Edge geometry

Oops, I wanted to come back and point out that I do realize no hunter is going to slice an apple up quite like a 0.050" thick X 0.500" wide paring knife will--- but the point was echoing what Ed said about testing your knives. Until you start actually trying to use your knives for what they're intended for--- it's all just theory on whether or not they're any good. :)
 
That was one thing Ken Onion told me, he makes knives with considerably thinner edges than many or even most makers- a knife should cut. That's what it's for. Many of my early knives had super thick edges. I just finished a fighter with a blade .030 at the edge. That's about normal for me.
 
I measure the edge thickness before sharpening and list that measurement as a stat when selling the knife. Also, some hollow grinds can go above tangent, an example of this is some straight razors, and most of my skinners, I list this detail as well.

I've been surprised how thin you can push it without trouble. I reground a relatively inexpensive CRKT folder I picked up at Blade last year and cary around the shop and use everyday. A machine shop. I cut metal with it all the time. I honestly can't remember the last day I didn't deburr some part with it, and I ground it all the way down to .010. It was .015 the last I measured it, due to wear. It gets sharpened around 15 deg per side.

A typical edge geometry for one of my skinning knives would be a .015 grind tapered to .010 at the very tip, and ground above tangent at the belly. So the belly might measure .012", and .020" 1/8" up into the grind. You don't need a brass rod to "measure edge flex", a thumb nail will do. These are generally sharpened around 13 per side. I have one I keep around the shop, and I can't tell you how many times that .010" tip had popped through a box or something and wacked the bench underneath without trouble. This is in D2, I might go even thinner in a steel with less carbide.

I think the thick edges we sometimes see today might be a product of soft crummy stainless steel, and manufacturers avoiding product failure at all costs. But decent steels and HT can certainly be much thinner, both in thickness directly behind the edge, and also a ways up the edge too.


I have a relatively standardized test I use to judge edge retention. It involves cutting certain media and comparing the edge (under bright light and magnification) to "standards" which are blades that get run through the test at the same time to provide a reference point (on that particular day, in that particular media). The media are cardboard, leather, then hardwood (osage orange). The cardboard and leather eat up stuff like 1095, the osage orange kills things like S30V. But when comparing apples to apples it is a good test to compare changes to HT etc.
 
excellent info, can anyone post pictures of the diff types of edge geometry for reference?
 
This is an area that I really think plain carbon steel shines. I have ground some stainless (154cm and d2) blades the same way I ground a plain carbon knive have them completely chip out during testing (2x4, oak, micarta). I think that alot of the big secondary sharpening bevels that I notice are on stainless as well. It seems that most of the experianced makers that use plain carbon steel are able to achieve better geometry. I may be totally wrong on this but it is just an observation. I have recently made some japanese style kitchen knives from w2/ 15n20 at about 2.2 millimeters at the spine and .005- .008 at the bevel and used them in the kitchen, whole nother ball game than my whustoff's and customs with thicker edges. although I have not yet tested them on hard wood or micarta...yet
 
plus 1 Nathan on the edge flexing! When I see an edge flex over my thumbnail or a wooden pencil I know it will cut (provided the rest of the geometry is good... for the intended purpose)
 
It's a good topic, Matt.
Like Kevin Cashen says when he quotes Roman Landes, "Geometry cuts. Heat treatment just determines how long."
 
I have a Buck Scoutlite that I reground after nearly 20 years of use. When I reground it, I gave it a full flat grind to a zero edge. The blade is about 3/4" wide. Afterward, I sharpened it on the Sharpmaker. Whatever the thickness behind the edge, it cuts better now than when new. I don't have a way to measure it, but it's thinner than any of my other carry knives.

I also have a really small machete like blade that I tested various ways. I used to call it a puukko, but it's not. It's 5" long, 7/8" wide and 0.055" thick, with a clip point profile. It has no primary grind, just a 10ish degree convex bevel ground on the edge, just like a really small machete. It batons pretty easy, is strong enough to dig a hole through a 1 x 8 board, flexes 90 degrees without taking a set (not surprising at 0.055" thick), but fails the kitchen test. When cutting stiff vegetables and fruit, particularly apples, it splits them ahead of the cutting edge far enough that when the cutting edge comes through, it follows a different path and produces a very thin slice that sticks to the blade. I make one cut, and get three pieces. As an outdoor/utility knife, it works well, though I think I'd like it a little thicker to add stiffness. As a kitchen knife, it is just adequate.
 
