Edge geometry

[Matthew Gregory]

There's been a thread recently that, of course, has managed to take on an overall tone of angst and unrest, in fact there's talk of the good ol' days of Bladeforums Knifemaker's section being in the past. I say it's because all the usual topics of pinheads failing to listen to reason tend to hammer the old dogs down, wearing them out prematurely and causing them to leave.


...so i figured I'd start my own thread to discuss an overlooked aspect of what I think knifemakers here (or perhaps I should speak for myself?) are focusing on as critically important to the cutting performance of a knife: edge geometry. We're not talking about spine thickness, or the overall thickness of any other part of the blade except the edge and immediately behind it (I elected to use .050" behind the edge as the point of reference... if you have trouble getting a grip on how small that is, I totally understand... run to the tool box and grab a 3/64" drill bit. The width of the drill bit is how far up the knife I'm measuring).

When I make a chopping knife, my goal is to have the blade offer as little resistance to the material it's chopping as I can without failing (chipping out, cracking, bending, etc). If the angle is too obtuse, it requires greater force to achieve the same depth of chop. Greater force equals more work. More work means less work in a short period of time, or work fatigue... both of which can lead to accidents. It's the big reason folks in the know want sharp knives - they're safer! If you're forced to push too hard, you're bound to make a mistake.

Same thing holds true for any cutting tool... the edge geometry holds the key to what it will cut well! Now, there's other factors involved, that's true... placement of mass in relation to that edge, the amount of leverage created by the blade's point of impact in relation to the hand, even the velocity of the point of impact, the amount of drag incurred by the thickness of the blade (think of it - a triangular cross section is a wedge! You're pushing two surfaces apart with that cross section... if that cross section is thick, it takes a lot more force than a thin one does).

Here's the point I'm trying to make... the task is what determines the edge geometry. Many "test" their knives by cutting things that the knife will likely never see, and somehow by forcing it's way through these tasks (here's the question of efficiency I mentioned above...) this 'proves' the superiority of the knife.

For my own examples, I prefer to make chopping knives with long, widening blades with thin steel and thin edges that are resilient to flexing (something that has more to do with geometry than heat treatment, btw...). Handle shape & ergonomics play a huge role in user fatigue, as well, but all things being equal, a thin chopper will chop deeper into wood and brush better than a thick chopper, which will be predisposed to split the wood rather than chop (unless there is sufficient mass and force applied, it is not an appropriate means of delivery, hence hatchets and axes having longer handles than knife blades).

I never test my edges using hemp rope. I don't know of anyone that even uses hemp rope anymore. 2x4's? Sure, but they're pine... awfully soft stuff, certainly not a good test of chopping ability other than to be easy to get and move around. Maybe I'm strange with my testing medium, but I usually use a well seasoned chunk of hard maple, or oak. Something strong... likely stronger than anything my chopper will be required to face in its life, but not outside it's realm of responsible plausibility.

How have YOU come about your selection of edge methodology? I've just glanced at my own thinking, and about one specific design implementation... what are your thoughts?

 

[AcridSaint]

Folks, pay attention to this guy - I know Matt's knives are at least sharp enough to cut through one whole can of soup.

Backed into a corner, I'd have to admit that I don't measure such things. They get eyeballed for thickness and then get one of a handful of sharpening techniques depending on the knife. I learned how to machine sharpen a knife with an 8" wheel and the wire-edge method, then I learned how to do it on a paper wheel and then a slack belt. None of them lend themselves to precision, but all of them will get a knife awful sharp if you can do it right.

For most of my knives, I tell whomever ends up with them the same thing - buy a sharpmaker and sharpen after use. I like the system it uses, works well for most blades and even folks who have never sharpened a knife before can show off how good they are to their friends.

 

[Ed Caffrey]

My hat's off to you Matt!
You just brought up something that I feel is one of the most important aspects of a knife....but also one of the most overlooked by many makers. I see it anytime I look at a publication with knife images.....you see a small blade, with those HUGE bright lines along the edge....and you know the thing have the geometry of a cold chisel.

Before I go into "preach mode", I'll just say that I build my knives with the geometry being the baseline to start from.....not something I think about once I'm done finish grinding. Most of my grinds tend to be flat, with convex edges. I use a combination of edge geometry and variances in heat treating to achieve what I'm looking for in various blades...obviously a heavy chopper would get a heavier convex, where as a hunter might get a more gently convex on the edge. I personally prefer a convex edge because it produces much less cutting resistance that "hard" angles.
While there will never be any such thing as a standardized test for knives, I think it's important for a maker to test and use the product they produce...it's the only way to improve.

I'm sure this subject will raise some eyebrows.... This is one of those subjects that generally is not understood/considered, until a knifemaker has reached a certain experience level.....
GOOD ON YA for bringing it up!

