Edge Geometry Question

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KennyB

Joined
Jan 19, 2010
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Okay, so I've been thinking about something in relation to edge geometry. Everyone always says that thinner is better, and usually say something like 30* is thinner than 40*. Sounds good, but I'm wondering, isn't there lots of cases where a blade may have a 40* edge but still be thinner than one with a 30*?

So, for example sake:

Blade 1:
40* Inclusive
.025" directly behind bevel shoulder
.100" thickness at spine

Blade 2:
30* Inclusive
.040" directly behind bevel shoulder
.150" thickness at spine

So, will the blade at 30* still be thinner at the apex regardless of the otherwise larger dimensions, or is this a case where the blade with the 40* bevel could still be a lot thinner?
 
agreed.
a funny observation i made about kitchen knives. while still in apprenticeship a coworker lend me a suisin inox honiaki chef's knife. those are high end japanese knives, 19C27 steel HT'd about 62hrc and VERY thin blade with a VERY thin grind and almost nothing above the very edge. i worked with the knife all day long, and was amazed how well the knife held it's edge ... at the end of the day (very busy day) i was pretty sure that the knife was still razor sharp until i had to cut a tomato and noticed that the knife wasn't sharp enough to make a clean cut. i returned to my previous job (can't remember what it was) and the knife fell thru the food ...

long story short, the knife was almost dull but i didn't even notice until i had to cut something that really needs a sharp edge like a ripe tomato. this because it was thin enough behind the edge to perfom amazingly even with a less than perfect edge on it.

same applies to your analogy.
 
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I am frequently reminded of how a simple, THIN blade makes a big difference in slicing (for some uses), every time I use or see someone use a plain ol' paring knife. Edges can frequently be ugly & blunt-looking on these, but the blade itself is so thin, it often doesn't matter that much. Of course, I've also noticed that if one takes the time to put a pure, clean edge on it (especially at a low angle), they'll slice like a laser beam.
 
Obsessed with Edges said:
I am frequently reminded of how a simple, THIN blade makes a big difference in slicing (for some uses), every time I use or see someone use a plain ol' paring knife. Edges can frequently be ugly & blunt-looking on these, but the blade itself is so thin, it often doesn't matter that much. Of course, I've also noticed that if one takes the time to put a pure, clean edge on it (especially at a low angle), they'll slice like a laser beam.
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If the blade is thin enough it doesn't even need to be sharp to cut. :D

I have used DULL blades that had .010 or less behind the edge and they still cut like crazy.

That's the reason why most kitchen knives never really get sharpened, they still cut when dull and they can be butter knife dull and still cut.
 
Cutting performance is dictated by many factors related to blade geometry (which includes more than just the tiny area around the cutting edge).

To your point, care needs to be exercised when discussing these things or confusion sets in quickly.

It is (in my opinion) wise to state things clearly as indicated with the bold text below:
This knife is thin behind the cutting bevel, or better still,
This knife is 0.xxx inches thick at the interface between the single edge bevel and the blade face.

Better to say "The angle of whatever bevel we are discussing (primary, secondary, micro, whatever) is XX degrees inclusive" Instead of "this knife has a thin bevel angle" (which is absurd)....angles can not be thick or thin...they are angles that are quantified by numbers that have a value that can be qualitatively discussed as lower or higher, but not thicker or thinner.

There will probably always be confusion in these areas considering the diversity of people working on blades. It is all good, just confusing at times.

I am not claiming that I have not used BAD terminology at times...I am reasonably confident that I have;)
 
It just depends on the intended use for the blade. Most knives are meant for cutting. I made the mistake of convexing an Ontario RD4. I went past knocking off the shoulders and made the edge too weak.
It does cut better, but I chipped the edge trying to baton through a golf ball.
The RD4 is a hard use knife, I should have kept the factory edge angle, and more metal would have been left behind the edge. Live and learn.
 
unit said:
Cutting performance is dictated by many factors related to blade geometry (which includes more than just the tiny area around the cutting edge).

To your point, care needs to be exercised when discussing these things or confusion sets in quickly.

It is (in my opinion) wise to state things clearly as indicated with the bold text below:
This knife is thin behind the cutting bevel, or better still,
This knife is 0.xxx inches thick at the interface between the single edge bevel and the blade face.

Better to say "The angle of whatever bevel we are discussing (primary, secondary, micro, whatever) is XX degrees inclusive" Instead of "this knife has a thin bevel angle" (which is absurd)....angles can not be thick or thin...they are angles that are quantified by numbers that have a value that can be qualitatively discussed as lower or higher, but not thicker or thinner.

There will probably always be confusion in these areas considering the diversity of people working on blades. It is all good, just confusing at times.

I am not claiming that I have not used BAD terminology at times...I am reasonably confident that I have;)
Click to expand...
Um, I'm not sure what you mean by this. By "thinner" and "thicker" I'm not actually talking about the angle, I'm talking about the apex of the edge itself. I've seen it asserted that a 30* angle will lead to this apex being thinner because it's more acute.

Oh, and while I appreciate the input from others, it's not exactly what I was wanting to know. I figured that blade 1 would be a better overall cutter, but really what I'm wondering is more of question about the edge and the apex itself.

Basically, does more acute always lead to the apex being thinner? I put up the other dimensions of the blade because I wasn't sure if they matter at all to this part, but I'll compare it the other way

If Blade 1 has:
.050" Wide Bevel Faces
.040" Directly above bevel shoulders
30* Inclusive grind

and Blade 2 has

.050" Wide Bevel Faces
.040" Directly above bevel shoulders
40* Inclusive grind

With those dimensions, the 30* angle would lead to a thinner apex, but what if instead of .040" Blade 2 was .020" or under? Since the angle itself is only 10 degrees more, but the point from which it starts is .020" thicker, wouldn't the actual apex and edge of Blade 2 be thinner? Would the difference in thickness at this point even be significant?

Or does it just make the rest of the blade behind the edge "thinner" and lead it to cut better?

I'll try to ask the question in a practical context, even though it kind of changes the circumstance. Say I have a knife that I put a 30* bevel on, and decide, "Well, this is a little too weak, but I don't want to go all the way to a 40* bevel," and you decide to put a 40* microbeel on it instead. How does the thickness of the edge apex change now that there's a 40* bevel on it? Is it now "thicker" than the apex that was there with the 30* grind, and if so only be technicality, or by a significant amount?
 
an apex is an apex it's size depends only of how well it is sharpened. if both faces meet whatever the angle is, the very apex will be the same width wich is probably measurable but i guess you'll need a microscope. what will change with angle change is how fast the edge will thicken back to shoulder's thickness

you should see paul's verhoeven study for apex measurements and chad ward tutorial on egullet about kitchen knives, how to use why they cut ... he says something quite interesting about how the thickness at the first inch or so (not sure) behind the cutting edge is the most important in a kitchen knife. quite instructive.
 
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Without experimentation, I doubt anyone can give you the correct answer. When you start changing multiple variables, edge angle, blade thickness, primary grind thickness, etc., it's hard to predict what has more and what has less effect.
 
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