What happened to the convex edge?

I'm sorry if it sounded like a "duh" comment. I'm trying to keep things simple. Things that are thicker are stronger than those that are thing. Sometimes we try to make things complicated by calling the hollow grind an "I" beam or something and that it is stronger or some nonsense like that.
 
Certainly an I-beam is tougher than a flat plate at resisting lateral forces....however, I've yet to see a well-sharpened I-beam suitable for hunting...


Any structural engineer will tell you that if you remove material from the middle of a plate of steel, you weaken the steel in every dimension. That is why a hollow-ground piece of say, 1/4" thick steel will always be weaker than a solid 1/4" bar.



Let's dig into this I-beam idea for a bit.....


Let's say you have two pieces of steel - 2" tall and 12" long. One is 1/2" thick and the other is 1/4" thick. The 1/2" thick piece is better at resisting lateral forces. An engineer would rely on a ratio of the length to the thickness to determine this. Typically, in a building, you want a 20:1 (or less) ratio. That is, for every 20 feet that a beam has to span, you want it 1 foot (or 12 inches) deep.

In this case, the ratio (length:thickness) for the 1/2" piece is 12" : 0.5" - which is 24:1

The 1/4" piece would be 12" : 0.25" - which is 48:1

Clearly the 1/4" piece is weaker than the 1/2" piece.



Now, let's say that we have two 1/4" thick pieces. One is convex ground and the other is hollow ground (both from 1/4" barstock).

The convex grind is going to have more material to it and thus, a higher "average" thickness.

So, why hollow grind?

Let's go back to our first example - say you have a 1/2" thick piece - way too thick for a knife....but you want something that is more sturdy than your 1/4" thick piece.

So, you hollow grind the 1/2" thick piece down to 1/4" (or even 3/16") in the middle. You get the sturdiness of the 1/2" thick piece at the top and bottom - despite it being 3/16" in the middle.

Sounds like a great idea right? Let's call it the I-beam idea.

The one big problem with this line of thinking is that your typical I-beam is 10-12 inches wide and 14-24 inches deep.....which in knifemaking terms would mean that if you wanted an "I-beam Bowie" with a 2" tall blade, it would have to be at least 13/16" thick at the spine! (using the the most slender ratio: 10 inches wide, 24 inches tall I-beam)

What we're talking about with knifemaking is a very minor difference. Using the 2" tall, 1/4" thick Bowie blade example....It would be like having a 24 inch deep beam that is only 3 inches wide! Not really an I-beam any more is it?!


The most important factor in determining lateral strength in a blade of steel (all other things equal - heat treat, steel type, etc.) is the overall cross section. The greater the cross section area, the more steel, the higher average thickness, the lower the slenderness ratio.....the tougher your knife will be.

Your knife doesn't care if it's convex, flat or hollow ground. If the cross section is equivalent (all other things being equal) your knife will perform the same in a lateral forces test (within reason).


The reason that engineers specify I-beams is a lot more complicated than hammering a knife into a piece of wood and trying to bend it. There are many calculations involved including floor load, twist, shear, wind load, cross bracing, earthquake load, live load and so on.
 
That all said.....lateral bending is not the only test we subject knives to...and it really is only one of many other, more important factors to consider when designing a knife.

Too often, we only rely on what we have seen others do, rather than sit down and try to figure out what should be done for this particular application.
 
Was I the only one who had to read that twice? Good stuff Dan!

If it is possible to build a knife with a spooky sharp edge with a stronger geometry...like the convex (not only edge but also grind) why is the current trend for flat ground thin blades?

Is it the whole compromise deal between ease of sharpening, cool grind lines etc?

Or is it something else?

Somebody mentioned that different knives require a different edge. When I make a little knife, say a 3" blade I still use a convex grind that flows from spine to edge...and those little buggers get scary!

