Cotis v Synthetics

[mbrossar]

I've been re-reading posts and tutorials on honing in hopes of improving my understanding of what's going on in hopes of improving my results, and something strikes me as I compare coticules and synthetics. BTW, I have a Coti/BBW combo and a 3K/8K Naniwa combo.

It strikes me that honing with a synthetic like a Naniwa, you're actually creating about as sharp an edge as you will ever get on your first step (e.g. bevel setting). Then, you are progressively polishing out the scratch marks of the previous steps, making the edge smoother. So the trick here, once the bevel is set, is to know when the scratches from the previous step have been replaced with the scratches on the current step.

Conversely, when setting a bevel on a coti, because of the high concentration of loose garnets in the thicker slurry, the edge of the blade is actually quite dull. The successive steps on the coti (i.e. dilution) is geared more at diluting the garnets thus allowing the edge to get sharper/keener. So, the trick here is, to know when you've reached maximum keenness at any given dilution of slurry.

Honemeisters, would you say this is an accurate summarization?

 

[chess1]

When you hone never pay attention to the bevel scratches. will not help you on long run.
Try to see progression on the edge of the blade.
Lets say.
you have used course stone in early setting bevel level.
you can make blade shave ready without removing that scratches.
Basically you can see scratches from bevel setting level on finished blade.
i cannot say anything about Naniwa's haven't tried ..
Every single stone has it is maximum.
you get that maximum and move on to next level.
As soon as you learn this process in any hone you will be in good shape.

 

[SliceOfLife]

I think that's pretty much jibberish. I'm of the thought that there are two problems with razor edges. Either it's dull or it's damaged. The old "wouldn't want to shave with broken glass" argument. This whole "sharpening vs polishing" debate seems like some honers trying to sound smarter than they are.


Honing improves the edge in two ways. First, viewing the edge as a mountain range sitting atop the bevel, it reduces the size of the mountains. A more uniform, less damaged edge cuts better (think of a chefs knife vs a bread knife) and irritates less. However, the primary determinate of cutting is a combination of the maximum width of the edge itself (at the edge, this is measured in microns or perhaps even fractions of microns) and the rate at which the edge widens as it moves into the bevel (matters only up to the width of what you are cutting), that's just the grind angle. The depth at which the stone you are honing on cuts is what affects that. There are other variables which actually get very complex having to do with how uniform the cuts are, how they are aligned and spaced, how fine they can be before the metal is no longer durable enough to survive performing its function, etc.

Make no mistake, a tool sharpened on a 1000 then 4000 grit stone will be much sharper than one sharpened just on the 1000.

 

[Bart]

I've been re-reading posts and tutorials on honing in hopes of improving my understanding of what's going on in hopes of improving my results, and something strikes me as I compare coticules and synthetics. BTW, I have a Coti/BBW combo and a 3K/8K Naniwa combo.

It strikes me that honing with a synthetic like a Naniwa, you're actually creating about as sharp an edge as you will ever get on your first step (e.g. bevel setting). Then, you are progressively polishing out the scratch marks of the previous steps, making the edge smoother. So the trick here, once the bevel is set, is to know when the scratches from the previous step have been replaced with the scratches on the current step.

Conversely, when setting a bevel on a coti, because of the high concentration of loose garnets in the thicker slurry, the edge of the blade is actually quite dull. The successive steps on the coti (i.e. dilution) is geared more at diluting the garnets thus allowing the edge to get sharper/keener. So, the trick here is, to know when you've reached maximum keenness at any given dilution of slurry.

What you say sound logical to me, but you're overlooking something. Most edges produced by a lower grit synthetic hone, that I've ever looked at under a microscope, carried a sort of sawtooth pattern. These micro-serrations lend an edge a great deal of power to penetrate the material being cut. The serrations act as spears: all the cutting pressure is delivered at a few spikes, instead of spread over the larger contact zone of a smooth edge.

That gives, e.g. a 1K edge, a lot of performance and often it's fairly easy to pass a HHT at this level. But these micro-serrated edges are not very easy on the skin, and that's why such an edge requires further refinement. Newbies who pay close attention often observe how an edge seems to initially loose sharpness when they've started working at the next grit. That happens because the teeth are formed by the higher ridges of the scratch pattern, while the lower grooves form the recesses in between the teeth. The finer hone initially reduces the ridges, hence reduces the teeth, and one ends up with the less keen line at the level of the recesses.
Sharpening is not entirely the same as merely removing scratches. Allow me an example: a good razor edge has no teeth and both bevel sides meet at the cleanest possible line. This line is typically about 0.5 micron thick. That's not a number I'm making up. It was measured by scientists in studies about "sharpness".
A 1K hone has a particle size of about 12 micron. Of course, the particles penetrate the steel only partially, but even if the particles penetrate only 1 micron deep, they will cut right trough at the thinest part of the edge, where it becomes less than that 1 micron thick. The teeth can be smaller, because they're basically fragments of "left over" steel, but the recesses where a particle passed through, will have a median size of about that same 1 micron. Imagine we would now use a progression of increasingly finer hones, only to merely remove the ridges. We would end up with a completely polished bevel that has a tip width of 1 micron. Good, but not good enough for shaving.
It seems generally accepted that you can hit a sufficient "recesses width" at about 4K, after which a mere polishing would suffice to make the edge smoother. But it also depends on the hone, and the pressure you use for sharpening, because that pressure plays an important role in how deep the particles are going to scratch the steel.

The Belgian whetstones do not cut with spiky particles but with larger faceted spherical "garnets". Because of their width and shape, and the way honing pressure spreads out, these garnets tend to leave wider yet shallower scratches. And when present in a slurry, they also have a slight rounding effect on the tip of the bevel. The result is an edge that carries no teeth, hence without the performance gain of a micro-serrated edge. That's why Coticule honing is a matter of refining till the very end. The typical smoothness of this type of whetstones rides along for free.

Kind regards,
Bart.