grit size, lapping stones, and the finish they provide

[bgentry]

I'm not trying to pick on Knife Outlet here. I just thought this discussion should be continued and it was off topic from the original thread, so I made this one.

In another thread Knife Outlet said:

Sorry, if you are trying to say that you lapped a stone and it made the grits smaller or provided a better polish without burnishing, then you simply don't understand what is going on. You keep thinking I'm fighting you over your observations. I'm not. I'm trying to explain the what is behind your observations.

Originally your argument was:

Lapping a Sharpmaker stone won't change the grit size. It may smooth the stone so that it cuts slower but it won't really affect the sharpening job to any meaningful degree. If you want less abrasion, get smaller grits.

You now seem to believe that lapping has more of an effect than "just cutting slower", as later you attribute the results you saw to a "burnishing effect".

So a lapped ceramic stone *does* polish the edge differently then right? If so, it's a distinctly different sharpening tool than the original unlapped version. It does a different job.

As for the mechanism behind it, I guess it really doesn't matter, but I have my speculation based on what Sal of Spyderco said in a thread over there.

http://www.spyderco.com/forums/showthread.php?t=31188

About half way down the page, Sal explains that *ALL* of the spyderco ceramic stones (not the duckfoot, the stones) use the same grit size! Further, he explains that the UF stone is actually a "surface ground" fine stone. AKA lapped. Later he says he had made his own UF stone back before they were available, by lapping a fine with a DMT diamond stone.

How can this be? I can only speculate as I'm not a materials expert. Hell, I hardly know anything at all about that field. I'd throw out a guess that the carrier that is binding the "abrasives" (the synthetic sapphires) is actually abrasive itself. Maybe when they "surface grind" the fine stone, the carrier gets smoothed out and becomes a finer abrasive. Like I said, I'm just guessing.

There is a very obvious difference between the Spyderco medium and fine; I don't think anyone would argue that point. But they use the same grit size. I don't have a UF, but those that do seem to think it's finer than the UF. Carrier, firing (Sal's words), and apparently lapping all have an effect on the ceramic stones' cutting or polishing abilities.

Discuss.

Brian.

 

[David Martin]

Gentry, I don't know much on lapping as well. However, I've been lapping stones for about a year. First to level, then I noticed a difference in the edge the lapped stones left on the blade. So, when I read Sal's comments on that topic, he helped to fill in the blanks. Yes, I think (and am gaining more evidence) it does make a difference in the fineness this imparts to the edge.
I've noticed this is the second very knowledgable persons writings some will dismiss. But its a free country. DM

 

[hardheart]

Lapping the stone does not reduce the grit size, but it does make a finer polish. It's the same as using a file or using a steel. They're the same material, and the composition and grain size is the same, but you get a different finish. I don't know what the original thread was, so I can't quite see where the disagreement stems from. The alumina is still alumina, so has the same cutting power. The surface is in a different condition, so the finish on the edge is different because of how much is exposed. Kinda like using a coticule. Raise a slurry and the edge will be covered in scratches. Use it with plain water and it's a mirror polish. The garnets are not friable (always the same size), there's just less exposed when not rolling around in the slurry.

 

[Brutus013]

Could you make the surface less polished in order for the Sharpmaker ceramics to cut faster?

 

[hardheart]

I think so, some razor honers tried to lap the brown medium stones and said they ruined the surface (meaning it not only didn't get finer, it also didn't return to the original level either)

 

[bgentry]

I don't know what the original thread was, so I can't quite see where the disagreement stems from.

There were (at least) two different disagreements in the thread. The one about lapping starts at post 27 or 28 of this thread:

http://www.bladeforums.com/forums/showthread.php?t=686185&page=2

The alumina is still alumina, so has the same cutting power. The surface is in a different condition, so the finish on the edge is different because of how much is exposed. Kinda like using a coticule. Raise a slurry and the edge will be covered in scratches. Use it with plain water and it's a mirror polish. The garnets are not friable (always the same size), there's just less exposed when not rolling around in the slurry.

