"Double-edging"

[idanp]

I've just finished reading The Razor Edge Book of Sharpening and all through it the author supports grinding a secondary edge in a desired angle, followed by grinding the primary edge in a larger angle.
This is a bit counter-intuitive to me since increasing the angle of the first grinding makes the edge less sharp. Also, most YouTube videos I've watched on knife sharpening do not mention a second grinding phase; yet the book still claims this is the most common grinding.
I couldn't find anything on it in Google while searching the term double-edging.
What exactly are the trade-offs between having one edge or two (see image) ? I am mostly interested in knives, but if an answer is relevant to other tools that would be fine.
Thanks in advance.

Edit: Sorry for not being clear. Added a proper picture for clarification. In the book he uses this grind on a sheath knife where he puts an angle guide at the furthermost part of the spine to grind a relief angle (primary in picture), and then he moves the angle guide a small amount towards the edge and grinds the actual cutting edge (secondary in picture).

 

[FortyTwoBlades]

Do you mean he suggests grinding one part of the blade at a thinner angle and the rest at a thicker one, or do you mean converting a single-edged knife to a double-edged one, making a previously un-edged region sharp? It sounds to me like the former, but "double-edging" is a horrible and confusing term if so. It also sounds like you could be describing micro-beveling, whereby you set the majority of the edge at a low angle and then hone at an elevated one to lend a little extra stability to the edge without it affecting the cutting performance too greatly. The description you've given is a rather vague one.

 

[Ernie1980]

From your description it sounds like a micro bevel, do you have a picture?

 

[HappyDaddy]

I couldn't find anything on it in Google while searching the term double-edging.

I doubt that you could share here what you did find.

 

[danbot]

Yes, I think you are describing a micro bevel. Very common with woodworking chisels.

 

[Petunia D. Feeble]

I think this is what he's referring to. Essentially accomplishing what a convex grind does, but in 3 defined steps rather than a gradual curve. Maybr useful on a real beater in a crap steel, but completely unnecessary on most modern knives.

 

[FortyTwoBlades]

Ah. Faceting.

 

[lambertiana]

I have seen a number of people talk about using this secondary microbevel, but I have never felt the need to do so.

 

[MtnHawk1]

It also sounds like you could be describing micro-beveling, whereby you set the majority of the edge at a low angle and then hone at an elevated one to lend a little extra stability to the edge without it affecting the cutting performance too greatly.

I read the book and this is also my understanding.

T.L.E. Sharp, the author teaches sharpening in 2 steps, not 3. Using the terms in your diagram there would be the relief angle and the secondary edge would go all the way to the apex. There is no primary edge.

Why do you say this method is completely unnecessary on most modern knives?

idanp, I think the term "double-edging" refers to the 2-step process the author teaches: the relief grind, then finishing at a higher angle. Unfortunately there is a lot of unnecessary confusion with knife-sharpening caused by non-standardization of terms. The same word or term often means different things to different people. That's why photos and drawings are valuable.

 

[FortyTwoBlades]

Why do you say this method is completely unnecessary on most modern knives?

I suspect the reason he stated this was because many modern steels can take and hold much thinner geometries than knives of years past without suffering from rolling or buckling. However, I'd offer the counterpoint that it means you can go thinner still with the relief angle for even greater improvement in performance. The old engineering conundrum with material advances: if a new material comes along that's twice as effective as the old, do you use the same amount to make it twice as resilient, or do you use half as much to make it just as resilient, but with less bulk? The answer depends on the context of the application, and is usually somewhere in between.

 

[DocJD]

I suspect the reason he stated this was because many modern steels can take and hold much thinner geometries than knives of years past without suffering from rolling or buckling. However, I'd offer the counterpoint that it means you can go thinner still with the relief angle for even greater improvement in performance. The old engineering conundrum with material advances: if a new material comes along that's twice as effective as the old, do you use the same amount to make it twice as resilient, or do you use half as much to make it just as resilient, but with less bulk? The answer depends on the context of the application, and is usually somewhere in between.

^^^ That's kinda sublime !

 

[MtnHawk1]

FortyTwoBlades, I agree with DocJD. Good points and well said!

 

[idanp]

Do you mean he suggests grinding one part of the blade at a thinner angle and the rest at a thicker one, or do you mean converting a single-edged knife to a double-edged one, making a previously un-edged region sharp? It sounds to me like the former, but "double-edging" is a horrible and confusing term if so. It also sounds like you could be describing micro-beveling, whereby you set the majority of the edge at a low angle and then hone at an elevated one to lend a little extra stability to the edge without it affecting the cutting performance too greatly. The description you've given is a rather vague one.

From your description it sounds like a micro bevel, do you have a picture?

I apologize for being vague. Added a picture and an example in the first post to make things clearer.

I doubt that you could share here what you did find.

A bit of laziness on my part. Found some information about single or double edged blades so I thought I am probably not using the correct terms.

 

[idanp]

It also sounds like you could be describing micro-beveling, whereby you set the majority of the edge at a low angle and then hone at an elevated one to lend a little extra stability to the edge without it affecting the cutting performance too greatly.

