So, CBN free abrasive sucks?

[RadialBladeworks]

Alright, alright, I admit it, it's a click-bait title, but there is some weird shit afoot here, gents.

Allow me to show you the incredible polishing power of... sixteen micron abrasive?

Above you can see my MDF strop loaded with Bark River 16u emulsion.

Why a hard strop, you ask?

Because at 16 micron, I am clearly sharpening the edge, so I want a strop with zero compressibility. If I were to use a soft strop, like leather, the abrasive will be unable to cut as deeply into the steel, which would defeat the purpose of using 16u abrasive, because the edge finish would be somewhere higher than that.

So anyway, my CS Drop Forged Hunter in 52100 was in need of a sharpening, so I hit the stones, took 'er up to 1k, then wanted a coarser finish, so I figured I'd use 16u CBN on MDF as a finisher. Well... check out my mirror-polished 16u edge?

Yeah, pretty weird. The pic sucks, but you can see my phone reflected in the edge, which should not be the case with a 16u edge. I was looking for a nice matte finish with a distinct scratch pattern.

Puzzled, I thought maybe I was experiencing some weird interaction between the strop substrate and the compound. So, against all rationale, I thought I'd better try leather, and maybe then I would experience more typical results.

Yep. Nope. Due to the compressibility of the soft strop, the polish was increased, as to be expected. At this point, it was actually a rather impressive mirror.

And it was keen enough to split hair root-to-tip, which appears to me to be more difficult than tip to root, as your edge is unable to catch on any scales. You don't seem to pull curls off in this fashion, however. Instead, the edge tends to bite in deeply and if you keep pushing, you just cut the tip right off. Flipping the hair around, it easily tree-topped the hair. So, needless to say, we ended up with a proper keen edge.

So I thought I might make some external comparisons. Close at hand was a nearly analogous diamond paste (likely polycrystalline, but unconfirmed) rated for 20 microns.

As you can see, a proper 20u finish. It is matte, with a distinct scratch pattern, and you can see I even raised a burr with the paste.

Below is the other side of the edge, mirror-polished from the 16u CBN, for more direct comparison.

But here's the thing: in the pic below, you can see the stuff does cut pretty effectively, even on a soft strop. That black on there is all from the CS Hunter. This makes me doubt any kind of burnishing issue.

I reset the edge finish on the "16u" hard strop, which did produce a coarser scratch pattern than on the leather. Still mirrored, but not quite as clean.

Essentially, I was seeing results somewhere along the lines of 1 micron performance. So I thought I would compare the results with a different brand of CBN in the approximate range of the 16u performance. I was out of 1u Schwartz, but I still have a full thing of .75u emulsion.

Well, the cutting speeds were definitely similar, with the .75 coming in a little slower. The polish was excellent. Excellent. If CBN does prove to be a poor low-grit abrasive, I will absolutely be picking up some high grit CBN as a finisher. Some 1u, some .5u, my mind is already swimming with the potential. Soon I'll be complaining about the shitty factory edges on scalpels haha!

So anyway, we got the CBN checked out. I then felt I needed to compare it to a different abrasive type in the 1 micron range, so I grabbed the Norton 1u MDF strop.

The diamond without a doubt cut far more aggressively. The steel swarf in the pic above was accumulated after 10 strokes, and I used noticably less pressure than with the 16u and even the .75u CBN. I couldn't determine any difference in the edge, so I didn't take a pic. I'm sorry, but my eyes are not good enough to detect that quarter micron variance lol

Finally, it was time to test out an abrasive with a wider range of particle allowance. Some quality green compound, freshly applied to an MDF hard strop.

The polish was a nice mirror, as is typical for standard ~.5u CrO

I then took it straight to the 16u hard strop, not even the soft one which would've polished even higher. The 16u CBN increased the polish off the CrO. It's difficult to see, but in the pic below, the polish is somewhat cleaner. I need an actual camera. The phone doesn't cut it for stuff like this. Either way, however, all of these polishes are very similar as far as edge finishes go. In normal lighting and not under my construction lamp, they all just look like nice mirror finishes.

Then I took it to the .75u emulsion and the polish was, at least in real life, visibly the best of the three.

So, like, what the Hell?

The mystery remains.

Is it mislabelled product?
Occam's Razor says yes. There is even precedent regarding BRK and mislabelling.