Some of that makes sense to me, but I didn't even know what a coticule was until I looked it up so I obviously lack experience in this area.

Brian.

 

[hardheart]

Funny how a thread discussing steeling so much ignores it when discussing lapped stones. Just like I said, steels versus files. Lapped ceramic versus unlapped.

I still don't see how 8000 mesh diamond is going to produce an edge the same as 8000 JIS. The particles in the waterstone are half the size. Not only that, they are a different shape, and the diamonds are the ones that cut deeper because of the cuboid shape. Once you reach a level where the scratches no longer are visible because they are smaller than the wavelengths of visible light, you can't say which is finer from images. Then there is the mechanism by which the stone abrades metal. The diamonds are most certainly not as fine an abrasive as the waterstone particles, but there has to be something to the way they are mounted to the plate as opposed to be in a binder. Or, the diamonds have been reduced in size when 'broken in'. Perhaps not just the loose diamonds being removed, but the remaining diamonds being cleaved.

And I can't agree with Fred about the same size particle leaving the same finish, for the reason I just mentioned. A particle that is almost round will not cut as deeply as one that is cubic or cuboid, and particularly not like one that would resemble a pyramid. The corners cut, and the more acute the angle, the deeper the cut. Also, different particles have different hardness/ability to cut into the steel. And some break down, you start with one size, and finish with something smaller.

 

[Jason B.]

I will correct myself now on two things, first I had 8000 stuck in my head when really its more like a 4000 grit finish, thought with some steels my 8000 grit waterstone and DMT xxf do produce very similar finishes. I sharpened some INFI the other day and for whatever reason the DMT XXF was leaving the finer finish than the waterstones but when I sharpen ZDP the waterstones win.

Second, the picture I showed in the last thread of the finish that a unlapped stone produced was not that great, I found a picture of the finish my UF ceramic produced before lapping. This is the finish my UF ceramic produced before going back to spyderco.

An this is the results after it came back.

 

[FlaMtnBkr]

Knifenut:

What did Spyderco do to your stone?

 

[Jason B.]

Re-finished it because I was unhappy with its original surface finish, I felt the mill marks were effecting my sharpening and I also suspected that the surface was not level.

 

[jimnolimit]

Lapping the stone does not reduce the grit size, but it does make a finer polish.

i don't know if this is true but here's an idea :

the diamond particles are harder than the ceramic stone particles , because of this the diamond particles might be sheering off smaller bits of the ceramic particles creating a "finer" grit .

 

[Jason B.]

i don't know if this is true but here's an idea :

the diamond particles are harder than the ceramic stone particles , because of this the diamond particles might be sheering off smaller bits of the ceramic particles creating a "finer" grit .

He's not talking about what you think he's talking about.

 

[jimnolimit]

He's not talking about what you think he's talking about.

i understand that he was talking about shape effecting finish , i was just throwing out an idea for the topic of why lapping stones might create a fine grit . i will change the quote i used in my previous post .

 

[hardheart]

This may be part of the effect, but the diamonds used to lap the ceramic hones are generally much, much larger than the ceramic abrasive. Like, 10 to 40 times larger for DMT coarse to extra extra coarse. Even the extra fine diamond size is still a couple times bigger.

 

[John Frankl]

How might lapping on a DMT XFine ruin the stone? If diamonds are harder, won't they just cut the ceramic without any damage?

Sorry for my ignorance.

 

[John Frankl]

"This may be part of the effect, but the diamonds used to lap the ceramic hones are generally much, much larger than the ceramic abrasive. Like, 10 to 40 times larger for DMT coarse to extra extra coarse. Even the extra fine diamond size is still a couple times bigger."

I also find this very interesting. I am not disagreeing with you, but I am lapping my UF right now with a DMT course, and, under magnification, the finish I am laying down now is much finer than the one from the factory(?)

Any ideas?

