Thin Behind The Edge

[Sharp24/7]

I’m not quite clear on this concept. Does it refer to the edge bevel or the stock just above the edge bevel? If the bevel, does it simply mean the width or lack there of, from the apex all the way to the raw border of the edge bevel? Thanks!

[Evil D]

Thickness measured just above the top shoulder of the edge bevel.

[Sharp24/7]

Thickness measured just above the top shoulder of the edge bevel.

Thanks!

[Deadboxhero]

I’m not quite clear on this concept. Does it refer to the edge bevel or the stock just above the edge bevel? If the bevel, does it simply mean the width or lack there of, from the apex all the way to the raw border of the edge bevel? Thanks!

Its the thickness at the shoulder of the bevel.

It can change depending on the edge angle used.

A 0.008" BTE (behind the edge) with a 25° degrees per side (DPS) bevel is not as thin of a cutter as a 0.009" at 11° degrees pee side.

Since if you drop a 0.008" at 25dps to 11dps the bte will grow.

Also a 15° degree primary grind with lower BTE is technically thicker than the a 2° primary grind with slightly thicker BTE.

[Wandering_About]

Here's a demonstration picture I did of myself trying to measure the "behind the edge" thickness of a Police 4 for demonstration purposes. Hard to do one-handed, so the caliper isn't exactly at the shoulder of the final bevel. But it gives you the idea, I think.



In this picture I got the calipers at the shoulder as well as I could for a passably decent measurement. .018" is pretty good, although I like thinner at times.



It's best to measure several spots along the edge as grinds often vary a bit.

[elena86]

Here's a demonstration picture I did of myself trying to measure the "behind the edge" thickness of a Police 4 for demonstration purposes. Hard to do one-handed, so the caliper isn't exactly at the shoulder of the final bevel. But it gives you the idea, I think.



In this picture I got the calipers at the shoulder as well as I could for a passably decent measurement. .018" is pretty good, although I like thinner at times.



It's best to measure several spots along the edge as grinds often vary a bit.

Maybe you own a Stretch XL … I am so curious !

[vivi]

Thickness measured just above the top shoulder of the edge bevel.

Which confuses me. The edge is the edge. The bevel is the bevel.

[vivi]

Maybe you own a Stretch XL … I am so curious !

Reprofiling a VG10 FFG knife can be done in half an hour. Mine was really uneven too but edge angles are easy to fix.

[Evil D]

Thickness measured just above the top shoulder of the edge bevel.

Which confuses me. The edge is the edge. The bevel is the bevel.

I guess the debate is "behind the bevel" vs "behind the literal edge". Behind the literal edge apex has no defined place to measure so it's not really something that can be measured and compared. How far up the bevel from the edge do we measure? It just doesn't work for comparisons sake as well as behind the bevel does unless we all agree on a predefined distance up from the apex and then good luck actually measuring such a tiny bit of blade consistently with calipers.

There's more to all of this anyway, it's all just one measurement out of many. If we compare two blades, blade #1 is 0.0010 behind the BEVEL and blade #2 is 0.010 behind the BEVEL, but the blade #1 has a 90 degree inclusive edge angle and blade #2 has a 20 degree inclusive edge angle, I'll take blade #2 every time.

[Wandering_About]

Maybe you own a Stretch XL … I am so curious !

If I recall, the edge on my Stretch XL was in the .020-.025" range. But it was quite uneven which made it a bit difficult to measure well.

[vivi]

Thickness measured just above the top shoulder of the edge bevel.

Which confuses me. The edge is the edge. The bevel is the bevel.

I guess the debate is "behind the bevel" vs "behind the literal edge".

I don't view it as a debate. I'm not saying measuring right behind the very apex is a more useful measurement than behind the bevel, merely being pedantic and stating that IMO the terms are being misused.

I don't follow the BTE thread due to that and the fact that changing that measurement is one of the first things I do to any knife I own.

Personally I don't care about behind the edge or behind the bevel measurements, just overall geometry of the entire blade profile. The edge always gets thinned out on my knives.

Behind the literal edge apex has no defined place to measure

As close as you can get to the apex. Whether or not that number is more useful to others I don't know. What I do know is whenever that thread gets bumped the pedant in me thinks "ya know, it should be behind the bevel database

[JRinFL]

Thickness measured just above the top shoulder of the edge bevel.

Which confuses me. The edge is the edge. The bevel is the bevel.

I guess the debate is "behind the bevel" vs "behind the literal edge". Behind the literal edge apex has no defined place to measure so it's not really something that can be measured and compared. How far up the bevel from the edge do we measure? It just doesn't work for comparisons sake as well as behind the bevel does unless we all agree on a predefined distance up from the apex and then good luck actually measuring such a tiny bit of blade consistently with calipers.

There's more to all of this anyway, it's all just one measurement out of many. If we compare two blades, blade #1 is 0.0010 behind the BEVEL and blade #2 is 0.010 behind the BEVEL, but the blade #1 has a 90 degree inclusive edge angle and blade #2 has a 20 degree inclusive edge angle, I'll take blade #2 every time.