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I've been improving my geometry since I began and I suspect I will forever be tweaking it. Thinner, Thinner, Thinner is what Wally Hayes beat into my skull. Like Nick says, you will be afraid at first, but once you realise how thin you can go, it will all be worth it. I am somewhere between .020-.035" dpending on the intended purpose of the knife. My bush knives have a shoulder that increases as it approaches the tip... over .062" on some. This gives me great slicing down at the .025" section and good tip strength for those "abusive" moments. It is always a compromise... there is no "perfect fit".

I suggest taking the edge down by .005" increments and testing until it falls apart. You will learn quite a bit.

Here are some good shots of the progressive edge geometry on a small woods knife, before I blend it all in.

Rick

IMG_0020-7.jpg

IMG_0021-5.jpg

IMG_0022-4.jpg
 
I have a question, since edges are being discussed: I know "toothiness" comes into play when slicing, but how much, if any, does it affect chopping? (Forgive my ignorance, noobie here)
 
i have a hell of a time making choppers as i always end up grindig them thi8nner theni shoudl . then again i do 99% of my work in kitchen knives and razors. specs liek behind the edge thickness of .005 are normal and even thinner on my razors (i can set a caliper to .010 and go more then half way up a razor blade )

matt let me be the one to envite you to the ECG as if you want to see kitchenn knives and talk to kitchen pros thats the place to be (PM inboiuund)
 
Matthew Gregory said:
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...
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Matt this is hilarious,
I have been making kitchen knives for a few months now and I am not the "expert" you consulted, but the stock I start with is .090" and my edges before I sharpen are .004". At blades over 2 inches wide, that makes a difference too.
I definately use one of my knives in the kitchen to test, I have since september.

http://youtu.be/-uYfSihXbFo

One from the last batch.
Del
 
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All good info.

My first knives were thick and had very fat edges. Now the blades are half as thick ( or even thinner) and the edges go to near zero before the final edge is put on.

The blade finish will have a lot to do with the smoothness of the cut,too. I sand and polish the blade to a clean and smooth surface and the last thing I do is put the tiny sharp edge on.I does not need to be a mirror, but it needs to be smooth. The edge thickness is often only a few thousandths before the few strokes to sharpen it.

Today, I consider .10" as thick stock for any knife that will slice, and .16" as more than thick enough for most hunting/camping knives. Fillet knives often start out as .060" stock.

As Matthew said - If you make the sides taper to the edge evenly, and the blade tapers distally from ricasso to tip, such a blade will be light in the hand and function well.
 
I agree that edge geometry is what dictates how sharp an edge can become.
If you are working on two identical blades with the same edge thickness at the same point on the blade; the only thing that would make one blade sharper than the other would be its secondary angle and at what degree that angle was ground.

When someone says, full flat grind; does the term mean there is no secondary angle at the edge or would a better choice of terminology be Scandinavian grind. If the term scandi grind is used to describe the bevel, then there is truly no secondary angle at the edge; the blade is resharpened by laying the knife on its bevel and grinding away both sides until the edge is sharp again. I refer to a blade that has no secondary angle at the edge as a scandi grind and not a full flat ground blade.
If the blade is sharpened by raising the spine of the blade above the abrasive, when sharpening, then the blade cannot be described as a full flat grind blade; its just a flat ground blade with a secondary angle at the edge.
Looking at it from the scandi side; if a scandi ground blade is sharpened by raising the spine of the blade above the abrasive, then it is no longer a scandi ground blade; it is a flat ground blade with a secondary angle at the edge.
It seems these days the excepted sharpening angle is 10 to 12 degrees per side. How much sharper is a knife's edge if the sharpening angle is say 6 degrees or even 5 degrees per side?
How sharp is a 5 degree scandi ground blade? Why settle for twelve.

I hope this is not diverting the topic of this thread but adding something to it. We all want the most out of our knives; how we get there is always up for discussion.

I have been grinding edges at 6 and 8 degrees per side to see how sharp the edge becomes. In so doing it has caused me to question the wisdom of using the standard 10 and 12 degree grinds.

Just a thought, Fred
 
fred while you are right about the 0 edge things such as the grind behind that made the edge can have a massive effect on how the blade cuts
with straight razors if you get under 15 degrees total you have an edge that many times will flake away and thats why razors are sized liek they are (again like a scandi layed flat on the stones ) then ther is the world of sushi knives that are asymmetrically ground chisels both grinds are hollow ground one a wheel between 3 and 4 foot around and again layed flat on the hones for sharpening

we all know that with all the specs being the same behind the edge hollows are the weakest then flat then convex and its jsut cause of the mass behind the grind its self but conversely they have the hollow has less width to "split/wedge" the cut open for less force needed

how bout that dam super clean 90 degree edge on a sheet of PG steel who has cut themselves good on that

