cryogenic drill bit life extension

[KL27x]

Who says what works better with a burr? The burr is a side effect, not the intent. And in most cases, it is detrimental to have it.

eKretz, I can't find it. But in 2016, I watched a YT video tour of the Dovo factory. The guy who recorded it was a razor shop owner and distributor of a variety of razors and accessories. He was particularly curious about how they sharpened them, since he sold a variety of stones and such.

At that time, the razors were hand sharpened on a 12" long coticule that was shaped nearly like the top say 20 degrees of the arc of a football. A big hump in the middle. He got plenty of footage of a lady sharpening a razor with it. Draw strokes base to tip from the front to about the mid point of the stone. Then left and flip and do the reverse starting at the back end of the stone.

I know the fine layer is only so thick on a coticule, but it was curved enough to use up near all of that layer on the corners of the rectangular-footprint stone. He asked a lot of questions, and they informed them that the sharpeners shaped it like that, intentionally, and they would have to touch it up so often to maintain that shape.

So the coticule is hard enough to be used like what I'm calling a hard or non-lapping/slurrying stone, as well as the way I'm guessing you are using it. According to this shop owner, he remained in contact and was told that they later switched to similarly shaped Arkansas stone, because it required less maintenance, so overall it saved time/money. I don't know how old the video was, so if this is correct then Dovo must have made the switch in 2016 or earlier.

Dovo is apparently the oldest razor company still in existence that has been in continuous operation since it started in w/e century that was. According to the info this guy got on the tour, this is the way all razors have ever been sharpened in Solingen. At one time, it was claimed this was considered trade secret information.

[coppercone2]

I don't know if he is saying that the operation to remove the bur with a stone destroys the edge by chamfering and its impossible to remove as a post process of 'factory sharpening' or if the bur itself is helpful to the cutting operation (mc###)

it makes me think about grinding and burnishing and if this is an application where finishing might need to be conducted with a unconventional material type (I see there are CBN, AlO2, VC (rare? I swear I saw someone selling vanadium or something grinding wheels), SC and other types of refractory ceramic wheels out there). Maybe one of them smears less during final removal. I always wondered what the odd ball ones were for.



based on what he says it seems like you want to cool it down. I kind of wonder if they sharpen the CNC drills that have holes in them while pumping LN2 through them.

[KL27x]

I don't know if he [mcxxx?] is saying that the operation to remove the bur with a stone destroys the edge by chamfering and its impossible to remove as a post process of 'factory sharpening' or if the bur itself is helpful to the cutting operation (mc###)

You might want to address that more specifically. Quote some section for context or address mc172. Your posts are otherwise very confusing.

mc172 just misunderstood what was intended by words that can be ambiguous and concepts so insignificant to everyday life that only dweebs like us are still talking about them.

Regarding that short video:
Great, but I think I know a bit more about burr formation than that single two concepts. Maybe I'm wrong, but to me it is so far a pretty obvious truth. The greater the area of contact with the stone, the greater the bur formation. So on the bench grinder, if you grind a chisel to a hollow grind to match the shape of the stone, and slide the chisel only left/right across the face of that stone, you will get a massive burr should you continue grinding all the way to the edge. Even on a wet grinder. This is my contention, anyway. Now if you follow that up by increasing the angle and touching that edge to the exact same stone, you will remove that burr and produce a very much smaller one.* This is presumably because there is less surface area of contact which leads to less burnishing. This is my contention.

You might think that at one point in time at least one other person made this connection. Seeing that "burr" and "burnish" are etimologically related.

*This is not because of the more acute angle. Firstly, this crazy burr will form in either grinding direction, but worse if on the trailing edge. And it will be proportional to the area of contact or wideness of the area of the bevel in simultaneous contact with the grinding wheel. Even if you go and put a 45 degree angle on the sole of the chisel, then come back and grind that deep hollow by pressing that entire bevel across the stone, the crazy big burr will form despite this much more obtuse edge angle.

Another way to reduce the burr formation would be to round over your grinding wheel or reorient your fence to favor one corner of the wheel (then the other, then back to the middle.... ending up rounding over the wheel through use). So that when you run the chisel across the wheel, a smaller slice of it is against the stone. You might think that it should be the same, since that area of contact is still the entire hollow grind, just a smaller slice of it. But it does matter. This will produce less burr, because the abrasive can cut more efficiently. The bigger the area, the more teeth are trying to get their bite. But there are so many going for this bite at the same time they can't quite get their teeth sunk in. And they burnish, instead.

Added to the above: It's not that the metal that was displaced by the burnishing comes from all the way at the back of the bevel to the front, as if you get a cumulative deposit of that area of steel to that section of the edge. This burr forms in every direction where there is a sharp edge; let's say an edge of around 90 degrees sharp or better. And the burr formation is essentially a result of the total area of contact (for that particular material on this particular wheel). So for instance, if you were to cut a few grooves into the bevel, parallel to the edge (and you perhaps widened the blade just a skosh to make up for the loss of surface area), and then you were to grind it by holding the entire bevel against the stone in a perfect mating fit, that nasty of a burr would form on each of those additional edges you created at the ledges of those grooves you cut. Essentially. Holding other factors the same, which might not be the practical case, all of those edges will get the fully bad burr, not a respective fraction of it.

[coppercone2]

this is solid because most people on this forum are literary worrying about a handful of electrons that don't even stay where you want them unless you are paying for electricity

this bur will last without power, its a serious problem, it probably won't rust off either. at least you can turn a meter/computer off and the ram eventually resets

[KL27x]

It usually won't last too long. It's more a matter of:
1. Does it do damage when it gets ripped off
2. (Flash Bonus Round) Does the size of the burr indicate the quality of the edge, in and of itself.

I suspect the answer to the first question is yes, but it might not matter much.
I suspect the answer to the second question is yes. With grinding/filing, I believe the quality of the burr has corelation to the quality of the edge. I believe burnishing causes the burr, and I believe that burnishing changes the structure of more than just the very surface of the steel. I believe it might cause damage, like mashing the top of the tree to the point some of the roots are disturbed.

I don't know if lapping produces a superior edge or not. But I think lapping tends to remove the burr as it forms, so it might not be a reliable indicator in this case.

The burr can stick around for awhile with say a razor. When using a razor, you don't slice into anything in a way to rip the burr off. In many tools it might not be as critical. It has long been repeated that butchers leave the burr on because it slices meat better. This is older than I am, and I don't know if it's even worth repeating. To me that's just eventual metal flakes in the meat.

[eKretz]

KL I am tired and therefore don't feel like selectively quoting your post, but on the subject of the stones beginning to sort of "skate" and burnish as they smooth out - or "glaze" if you will - you'll brook no argument from me. I find this to be true in many instances. There are ways around this as well however - one, as you've already mentioned, is using a slurry. Another is to keep the surface area of the bevel very small so that applied force per unit of area (local pressure) is relatively high. Another is to create a "smiling" edge on the razor,  which accomplishes pretty much the same - and mimics your convex stone approach - without needing a convexed stone. There are more.

