Hollow grind vs flat/convex

M

miden

Joined
Sep 9, 2005
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Hi Ed,
I decided to open a new thread 'cause we got accused of hijacking the quenching oil chat. :o

I found your experiment with the different grinds very interesting, this answers my question perfectly. Hollow grind for show blade, not for work blade, unless specifically requested by customer!
I would really appreciate it if you could remember the exact issue of "Blade" in which your article appeared, I can get it on back order at my local book store.
Thanks again for the kind words and the info.
I would love to attend one of your courses but it is just not possible for me at this point in time but one day....
Take care friend
Mike
 
A message to anyone that is interested in this subject.
Ed Fowler and I were discussing steels etc. in the thread "what quenching oil to use" but we digressed pretty badly off the original topic.
Ed gave a very interesting description of an experiment he did for Blade Magazine, well worth the read.
You will find the thread just below this one (page 3)
 
This subject has been beaten like a rented mule on many an occasion.

Usually ends up in hurt feelings and the same conclusion every time.

" All things being equal, nothing is."

It ends up that everything is a trade-off .
 
Without beating the rented mule some more Mark, what is the advantage of a hollow ground work knife? I would have thought it would cut better because of the very shape of the edge.
Sorry if I am opening a can of worms but I hope everyone has an open mind, I am just on a learning quest and I understand that even with facts, there are personal likes and dislikes.
I agree with your comments about things being equal and the trade off.
sincerely
 
Without beating the rented mule some more Mark, what is the advantage of a hollow ground work knife? I would have thought it would cut better because of the very shape of the edge.
Click to expand...

I'm not Mark, but hollow grinds are good if you're cutting something that's not thick, like leather. Hollow grinds can be the sharpest of all, but if you want to cut through something that is thicker then the bevel is wide then the hollow bevels will want to wedge in and get stuck.
 
Hello Minden:
Sorry to take so long to get back on this topic, Shop has beeen full for a few days.
The article comparing the strength of blade grinds appeared in the April issue of Blade, starting on page 34.

I cut six equal lengths of mild steel from the same bar. Then ground part of the bars to various geometries, leaving one without any grinding.
The resullts of this test were as follows:

weight reduction in Torque in foot pounds
blade mass
Control bar .55 pound 0 31
Hollow Grind .35 .20 pounds 8
Flat Grind .40 .15 10
Light Convex Grind .40 .15 22
medium convex grind .45 .10 25

We ground about 1/2 of the bars to represent the blade, the back or "tang"
was left as a 1/4 x 1, each bar was 7 3/4 inches long. We built a jig to attach the torque wrench and placed it at the exact same spot on the tang for each blade. The blades were placed in a bench vice at exactly the same
depth.
The most suprising result was the increased strength of the light convex grind over the flat grind, it doesn't take much convex geometry in a blade to make a big change in the strength.

There is a book "Cats Paws and Catapults" by Steven Vogle that describes why this kind of event is perfectly natural.

I hope this answeres your question.
Thanks and Take Care
 
The stuff I wrote did not come into the reply as I wrote it, The first number you read is the weight of the bar, second number the reduction in blade mass and the third number the required foot pounds to flex the blade to 90 degrees. Hope this makes it a little more undestandable.
 
Here's some thoughts from Mike Lovett...with regards to blade geometry and the fighting knife...It seems Mr. Lovett has some experience making knives for military members involved in combat operations...

These are Mr. Lovett's words...."A lot of enemy carry , body armor, and equipment that can impeed penetration, and catch a blade. One of the reasons a distal taper is so important. And why I will "NOT" build a flat or convex fighter, They are heavier, much slower, and poorly balanced.. Exactly the opposite of what is usually claimed. Handel the two types side by side and you will see what I mean.A properly ground Hollow, will not only penetrate much deeper, and quicker, It withdraws much, much easier as well. As to strength, pound for pound, the simi- hollow is mucha stronger as well. I know this flys in the face of the writtings of a few "VERY"out spoken writers. Irregardless of what a few "Baggs" of hot air write in the Infamous Armchair, and Ninga Warior raggs This can be proven by historic research. What we here have wrongly called a blode grove in knives, is actually, properly called a fuller. It is a grove either ground or forged into a blade to give it rigidy, while a the same time making a lighter, and much stiffer and stronger blade. In modern times, one needs only to look at the structural I Beam Or H Beam. Load bearning beams arn't rectangles, squares, or enlongated pyramids (ie; Flat ground, or convex)for a very solid engerneering reason. Strength, and weight. Light weight and strngth make a fighting blade both dependable and lively in the hand."

Just another opinion from another knifemaker about grind geometry...and I hope he will not mind that I posted it here in this context. That is also why I posted only the part of his comments that referred specifically to grind geometry.