Or is CBN, perhaps due to its specific particle shape, a poor free-abrasive. But given a cursory comparison of extreme-zoom photos found on google, diamond and CBN particles look the same (to me, at least). I know that CBN is typically intended for electroplating on motorized sharpening implements, as it is a better high-heat abrasive than diamond. So is this simply a case of using the wrong tool for the job?

Ultimately, high-grit CBN performed its intended function extremely well, but the low-grit results are peculiar, to say the least.

What do you think?

 

[Obsessed with Edges]

Sounds as if this particular CBN (assuming it's not mischaracterized) might be breaking down or fracturing in use. This is also known to happen with SiC abrasives, which fracture pretty quickly and eventually trend toward a finer finish. In searching the web about friability of CBN, I'm finding there are mfrs producing different grades of CBN, some of which are engineered to 'micro fracture' (small pieces breaking off) or 'macro fracture' (larger pieces breaking off) in use. It keeps the cutting edges of the abrasive fresh and sharp, while at the same time the grit is reduced in size as it breaks down. Like some grades of aluminum oxide, which can also be engineered for varying degrees of friability, such abrasives can be made to start out fast-cutting, then breaking down into a polishing-grade finish. Other grades are designed to be tougher, in retaining their size/shape longer for consistently aggressive cutting, but not polishing as finely.

With minimally wear-resistant steels like this and 1095, CV, etc., if a coarser finish is desired on the edge, it's much simpler to just do that on a stone, and minimize any loaded-strop refinement afterward, which almost always tends to polish anyway. If doing any stropping at all on such steels with a toothy finish, I usually limit it to bare leather only, or a piece of plain paper laid over the stone.

 

[Dangerously]

There is even precedent regarding BRK and mislabelling.

We’ve replaced their honing compound with a fresh batch of folger’s crystals. Let’s see if they notice.

 

[RadialBladeworks]

Sounds as if this particular CBN (assuming it's not mischaracterized) might be breaking down or fracturing in use. This is also known to happen with SiC abrasives, which fracture pretty quickly and eventually trend toward a finer finish. In searching the web about friability of CBN, I'm finding there are mfrs producing different grades of CBN, some of which are engineered to 'micro fracture' (small pieces breaking off) or 'macro fracture' (larger pieces breaking off) in use. It keeps the cutting edges of the abrasive fresh and sharp, while at the same time the grit is reduced in size as it breaks down. Like some grades of aluminum oxide, which can also be engineered for varying degrees of friability, such abrasives can be made to start out fast-cutting, then breaking down into a polishing-grade finish. Other grades are designed to be tougher, in retaining their size/shape longer for consistently aggressive cutting, but not polishing as finely.

With minimally wear-resistant steels like this and 1095, CV, etc., if a coarser finish is desired on the edge, it's much simpler to just do that on a stone, and minimize any loaded-strop refinement afterward, which almost always tends to polish anyway. If doing any stropping at all on such steels with a toothy finish, I usually limit it to bare leather only, or a piece of plain paper laid over the stone.

Interesting perspective... admittedly, if that is the case, I think the product is absolutely ridiculous lol. I might try reloading some strops and looking at the edge after every single stroke. That being said, though, the stuff is truly useless as a low grit compound, even if that is the case, because the breakdown rate is insanely fast. I was noticing high-polishing after checking the edge at 10 strokes. Legit picking up some BRK 1u and .5u though. More playing around with that stuff is definitely in order

 

[RadialBladeworks]

We’ve replaced their honing compound with a fresh batch of folger’s crystals. Let’s see if they notice.

Next post? Gonna make some Folgers strops!

 

[Blues]

Next post? Gonna make some Folgers strops!

Speaking of grinds, if you use espresso the process will go quicker.

 

[RadialBladeworks]

Speaking of grinds, if you use espresso the process will go quicker.

Smart... typically I like an Americano edge for my EDC blades though

 

[Obsessed with Edges]

Interesting perspective... admittedly, if that is the case, I think the product is absolutely ridiculous lol. I might try reloading some strops and looking at the edge after every single stroke. That being said, though, the stuff is truly useless as a low grit compound, even if that is the case, because the breakdown rate is insanely fast. I was noticing high-polishing after checking the edge at 10 strokes. Legit picking up some BRK 1u and .5u though. More playing around with that stuff is definitely in order

It would be interesting to hear what Bark River says, if asked, about the intended performance and finish of this 16µ compound. In other words, if it was intended to be a polisher or a cutter, and on what substrates (mdf, wood, leather, etc)? That might lend some insight into what it's actually doing (breaking down, or not), if they use terms like 'fast material removal' (strong cutting) and/or 'high polish', or some combination of those traits.