On the subject of the strop and lingering abrasive pastes: none have ever touched my plain leather strop. I use separate strops for any abrasives and clean my razors with acetone after using them with those strops before they go anywhere near my plain leather strop. Precision lapping in the machine shop taught me to be very picky like that so as to prevent cross contamination. My plain leather strop has no sign of any black residue on it, and it's nearly a decade old.

[KL27x]

On the subject of the strop and lingering abrasive pastes: none have ever touched my plain leather strop. I use separate strops for any abrasives and clean my razors with acetone after using them with those strops before they go anywhere near my plain leather strop. Precision lapping in the machine shop taught me to be very picky like that so as to prevent cross contamination. My plain leather strop has no sign of any black residue on it, and it's nearly a decade old.

Yes, you surely are making a very good edge. I have no argument that lapping doesn't make a good edge. Grinding/filing also makes a good edge, but this is where you have to pay attention to more of the details which you and I have mentioned. It's only here where you will see the true extent of a burr and the negative consequences of creating fat burrs to edge quality. The finer the grit of the stone, the less amount of surface area you can get on there without burnishing.

I'm not saying it's the only factor in burr formation. Type and hardness of steel matters. But for the given material and given stone, the thickness/speed of burr formation is related to surface area of contact of the bevel to stone. If you pay attention you will find this to be true. So we (at least I) know more than "burr formation is mysterious and not well understood."

This is conjecture, but I find it intriguing that an entire city thriving on razor and knife manufacturing with a height/boom lasting several decades sharpens their stuff on a hard curved stone and also managed to keep that a secret at least well enough to want to try to keep that a secret. And now that we live in the internet age where everyone has a cell phone, social media, and forums, this knowledge is still basically secret. If you wanted to learn this method of using that "hard" stone in your collection, the one that seems to be completely worthless unless the goal is to make an impressive burr and a crappy edge, you would probably not find it. You will only find a couple redneck stressing how important it is to lap this hard stone perfectly flat. And then to dress it will a diamond plate every few uses to get it to do anything at all useful. It will still be miles from peak efficiency in this weird practice which is obviously not how these stones were used for the vast majority of human history. We didn't have diamond plates way back when.

[eKretz]

Yes that is true. Theories abound, but there really are no definitive answers. This is partly because there are so many variables involved, as you've alluded to. If there were a definitive solution we wouldn't be having this conversation, because burrs on cutting tools wouldn't exist!

[KL27x]

we wouldn't be having this conversation, because burrs on cutting tools wouldn't exist

I agree with one thing in coppercone's vid, though. Where the guy says we can reduce it but not completely eliminate it. That's the goal, to reduce it. And if you want to understand how/why they form (or at least understand the factors that affect it) you have to keep track and pay attention to those variables, not just throw your hands up and declare voodoo.

Where you get the point you can wipe the burr off on your palm or plain leather strop like you do, it's indicative of a good edge.

Well, with the one caveat. If lapping, there are some lapping stones that overdo it. As I said, I have one that is so muddy, you can't get a sharp edge; no burr, but also no sharpness. I'm sure there are muddy stones with the right amount to not overdo it. With the coticule, I have read how one afficionado actually titrates the slurry with dribbles of fresh water as he sharpens. His personal preference was to leave it more concentrated to start with and to dilute the slurry for the finish. Dunno if that's just sharpener version of "hint of oak and undertones of vanilla" or if it's actually making a practical difference.

I have no dog in the fight. I wanted to learn how to use the old arkanas stone, and I eventually did. I know for sure that flat lapped coticule and proper synthetics work great. But IME having used them both, grinding is faster than lapping, when you sort it out. And much less maintenance of the stone. So what started out as curiosity and perhaps some romanticism of old fashioned stuff and to some extent "lost" knowledge taught me something completely practical. And it is something that is really not on the web. It's crazy just how much this info is not on the web.

One of the cool side effects about using the shaped ark stone, the oil doesn't wipe off. On a flat stone, with a flat edge, you end up squeegeeing off oil (or water if you like), and the stone dries out faster. Using the curved stone, the layer of oil stays on the stone. I don't like to stand still, and I don't use a bench when sharpening a knife or razor. I just pick up the stone and hold it in my off hand and can walk around or even sit in a comfy chair without carrying also a bottle of oil and having oil dripping everywhere. I've used a stone multiple times over weeks without wiping off and adding more oil. Just when it gets too dirty.

Another is to create a "smiling" edge on the razor,  which accomplishes pretty much the same

Yes, it does. But there is a fundamental difference. If you sharpen a smiling edge on a flat stone, what spot on this flat surface do you run the contact patch over? You can't get too close the the edge, cuz then you will "fall off" your plateau and dig the blade into the corner of the stone. Anywhere you do it you will put a dip in the surface. And when this dip forms, it will tend to wear faster. Cuz when the blade gets over the dip, it will wear the stone when it run over the back end of the dip. This is in some way related to the reason why perfectly flat stones (and uniformly concaved ones) are rare to find in nature. But very smooth and regular convex stones are plentiful. I'm sure there are other reasons for that, of course. (Like every object is essentially 360 degrees in cross section after you add it up, so more of it has to be convex than concave, lol.)

This dip/wear thing is the reason my stones are slightly convexed in the long axis as well as side to side. It doesn't necessarily improve the current performance. The main reason to ensure continued performance. If you start with the stone straight/parallel in that axis, you will form a dish in the side of the stone. (You are always favoring the side of the stone a bit, because that's where you get the best curve/radius, at least until you wear the stone way down into a dome, and also because that's the only place you can get the heel of the knife/razor onto the stone. Then you flip it occasionally and use the other side and/or use the far side to sharpen just the tip end of the razor/knife). Once you form that dish, the wear of that dished area accelerates. (It is crazy how fast the stone can wear when it starts to dip. You can feel even a super dense translucent stone grind/wear as you draw the edge over the dipped area, and the wear is rapidly accelerated. It's like the stone creates a sharp edge/burr, and then this edge/burr saws into the stone at the back end of the dip where the angle puts the edge into the stone.) If you start with a slight convex dome, any dip is easily worn back out and this problem does not occur. That dip is more self-healing. It tends towards spreading out and disappearing rather than increasing. It took years of use to recognize the extent of that problem; it's significant. You could say the convex shape is more stable under wear/erosion. It tends to maintain that shape. The flat shape is unstable and will only remain flat through input of work. And a small dip over 30% of the flat surface means you have to remove 70% of the stone (and eventually 99.99% as you start getting out the last low spot!) to remove the dip. A small dip in a convex stone, you can wear down just the ends/corners/edge of the stone in that area and the stone is still close enough for horseshoes. It doesn't have to be perfectly symmetrical all the way around to work fine. Small ceramic rods work. Fat ceramic rods work. There is some difference, but it's all good. You just want to avoid dips. The flat shape has only one proper form and furthermore the only way to achieve and maintain it very exactly is through lapping, and lapping is slow*. Also some tools, like a plane blade, need to have a straight edge.