I can't remember if anybody ever threw in this kind of thought in the previous geometry thread/s...so I took the liberty.

That thread has covered a lot about shapes, styles and designs of specifically what is being called a "fighting knife".

Good stuff from somebody who has put knives in some tough places.

Food for thought anyway.

Shane
 
I would have to say it depends on the purpose and length of the blade.

The ultimate cutting comp bowie with a blade of 10" has to have a flat grind or a convex that's real close to a flat grind. This gives the best combo of chopping and cutting ability.

Whereas a scalpel with a short and very sharp blade is better off with a hollow grind. Don't see many scalpels with convex grinds.

For penetrating body armour, I would say a tanto with a flat saber grind would do well. How many times can a hollow ground knife be rammed through body armour before damage is apparant? A tanto with a fuller (to ease removal) would do much better. Again, for all intents and purposes ...
 
I figure it sounds like the same old dilemma of finding the compromise that works best for your situation/ what you're trying to do.

Ed-
I was glad to see that test you did in the magazine. I had always known a convex grind would be stiffer than flat, but was surprised to see just how big of a difference it makes by your numbers. That's some great info.

I would like to use a convex grind on some of my great big knives for more edge support, but they just get way too heavy, so I compromise with a flat grind and convex edge. I've been thinking about a great big persian style blade that has convex grinds with large double fullers to bring the weight back down. Since I focus more on chopping than stickin', I don't believe I could achieve the necessary edge strength in an efficient manner with a true hollow grind.

I have respect for Mr. Lovett's ideas, and I can agree with him on the part about distal tapers helping to prevent sticking, but am not sure about the hollow grind part. I would like to hear more of his thoughts on this. Is he talking about getting stuck in armor & gear? Or bone? Larry Harley has stuck a big blade in a lot of living things, and has never mentioned this that I recall. If I'm not mistaken, his design for hog hunting is flat ground.
battlebowie350.jpg

I'm trying to think of the times one of my knives got stuck while stickin' an animal, and out of the hundreds of times I've done it with various blades, I can only think of two occasions off the top of my head. Incidentally, both times were with my convex ground Blackjack 1-7. Once with a coon, and once through the spinal column and both shoulders of a coyote. Have never had any such problem with my flat ground bowie that I can recall...
 
I love the "I-beam" argument. Comparing a knife to an I-beam really makes me chuckle. Talk about two completely different ideas. Anyone have a discussion with a structural engineer about this I-beam idea?
 
Posum: I also had heard about blades sticking. I purchased some sides of some cows that came thru the plant for stew meat and were to be rendered to 1" pieces. I stuck cows in (through) the ribs, hind quarters, headed and did it all, no blade hung up. These blades were all convex grinds.

The only blades I ever had hang up was one with a fuller that caught between the costal cartilige and a ribb, the other was one with a droped edge. I have never carried those kind of knives since.

There is a whole lot to blade geometry that some understand, putting it in words that are easily understood is another story.
 
Ed,
Mark Williams warned me I would open a can of worms with this debate but I am glad I did, I am learning a lot again. You advised me to keep an open mind I am doing just that. Thanks to everyone for your input, you all clearly know a lot more about this than me so I will just sit and take it all in.
I have never plunged a knife into anything, living or dead but if I want to make good hunting and skinning knives I will have to at some point to get the first hand experience. It will definitely be an animal!! Preferably already dead. Ya, ya, call me a sissy. :D
Ed, thanks for the info regarding the test you did, much appreciated for your efforts and the results are VERY informative.
Mike
 
the possum said:
I would like to use a convex grind on some of my great big knives for more edge support, but they just get way too heavy, so I compromise with a flat grind and convex edge.
Click to expand...

I find this quote a bit confusing. While I've seen convex grinds on knives that look more like axe blades to me, I've also seen convex grinds that I had to put a straight edge on to tell from flat.

If Ed's tests demonstrate that even a slight convex adds a lot of strength maybe you just need a slimmer convex grind to get the right weight.

Dan Pierson
 
I would like to mention and ask?
A working knife is what kind of knife? and used for what?
butchering?
camp site chopping?
filet,
Boning,
you'll never have One knife that will fit the bill for efficient,
one knife can do it all but it has some lost of efficiency ..
One knife built for and suited a job it was built for, is good....