So what kind of knife requires a different edge/grind? Before we go all wonky let me narrow it down...What kind of "WORKING " knife. Lets define that as an EDC type for genral purpose chores. Otherwise Somebody will start looking for hoof picks and fruit knives and we'll get stifled.

Where would a flat ground knife out perform a convex ground knife...all things being equal?

Shane
 
I know you wanted EDC types....but I put a full flat grind on my santokus. I did one with a full convex grind, though...but it was intended as a small cleaver-type knife and not a thin slicer.
 
Knife makers who devote a little cognitive energy and some experimentation into the development of their knives can do very well with any grind they chose. Each has its own limits and potential. Some simplistic grinds result in simplistic knives, the disposable kind much of our economy encourage.

There is no single convex grind, each will vary with the maker.

When a maker devotes some time and steel into getting to know the performace characteristics of his methods and steel, he will become confident to the point that he can push the limits of design and materials to the apex of the potential of all three. (maker, design, material)

To me, there is no greater potential than the physical characteristics of the convex grind. The frontiers that await exploration are as extensive as the intelligence, and drive of the maker. I have been exploring this arean for over 40 years and still have more questions than I have answers. I have developed many answers and hope that I never see the end of the trail.

Convex grinds do not have to be uniform, they can vary in radius throughout the blade. They can take advantage of the internal make up of the blade.
On previous threads I have mentioned the martensite cone inside of the blade that is mirrored by the transition zones that we see when we etch the blades of differentially hardened steel.

Complex convex blades are not new, they have existed for as long as man has made knife. Many knifemakers have left us a record of their knowledge in their blades, all we have to do is look at as many antique blades as possible and open our hearts to their message. Whenever you as a citizen of the world of knives see a blade that is different, take the time to explore her as carefully as you can. Some knives take years to understand, others will have a new thought every time you pick them up.
 
Thank you Ed! I think the convex grind is so misunderstood because the majority of knives made today, production and custom are ethier hollow or flat grinds. You don't run across that many convex ground knives. I didn't even own my first convex until 1993 when I bought a Blackjack model 1-5. That's when my love affair started with the convex grind. I've had nothing but positive feedback with my knives. (knock on wood) :D
Scott
 
Japanese Swords are a notable example of an 'old' use of the convex cross section. But, I thought most western knives/swords were 'hollow' ground because they were made on big grinding wheels. At least over the last several hundred years.

So, is there much common place convex western cutlery?

Steve
 
Dan,I flat grind my santokus ,also.I put a convex on them at about 12" blade length.As you said,at that length they will be used fro chopping.
SA
 
I do it pretty much the same as Nick does. Flat grind all the way to the spine and then convex the very edge on a slack belt. Edge thickness before slack belting depends on intended use with big blades being around .020-.025 and hunters .005-.010 On some of my small light duty knives I will flat grind all the way to sharp then put a slight secondary bevel on with a fine ceramic. I do all my final sharpening with a ceramic crock stick.

I used to do a full convex grind on bowies and camp knives, but I've found that the extra meat isn't really necessary. As long as you've got enough metal right behind the edge to keep it from deforming on impact anything more is only for bending strength. If my customers want a prybar I'll pack one with the knife :D
 
Steve: You won't find convex grinds in many factory knives, the convex blade requires a level of craftmanship that makes mass production impractical.
Some custom makers produced some great convex grinds. Scagel is fairly recent, Bill Moran made some great blades with convex grinds. I believe they both studied historical knives thououghly. Although Scagel could have figured much of it out on his own. There are many more, but their surviving knives are few and in private collections.
A great many knives that were made my men who knew what a knife needed to be used the convex grind to achieve knives that will always stand above and beyond the simple knife. Unfortunately many are unsigned and they lack stuff like birth certificates and titles. I wrote about some in the last few issues of Blade. I will try to take them with me to shows where others can explore their essence.
Should you wish to visit some fine knives, Madison Grant 't Book, 'The Knife in Homespun America" and Gordon Minnis's book "Primitive Knives" are great places to explore.
Many museums have some great knives on display, unfortunately they do not usually know what they have. For any 1,000 knives you visit at antique stores and shows you may find one great knife if your lucky, when you do enjoy them as much as you can.
 