This is all stuff that you can only learn through years of paying attention. And when you die, your grandkid will go on the internet and figure out how to fix all your stones that are out of flat, lol.

*Taking out even a 5 thous dip in a 6" by 2" flat ark stone is not fun. Go to the interweb to watch people do this. And see how many actually finish removing the dip of the noob-used ark stone to the noob-ideal flatness before saying "I could remove the rest, but I don't need to." They just have to avoid hitting that spot forever, should they be sharpening a smiling edge. Every common knife has a belly and will dish a flat ark stone when used in the same way that which is actually proper/ideal for a friable/lapping stone. The friable lapping stone is easily flattened. The ark stone is not.






Thus, I find it so damn interesting that this info is not findable. It's just not out there unless you dig deep and read between a lot of fuzzy lines. So after years of experimenting you might figure out more and more of the reasons that the western world's leading cutlery manufacturing city for 100 years actually sharpens their blades this way, and that it is for good, practical, applicable-to-even-you reasons. But that's where it ends. It's as if the world is dominated (for strong financial reasons and modern thinking of people obsessed with flatness and exact edge angles and thinking that these are essential factors of sharpness?) by synthetic friable stones, and anything you would ever try to add to a more common body of knowledge will just get put in the wrong box and dismissed. Hence, the secret remains a secret despite not being a secret. All "they" had to do was to offer the common person a shinier, flatter, "truer" method that makes more sense on the surface, and ba-ding. They know better.

I didn't learn the dishing issue by being smart. I didn't even learn it by copying what Dovo did to their stone. Like any other idiot, I bought a stone, and it was nice and perfectly machined and lapped into a rectangle, and I didn't want to "mess it up." I started out just rounding only one long edge of it, so as to leave the rest in such a pristine and perfect rectangle. And after a year, I learned why this is undesirable. That edge of the stone turns into a saddle shape, and the wear accelerates and doesn't do any favors to trying to get a good edge. So in the end, the ideal shape is very nondescript, not perfect in an ideological sense, and looks like not much thought or effort went into it. It's just sort of rounded everywhere and the opposite of sexy. I have a stone that is kinda shaped like a pentagon in outline, and simply judicously rounding it works just fine. I can draw a blade over the the corner where 2 sides of the polygon meet and and it doesn't matter. The corner is dropped a farther, and the line the blade follows turns into a gentle curve rather than a sharp turn. W/e shape the stone starts at, you just have to round it over a bit on all directions, which is very easy and why stuff tends to get rounded over by entropy, naturally. Just w/e there is a straight line, in any axis, get rid of it by working down either end. If the the beginnings of a dip (or too much straightness) start to develop along a long axis, I can just turn the stone and sharpen over the corner, same drawing motion. The "imprecision" of all this just creeps out the modern world. And it lacks the distinguishing features of a successful commercial product. It doesn't have a "new car" aspect that is easily lost and difficult to regain. It doesn't wear out (or it at least takes generational cycles of kids "fixing" then later "unfixing" these things to wear them out). It is a commodity that is not easily branded.

[KL27x]

https://youtu.be/3ACJrAI3SxM?t=247

Interesting that this vid shows what looks like the "normal" use of a muddy looking synthetic or perhaps coticule?

The video I watched was not like this at all. If you watched something like this, you might think I have a fanciful imagination or I'm a liar.

After looking around, I'm pretty sure the guy that posted the vid was "the superior shave" guy. And the lady sharpening was much faster. She was sitting at a table, not standing. And the razor moved like a blur. It was not a professional done video, just some footage from a guy that talked up with the people on a factory tour, and then posted vid with some details in the video description.

Not only that, but he has posted video showing him recreating this curved stone. He "sacrificed" a much smaller piece of coticule for this experiment. And he went way overboard. I think he obtained a piece of granite or something with a gentle dish it in, and then he lapped the coticule with the granite. I don't see either of these vids in "the superior shave" guys videos, but I am pretty sure it was him. I remember distinct details, like the coticule was only about 4" long (yeah, it was painful watching him try to sharpen a razor on this short a stone), and the fine layer got so thin on one end that when he was done it had worn through in a spot near that end, revealing the coarse layer underneath. He pointed that out.

Perhaps the secret guild of illumiati stone cutters saw it and got Dovo to ask him to take it down. Or perhaps his viewers complained how stupid this was and that the stone has to be flat and/or he got tired of talking about it to the people buying expensive coticules from him.

Or maybe Dovo customer support costs increased due to people calling in and asking about the video. And complaining they can't recreate the Dovo edge if it's made on a hoky hand-shaped humpback stone, and if that's how they sharpen them then where can they buy this hand-shaped stone for themselves?

OTOH... he is making convex arkansas stones for sale. So yeah, it was definitely this guy. Just I don't see these two exact vids.


The way I do it, I'm not concerned too much about the center. I would blend it in, but I wouldn't care too much if it gets nearly flat there. I just would use it in a way that the wear is around the edges, and the stone gonna get there by itself, eventually. An 8x3" is a lot of shaping and a huge workout to get the entire thing to a decent curve, even using diamond plates.

Actually, if you take your precision flat ark stone and rub it on a flat piece diamond plate or a piece of sandpaper adhered to a precision flat granite... guess what. You won't put a scratch in the center. There will be a bare oval area that doesn't get touched. About time you reach the point where the scratches reach all the way to the center is where I'd be done curving the middle, and it looks like maybe he is doing something similar, although knowing his previous video I bet he precision ground the glass to be dished, too, lol.   

Addendum: in fact, I know his lapping plate is dished, just by seeing the frame of the scratch pattern on the stone. His untouched spot is the entire third of the stone lengthwise. If he were doing this with the sandpaper over a flat platten, the oval would match the outline of the stone. Starting with nearly the entire surface, then receding from the borders at essentially the same rate. It wouldn't shrink from the ends of the stone, only. This is assuming the stone started out flat, which is pretty safe to assume from my experience with a few stones from common sellers. I think it would be harder for a manufacturer to not make them flat, these days.

[eKretz]

Yes I know that guy, IIRC his name is Jared and he does use a concave plate for shaping those stones. There has been a considerable amount of discussion regarding his convex stones at razor forums. Personally, for razors I will stick to flat honing surfaces and a slightly smiling edge. It is easy to renew the surface of a flat hone/stone, and checking for flatness is much easier than trying to reset a large convex surface - there's no easy way to check such a surface for correct radius throughout - unless you do like Jared and make a concave master to use with wet/dry.