Might also be interesting to see how it compares in (1) working speed AND (2) final finish, when compared to a same-branded CBN compound at a finer starting grit size (maybe 5µ or smaller). If it's a coarser compound breaking down into something finer, the working speed may be faster to a given finish than a finer-grit compound going to the same approximate finish. That might indicate some value in the larger compound, in enabling polishing on finishes that might be a little rougher to begin with, without having to do as tight a progression in advance prep work.

 

[RadialBladeworks]

It would be interesting to hear what Bark River says, if asked, about the intended performance and finish of this 16µ compound. In other words, if it was intended to be a polisher or a cutter, and on what substrates (mdf, wood, leather, etc)? That might lend some insight into what it's actually doing (breaking down, or not), if they use terms like 'fast material removal' (strong cutting) and/or 'high polish', or some combination of those traits.

Might also be interesting to see how it compares in (1) working speed AND (2) final finish, when compared to a same-branded CBN compound at a finer starting grit size (maybe 5µ or smaller). If it's a coarser compound breaking down into something finer, the working speed may be faster to a given finish than a finer-grit compound going to the same approximate finish. That might indicate some value in the larger compound, in enabling polishing on finishes that might be a little rougher to begin with, without having to do as tight a progression in advance prep work.

Well, I’m gonna shoot BRK an email, that’s a good idea. I just assumed it would be for coarser finishes, but maybe they are indeed intending it to be some kind of high-finish tool.

I’ll also be placing an order for some 1 and 0.5 micron grade of the stuff, but that update will not come for a while as I’m up in Canada.

 

[HeavyHanded]

CbN in a loose abrasive form will often align on its sides. In this case it becomes more a machining process as the edges cut instead of the corners. This should result in a brighter finish as well difficulty comparing one to one with more traditionally shaped abrasives. You would need to take a closer look at the scratch troughs and see what they're doing relative to each other.

 

[RadialBladeworks]

CbN in a loose abrasive form will often align on its sides. In this case it becomes more a machining process as the edges cut instead of the corners. This should result in a brighter finish as well difficulty comparing one to one with more traditionally shaped abrasives. You would need to take a closer look at the scratch troughs and see what they're doing relative to each other.

Fascinating! Thanks for the perspective, man. That definitely supports the proposition for high grit finisher grades then. Gonna be money well spent!

 

[ToddS]

CbN in a loose abrasive form will often align on its sides. In this case it becomes more a machining process as the edges cut instead of the corners. This should result in a brighter finish as well difficulty comparing one to one with more traditionally shaped abrasives. You would need to take a closer look at the scratch troughs and see what they're doing relative to each other.

Are you suggesting the CBN abrasive is cube-shaped?

 

[HeavyHanded]

Are you suggesting the CBN abrasive is cube-shaped?

More so than many other abrasive materials.

Not perfectly by any means but tends to have fewer sides, longer edges, larger facet surfaces.

 

[ToddS]

The CBN I've analyzed has all been sub-micron and very irregular shaped.

There are some images of larger grits in the link below. Maybe the 55micron is cubic-ish, but I'm not seeing it in the others. (the 55 is poly).

https://ceratonia.com/en/first-clas...synthetic/abrasive-grit-cbn-cubic-bornitride/

 

[RadialBladeworks]

To complicate the issue, I do feel like it's now appropriate to add that although the 16 micron CBN mirror polished, it did have a few errant scratches atypical of, say, a properly graded 1 micron compound. It behaved like 1 or 2ish micron compound with a couple lower grit particles to apply random deeper scratches on the more mirrored overall polish. This is why I am suspicious that it isn't a mislabelling problem, because then what would explain the random errant scratches? Even the cheapest abrasives I've used are quite accurately graded.

I thought that maybe the CBN particles were of such a shape as to have a tendency to roll. Or, as Heavy suggested, to sit flat, thus failing to offer the expected cutting action. So, I would be inclined to bet on something along this line, but the micrographs I looked at between CBN and diamond don't really look noticeably different to me.

Still no reply from BRK, so we'll see what they say. The high-friability theory also still stands.

