former position. To me this is "strong". I know that you can also get the rectangular bar to react in a similar manner so that is why I said "when talking knives" due to the fact that we don't usually refer to a sharpened bar of steel as a knife. At least this is how I see it.

 

[Rick Marchand]

The reason I ask is that Strength and Toughness are often misinterpreted.
Simply put…

Strength is the ability to resist deformation.

Toughness is the ability to absorb energy, usually under a rapid impact.

Ductility is the ability to deform without breaking.

Gradual loading or bending is often mistakenly thought of as toughness. This is actually a display of tensile, compressive and shear strengths and has little to do with toughness, steel type or even heat treating. That's why I threw in the ductility definition. How far the blade will flex before taking any sort of a set will be mostly affected by how thick or thin it is (look up Modulus of Elasticity). When load is applied suddenly, steel behaves in a more brittle manner because it does not have time to compensate with the internal mechanisms of deformation. Data sheets from the steel suppliers are much more useful in considering measured impact toughness than any test we as knifemakers could perform.

or I could be wrong... and drifting off topic..... lol.

 

[LRB]

IMHO, everyone here has a good point. However, IMHO, it comes down to, what is the blade you are making expected to do? Is it expected to be a fine cutting instrument, an armor piercing stabber, a pry bar, a chopper, a shovel, or what? It is impossible to make one knife that does all of those things well, and if trying to combine all, or a few of those features into one knife, none of those features will excell. That is just how I see it, but suit yourselves. I would allow that maybe a couple of those features can be employed together in one blade with success, but there is just no one fits all.

 

[Rick Marchand]

Very true, LRB.... everything is a compromise.

 

[james terrio]

However, IMHO, it comes down to, what is the blade you are making expected to do?

I agree completely. :thumbup: I apologize if I misconstrued or misrepresented anyone.

 

[Brian Avila]

This turned into a good discussion! It went down an alley that I can relate to, which I like. Lots of good stuff. Rick your explanation of elastic properties and the definitions I think was the right direction to go. The data sheets are a good way to go to get information on the as well. However, I think when talking about the elastic properties of the material it changes to a smaller level, but you can make the jump saying that if you had two different steels that where ground the same way and had the same thickness at all the different points (meaning the tapers where the same) then you could make a good guess which would hold up better depending on the force, and type of force applied. I really think it comes down to the cross section....

 

[james terrio]

This turned into a good discussion!

Heck yeah! It's important for all of us to get clear on what strength, toughness, flexibility etc really mean, so we're not talking at cross-purposes. I assure you, the folks who design and build skyscrapers and racecars would chuckle at how confused most of us knifemakers are on these basic terms.

 

[elementfe]

What Rick said.
If you ask an engineer what type of section is "stronger", he's going to think in terms of resistance to deformation, and a "blocky" shape wins.
If you ask him what type of section will approach failure in a gradual and non-catastrophic way, he'd be more likely to favor the distal taper.
Very literal they are, those engineers!
Great thread, it's good to think through those "everybody knows" assumptions.
Andy G.

 

[rocketmann]

This is why I like this forum, it makes us think about why we do things a certain way. I'm still not sure that defining strength in relation to knives is as simple as quoting a definition out of a book. I always think about it like this, you have 2 athletes, the first is a 300 pound power lifter that can squat 1000 pounds but can barely get out of his own way and the other is a gymnast that can climb a 50 foot rope with out using his legs and do 1 arm push ups while standing on his head. Neither can do what the other can do yet they are both very strong. I tend to correlate knives with the gymnast not the powerlifter. The powerlifter is the crowbar..and we all know how sharp they are. haha we have all seen the commmercial "I pick things up and put them down".. this is not meant to offend any powerlifers that may be reading this...

 

[tedinatl]

I assure you, the folks who design and build skyscrapers and racecars would chuckle at how confused most of us knifemakers are on these basic terms.

Ain't that the truth! I am an Industrial Engineer (Mech Eng undergrad), and did my fair share of metallurgy and structural engineering classes (not that I remember all of it, but the basics are all still rolling around in my empty noggin). It is fascinating sometimes to read what people write...

Rick did make some very good and basic points, though, as did LRB. Everything is a compromise, and you need to decide what the right mix of strength, toughness and ductility is for the specific application... and a lot of this is determined by a number of factors - geometry, material, heat treating, how the force is applied etc.