 

[RobbW]

Does anyone know the grit sizes Spyderco uses to hone their Sharpmaker sticks? I've used DMT hones and Lee Valley lapping powders to "refresh" the surface of my Sharpmaker hones. I assumed that I was cleaning the Sharpmaker hones of built up gunk, but never considered that I might be changing their polishing/cutting ability.

 

[Jason B.]

Does anyone know the grit sizes Spyderco uses to hone their Sharpmaker sticks? I've used DMT hones and Lee Valley lapping powders to "refresh" the surface of my Sharpmaker hones. I assumed that I was cleaning the Sharpmaker hones of built up gunk, but never considered that I might be changing their polishing/cutting ability.

Spydercos medium and fine stones are formed and the UF is milled. Techcically you can't "refresh" the medium and fine stones.

 

[John Frankl]

"Spydercos medium and fine stones are formed and the UF is milled. Techcically you can't "refresh" the medium and fine stones."

I'm not sure about the medium, or about refreshing, but you could certainly do exactly what Spyderco does, and turn the Fine into an Ultra Fine.

 

[Knife Outlet]

I'm not trying to pick on Knife Outlet here. .

You would have fooled me.

Try to imagine in your mind what would happen on the surface of an abrasive if it were lapped or milled. Do you think the abrasive particles would break up into smaller pieces and stay connected to the sharpening tool? That isn't possible, obviously. If you broke up the particles, they would detach from the matrix. The only thing that can possibly happen is that the particles become smoother by cutting off their tops. They are the same size - perhaps a little larger, in fact, because the parts that were cut off were likely of smaller diameter than the parts that were left behind. I hope I'm explaining this reasonably well.

If you accept that the abrasive particles can't be smaller after lapping, then explain what would cause them to leave a shinier finish on the edge. Do the particles "scratch" less because they are smoother? If they do, then how would the scratches be smaller since the particle size is the same or larger? The fact is there isn't any way that I can imagine for this to happen. Physics and geometry don't allow it as far as I can see.

Perhaps you have some experience with leather stropping. If so, you know that it is possible to drag a sharp edge across the surface of a piece of bare cowhide and see an increase in shininess. No abrasives, raw cowhide. What causes this? Is it smaller scratches? No, it isn't scratches at all. The leather can't cut the steel because it isn't hard enough to cut it. What causes it is burnishing. The steel is actually pushed into a shinier state rather than being cut into a shinier state. If you have ever seen a cabinet scraper, their edges aren't caused by grinding or abrasives at all. They are pushed into shape by rubbing them against another piece of steel.

Consider steeling as is common with kitchen knives. When steeling is done properly, there is no abrasive action at all. The deformed edge is straightened by pushing it into a straighter shape. I usually steel using a pot lid. Pot lids don't cut steel. They can only push it around a little.

So now back to the lapped abrasive. If we can accept the fact that the abrasive grits in lapped stone can't be smaller but just smoother, then it isn't hard to see that they will cut more slowly. They have to. They have had some of their aggressiveness removed. Diamond stones do this all the time. They lap themselves and get less agressive because the abrasive particles are permanently attached to the matrix. Do the particles of a lapped stone continue to cut? Sure they do. We saw an image in the other thread of a lapped stone with steel particles on it. But if the grits are the same size, how does the surface get shinier? BURNISHING! The abrasive surface is smoother. Most of the change in the finish comes from pressure, not from cutting - just like it does from stropping (without an abrasive) and from steeling (without an abrasive.)

How does burnishing help the edge? Pretty simple. In the first place it can straighten a warped or rolled edge and that makes the edge cut better. Secondly, it polishes the edge and that reduces friction between the blade and the object being cut. That improves the blade's efficiency. I'm not against burnishing at all. I'm trying to explain the physics behind what is going on in the sharpening process. Lapping an abrasive can't make it cut any way other than more slowly. The effect of using a lapped abrasive is a transfer of some work between cutting and pressure - between grinding and burnishing.

Whew! That about wore me out. More confused now?