I'd make a set of ....working knives
from a paring, utility to a full fledge chopper

to have one knife doing it all IMHO
you lose a little in every department your working in with the one knife no matter what you have for a knife..

would you use a backhoe in the place of a bulldozer ? you can, but it's not suited for dozer work, both are built for a propose
both can do the others work but very inefficiently,
I see knives no differently,
just to butcher a deer and do it well with ease and with efficiency IMHO you need 5 different styles of knives
hollow, flat and Convex,, a hatchet comes in handy too at times ;)
 
Daniel Koster said:
I love the "I-beam" argument. Comparing a knife to an I-beam really makes me chuckle. Talk about two completely different ideas. Anyone have a discussion with a structural engineer about this I-beam idea?
Click to expand...
Haaa! Heee! Well Yes I guess Have, both in studing metalurgy and Mechanical eng, it the U. More importantly, my shop was in Huntsville ala. for the first 10, years, and many of my friends, and customers were NASA, enginirers, those boys are sharp. and have all kinds of computers, to run test on. Ed, We met for a few monents at the Blade west show, at least I think this was the correct show, I was there on a sunday with Louis Chow, for a meeting with Bob Loveless the next day.. Nice work, and wonderful attitude. "Guy's" when I say, Stronger I'm talking about in a stright line, such as a penetration. And I stated pound for pound. But I'm not talking pounds here. I'm talking .oz's. Not size to size, or thickness to thickness. But weight to weight. People are always amazed at the lightness of my knives for thier blade thickness. Its all in the grind. Soldiers have a lot of gear to haul around, any wait savings in a god send. As Dan pointed out, all knives can not be expected to do all things well. My fighters are not for choping wood but will work, just not well, My combat blades will make do in a hand to hand situation, but can't be expected to excell. But they'll chop and pry all day long. I had one of my customers in Desert Storm, A capt. from here at Ft. Hood, Attacked while taking a dump behind his Abrams Tank. His M-16 was just out of reach. He fell back, (PE_UUU), while unsheathing his fighter. He swung up and over, with the speed and strength only a desprite man can have. The enemy's head hit the sand, behind the body that fell on the capt, Double Pe-UUU! But the Capt, lived to fight another day. Would a different blade shape, or grind have worked, most, more than likely. Better? whats better. Worse, don't know. Point it worked. One reason I don't get too excited withe whats better discussions. I've made untold knives, that were carried into combat, without a single failure. I think we armchair this aughment to death, just for the sake of what-iffing. Whats Better is what you have with you, have confidence in, and can use in a tight spot. Thank's Dan, Finnaly someone hit the nail on the head. Different tools. I cringe every time some one describes beating a blade through a pelvic bone with rock. Any one out there ever heard of an ax. The same folkes would never carry an as only, to field dress a dear. Or maby pry that troublesom rock out of the sleeping area with their gun barrell... :eek: :D
 
There is a big difference between a knife that will be used for killing a man compared to a knife that will be used for cutting up a dead moose. I dont understand some peoples mentality of knife testing. Yes its fun to chop through 2x4s but you would have to be an idiot to use a knife you considered your weapon of last defence and thats what a knife is " A weapon of last defense" to chop up anything in a combat situation. In any survival situation you wouldnt even want to use a heavy bowie for anything other than starting a fire and this would just be for scraping up some dry shavings or somthing so your fire will get hot and fast. The thing is, A combatant is prepered for combat and a survivalist is prepared to survive. You can be both things but it usualy demands two different tools.
 
with all respect, I still don't see the correlation between an I-beam and a hollow-ground blade. Yes, I see the hype...but not a scientific correlation. It's apples and oranges. I respect your studies in engineering and you're not the only person to ever make this comparison...

I too studied Mech. Eng as well as Physics, Math, and finally ended up with a Masters in Architecture.

An I-beam is not stronger than a rectangle, it is lighter and easier to make and more economical - that's the value. I-beams do not carry loads, they transfer loads to columns. As to the actual shape of an "I" beam, what gives it its ability to resist lateral deflection is the size of the flange, not the webbing. In other words, the wider the beam, the "stiffer". The webbing (vertical part) keeps the beam from sagging.

So, if you want to do lateral bends with a knife, ie. prying, bending to 90 degrees as Ed does, then what you need is larger "flanges"...but ones around 1/3-1/2 the width of the blade. On a related note, I've yet to see any hollow-ground I-beams....structural engineers missing something? (sarcasm mode off...)

I'm a fan of the hollow grind - I think it has it's place. You've proven that, as have your owners. I'm just weary of the I-beam comparison. Don't take it personally. It's like new-to-knives-folks insisting that "No way could you mirror polish that blade, must be nickel-plated!" It's painful to those of us who actually do this on a daily basis. Perhaps I'm the only one grieved by it...and I'll shut up about it....:rolleyes:

Your stuff is great, mlovett - I am in no way knocking it. I do agree that penetration strength is different. I wonder, to what extant distal taper plays a role in that? I imagine it would help.


I have knives of all three grinds in my shop right now.....why not? A little here, a little there...I do tend more toward what Ed calls the "light convex edge". Mostly because, of all the knives I've made, the ones I've given that grind to perform the best for the kind of work and testing I do. But I have friends who hollow-grind almost exclusively and that's ok with me too.
 
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