Lets Talk t.b.e (thickness behind the edge)

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Cushing H.

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I would like to invite a little discussion on thickness behind the edge. I have always been under the impression that for a knife to exhibit the ability to get really sharp and perform well, that a long, low angle secondary bevel was most important. Then ... along comes Stacy, who repeatedly says a small t.b.e is most important. I guess I can see that, as for example even a long low angle secondary bevel would probably not sharpen too well if the tbe were, say, 0.02 or 0.03".

Can we take just a little space here to elaborate on how/why tbe is so important? Also, maybe also explore the limits of that criteria (thought experiment - really short bevel, say 0.1", but taken to an extremely small edge thickness. One could say the tbe for that is really small, but the thing is a wedge, not a blade ... and would likely not perform very well......
 
I took a folder blade 0.5" wide and 0.080" thick and did a full flat grind. I sharpened it and made some cuts. Then i put a swedge on half the blade length and then made some more cuts. It was night and day difference and the edge was the same. I always do some kind of swedge now. I know everyone likes swedges for the lines and style but i personally like to smooth out the line and make it subtle.

So in my opinion it is more than just tbe. I aim for as close to a zero second edge bevel as possible. That's just me.
 
My comments were that if you grind the main bevel to a zero edge the metal behind that ZERO is too thin to last for anything but the most delicate cuts. As you grind and sharpen this edge, it will come off in microscopic chips and leave a ragged edge. By putting a secondary edge on at the desired angle, you create a TBE that has a little more meat and can support a very low angle edge. If this were not true all edges would be zero angle and controlled only by the thickness of the blade. However, on most knives that angle would be far too low to have any edge life. On a 2" chefs blade made in .100" thick steel a zero edge is less than 3°. That is too low, so you add a secondary to make the edge hold up. How far you sharpen that edge back determines the TBE. After many sharpening's on a heavily used kitchen knife, I often need to regrind the bevel a bit to thin the TBE back down.

Here is what I do:
I grind to a near zero edge on kitchen blades and then put on a secondary that ranges from 5° to 10° per side. On rougher use knives, I take the edge to around .005"-.010" and then put on the secondary at 10° to 15° per side. This will be more robust and survive chopping and hard use. I only sharpen the edge back sufficiently to create a smooth edge. Examining the edge with a 10 or 20 power loupe can help yo a lot in finding out how well you are sharpening and how rough/smooth your edge actually is.
 
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I was reading Randy3000's post and think there is some sort of nomenclature problem.
In much of the world the main bevel (spine to edge) is called the primary bevel, but there are places ( mainly Japan) where it is called the secondary (because the edge is the primary angle of importance).
The bevel that forms the cutting edge is called the secondary bevel because it is added as a secondary task (often in sharpening). In the places that consider the edge as the most important feature of a blade, they call it the primary bevel.

A zero edge is when you grind/sand/sharpen the primary bevel to come to an apex at the edge .. basically ero thickness.

The primary bevel is the main bevel of the blade. Often done as a full-flat-grind (FFG), but in some cases a partial bevel going not completely to the spine.
The primary bevel is sharpened to leave an edge of desired thickness, depending on the use of the finished knife. This can range from, .000" thick (zero edge) to as high as .100" for a heavy use chopping blade. This thickness at the edge is called the "edge thickness" when grinding.

The secondary bevel is the small bevel that creates the cutting edge.
The secondary is always a zero edge.

Thickness Behind the Edge (TBE) is the thickness at the top of an imaginary triangle with the edge as the apex and the sides as the secondary bevel. It is difficult to measure accurately, but a steel dial caliper will get a close measurement. Ordinary "C" micrometers will not work for measuring TBE.
 
There was a hread recently that discussed that bevel nomenclature ... and posited we use the opposite nomenclature (cutting edge as primary). Ugh. I like stacys logic, and will use that in the future.

thank you for posting your clarification stacy.
 
I see a lot of confusion over this, I've always referred them as Stacy does. It gets even more confusing when I see a member post about a poor grind...only to find out that they're just referring to a wonky edge bevel.
 
In my example, a half inch blade at 0.080" thickness ground from spine to edge with zero edge thickness results in an edge of less than 1 degree. Of course this would not last long when cutting. But when you sharpen it, you put that second bevel on it, whatever angle you wish. Instead of saying I do a "as close to zero second bevel as possible", I should probably say "I grind the primary bevel to make a sharpish edge (mostly safe to hand sand the blade, you would have to really slip up to cut yourself with it) and then sharpen normally". I think a lot of times people leave the edge with 0.010" thickness or more before sharpening, I was trying to say I don't really leave any thickness before sharpening. But my main point was that as far as slicing goes I think the spine thickness is very important, not just the edge, because the spine also has to pass through the cut.
 