 

[james terrio]

Thank you! Call me crazy but the more I learn, the more it seems that knife design should start with the edge and what you want to cut with it. Everything else should build from there: steel selection, HT, blade mass/size, handle shape and so on. And having said, that I'll shut up and listen.

 

[Justin King]

Ever notice that edge thickness is one of the things that is almost never specified on a knife for sale, custom or production?
It is, indeed, one of the most important anchors of a knife's performance and possibly one of the most variable from one maker to another.
When Aldo starts selling unobtanium, I will be zero grinding all my edges.

 

[Lorien]

the steeper the ramp, the harder it is to push things up it.

 

[wnease]

I used to measure edge thickness @ 0.0625 from the edge because it was an even 1/16". I'm happy to start measuring @ 50 thou... just not happy to do both. What's everyone's fav?

 

[kc custom]

Thanks for bringing up EDGE GEOMETRY though I don't build choppers I'm to the
point that I don't care to make a knife thats flat ground and not all the way to the spine
or very nearly so. Hollow grinds can help some at the expense of some edge strength.
Grind them thin (and they will cut-knives) Thicker and they will chop (Machetes, Axes).
Anyone been hanging off a cliff lately and needed a crowbar to hold their weight?
Ken.

 

[tryppyr]

Thanks for starting this topic. I know nothing about edge geometry, and am looking forward to learning from those that do.

The fact that we refer to it as geometry suggests that there is a mathematical principle involved. I am thinking along the lines of an algorithm that could be used to calculate the specific angles and the associated forces that could be withstood before deformation occurs. I'm almost certainly oversimplifying it. Is there something like an algorithm to determine optimum edge geometry for specific applications?

- Greg

 

[james terrio]

Is there something like an algorithm to determine optimum edge geometry for specific applications?

WAY too much math for me to figure out. A person could certainly do some interesting testing with a standard alloy/HT process and various cutting media and edges, though. Get to work, it was your idea

 

[Daniel Fairly Knives]

Matthew, great points you bring up, they are always on my mind it seems. I feel like I focus the most on the whole blades geometry as the most important part of a knife.

I just started a thread over in reviews and testing asking what people wanted to see in a knife test. As a (wanna be) custom maker I feel that my main job is to make what the customer wants. I'm finding it interesting that so far everyone wants to see some real life tests not just gratuitous "extreme" testing and destruction tests, at least so far. I'm glad, as this is what I'd rather do, if I wanted to chop cinderblocks I'd use my rock saw.

On a side note I'm glad you mentioned Oak as a test wood. Oak sounds like a much better test than the pine and aspens I have been cutting and I have tons of scrub oak to remove, lol. I did a short video where I cut into an aged hardwood pallet and thought it was a poor test subject, pallets vary too much.

I know I want a chopper to chop, a slicer to slice, etc. I just cut a grapefruit up with a big chopper and it was actually funny, the knife was super sharp but I felt like I was cutting it with a doorstop, lol. On the other hand it made short work of chopping big trees, brush, tiny branches I didn't hold, it whittled fire sticks quickly, I was very happy with how evenly and effortlessly it made cuts.

Style means everything too. I used to live in the Caribbean and carried a 30" machete just about everywhere I went. It worked great for cutting trails, big bamboo, digging up mango and palm trees, I even took cuttings of of plants with it. It cost about 10$ and I sharpened it with a file... sure was ugly though plus I'm sure the chopping end was all pitted up. It worked but I want my knives to impress people when they see and use them, not just "cut it."

Pardon my rambling...

 

[Matthew Gregory]

All great points, Daniel!

 

[BMK]

I would think that friction is one of the major factors that effects a blade's ability to separate material. Friction is a by product of a blade's geometry and beyond that I am not really sure what goes on.

 

[Ban]

Edge geometry is king. Match the proper geometry to the cutting tasks a tool is required to do and you will have a very optimal cutter.

 

[Darrin Sanders]

This is one of the things I try to explain to customers. I want to know what the knife will be doing the majority of the time. We all know a chopper will slice but not as good as a blade with thinner geometry. And vice versa. So when talking to customers I use the example of scalpel vs. splitting wedge. They are the extremes. You gotta figure out where you want your blade to be between these extremes for what you're gonna use it for the MAJORITY of the time.

 

[David Metzger]

After I bought everything I thought I needed to make knives, I quickly learned that I need to be able to quantify edge geometry much more accurately. Both along the entire edge and compared to other makers. I also keep records for knives so I can adjust the edge and area behind edge based on performance. What I read from good makers is that a kitchen knife could be .015 for a user that treats their knives well and .025 for a more abusive user. Some kitchen knife threads contain stats for about 12 measurements.
D

 

[Daniel Fairly Knives]

random stuff on the subject...

I used to be a chef and I think while a thin chef's knife can be nice is not the only thing to focus on.