That's close to my thinking as well. It's not really a metric to use in isolation from other measurements.

[ChrisinHove]

I suppose we should be measuring the blade thickness at the bevel shoulder and the depth of the bevel itself, then the maximum depth of the blade from bevel shoulder to spine, and the blade thickness at the spine.

This will then give us a series of simple ratio numbers to compare knives with and to argue about .

[Evil D]

Thickness measured just above the top shoulder of the edge bevel.

Which confuses me. The edge is the edge. The bevel is the bevel.

I guess the debate is "behind the bevel" vs "behind the literal edge".

I don't view it as a debate. I'm not saying measuring right behind the very apex is a more useful measurement than behind the bevel, merely being pedantic and stating that IMO the terms are being misused.

I don't follow the BTE thread due to that and the fact that changing that measurement is one of the first things I do to any knife I own.

Personally I don't care about behind the edge or behind the bevel measurements, just overall geometry of the entire blade profile. The edge always gets thinned out on my knives.

Behind the literal edge apex has no defined place to measure

As close as you can get to the apex. Whether or not that number is more useful to others I don't know. What I do know is whenever that thread gets bumped the pedant in me thinks "ya know, it should be behind the bevel database

Even a skilled person who measures tight tolerances for a living could measure an edge that way 10 times and get 10 results. I'm not saying it doesn't matter, I'm saying it isn't a reliable way to compare anything. It would be far more relevant in a discussion to also mention the inclusive edge angle along with the behind the bevel measurements so we can see that maybe there's a reason that the number is big or small, because again it doesn't tell the whole story.

[Deadboxhero]

The bevel is the edge, there's not a good way to measure behind the Apex which is beyond what your eyes could see.

(Currently the only way to measure the true apex features is to use a laser goniometer)

BTE is something important and completely different.

Definition has never changed and goes all the way back to the 90s with Wayne Goddard and Phil Wilson

The apex radius(the very point of the edge) can have different features at the same "15 dps" bevel width, if you freehand, it will be more convexed near the apex radius, if you used to fixed angle system it will be flatter from the bevel to the apex radius, if you use a tormek it will be more hollowed out.

Behind the edge thickness is a way to measure the cutting ability of the actual grind, it is independent of the apex radius.

The reason why the phenomenon was important to point out and label to people is because you'll have a knife that will cut easier with less force and sharpen easier at the same DPS, this was mostly due to BTE.

In Jim Ankerson's rope cutting tests they were reliant on lbs of force needed to slice and stopped the test when excessive lbs was needed to cut rope on a scale. The behind the edge thickness was a huge factor in allowing a knife to slice cut more with less force in lbs to separate that very difficult to cut Manila rope.

[Deadboxhero]

Here is a visual cross section of a knife reground to have thinner behind the edge features.





You can see all that changes are the sides of the triangle, not the base. By having the sides of the triangle more acute when you put the same angle bevel on that knife it will be smaller because the behind the edge thickness is thinner.



The base of the triangle(the spine) does not change.

People often confuse a thin, excellent cutting knife as just having a thin spine but you can have a knife with a thin spine with a really thick geometry behind the edge technically making it a horrible cutter. So, behind the edge thickness is more significant than spine thickness for the cutting ability in most cases.




Both of these knives have the same angle at the cutting edge, thinner behind the edge means the bevel is smaller at the same angle. This improves both The cutting ability and the sharpenability since there is less material to wedge in what's being cut and there's less interference from a wider bevel to reach the actual apex when sharpening.

[vivi]

BBB, you and me are on the same page with regards to everything you said. We have the exact same views when it comes to geometry, cutting ability, etc. I was just operating from a different definition with the words edge and bevel.

[Eli Chaps]

Here is a visual cross section of a knife reground to have thinner behind the edge features.

...

Outstanding visuals Shawn.

[Deadboxhero]

BBB, you and me are on the same page with regards to everything you said. We have the exact same views when it comes to geometry, cutting ability, etc. I was just operating from a different definition with the words edge and bevel.

No worries,

You'll like this.

I took a picture of a double edge safety razor with a metallurgical microscope.



The stock thickness of these are the same as the behind the edge thickness. 0.006" so its a floppy piece of metal. You could see the striations of the grind on the edge bevel, the edge angle is 5.5° per side.



If we zoom in to 1000x we can see the tip of the edge better, what you're looking at here is the coating that they put at the edge to help with corrosion and comfort. The shiny rainbow like light is due to the bifurcation of the light on the polymer based coating at the edge when observing with polarized light.

The apex radius on a double edge safety razor is in the ~250 nanometer range which is 0.25 microns according to a study by done (Roscioli 2020)

So, we can see there is not really a good way to measure the radius of the actual apex or slightly behind it with basic tools because size defeats us.

[vivi]

You must have access to quite a microscope. I thought the 200x one I got to use at a local lab was nice, but yours seems to blow that one away.

viewtopic.php?p=1635813#p1635813

It's incredibly interesting looking at edges under magnification. Really opens your eyes as to what's happening around the apex.