Randy3000 said:
In my example, a half inch blade at 0.080" thickness ground from spine to edge with zero edge thickness results in an edge of less than 1 degree. Of course this would not last long when cutting. But when you sharpen it, you put that second bevel on it, whatever angle you wish. Instead of saying I do a "as close to zero second bevel as possible", I should probably say "I grind the primary bevel to make a sharpish edge (mostly safe to hand sand the blade, you would have to really slip up to cut yourself with it) and then sharpen normally". I think a lot of times people leave the edge with 0.010" thickness or more before sharpening, I was trying to say I don't really leave any thickness before sharpening. But my main point was that as far as slicing goes I think the spine thickness is very important, not just the edge, because the spine also has to pass through the cut.
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Actually, an isosceles triangle with sides of .5"and a base of .08" has a 9 degree apex angle. As you said, the edge is too acute for normal use and needs a secondary bevel. On a folder, 12 to 15 degrees per side is a good edge bevel.
 
generally it has been advised here to leave .02 to .03 behind the edge for a heavy duty use knife like a camp knife. Anywhere from .005 to .015 for cutting tasks, like kitchen and light utility.
It is important because it is easily measurable and allows the knife maker to know where they are at, but is only one factor in many regarding the cutting geometry.
 
My personal experience tells me to go as low as you can with the TBE, that still allows the knife to perform properly. This usually means .002" to .005" behind the edge for most of my knives (smaller EDC style). I will admit, I have been lazy and left thicker edges before adding the secondary. The end result are knives that I know don't cut as well as they could.

When I do kitchen knives, they are taken to zero, dulled, and then a small secondary is put on.

I believe that TBE is something we all need to be focusing on. It's a distinguisher from production knives.
 
Timos - Was that a typo? - did you meant .003 to .005.

Maybe it is me, but .030" TBE would be for a very heavy chopper. That is thicker than many of my edges at HT.

I adjusted my thickness numbers a tad for heavy use knives when I considered that I go thinner than most and sharpen back to what holds up right.
 
My knives get the primary bevel (the full flat grind) taken down to “zero” on every knife and then lightly cut into a piece of 400 grit paper to establish a flat that will just reflect light and have a little meat to support a strong edge, maybe 0.005” max. Then sharpened anywhere from 10°-15° per side. The “BTE” thickness just depends on how thick the spine is and how tall the blade is, and I never actually measure it. Since 90% of my knives are hunters and kitchen knives, I rarely use blade stock greater than 0.090”.

Also I have played around with a little sharpening procedure where I sharpen at, say, 13° per side, and then lower the angle by a few degrees and knock off the shoulder that was created by the initial sharpening. Think of an inverted triangle with the top two vertices knocked off.

IMHO, Knives should be as thin as possible but still able to do what the knife is intended for. Thin spines. Thin grinds (low BTE thickness), and low edge angles. Get the heat treat nailed.
 
I probably shouldnt even comment on harder use knives b/c I really dont make any! yeah .03 would be waay thick behind the edge. The thickest Ive actually gone is .020 on a camp knife , i never tested it much but I imagine you could chop a small tree down with it. What is the standard for the competition cutters?
The more i think of it, another very important aspect behind measuring TBE is making sure your bevels are even. I always measure tbe all along the blade and that is how i decide how much pressure to be applying while grinding. Typically Ill always end up with a bit of a dished edge about 1-2 inches from the heel and an extra thick tip and heel. As I grind closer to finished dimensions, measuring really help to identify these trouble areas and catch the problem while its still fixable.
 
samuraistuart said:
Also I have played around with a little sharpening procedure where I sharpen at, say, 13° per side, and then lower the angle by a few degrees and knock off the shoulder that was created by the initial sharpening. Think of an inverted triangle with the top two vertices knocked off.
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Great idea. I am going to try this sometime.

samuraistuart said:
IMHO, Knives should be as thin as possible but still able to do what the knife is intended for. Thin spines. Thin grinds (low BTE thickness), and low edge angles. Get the heat treat nailed.
Click to expand...

This! IMO, this is one of the reasons people by handmade knives.
 
Stuart's method is the same basic way the Japanese start a convex grind. They sharpen at one angle and then a new angle several times. This gives niku to the edge (niku = meat. Japanese version of TBE). It is an excellent way of getting a razor sharp edge and thicker supporting metal behind it.

His comments on getting the most from HT and making blades thin is spot on. I know every new guy wants his knife 1/4" thick, but even 1/8" thick is very thick. If you measure some successful brands of commercial knives you will find the spine is between .040" and .10" most fall around .06-.08" .
Thin cuts, thick splits.
 
I have seen on forums discussing kitchen cutlery discussions about "thinning" the edge. What they are referring to is after a kitchen knife has been sharpened a number of times the TBE grows and to get it back to original performance you must reduce the TBE. This is done similar to sharpening but at a lower angle. Say the blade is sharpened to 20 degrees/side you would thin the TBE at 10DPS, not all the way to the cutting edge but enough to thin the TBE down to where it again cuts like it did when new. The numbers I gave above are just an example.
 
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