EDGE GEOMETRY !!! WHAT IS IT?

[kansas stik man]

I build knives like alot of the fellas on here and i can sharpen a knife pretty good but im at a loss when the edge geometry thing comes in.  it makes since but i dont know how to control it or how to tell what shape its in or what shape it needs to be in.  i have heard that you need 2 cutting angles and that you need 3 cutting angles but i just dont know how to achieve this while sharpening a knife.  as for myself when i sharpen a knife its sharp enough to shave hair right of but i does not hold a edge very long.  if i had to say what i do wrong id say i make the angle to steep and the edge is kind of like a straight razor edge.  how can i correct what im doing wrong but end up with a razor sharp knife?  look forward to seeing what some of you do!!!

[Lin Rhea]

Jerold,
         This is a good question.

First, we have to assume the heat treat on the blade's edge is right for the type knife it is. Because, after all, it's a combination of the blade's heat treat and edge geometry that determines it's over all performance.

There are two extremes of edge geometry. A razor with a deep hollow grind and thin edge and an axe with a pronounced convex cross section and rolled edge geometry. Both are suited to their purpose, but wont cross over at all within the other's realm.

A knife blade is somewhere in between. Then you can divide the geometry of different knives up into little sub groups. Each sub group will need to borrow a little fromt he axe or a little from the razor to take care of it's required role.

I like a flat grind on my blades. In other words, half way between an axe and a razor. It's not hollow and it's not convex.

The actual edge is where there is some confusion and where you have to think small. Is the knife going to be primarily a hunting knife, small of blade and for skinning? I envision a thinner flat grind with a convex edge with a low transition.

A Bowie or Camp knife with a slightly thicker flat grind and a convex edge that is supportive for heavy cuts and occassional chopping.

The actual word discription is difficult so I may have to clarify. But, once you get a good image and understanding of it, it works.

[kbaknife]

Yep - the grind itself has a lot to do with it.

This is no joke, I got this email from a customer of mine in Sweden just yesterday. They can do something crazy like shoot 6-7 moose per year. They have so many that they are required to shoot a MINIMUM of so many calves and cows before they can shoot a bull!
Anyway, Made him a knife of my clay processed W1.
This is copied exactly like he sent it yesterday:

"It really does cut well. It does not seem to get the same bitter sharpness as the D2 steel blade I have, but cuts better, for some reason I do not really understand?."

So, W1 out-cutting D2???!!??

I have an opinion on that. Kevin Cashen frequently quotes one of his good buddies, Roman Landes, who is renowned in the world of swords and such for his metallurgical prowess, and he says, "Geometry cuts - heat treat just determines how long."

So, does my W1 really make a better blade than D2?
I'm not sure.
But! I will suggest that the geometry I put on that blade is a BETTER geometry than the D2 blade he is comparing to.

[Bladepeek]

I bought a few pieces of Kevin Cashen's O-1/L6 damascus and he heat treated one of my blades in his salt bath set up. We talked a bit and he told me of a cutting competition where one of the guys showed up with a new, secret "super" steel that out-cut everything there. What was it? O-1 on which he managed to nail the heat treat exactly and had the right edge geometry.

[Lin Rhea]

The last two examples by Karl and Bladepeek illustrates what I mean by "think small". To the untrained eye or to the glance, the difference in the edges might not seem significant, but the small variation in shape makes a big difference in performance.

Some think that a primary bevel and a secondary bevel is all there is to it and, in fact, it will serve pretty well. There are two angles to blend from the actual edge to the blade sides. These angles are, more or less, faceted corners which hinders flow of the materials being cut.

If you were to knock the sharpness off those corners, you would essentially have a convex edge allowing material flow to take place with less effort while retaining the edge holding of a good heat treatment.

[kbaknife]

Nicely written and explained, Mr. Rhea.
I'm sitting here and thinking of the next thing to write - and I don't know where to start.
What we're talking about is "What makes a knife cut?"
Everything we do is involved.
Everything.

[Todd Robbins]

I use a flat grind as the primary grind on all the knives I make.  I grind them to a certain thickness at the edge, depending on the intended use and the steel used to some extent. Except on knives that are exceptionally thin, I convex the edge from whatever thickness I leave the primary grind at. Big choppers are left thicker before convexing and that thickness gradually diminishes along with knife size and steel type, to the point that some of my kitchen knives are ground to almost a zero edge.

[Lin Rhea]

Todd, I believe that is a good explination.

When I started making knives I had not used thousandths in my vocabulary very much having a builder background and not machinist. Since then I have changed my thinking quite a lot of course. But when we are talking about the difference in these very small thicknesses, I find it necessary to keep a dial caliper at least handy. .005 (five thousandths) one way or other in the thickness of a blades edge ,before convexing, makes a big difference in the performance. Also how high up the blade the convex reaches before it blends to the flat ground sides does too. It's this combination plus the heat treat that determines the performance.

Thickness where the convex starts - X - How high up the blade the convex starts - plus - heat treat = performance.  

So, in acuality, the thickness of the blade where the convex starts can be up or down from one knife to the next depending on it's type. For instance, my large knives. I like the edge to come to about .045, but I dont measure it right on the edge. I measure about 3/16 up. When I get that thickness right, I can roll the convex edge and expect it to be in the ball park. Small knives are scaled down similar to the way Todd described.