Thanks. I am interested in the science behind this. At least now I know what to look for.

idanp, I think the term "double-edging" refers to the 2-step process the author teaches: the relief grind, then finishing at a higher angle. Unfortunately there is a lot of unnecessary confusion with knife-sharpening caused by non-standardization of terms. The same word or term often means different things to different people. That's why photos and drawings are valuable.

Thank you. The terms are also often similar which is confusing.

The old engineering conundrum with material advances: if a new material comes along that's twice as effective as the old, do you use the same amount to make it twice as resilient, or do you use half as much to make it just as resilient, but with less bulk? The answer depends on the context of the application, and is usually somewhere in between.

Very interesting (and makes sense since the book is not that new).

 

[Sergeua]

See it's easier to make this chisel sharp again when you just have to touch up the microbevel versus the whole primary bevel.
It would take longer.
It's also easier to remove burr at this elevated angle.
But notice there is no changing of the angle at the bottom of the chisel. That part stays flat, otherwise you would have to raise the chisel to cut into wood.
So keep this in mind, when you work with wood the edge configuration on the left is best. If you cut meat and just want the edge sharp it doesn't matter and you might want the added strength.

I usually go for the one on the left for best performance, but because I freehand I might get a bit of a convexity and a microbevel or if I'm having tough time with the burr I will raise a bit to get it over with. Probably a bit of a strop at the end will increase the angle slightly also. But I still strive for perfection

 

[Tseg]

 

[FortyTwoBlades]

It's worth noting the profound difference between a microbevel and a macrobevel, as well. Many use the term "microbevel" to mean a smaller secondary bevel that comprises the apex, but it's more specific than that. A true microbevel should be so small as to be invisible to the naked eye unless deliberately looking for it under good lighting conditions. Most of Mora's outdoor and utility knives use a microbevel on them, and yet--because it's a true microbevel--most people don't even know that they do it. A microbevel should be set on a fully-apexed knife using only 1-3 light strokes per side on fine stone at a slightly elevated angle. It's really quite tiny, and thus deserving of the term "micro". If you can see it with the naked eye without having to deliberately scrutinize the edge, you've got a macrobevel.

Why this is important is that almost all materials, including pretty hard ones, do deflect somewhat under pressure, bringing the surface nearer to being square with the edge. With a true microbevel, the radius of the deflection is tight and slight enough that the presence of the microbevel has almost no discernable impact on your ability to ride the backing bevel as a guide when making cuts, as in the chisel example. To prevent a microbevel from growing into a macrobevel, during sharpening the edge has to be fully apexed again to remove the previous micro so a fresh one can be applied.

 

[numbersman]

I wanna hear more about the double edging! Purely for, you know, research purposes. However, isn't what the OP is inquiring about exactly what Spyderco suggests when using their SharpMaker? They call it a double bevel. I generally don't bother with it, but I used to. If I have to sharpen a very blunt knife I will usually do this, but for good knives, not so much.

 

[idanp]

Why this is important is that almost all materials, including pretty hard ones, do deflect somewhat under pressure, bringing the surface nearer to being square with the edge. With a true microbevel, the radius of the deflection is tight and slight enough that the presence of the microbevel has almost no discernable impact on your ability to ride the backing bevel as a guide when making cuts, as in the chisel example.

Very clear and informative, thank you.
Question: if you grind a primary bevel and then hone it on a ceramic rod once every few uses isn't this essentialy creating a microbevel? Isn't this better in terms of material loss?

To prevent a microbevel from growing into a macrobevel, during sharpening the edge has to be fully apexed again to remove the previous micro so a fresh one can be applied.

So basically, taking into account the time and number of sharpening sessions on a knife, we're weighing 1) relatively long sessions for a primary edge (only) against 2) very short sessions (microbevel) with a few relatively long macrobevel sessions once the micro- grows into a macro-bevel, is that correct?

I wanna hear more about the double edging! Purely for, you know, research purposes. However, isn't what the OP is inquiring about exactly what Spyderco suggests when using their SharpMaker? They call it a double bevel. I generally don't bother with it, but I used to. If I have to sharpen a very blunt knife I will usually do this, but for good knives, not so much.

I believe this blog helps explain the concept well (gain durability for an indiscernible loss of sharpness, which is also explained by FortyTwoBlades in his post)

 

[FortyTwoBlades]

Very clear and informative, thank you.
Question: if you grind a primary bevel and then hone it on a ceramic rod once every few uses isn't this essentialy creating a microbevel? Isn't this better in terms of material loss?

Except do remember that a chief reason for using a microbevel at all is for the increase in edge stability, which means running it without would risk edge damage.

So basically, taking into account the time and number of sharpening sessions on a knife, we're weighing 1) relatively long sessions for a primary edge (only) against 2) very short sessions (microbevel) with a few relatively long macrobevel sessions once the micro- grows into a macro-bevel, is that correct?

Microbevels turn into macrobevels very quickly.