When Is Grit High Enough?

[Rennd]

Does anybody know what grit it takes to get to a point where the particle size of the steel causes the abrasive the be useless in sharpening the knife any further?

 

[Obsessed with Edges]

So long as the abrasive's particles are hard enough to abrade the steel, it shouldn't matter anyway.

This is the basic assumption behind all the sub-micron abrasives in diamond, CBN, alumina or silicon carbide, which are all hard enough to abrade most any steel. Steels with very large carbides, like the chromium carbides in D2 (non-CPM variety in particular, which can have huge carbides in the tens of microns across), can still be refined at the edge to levels much smaller than the size of individual carbides. This is done by using abrasives that are often much smaller, but still harder than the carbides themselves. At such small grit sizes, the 'abrasion' taking place has the effect of refining/polishing/burnishing, as opposed to larger grit tearing out whole grains or clumps of grains, which would result in a much coarser edge. Assuming the abrasive gets smaller than the particle/grain size of steel, it should be shaping the steel's grains at that level (with some skill and care on the part of the sharpener, anyway). Abrasive particles larger and harder than the steel's grain size are more likely to tear out grains from the steel, as opposed to shaping or refining them.


David

 

[Rennd]

Really? Huh. Looks like I need to get some .0015 micron spray.

 

[Obsessed with Edges]

Really? Huh. Looks like I need to get some .0015 micron spray.

The real limitation most of the time, is in actually noticing significant improvement below a certain point. Other limitations of the substrate, variation in angle & pressure, the pre-existing level of finish (coarse & toothy, fine, shaving-capable or not, polished, etc.) and contamination by larger particles will start to seriously impact or completely negate any real benefit of going so small in abrasive grit. At that level, everything has to be absolutely clean & perfectly controlled to produce any real benefit from the compound itself. Has little or nothing to do with the characteristics of the steel itself, at that point, aside from the basic necessity that the steel can actually be abraded by the chosen compound.


David

 

[Rennd]

So, based on that, what grit is realistically as far as you need to go?

 

[Obsessed with Edges]

So, based on that, what grit is realistically as far as you need to go?

'Need' is entirely subjective, based on individual preference and what the edge will be used for. A true 'shaving edge', such as found on a straight razor, is usually said to be < 1 micron thick at the apex (at most), and often around 0.5 micron or slightly less. And with shaving, thin geometry (edge angle) has the greater impact anyway. If one doesn't actually need to shave with their EDC knife, great edges can be made at 1-2 microns thickness; even those can 'shave', albeit somewhat coarsely. Depending on the abrasive, substrate, skill & technique used by the sharpener, these edges can even be made with compounds that are larger than that, up to ~10 microns or more. A larger compound on a softish substrate (like leather) will abrade at a level far smaller than it's stated grit size. As the substrate under the grit becomes harder/firmer, the grit will cut more aggressively (digging deeper); therefore performing closer to it's stated grit size. This assumes the compound is still firmly embedding in the substrate. Things get kind of unpredictable if the compound doesn't embed, and is free to move around between the blade edge and the substrate.


David

 

[Lorien]

I can usually tell by the empty bag of Doritos and the short term forgetfulness.

 

[Rennd]

So, you would say .1 micron is plenty high for most purposes, other than a toothy working edge?

 

[ksskss]

David, my compliments on some EXCELLENT answers! Really hard to add anything else.

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Ken

 

[Obsessed with Edges]

David, my compliments on some EXCELLENT answers! Really hard to add anything else.

---
Ken

Thanks Ken. :thumbup:


David

 

[Obsessed with Edges]

So, you would say .1 micron is plenty high for most purposes, other than a toothy working edge?

To give my own perspective, I'm perfectly happy and sometimes even thrilled with edges I'm stropping using 2-5 micron compound (<-- edited: see my following edit in post #14). I routinely 'test' these edges by dry-shaving the fine hairs from the back of my neck, with no irritation. That suits everything I need from my knives, just fine.


David

 

[Rennd]

ksskss said:
David, my compliments on some EXCELLENT answers! Really hard to add anything else.



---

Ken

I must say, I agree. I had no idea there were people out there who knew this much more than I do!

 

[Rennd]

Obsessed with Edges said:
To give my own perspective, I'm perfectly happy and sometimes even thrilled with edges I'm stropping using 1-3 micron compound. I routinely 'test' these edges by dry-shaving the fine hairs from the back of my neck, with no irritation. That suits everything I need from my knives, just fine.


David

Really? That's interesting. What do you use to sharpen before you strop?

 

[Obsessed with Edges]

Really? That's interesting. What do you use to sharpen before you strop?

A little of everything, from diamond hones (DMT C/F, most of the time), to wet/dry sandpaper (usually finishing anywhere between 320-2000+ grit), and even on my relatively coarse Norton 'Economy' stone bought at Home Depot. Most of that is for setting bevels and establishing the basic geometry (usually 30° inclusive or less). Once that's done, I go to hard-backed strops of paper/fabric on plywood with some Ryobi 1-3 micron 'White Rouge' compound* (also found at Home Depot). That takes care of most of the refinement and follow-up maintenance as needed. Sometimes, but not always, I'll give them a few passes on a bare leather belt, mainly just to clean up any edge debris created in previous steps. I was using some 0.5 micron green compound (chromium oxide) quite regularly a while back, but I've noticed I don't even feel much need for that, anymore. The bigger difference, for me, was in finding a great combination between the plywood-backed paper/fabric strops and the white Ryobi compound. That's what I earlier hinted at, in emphasizing the importance of the stropping substrate, versus the compound by itself. The combination of them working together is what can make a radical difference.

* EDIT: The Ryobi 'White Rouge' compound I'm using is actually spec'd as 2-5 micron (as marked on the packaging).


David

 

[Rennd]

That's kinda funny, that's almost exactly what I use.

 

[ksskss]

"What do you use to sharpen before you strop? "

For me this literally has hundreds of answers. Echoing David's comments, it is very task specific. So for instance, I might decide to go only to a 4k stone and switch to a strop using 4 micron CBN or even a 1k stone and begin stropping with a 16 micron CBN strop.

On the other extreme, I might go to a 30k stone and switch to a strop with 0.5 micron CBN or polycrystalline diamond.

Or I might go through a sequence of Japanese natural stones and stop at anything from an Aoto or some type of nakado like an Igarashi and switch to a 4 micron CBN or a Nakayama Iromono and switch to a 0.25 micron CBN or Polycrystalline diamond.

What choice I make will depend on what I'm sharpening - either a carbon steel to bring out contrast between the soft and hard steel (jigane and hagane) or say an extremely abrasion resistant steel, etc etc etc.

I customize my 'lineup' in a task specific way.

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Ken

 

[mycough]

That's kinda funny, that's almost exactly what I use.

Try .25mic. Smaller than that can be tricky. There is a strong possibility of developing a wire edge, especially with the wrong substrate. Russ