I liked my main knife that I used all day as a sous chef to have a thick spine with rounded edges. The bulk of the blade should be thin. If the spine was thin after a while your hands get fatigued while pushing through tougher stuff. Imagine starting the day by cutting up about 60 half frozen chickens and it starts to make sense.

On the line you don't use a knife as often so a thin slicer usually works best as your go-to knife, speed is everything here.

Funny enough it seems like whoever is in charge usually uses a filet or boning knife the most because they are cutting the high end meats and fish.

You always want a very thin paring knife with a sharp pointy tip for detail.

 

[Makermook]

I don't know if it makes me sound like a rube, but I kinda like a Scandi grind on a general use outdoors knife. All the stuff I use one for, like cutting up grub, battoning through firewood, whittling, cleaning game, all seem to be passably well done with that kind of edge geometry.

Now, I'm definitely not one of those Bigger-is-Better guys who thinks a 4 1/2" long blade needs to be 1/4" thick. That's just silly. A 2mm thick blade with the right temper is plenty.

Plus, as a user who doesn't use a lot of complicated rigs for sharpening, I've found that a nice flat Scandi grind is easiest for me to reliably sharpen the same way every time. I use flat waterstones, and I can get a really amazingly sharp edge in no time at all. But put a convex ground blade in my hands, and there's no telling from one stroke to the next what I'm gonna do to it.

Of course, I'm talking about compromise. If I want to chop, I'd prefer a beefy convex grind. If I'm slicing tomatoes or skinning game, a nice thin hollow grind is good for that. For me, the Scandi grind (again, on a general use outdoors blade) seems to be the best compromise.

[Having said that, I still am not really a fan of deep hollow grinds; I like to imagine that I have more knife than I might need, rather than less, and I can't see trying to baton firewood with a hollow gring knife when hitting a hard knot might mean chipping out a big divot on the cutting edge. But that's just me.]

 

[Matthew Gregory]

Okay, here goes... this is where I start to upset the apple cart!

My choppers generally will have a 9" blade, distally tapered with a spine starting @ .25" or just slightly thicker, and the measurement .050" behind the edge will be .015-.020". The steels I prefer to use are considerably tough, and with a proper differential heat treatment I have excellent results and no deformation of the blade under torsional stresses.

My big fighters share similar traits, however the blades will have more pronounced distal taper, as well as a clip capable of being sharpened, and behind-the-edge measurements @ .015". The spine is VERY resilient to impact stresses, and the blade moves like a feather and will effortlessly pass through 1.5" thick grapevine (tough stuff, and lots of it in the forestry around here).
My previous designs were considerably thicker, but with new steels and better heat treatment practices, I found I could defy convention and really thin things up with no appreciable loss in strength while improving chopping and cutting capabilities immensely.

You're more than welcome to call bullshit on me, cuss at me, say I'm out of line, call me names, say my knives suck, etc. Or, perhaps, you might consider trying it yourself. Be advised, however, that there's more to it than just geometry, at this point. Steel selection and heat treat becomes far more critical. But I suppose that's what all this is supposed to be about, isn't it? Making a better mousetrap?

It's my intention to enter into the kitchen knife realm in the near future, and after speaking to someone that I deem is an expert in the world of chef cutlery I've concluded that the first blade I'll attempt will be a 9.5" long, distally tapered blade approx. .090" thick and ground to an edge @ .005" thick. Stay tuned...

 

[NickWheeler]

I like to chop through steel drums and drop the blade tip first into concrete from 120 feet up. I really have no idea what all this "thin edge" talk is????


I had made several knives early on where I focused on trying to make them look and feel well made, but was scared to grind the edges very thin because a.) I knew I'd screw them up, and b.) there seemed to be a lot of talk about ruining blades by grinding them too thin and "ruining the temper."

Then one day I tried to cut up an apple with one of my hunters. It was more like driving a splitting maul through wood than it was like slicing an apple. It was at that very moment that I realized just because a knife looked pretty decent and had a professional heat-treat (Paul Bos at the time) didn't mean it was a good knife.

I keep getting them thinner and thinner ever since then.

I think there are a lot of people that think just because a knife doesn't look super thick at the edge with a huge secondary bevel, that it's thin---- but if the blade is over 0.250 thick at the spine, and the bevel only goes halfway across the width of the blade, and there's no distal taper--- it's still a thick knife.

And I'm definitely with Ed on this--- it kills me when I see a photograph of a $5,000 folder and it's got a secondary bevel on it that should be on a camp axe. A knife like that could (and should IMHO) be almost a zero edge. Not only does it show the knife isn't really very functional, it completely takes away from the art in the piece as well--- It's like seeing a beautiful, tall, slender super model--- with a giant, harry, beer-gut sticking out over her bikini. :barf: It just ain't right.