And yes you are correct that there isn't a lot of info out there on the subject of proper use of some sharpening stones. For quite a lot of years even a large percentage of the guys on razor forums were saying that Arks were no good for razors.That's just bunkum. I started with an Ark I got from my grandfather and used that stone for a very long time before starting to collect, use and experiment with others. Arks are excellent razor stones.

[KL27x]

It is easy to renew the surface of a flat hone/stone, and checking for flatness is much easier than trying to reset a large convex surface

When the stone is appropriate to this type of use, then yes, it is easy to maintain it flat. Your coticule is way more friable than an arkansas stone. It is a "tweener" stone and can be used flat or curved. Many more friable synthetics cannot be used curved and have to be flat. If you tried to curve them (I have tried everything), the curve makes the lapping paste just push away from the contact area. The small contact area erodes the soft stone very quickly making more paste. And the stone does not work well or hold its shape and will eventually conform to the shape of the blade and start working again, but you just wasted time and stone. It's with the ark stone in particular, and perhaps some other harder stones like slate, and the hard sintered ceramics which really don't work as sharpening stones when they are flat lapped. Some folks have granted them a mythical position of "finishing" only, and some people use them flat and can still sharpen stuff on them because they unwittingly have developed the habit of favoring the edge just slightly. But they could do much better in either case.

- there's no easy way to check such a surface for correct radius throughout - unless you do like Jared and make a concave master to use with wet/dry.

Despite Jared being very careful to precision grind his lapping plate into a perfect sphere, it is not necessary just because he thinks it is. Well, from a marketing standpoint it is actually great, and I'm sure his customers are in awe to receive such a perfectly spherical shaped stone.

I understand completely that if you put a razor on a stone, the curve/radius of that stone affects the sharpening angle, just a tad. If you increase the radius, you will start hitting more on the apex. And if you decrease the radius, you will start hitting more on the back of that bevel. But the differences here are minuscule*. The main reasons to curve in all axes is to reduce the area of contact and ensure the entire surface is being of the stone is being used, one spot at a time, to retain a regular surface with relatively sharp particles. And IMO one of the other practical purpose of curving the stone in the long direction as well as side-side is wear pattern. That is my experience. You want the stone curved enough to be able to handle normal wear without tending towards developing dishing. Is does not have to be spherical all over, and I actually wouldn't want this on a long rectangular stone. I would prefer it have a tighter radius side to side and a larger radius end to end. Hence to end up with something more like a section of a football. That's just my personal preference.

Flat stone, anywhere you put wear on it, ba-ding, you have a dish, and the wear accelerates. Curved stone, anywhere you put wear on it... it's a little flatter there, but still no dish. And where you put this flattish spot, on either end there's now a bit more radius and those spots will wear faster, returning the flat spot back to curved. Self-maintaining, glaze never happens, stone cuts more and burnishes less, no wasted hours of trying to flatten a stone so hard it doesn't want to be flattened.

Yes I know that guy, IIRC his name is Jared

This guy's demeanor is awesome. He's just so disarming. AFAIC, he lulled Dovo employees into giving up trade secrets, then they later got him to take down the video.

*The important thing is the the stone will cut more and burnish less. Wherever it does grind the bevel it will be able to cut. And this means it will be able to reach the edge, even if it has to cut a bit more from the back of that bevel (because maybe you used a flatter stone to set the bevel) it will cut that down and keep cutting until it reaches the apex. If it doesn't hit the exact spot on every pass, it is of no consequence, just that when it does touch the apex it is not burnishing and decreasing the durability and sharpness of the edge. When I first started to use these stone, I followed the info available to me. If you use it the way most people do, this just doesn't work. It only can add a slight microbevel, at best, by lifting the angle and touching only a small area of steel to the stone. If you ran the blade over the stone and weren't hitting the apex due to difference from the pervious coarser (actually doing-something-useful) stone, then the apex would never hit no matter how much you slide the blade over the stone. Most all you could do is eventually grow a burr and still have a bad edge. This is especially true of the harder black and translucent stones.  Once you curve the surface, they are no longer a mystical burnishing/finishing (the blade was razor sharp , already, but can you feel the difference after watching me do 1000 passes? No? Well, I can!) stone. They sharpen things. They will put on that razor edge from nothing, if you have the time. And no that is not the case if this stone is flat; given infinite years of labor, it will still not produce a very sharp edge from dull... well, unless you finish with more obtuse "micro-bevel" passes to cut off that burred and mucky steel from the edge. IME, you need to raise the angle about 30-40 degrees for this to work on a flat ark stone.

There's nothing "wrong" with doing this microbevel, but it is necessary due to the burnishing going on. When I first learned to get a sharp edge with an ark stone by doing this high angle microbevel, I thought it was wonderful. I mean, I finally got a razor edge off this stupid stone. Maintained my kitchen knives on this flat ark stone and it is all great, at first. But fast forward 4 months later after you've repeated this procedure 4 times, and all your knives are still razor sharp?... but that edge starts to flop and fold over, and now you need a proper sharpening stone. One of the first things you might have noticed is that the burr isn't being removed by the high angle microbevel passes, anymore. Not as easily. The distinction between burr and edge is blurring, and you have a burr-edge. Used the way everyone else uses it, the ark stone is extruding a bunch of weak metal into the apex through burnishing, so it is good for "light maintenance" until the edge turns into a limp biscuit. With the curved stone, the edge is sharp at any angle you want to sharpen at, and the burr just falls off on your palm, and it's a hard, durable, new edge everytime. To some of the most prominent sharpening and vocal students/enthusiast/experts on the web, it is considered normal to have to grind the edge at 90 degrees and start over, every now and then. Some claim they do this every time they sharpen; and I've even heard a strange technique called plateau sharpening. But I'm telling you, if your stone has a high cutting:burnishing ratio, this is not necessary. It will remove material without extruding as much new burr, revealing a fresh edge.

**There's really no other good way around this. For the "hard" stone to be able to cut that steel without burnishing so much, it has to have widely dispersed cutting points with some more breathing room between them. But the finer the grit, the closer those points get to each other. Diamond plates can be engineered to do this to some degree, by altering the distribution of the abrasives. This is similar reason we don't flatten stuff to a fine finish with a giant belt sander. We plane them with a blade that rotates in an arc and cuts a tiny section at a time. We run them through drum sanders and surface grinders and such. Tiny areas of cutting.

Also, IMO, the true hard (surgical black and translucent) ark stones still burnish a bit more than ideal even when convexed. For a hard hone, sintered ruby is a bit superior in this grit/fineness range. But the true hard arks are pretty close to a no hassle sharpening stone when convexed.


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BASIC INFORMATION (that in hindsight I should not have assumed is commonly known.)
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It's not like you burnish the steel and make a burr, and then at some point you've burnished all the weak steel away from that surface. Initially, most of the metal may be the "looser" or "unimportant" stuff, but removing it exposes the important structures. These come tumbling down, and the burnishing continues. That metal just keeps oozing/flowing out past any acute edge connected to this area being burnished (the contact patch), namely the apex of your blade. If the contact patch ends over a flat surface or end in an obtuse angle, any of the burnished metal flowing there will stop, because it's supported there; it has more interaction. It has buddies saying "hey don't jump off that bridge, yet, cuz we're having a party." But over the acute edge, the burnished metal just happily flows right over.