The first time I ever heard about the possibility of using a different grind on the same blade was in John "Lofty" Wiseman's book SAS Survival Handbook. He talk about sharpening a parang in different ways for different jobs.It made sense. He sharpened the blade closest to the guard for fine jobs, the belly for chopping and the tip for really fine work. (Page 20 on my version) I believe this was on a flat ground blade, but still the idea of different bevels on a cutting edge got me to wondering.

The first time I ever saw this type of grinding was sitting at Ed's kitchen table. He brought out one of his Large Camp Knives. It had four specific grinds in that one blade.I have not seen anything like it before or since.What was really cool was ther transitions between the different geometries was so smooth. Flowing is the right word.

If you don't believe the validity of the multi-grind approach take a look at the Tracker Knife designed by Tom Brown. Mr. Brown is a real time user of knives. He must have had some idea of what he wanted in a tool.

That prybar statement popped up again. We all agree there is no way to know what our knives will be used for. Even little knives are useful as chopping tools when used with a baton. Splitting wood or chopping in this manner does exert a lot of pressure on a blade. Especially if you get her wedged deep. What happens then? Ok, I know...different blades for different jobs...but in an EDC knife...one that we have with us...what are we looking for? Until somebody makes a really good sheath for a prybar, we may be left hanging with only a knife on our belts. :eek:

Dan's example of two similar knives with different grinds was a good one...and one often easily misunderstood by a novice,the cleaver. Even though we see a familiar profile, cleaver is a specialty tool. It can be ground for any number fo jobs. With different grinds and geometries cleavers can be used for everything fromm fine slicing to splitting beef carcasses. (Anybody who has seen an old time butcher half a beef with a three foot cleaver will know he has seen something.)However the different cleavers are not interchangeable. DO NOT use that fine Chinese Cleaver to chop through bone. Right Dan? ;)

Last, I apologize if I took a bunch of room on this thread. My old computer is so slow I have to reply to different point all at one. Otherwise I'd be here all day.

I'd really love to hear about testing of flat ground blades by anybody...

Shane
 
Thanks Ed,

I'll keep my eyes peeled. But I haven't found that 1 in a thousand ...yet.

Steve
 
Dan, Thanks for the I-beam explaination. Makes sense the way you explained it. I guess I was thinking that the mass would be one of the "all things being equal" things. But in the real world profile geometry would have to change between the three knives if the mass is equal and each knive had a different type of bevel grind.

Shane, To your prybar point... Aron Ralston (the guy who cut his own arm off after being pinned to a canyon wall by a fallen boulder) probably wished that he had a myriad of different tools with him on his fateful day. A prybar comes to mind as does a boulder sized ice pick and a "Sharp" knife. Like you and many other have stated, we never know what the tools we have at hand will be called to do.
"I'd really love to hear about testing of flat ground blades by anybody..." The only flat grind versus convex grind experience that I have doesn't include knives with similar profiles or steel chemistries. But for what it's worth...
Forged 5160 blade, convex ground, multiple edge quenched -vs- stock removed O-1, flat ground, commercially heat treated. Different profile geometries to boot. The forged blade out cut the stock removed blade by 4 to one in side by side hemp rope cutting. Not exactly a fair comparison but it's a start. Maybe somone else will share their own experiences.

Rick
 
Steve:
Many great blades were forged by the Phillippene Bladesmiths. Many of their blades manifest design,workmanship and knowledge that puts a whole lot of other knives in the back of the pack. I believe that they deserve a much higher level of praise than our market recognises.
When you look at them, don't seek the flaws, look at what the intention of the blade's function was. Ask Questions and a greater understanding will follow.
 