i used to think i made sharp knives when i made hunters then i started making kitchen knives and proclaimed that i know what sharp is, now i make straight razors and have stopped proclaiming any thing (im ready for the next level of sharp its now a sickness)
 
butcher_block said:
fred while you are right about the 0 edge things such as the grind behind that made the edge can have a massive effect on how the blade cuts
with straight razors if you get under 15 degrees total you have an edge that many times will flake away and thats why razors are sized liek they are (again like a scandi layed flat on the stones ) then ther is the world of sushi knives that are asymmetrically ground chisels both grinds are hollow ground one a wheel between 3 and 4 foot around and again layed flat on the hones for sharpening

we all know that with all the specs being the same behind the edge hollows are the weakest then flat then convex and its jsut cause of the mass behind the grind its self but conversely they have the hollow has less width to "split/wedge" the cut open for less force needed

how bout that dam super clean 90 degree edge on a sheet of PG steel who has cut themselves good on that

i used to think i made sharp knives when i made hunters then i started making kitchen knives and proclaimed that i know what sharp is, now i make straight razors and have stopped proclaiming any thing (im ready for the next level of sharp its now a sickness)
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Butch,
That makes sense especially on knives that are very thin just above the edge. When I worked as a wall paper hanger and the fad was foil on your walls I was always going around with sliced up hands; God that stuff was sharp.
What passes for sharp these days is a far cry from what I once thought of as sharp. Its hard to believe all that goes into a truly sharp edge.

I know someone who has found a way to put a hollow ground edge on a flat ground blade. Not very practical; but sharp it is.

Fred
 
Since nobody chimed in with an mathematical algorithm for edge geometry, I have to assume that for most folks it has become simply a matter of trial and error over the years. Please correct me if I'm mistaken about that. If so, that's fine, and I can certainly understand why that would be the case.

I've done a little digging to try and understand more on the subject. What I've come up with is more questions than answers. I'll start with the most basic question first. There seem to be about six "main" types of grinds used in creating knife blades:

1) Hollow (concave)
2) Flat
3) Sabre (Scandi?)
4) Chisel
5) Compound (double bevel)
6) Convex

The question, then, is what are the strengths and weaknesses of each type?

My assumption is that hollow grind is much sharper (strength) but more prone to edge damage (weakness), and harder to maintain (weakness).

I suppose the follow on question is what sort of applications each grind type is most suited for. "Chisel" grind seems to speak for itself, but perhaps that is only one of the main applications for that grind. The others are less obvious.

Also, I acknowledge the above is an oversimplification, and that there are other cross sections and grinds (especially for swords), but I think this gives us a basis for discussing knife edge geometry in more detail.

One last thought... presumably, these grind types were developed, at least in part, because the steel of the day was limited in strength and flexibility. Do the new modern steels make any of these grind types less relevant to modern knife design?

- Greg
 

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tryppyr said:
I suppose the follow on question is what sort of applications each grind type is most suited for. "Chisel" grind seems to speak for itself, but perhaps that is only one of the main applications for that grind. The others are less obvious.

- Greg
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As as big fan of the chisel grind I'll bring up a few points I have thought about...

The chisel is almost like a zero degree flat grind with no bevel on the cutting edge.

This makes is very easy to sharpen and for some types of cutting it works very well such as whittling fire sticks. The "zero bevel" cuts straight and true like a planer.

My first knife and edc is a chisel grind and I have yet to sharpen it, I just strop it on one side and it is good. I like the low maintainence.

When you slice things like food the slices fall to the correct side more easily. When rapidly slicing veggies to size, the flat side is a little more comfortable against my pivot finger since it is a smooth surface. I have a few chisel grind santoku kitchen knives I really like for food prep.
 
We have a knife we call the "Adventure Knife" it was made for folks to try out one of our knife designs, use it and return it, the only charge is a deposit to guarantee we get it back and a request that they write up their experiences. So far no one has written up a response, only verbal discussions about what they thought of the knife.

We have learned a lot through that knife that pertains to this discussion:

One edge is not suitable for all users. There are folks who could dress out, skin and bone out a large bull moose with a single razor blade and folks who would obliterate the edge on an axe skinning a gold fish.

Edge flex testing for chip is an important predictor of performance of the utility blade.

When it comes to predicting the blades ability to cut nothing I have used beats the old hemp rope slicing test.
 
The chisel is almost like a zero degree flat grind with no bevel on the cutting edge.
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Hi - there's actually nothing stopping one from putting a secondary bevel on one or both sides of a chisel grind. Mine are rarely zero ground.
 
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