So it's important to understand this bit:  the burr is not a bit of metal leftover after grinding in the apex. It is not formed through a subtractive process. It is being constantly extruded. It grows. These pressure points of the abrasive pushing into but not cutting do some kind of japanese pressure point kung fu to the steel, and parts of the metal temporarily act like a fluid. To get a sharp edge, you want a stone that will cut the burr away faster/better and grow new burr less quickly. If you have too much burnishing and too little cutting you cannot get down to "good" steel unless you go 90 degrees to wipe the edge off and start over, and you can still have issue by the time you form a new edge (and this is where Cliff Stamp's "plateau sharpening" theory comes in to save the day; the theory is to cut off the edge at 90 degrees, then sharpen it only until the edge becomes nearly sharp, but stop before the burr gets all the way to the apex. But if you use the proper technique, of which there is more than one, you can forget anyone ever came up with this. When it comes to the methods that aren't lapping, this essentially boils down to controlling surface area of contact when on the edge. Make the bevel smaller or reduce area of stone touching the bevel).

The burr could be considered as a glacier flowing over the edge of a cliff. If it flows faster than it is ground away, it will obscure the riverbed below, and the apex you make is made out of the glacier. If you can remove it faster than it grows, you can grind an edge into the riverbed, proper. This burnished, fluid metal is covering the entire surface of the blade, both sides. It's just where it is allowed to flow past the edge, there's no "solid" steel underneath it, anymore, and it's all burnished metal or burr.

The surface of steel always has this crappy stuff on the surface that is not as strong. This why you can't completely remove the burr. When you expose that new edge, the surface will still be relatively weak. But you have the minimum thickness of this weak stuff. Unchecked burnishing at the edge will thicken this layer of weak metal.

It's not that this info isn't out there. It's just the signal to noise ratio isn't very good. Due to internet experts like me, except some of us got it right and the rest got it wrong. When everyone has a say and everyone has feelings, truth is a popularity contest and everyone gets to be right.

It is obvious to me what most people are doing with the flat lapped arkansas stone (and/or flat sintered ceramic stones). You have 3 basic categories of time-wasters.

1. The "finisher."  The "finisher" makes an actual sharp edge on a friable lapping stone. This slightly rounds the apex and produces a razor sharp edge. They then "finish" the edge on the flap lapped hard stone. This burnishes the bevel, refining the scratch pattern. And because the apex was slightly rounded by the lapping stone, they can stop before pushing burr all the way to the apex and/or they are putting some lapping compound on the stone. The notion that the scratch pattern on the bevel is what makes a blade sharp is ridiculous. The apex is what makes a blade sharp.
1.b. or he uses another stone or abrasive to form a slurry on the stone.

This is the kind of guy that talks about lapping hundreds or thousands or times on a razor that is already sharp. His favorite 2 words are "JNAT" and "slurry." Indeed, there are people who believe a slurry is the only way to get a blade sharp, despite people have sharpened knives, machetes, swords, and axes with files for centuries. If you sold this guy a block of glass, he would find it exquisite. As long as you charged him $200.00 and told him it was a rare natural stone from Japan.

2. The "microbeveler." This guy takes the already sharp blade, and he burnishes the bevel to polish out the scratches. But he doesn't stop before the burr gets to the apex. So he does a high angle "microbeveling" pass on the flat lapped stone to cut the burr back and reveal a good apex.

A good example on YT is Richard Blaine. I believe he sells kitchen knives and sharpening accessories. He demonstrates this way of using a flat ark stone. Ironically, he demonstrates the difference between the "western stroke" (more of a drawing stroke) and the "eastern stroke" (back and forth shush-shush slave labor on a sandy stone). I believe the difference stems from using hard/file stones vs lapping stones, the former benefitting from a drawing motion over a smaller area. But he fails to make that connection, and his version of a "western stroke" has the rotation but has only minimal drawing motion.

3. The unintentional curve guy: This is the guy that may or may not lap his stone truly flat. But when he sharpens, he is intentionally or subconsciously favoring the edge of the stone when on the straight part of his knife and is drawing the blade across the edge of the stone. When he gets to the belly/curve, he starts to lift to get onto the flat of the stone and that's fine, too. Then this person will typically periodically re "dress" the stone with a diamond plate, to take out the dip/wear and this actually maintains a slight curve to the stone. This is the guy that is actually sharpening the most efficiently of the 3. But he erroneously thinks the stone must be "flat" and that flatness does him any favors, and he is wasting a bunch of time to maintain the stone in this "flat" shape. He has learned how to get the best edge using the wrong paradigm for the stone, because everyone else is doing it.

Good example on YT is Jeff Jewell. He is just some guy posting videos with no financial incentive, AFAIK.

The first two guys are just wasting time. And if this is the only stone you had, you could not maintain an edge doing just this no matter how delicately you use the blade and/or how often you touch it up. The burr would eventually push out into and become the edge. You would have to use an actual sharpening stone and/or to cut the edge off at 90 degrees, periodically. The third guy sharpens great; he is just delusional.

The reason I use a strop touched with compound following sharpening of my razor is not to remove the burr. I don't need anything special to do that. It is to polish over the microserrations on the apex. These microserrations are a natural result of sharpening on the curved hard hone, and in most cases they are advantageous. On a razor, they can cause razor burn, cuz you actually angle the edge towards your skin. Right off the stone (convexed translucent arkansas or ruby), I can strop (a razor or any thin knife of decent razor steel) a few times on the edge of my hand and get an amazing, very close, very nice shave without cutting myself. By most objective criteria, this could be the best edge, possible. The closest shave with essentially just a single pass without missing any hairs. But 3-4 days of this in a row and some areas of my face have been thoroughly exfoliated and can start to get a bit of sting or burn. A few swipes on a strop with compound guarantees this doesn't happen. Any stone that burnishes too much will obscure these microserrations; this is where you get a razor sharp edge, but after little use it can't cut a tomato.*** Only a stone that has a high nuff cutting to burnishing ratio can form/reveal these microserrations in the "hard edge." The other reason to use a bit of compound is to remove the new schmoo that forms on the edge over time. A bare leather strop just pushes the schmoo back into place rather than remove it. The blade will be re-aligned, but it will require more frequent stropping as you go, because the edge will get weaker each time and get more quickly out of shape. Before figuring out all this stuff I used to have to re-sharpen my razor more frequently. The more you learn, the less work you have to do. On most blades, other than razors or chisels and plane blades, I don't use a strop with compound. The reason I use compound on chisels and plane blades is because those tools are only sharpened on one side and the flat/unsharpened side tends to develop a bit of back bevel through wear. It's not always worth it to me to grind this tiny wear-bevel all the way out every time I sharpen these tools. I hone the flat side but maintain within a couple degrees of flat. The apex might be only 80-90% complete (still sharp enough for most things), so there are usually parts of the burr that don't come away with a bare strop. Micro chips/scratches that go to the apex get filled in with burnished metal, so the burr has spots of thicker root that doesn't separate, easily. Compound gets in and cleans the burr out of the low spots in the apex, so that the burr will come away.