A blade grind I'm becoming more interested in is a combination hollow/convex grind. The top 2/3 of the blade is hollow-ground and the bottom 1/3 is convex. I had a Thai folding knife that was ground like this and was a great cutter.

Anybody have any arguments for/against this?

I should also note that this kind of grind is found in some of my most favorite knives - khukuris. We call the "depressions" fullers....not to be confused with the more thin fullers found in bayonets, daggers, etc.

Here's a picture of two khukuris with significant fullers.
 
Hello Dan:
The knives you illustrate show one of the most important aspects of knife, ----ethenic foundation.
I love knives that are true to their heritage. If you like that kind of knife go for it! It all depends upon what for and why? We and a lot of others could debate merrits of design for months and still not reach any conclusions.
I have been working on basic geometries and the influences between steel, method of hardening and geometry. It readilly got the the point that it was over the heads of most and those who figured they had some valid thoughts got a little shy of comitting. There are a few who want to play, but that is what it is is play. That is what knives are for most, play.

What I would not give to have been able to make knives for someone like the mountain men who skinned thousands of beaver a year. Not the Bowie boys, but those few who lived with a knife. For most those times are over, there still remain a few who know the difference and can appreciate a great knife, the rest must rely on tinsel. It may shine but doesn't make Christmas.

Find the frontier you like, explore it as far as you wish to go. My advice, don't waste time, all too soon you may find yourself limited.

There are many regions in the world, if each man found something in his realm and made the most of it we would have some honest diversity in the world of knives and each could shine on their own merits.

Make the New Year Your Dream and fly with it.
Take Care
 
Ed Fowler said:
Hello Dan:
The knives you illustrate show one of the most important aspects of knife, ----ethenic foundation.
I love knives that are true to their heritage. If you like that kind of knife go for it! It all depends upon what for and why? We and a lot of others could debate merrits of design for months and still not reach any conclusions.
I have been working on basic geometries and the influences between steel, method of hardening and geometry. It readilly got the the point that it was over the heads of most and those who figured they had some valid thoughts got a little shy of comitting. There are a few who want to play, but that is what it is is play. That is what knives are for most, play.

What I would not give to have been able to make knives for someone like the mountain men who skinned thousands of beaver a year. Not the Bowie boys, but those few who lived with a knife. For most those times are over, there still remain a few who know the difference and can appreciate a great knife, the rest must rely on tinsel. It may shine but doesn't make Christmas.
Click to expand...
The above paragraph is so true. I think alot of knives today end up as pieces behind glass or in collections. It's a shame that more knife buyers aren't putting man's oldest tool to work in the woods and fields. There is so much emphasis being put on the "look" of a knife and not it's function. I'm not saying anything against people collecting, but when I make a knife for someone I want them to use it for it's intended purpose. Alot of knives out there may "look good" but do they perform.
Scott
 
thanks for the encouragement, Ed. I've got a few "in progress"....I'll send you pics when they're done.


But what about the grind? How would it compare to a full convex?

On the knives I have made full convex, I usually make the spine slightly thinner than the blade. Seems like it would help in passing through material better.

I wonder how it would compare to a hollow/convex grind....? That would also be good at letting material pass.....but would, perhaps, be a lighter, potentially weaker blade.
(I'm thinking of a knife the size of your hunters, Ed)
 
I had a good talk with Bill Burke on the phone l;ast night. We got to talking about rope cutting.

We were chatting away about steel and rope. Bill mentioned he was working on his third "box" of manila rope. He said he was somewhere around 1200 feet of testing!

And that brings me to my point...

Make it, use it...and then test the mustard out of it! Honest comparative tests will tell a maker a bunch about what he is doing.

SO Dan...if it is what you love...ROCK ON! Let us know what you find out.

Wish Ihad some shop time to test right now...I want to do two blades ...identical HT...one flat and one full convex...then test them a bit..

Shane
 
You must log in or register to reply here.
Back
Top