 

[Obsessed with Edges]

With my curiosity piqued by this discussion, I've been hunting around on the web and looking at some different mfr's sites for CBN grit. Manufacturing seems to be going the way of aluminum oxide grit production, in that many, many variations of CBN grit are being engineered for for a range of shapes (within some limits) and for varied toughness, friability, thermal stability, etc. Some have pictures of different grades/types they make; of those, some are more blocky, some are more angular/sharp, and others look flatter and plate-like, with sharp edges. So, I wouldn't discount the possibility that each might behave in radically different ways, especially as a loose, unbound polishing/lapping abrasive, where they're allowed to move around and change their cutting orientation relative to the piece being worked. Most or all of the mfr sites seem to be geared toward marketing the product to mfrs of tools like grinding wheels, implying the grit will be firmly bound in some manner (vitrified, resin-bound, metal-bonded, etc). But as a loose, unbound grit, who knows(?) how they might work.

And then the big question, when a vendor such as Bark River or anyone else buys their raw CBN grit for use in an emulsion:
From whom are they getting it, and which choice of (possibly) hundreds or thousands of permutations in manufacturing are being selected for the product being sold? Seems like most anything would be possible.

Just one example, from a Chinese company, is linked below. I referenced this one simply because their pics (micrographs) show a pretty wide variety in the individual grain shapes of one of their product lines (called 'CBN mono-crystal'), presented in a centralized and concise manner.

http://cbnmaterial.com/1-1-1-cbn-mono-crystal/194423/

 

[HeavyHanded]

This is typical of most images I've seen.

and another showing process CbN:

And this typical mono diamond:

And this is pretty typical of the steel surface from CbN machining. I've seen images of pretty longish ribbons removed via CbN that have cleaner boundary edges and tend to be much longer than the ribbons removed via other abrasives. I'm not claiming night and day, but enough to create trends. I recall my father in law discussing CbN and how the viscosity of the slurry when used on a wheel really changed its cutting characteristics - too thick and it wouldn't align along its facets. Admittedly everything I know about it comes from other sources.

 

[ToddS]

From what I see, the various CBN grits are classified as "irregular" "semi-blocky" "blocky" and "very blocky"
It looks like 30 microns and below is typically "irregular" - I would speculate that the OP's 16 micron CBN is going to be irregular, not blocky.

This is a DMT fine plate (below) I'm not sure the CBN looks any more blocky than the mono-diamond on this plate.

Also, I would be astonished if we ever see the sort of pressure while stropping on wood that is required to fracture these type of abrasives.

Rolling grit is an interesting thing. In my experience blocky grit won't roll the way irregular grit particles do. Also, rolling grit = broken apex. This is why sharpening with mud/slurry avoids the formation of a burr.

 

[RadialBladeworks]

Well, still no reply from BRK.

Tentative conclusions: avoid low grit CBN.

Practical conclusions: avoid CBN free abrasive entirely, as we are talking about sharpening a god damn edge here, this isn’t rocket science, and therefore I have an extremely low “weirdness” tolerance. Diamond just cuts, no questions asked.

Tested a few different steels, similar results. VG10, PM A11, M4, K720. All the same scratch patterns. More wear resistant steels took longer to develop the scratch pattern.

I still like the high grit stuff, but having experimented a lot more with the .75micron CBN, I find that it doesn’t give me any results I can’t achieve on 1 micron diamond, and the diamond cuts noticably faster.

With the money, I would buy a CBN wheel in a hot minute, but I think I will just stick with diamond pastes for my own typical hard and soft strops in freehand use.

 

[ToddS]

People tend to focus on the grit size and hardness, but the shape of the grit plays a huge role in its performance. I would like to see these stropping compound suppliers give that information. From what I can gather, CBN above 40-50 micron is shaped for efficient cutting, but the smaller stuff maybe not so much.

The other issue is that a "mono-dispersed" sample of 16 micron grit (eg particle size distribution centered around 16 microns) will behave very differently from a sample of 16 micron and finer. The finer component will dominate the stropping behavior. Both could legitimately be called "16 micron" but without a PSD (particle size distribution) there is no way to distinguish them.

I think of grit on a piece of wood as an inexpensive way to make a high grit hone. Pennies worth of metal polish on a scrap of MDF has obvious appeal over a $100 stone - cheaper, no soaking, no lapping, no cleaning. However, putting $30 / ounce spray on wood isn't really economical compared to buying a high grit Shapton (or comparable), and the Shapton can be used edge leading.