***This is exactly the kind of edge you make on a flat-lapped arkansas stone. Stropping with compound makes such an edge actually sharper. In this case, the strop is removing some of the burnished goo to reveal the hard edge, warts (natural serrations) and all. And the edge gets some bite and lasting sharpness. In the case of sharpening on a curved ark stone, the main reason to strop is to smooth the edge. The strop doesn't make that edge appreciably sharper, cuz it's already good to go. Except for shaving. If you were to try it, it just might change your perception of what the burr is, and it might give you better insight into what the hocus pocus you might have learned from others is actually doing and why it is "necessary" in the first place. When I say "you," I don't mean you, eKretz, but anyone else that might be reading this. There is some case where the smooth burnished bur-edge is desirable. Some of the eye surgeons of old experimented with burnishing steel scalpels and claimed this decreased healing time and reduced scarring of the cornea. And that's fine, cuz an eye surgeon doesn't cut very much, if he's doing it right. But for most things, including even a razor, any of this "smooth peanut butter" filling in the valleys in the apex just reduces the useful life of the edge.  If the apex isn't perfect to begin with, the flat lapped ark will make scratches and voids/chips disappear - by filling them in with goo. This is mostly cosmetic and it degrades the edge, in a way. Keep on sliding that bevel over the glassy flat stone, and you will just extrude more burr over the apex. The convex ark will keep on cutting until those defects are removed and replaced by the finest defects it can leave behind. Once it has cleaned up all the apex and reached most of the original low spots, any tiny burr fragments that are still clinging to the apex will fall off if you even look at them wrong.

Lapping stones can automatically do what the strop with compound does. You shouldn't need compound if you use a coticule, properly. That is a stone that just happens to have the properties to give a good shaving edge, right off the stone, when used right. But for maintenance between sharpenings, you might find a bit of chrome ox will extend the life of your razor better than a plain strop. The plain strop will tease off a fine burr immediately after sharpening. And it will realign the edge as it takes damage through use. Add a bit of chrome ox, and it does these two functions plus is removes new schmooey metal that forms over time and becomes part of the edge.

[coppercone2]

in regards to weak steel particles scattered on the surface, can it be etched away with acid prior to sharpening in order to reduce the complexity of the sharpening operation (this is getting hairy)? like increase roughness but decrease burnishing if one species is susceptible to making valleys on exposure to acid that leave the non burnishing part elevated?

\perhaps some kind of chemical/peening/ultrasonic/(laser blasting??) process (or mix of multiple processes like acid etch in ultrasonic) can etch/remove this from the surface some how prior to the final sharpening step if done carefully?

re-heat treatment maybe like induction edge hardening, even if its already hardened? or thick nitride deposition and diamond sharpening (where the nitride acts as a gap filler or something on the weak steel?)

with healing I think the increased surface area of a cut allows for easier coagulation/platelet buildup, so you want a 'rocky coast line' for healing I think.. the platelets should have anchor/'safe harbor' points to attach to I think? a ship is happier in a natural harbor vs the side of a  concrete city sea/lake-wall (vertical sea walls are just fucking creepy anyway, all the good dams have their walls built like zigarauts now to reduce this effect IMO).

[KL27x]

1. In theory? Maybe, maybe not. The "fluid" surface metal is still interacting with the solid steel. If you "surgically" removed it bit by bit, the remaining structure of the surface may no longer be stable. There might be a minimum amount of it, anyway, that is necessary to be there. And even if the surface was stable, as soon as the exposed structures of this surface are damaged during use, then this "releases" a new layer of this fluid surface. The surface turns to this fluid metal, that flows like a glacier under pressure, again. I think of it as a surface effect of steel. Steel can't be good at everything; it's very strong, but where it just "ends" some of the things that make it so strong don't have anywhere to attach/finish. Just cables going nowhere. Well, many might form alternate bonds, but end result is the surface has a different structure than the interior.

So if you press hard enough on the steel through small points of localized contact, e.g. burnishing, you can damage the solid steel underneath and make this layer thicker. (More accurately, maybe the layer is only ever very thin where it is being formed; but it CAN definitely be moved/flowed and become thicker in another area). But there might be a minimum amount that is always present.

2. Yeah, maybe laser. Sure what not? Etching a file in phosphoric acid is said to sharpen it. And it's not just a home-remedy folklore. There are businesses that offer this service to other businesses, along with other sharpening services.

3. Perhaps modern nitride coating already effective achieves this? I dunno, but it sounds plausible to me.

4. I dunno about this. I have heard it both ways, that a cut heals better/faster if the blade is sharper/smoother. This is what I tend to believe. The closer we can put humpty back together, the less new tissue has to form to glue it back together. Like putting a broken vase back together, kinda. I cut my finger once where it wouldn't stop bleeding for a day or so. That was a very sharp knife, but I think the bleeding wouldn't stop because I hit an artery.

[coppercone2]

I wonder if parts burnished vs ground would show a difference in a short term acid etch. is there an 'air gap' when the peaks are pushed down between them (so its like deformed) or does it actually bond it. Is it like 'combing' metal, or do the peaks 'weld' to each other?

[KL27x]

is there an 'air gap' when the peaks are pushed down between them (so its like deformed) or does it actually bond it. Is it like 'combing' metal, or do the peaks 'weld' to each other?

As long as it is not overdone, the burnished surface is continuous and connected. If you overburnish (not really possible with a sharpening stone or a grinding wheel), it is possible for a layer of burnished metal to start to separate from the metal underneath. If you flake off the burnished metal, the metal underneath will still have a "fluid layer," you just made the layer so thick and mashed it around so much that some of the excess fatigued and tore away. After it gets so thick, burnishing won't reach the solid metal to turn it into more goo, and you're just mashing the goo until it breaks.

What difference will that make after a short acid etch? Well, the smoother the metal to start, the less variation it will have when it's done, regarding pits and whatnot. Intentional burnishing is usually done to improve corrosion resistance, AFAIK. Well, where it comes to making edges, many people tend to equate bevel shininess with sharpness, so they may intentionally burnish for that reason and/or simply for the esthetics.

Maybe this is related or useful to you in some way. I have mentioned before of not removing ALL the oxides when cleaning off steel things. If it's going to go back to an environment where it will rust, again, just knock off the loose stuff and leave the patina. One of the ways to do that is to use a smooth very slightly convexed and arkansas stone, translucent/black preferred. It will knock off the loose stuff. But if the surface is very smooth, it will leave the patina. If the surface is not very smooth, it will smooth it out. And next time, it will leave the patina. Once the patina is flat and contiguous, the silicon dioxide of the ark stone won't easily scratch through, as long as you use light pressure. This is what I do to my carbon steel kitchen knives when orange spots appear in the patina. The patina is a hard candy layer over a softer M&M.

Of course on large objects, it's much easier to use a wire wheel.

I imagine 500 years ago, burnishing was done by hand to make silver mirrors. And possibly to maintain the patina, natural or otherwise, on steel and iron tools and implements... although it might be a different sense of the word in that context. I've read that in Japan, burnishing was done to swords. After polishing with progressively finer grits, the burnishing was performed by manually rolling hardened steel pins/rods over the metal. This makes the final fine scratch marks disappear. This might sound purely cosmetic, but I bet it was a practical way (at the time) to improve the corrosion resistance of something that took many weeks of labor to make and could have cost more than a house.

[thesuperiorshave]

hi

I just registered to clear up a couple of errors here.  Most of this forum's way beyond my capacitors' capacities, but the convex stone thing, I know well, from taking a knee to those who created it.

First, the German master grinders don't care about our shop 'telling their trade secret'.  It isn't like that to them.  They care deeply about hiding their particulars of heat treat and temper and if I were to post a picture of a sheet of paper with those specifics on it for a batch of razors about to be addressed, they'd have a massive problem (I'd lose the trade partnership, actually).  The grinders' view of me being Convex Johnny Appleseed's more along the lines of "you can tell them, but they won't understand it, and can't achieve it" or something of that approximation.   As a lot, they don't think the North American buyer can come to understand 'the concept's irrefutable superiority' (their words, or more specifically the words [via translation] of the most senior fellow who first explained this to me ~9yrs ago, a man in his 80s who'd been a master grinder for >50yrs). 

I'd taken the stones we've convexed to show them for their blessing, which they gave.  "Have fun, they'll think your crazy, or stupid" was their general reaction.  They're bemused that I'd try to propel the notion.  I would like to point out that getting even a tiny chuckle (or in this case a little smirk at the corner of the mouth) from German engineers is something to be proud of.

Second, there isn't a burr being created when properly convex-honing.  You may confirm this with your e-scope.  This is due to the angles used and the thinness and flexibility of a properly hollow ground straight razor's bevel form.   A burr occurs when one repeatedly rubs one side of a bevel form with a steel cutter with a stone-to-blade angle of approach that is either equal to the incumbent angle dictated by the bevel form (such as when you hone a flat razor flush to a flat stone) or slightly more *obtuse* than the blade's incumbency (such as when you apply tape to the straight razor's spine, and only move on one side of the bevel awhile).   When we take an angle of incidence of stone to blade which is more *acute* than the blade's incumbency (such as when a ground-to-flat razor's bevel's taken to a convex stone), first of all we wouldn't be touching the apex of the cutting edge if the razor's incumbency caused physical obstruction between the spine and the convex hone's acute angle of address; the grinding would start to appear on the rear (closest to spine) side of the bevel form until such time that the physical obstruction was no longer present.  When the obstruction is gone, if a knife's thin enough and of a flexibility sufficient to perform it, it will move from the stone in a way not unlike how you can bring the edge upward with your thumb nail pressure and watch it go back down when you remove your thumb.   

With a properly shaped, tempered, and hollow ground straight razor, you use an angle of address with a convex hone that's extremely close to but slightly more *acute* than the blade's incumbent angle.  The small angle change combined with the thinness and flexibility of the bevel causes the bevel form itself to flex at the very edge for the moment that particular bit of contact's being honed, the terminus of the cutting edge (the grinders call this 'microteeth') gently extruding away and upward from the spine position - and return to incumbent position when its' time on the pass of the honing stroke is complete.  You're trying to impart a slight concave shape to the bevel itself.  All the grinders I've talked to either use a barely convex or flat bevel-setting tool, and then refine on something that's at the limits of what shape the steel will hold. 

Hopefully I did this correct but here is an attachment from a Böker catalog where they confirm that they're using convex stones (how would it be possible to impart a small hollow grind to the edge itself with a flat stone unless you held your spine aside of and within the plane of the flat hone and did some strokes like that?)

I didn't delete any YouTube videos on this topic, and I don't care if everyone makes their own concave plates.  They should.  Nobody's getting rich propelling this notion, if anything it is better to not say it.
 
Knowing what I know now from experience, if I were reading this and wanted to make a quick permanent concave plate for the use with razors, I'd take a ~8x2" hard cheap stone of some sort like a big carborundum, some 60-grit silicon carbide grains and water, and take that to a 12x12" sheet glass at least 6mm thick over and over.   Flatten one side, flatten the other.  You might have to buy two of the cheap stones.  After not too long, you'll start getting a spherical curved pattern cutting in to the glass,...you could also do this with the technique you see here using two sheets of glass  https://youtu.be/P_u9zjamBvs?t=126   but personally I'd rather start with one plate and cheap hones which cost me less to get into my life than a second plate of glass and do not threaten me with laceration.  Keep on with your grinding until your center point is at the very least 0.7mm deeper than the edges and you'll have something that'll work, but if you press on to about 1.25mm it will be better. 
- The plate's curve intensity should be more curved than the least straight razor's deviation from true, such that the hone-to-be cannot touch the razor edge twice w/ a gap in contact.
- The plate's curved intensity should not be so intense as to cause the razor's edge to be unstable.
- What exactly is the best position would depend upon the uniformity of the production values and metallurgical structure of the razor at hand. 

As I've found even granite tiles do start to increase their sagitta if used flush to whetstone with grit (originally I did this with two granite tiles and that took an astoundingly long time), I'd only recommend using with silicon carbide sandpaper affixed to your tile, with some grit sprinkled on the surface at least as fine as the backing paper.  I use hairspray just a spritz or two to the backside of the paper, then use up a whole paper's lifespan all at once with grinding, it removes easily and any residual tackiness can be easily washed with soap and water.   A pair of cheap but reasonably flush to one another tiles from Lowe's cost me $6 and I got the top one to >1mm deep in about 2hrs of labor, that I'd only have to do once as the sandpaper fronting it keeps the shape in place. 

[KL27x]

I dunno how you found this thread, but thanks Jared/Jarrod for taking the time to join the forum just to clarify. I hope I didn't misrepresent what you put out there too badly.

"you can tell them, but they won't understand it, and can't achieve it" or something of that approximation.   As a lot, they don't think the North American buyer can come to understand 'the concept's irrefutable superiority'

"Have fun, they'll think your crazy, or stupid" was their general reaction.

Bingo. This is exactly what I feel, but is it really just NA? I wonder if it's any different to average Germans, these days.

This why in one of my posts I lamented the fact that after we die, some kid is going to acquire your excellent stones and go on the internet to figure out how to flatten them. And on the (english speaking) internet, they will encounter exactly nothing to dissuade them from their purpose! Except for a few YT videos from a lone guy who does it for the pure passion of sharing this insanely better way, even though there's basically no money in it. (That would be you!) Everything else on the modern english speaking internet is just "dish = bad. Flat = good. Flatter = gooder. Let me show you how to flatten your stone."

And it wasn't always like this. I remember a webpage showing and describing the benefits of curving an arkansas stone in very good detail. I also remember a webpage showing how you can (actually) flatten two stones against each other without making one concave and the other convex. That was probably over a decade ago, though. Our modern internet is more like a popularity contest. The most popular view is correct. And I think there is also some influence from $$$. There's huge money in making people forget how to do things and then selling them a solution with a high profit margin that you import from Asia and sell at many times the cost. Lots of money leftover for marketing and salemen. In any online discussion about sharpness, any real discussion is discouraged. There is always way too many peeps talking Naniwa 6000 GTX vs Shapton 8000 GSXR-7 Pro. And I'm sure many of them are real folks, but I bet there was plenty of help getting this situation created in the first place by internet/social media marketing teams. I remember 10 years ago, there was a couple year period where in every discussion there were multiple guys interjecting any discussion just to say how amazing the Worksharp is, and only $XX.99... And again, I think many were real people, but I think there were a lot of marketers pushing it at that time.  Do you think a video showing someone sharpen a cheap knife on $1000 (retail) of waterstones gets 7 million views naturally? I doubt it. But other aspiring Youtubers will see that and copy it. And they might even get a bump in views from marketing bots, if their video promotes someone's agenda well enough.

Second, there isn't a burr being created when properly convex-honing.

You know, I have never been able to feel a burr after sharpening my razor on the convexed ark stone. I just always assumed it there but too small to feel. I will take it from the guy looking through an electron microscope, though.

- The plate's curve intensity should be more curved than the least straight razor's deviation from true, such that the hone-to-be cannot touch the razor edge twice w/ a gap in contact.
- The plate's curved intensity should not be so intense as to cause the razor's edge to be unstable.
- What exactly is the best position would depend upon the uniformity of the production values and metallurgical structure of the razor at hand.

Interesting, the kind of details what can eventually be noticed by a company/culture that has done this for centuries. I imagine it started out as "oh it works better when it's sorta rounded." And 200 years later, this.
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Edit:

All the grinders I've talked to either use a barely convex or flat bevel-setting tool, and then refine on something that's at the limits of what shape the steel will hold.

Interesting, but I'm not sure if you are reading too much into this. For one, IME, hard (non-slurry) stones just don't work well in coarse grits. The stone gets dull, and then it just sucks (exception being diamond plates).*** The coarser the grit, the more friable it needs to be, and the more friable, the more it just doesn't work unless the blade fits to the stone to get enough surface area of contact to get liftoff. Finer grits is where the hard/dry convex stone really shines.

***I have found the coarse side of many stones to be very firm and very coarse. This includes a very cheap silicon carbide stone I have, as well as the Norton Combo India, for examples. And I have found this side of the stone most useful for shaping (including flattening) other stones. Do they cut steel? The first doesn't do it so much as just make crunchy noises whilst giving the feeling of dragging a steel part over gravel. The Norton cuts steel ok on the coarse side, but it can rattle your teeth, and it's almost slower than just using the fine side and getting a better finish and/or edge. But rather than be disappointed with the performance, I found that what is too dull for cutting steel is still fine for shaping of other sharpening stones, and maybe that is what some of these "coarse sides" were designed to do from the start. The larger chunks of firmly bonded abrasive can chip apart the other stone without taking as much wear, themselves. Grind wheel dressing stones come in even coarser grits in a very hard, firmly bonded stone, too. But the surface speed and inertia of grind wheels and the way you use them (better hold on somehwat firmly, at least, to your part) means everything is coarser and still works out fine.

Another way you might think about it, the fundamental reason for curving the fine stone (I think) is to reduce surface area of contact (aside from razor-specifics). And the coarser stone has way less points of contact per unit surface area. So even a gentle curve might be overdoing it, leaving too few points of contact and not being able to do what it's supposed to. When curved it would be like, say, putting 3 points of contact against a spot on the bevel and gouging it. The teeth could sink too deep and get stuck. It would be grabby and ineffectual. The law of averages is working with a much smaller sample, and your blade would be hitting higher points and banging up the edge and just bouncing around.

Analogy would be sanding a large flat area of wood, you would need to use a really coarse grit. But if you wanted to sand a very small flat spot, you would start with a much finer grit. If you try to sand the end of a toothpick on 36 grit, it won't be rougher but faster. It would be rougher, yeah, but it might not even do what you want it to. Instead of cutting away material, you would just splinter the toothpick and break it.

Even here, in our backwards country, ceramic rods are way more popular than flat ceramic sharpening stones, and we don't have super coarse rod sharpeners. I wonder if for every sale of a fine ceramic flat stone, there is a German engineer smirking, thinking it's the perfect tool for rounding over the point of a needle.

Edit2: there are multiple reasons this info is never going to take root in the modern US. One of them is many of us can't even tell what is flat. Seen a guy take a brand new King waterstone and "flatten it" to improve it, right out of the box using sandpaper over a piece of glass. For one, King waterstones are flat, out of the box. But more importantly, even most waterstones are too hard to flatten like this over that large of a surface area (7"x3"ish). What you will end up with once the scratches end up reaching the center is a convexed surface. A straight edge will easily verify this. Coppercone2 experienced this when attempting to flatten a 3x3" aluminum surface with a file. The result is a convex surface. Many people believe this just a result of imperfect biomechanics, but the file just can't cut that much surface area at once.    So if an average person can take a flat object, make it curved, and then call that "flattening," it's a steep curve from there.

[KL27x]

Oh, and...

Jarrod, if you do read this:
Can I suggest to you two words? Sintered ruby.

Apparently, this stuff was discovered/created by a German company some 30 years ago. The patent expired in 2013, best I can tell. So now it is pretty much dirty cheap. Much cheaper than translucent arkansas stones.

The fine sintered ruby is maybe even finer than translucent arkansas. And after it is gently convexed (diamond plates; no sandpaper, here!) it leaves a most excellent razor edge, IMO. It seems to cut a bit more efficiently with less burnishing.

What I have found, and it might be wrong, is that sintered ruby is made the same way as white ceramic crock sticks, except a bit of chromium oxide particles is added to the aluminum oxide particles before sintering. The end result is it cuts better than white ceramic (IMO), and it is also 30-40% tougher. I have dropped a new 6" long, skinny machinist stone onto concrete and cringed. But the thing bounced and rang like a bar of steel, and where the sharp corner hit the concrete was some concrete dust... but the corner wasn't even fractured. I tend to sharpen razors on regular ark and then finish with the sintered ruby, leaving out the translucent ark, these days.