Sharpening on dished stones

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ian

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This is prompted by a comment by @Brian Weekley in this thread.

So, sharpening on dished stones often works, but here's what I imagine is happening.

Say you're sharpening with a consistent 15 degree angle (relative to the tabletop) and you have the edge facing toward you when you sharpen, as I always do. If your stone is dished, then at the beginning of each *push* stroke, you'll be hitting the edge at a > 15 degree angle, which decreases as you push, giving a <15 degree angle toward the end of the stroke. If you're scrubbing back and forth on different sections of the blade, you'll probably hit every part of the apex at some point, and will end up with a slightly convex edge, which is fine. It's kinda like the beginning of each push stroke is sharpening and the end of each push stroke is thinning a bit.

I find flat stones useful when doing full edge-length deburring strokes. For then, if you hold a constant angle with the edge facing you, and push away from you while sweeping all the way from tip to heel, you'll hit with a >15 degree angle at the tip and a < 15 degree angle at the heel. This is problematic, since then you might not properly deburr near the heel. Of course, you can correct for this by altering your angle slightly as you sweep or something. You'll probably learn to do this automatically if you're always using the same stone, but if you're using lots of different stones at different levels of dishiness, it can be hard to adapt.

Another subtlety: in the model above I'm sort of assuming stones dish in a U shape, where it's higher near the short edges of the stone, and lower away from the short edges. Of course, that's not really what happens, because the long edges of the stone don't really get worn down in the middle and your stone really looks like a bowl, not a halfpipe.

Screen Shot 2022-02-23 at 2.29.16 PM.png


While the discussion above still applies to the bowl-shaped stone, you run into additional problems where your angle of contact is even more unpredictable, since it depends on which part (left, middle, right) of the stone you're using. And when you're doing full length deburring strokes, it's easy to accidentally tilt your hand too much while you're sweeping the knife across the stone, so that the contact point sort of skips out of the bottom of the bowl and suddenly hits the edge of the bowl, near the long edges of the stone. This can lead to inconsistent deburring.

Finally, if you have a suuuper dished stone and a really flat profile, you can really run into problems, since the profile doesn't bend enough to even allow contact with the bottom of the bowl. Most of the time that won't be the case, but if the stone is sorta dished and the profile is pretty flat you have to be more accurate with your point of contact than you otherwise would.

Anyway, feel free to object, trash, scream, applaud, or laugh-react.







**The author of this post approves of halfpipes in extreme sports, just not as a model for dished stones.
 
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Wagnum

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Thanks for writing this. Some interesting points in there, a good read for anyone getting into sharpening
 

tcmx3

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lots of things that are bad ideas "work". often they even come with people telling you "it works for {some other idiot} so it works for me" or "I dont take it that seriously yet Im going to make sure to give my opinion at every opportunity. casually, of course".

unless you're using a method that relies on specifically shaping a stone (e.g. togishi), in which case you probably dont need much advice, you should keep your stones as flat as is practical. this has the added benefit of working faster, in addition to all the above.

I particularly enjoy Hi-Uni B pencils to mark my stones for flattening. I dont just draw an X though, I like to scribble on the whole face to make sure Im flat.
 

M1k3

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I tend to convex my stones. Not intentionally. Need to make an identical set as @stringer so we can switch stone sets?
PXL_20220110_193106152.MP.jpg
 

M1k3

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Eh? If you're doing that while sharpening, you have some unique skills!

What do you flatten on usually? Still using that huge JKI flattener?
I guess? 🤷‍♂️

That's my Venev, so I use my SG4k to flatten it.
 

Pie

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This is prompted by a comment by @Brian Weekley in this thread.

So, sharpening on dished stones often works, but here's what I imagine is happening.

Say you're sharpening with a consistent 15 degree angle (relative to the tabletop) and you have the edge facing toward you when you sharpen, as I always do. If your stone is dished, then at the beginning of each *push* stroke, you'll be hitting the edge at a > 15 degree angle, which decreases as you push, giving a <15 degree angle toward the end of the stroke. If you're scrubbing back and forth on different sections of the blade, you'll probably hit every part of the apex at some point, and will end up with a slightly convex edge, which is fine. It's kinda like the beginning of each push stroke is sharpening and the end of each push stroke is thinning a bit.

I find flat stones useful when doing full edge-length deburring strokes. For then, if you hold a constant angle with the edge facing you, and push away from you while sweeping all the way from tip to heel, you'll hit with a >15 degree angle at the tip and a < 15 degree angle at the heel. This is problematic, since then you might not properly deburr near the heel. Of course, you can correct for this by altering your angle slightly as you sweep or something. You'll probably learn to do this automatically if you're always using the same stone, but if you're using lots of different stones at different levels of dishiness, it can be hard to adapt.

Another subtlety: in the model above I'm sort of assuming stones dish in a U shape, where it's higher near the short edges of the stone, and lower away from the short edges. Of course, that's not really what happens, because the long edges of the stone don't really get worn down in the middle and your stone really looks like a bowl, not a halfpipe.

View attachment 167043

While the discussion above still applies to the bowl-shaped stone, you run into additional problems where your angle of contact is even more unpredictable, since it depends on which part (left, middle, right) of the stone you're using. And when you're doing full length deburring strokes, it's easy to accidentally tilt your hand too much while you're sweeping the knife across the stone, so that the contact point sort of skips out of the bottom of the bowl and suddenly hits the edge of the bowl, near the long edges of the stone. This can lead to inconsistent deburring.

Finally, if you have a suuuper dished stone and a really flat profile, you can really run into problems, since the profile doesn't bend enough to even allow contact with the bottom of the bowl. Most of the time that won't be the case, but if the stone is sorta dished and the profile is pretty flat you have to be more accurate with your point of contact than you otherwise would.

Anyway, feel free to object, trash, scream, applaud, or laugh-react.







**The author of this post approves of halfpipes in extreme sports, just not as a model for dished stones.
Great post!

One thing to add, from unfortunate experience. If you flip your knife to sharpen the backside (vs switching hands), the heel is generally sharpened with the edge perpendicular to the long side of the stone, as to not hit the handle.

A sufficiently dished stone will only contact the knife in small patches (lines?) at the sides of said stone, effectively cutting two grooves into your edge. Or entire bevel, if you’re thinning. Like I was 🙄. I imagine this would also really screw with some deburring techniques that involve running edge across the stone from one side to the other.
 
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cotedupy

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I personally hate using dished stones. I know that if I was better at sharpening I would be able to use them to good effect, and maybe even do something clever with convexity in the bevel or edge. But I'm not, and I can't.

This is just a hunch, but I believe some old stones were either concaved intentionally on one side by their owners for different kinds of sharpening, or possibly even sold that way. I've seen quite a few old stones where the dishing looks pretty much like it's been cut to a specific shape. Particularly some old UK market Washitas.

And I've definitely got one stone (an old India) where the previous owner has intentionally convexed one side and concaved the other. Like a banana.
 

KingShapton

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I personally hate using dished stones. I know that if I was better at sharpening I would be able to use them to good effect, and maybe even do something clever with convexity in the bevel or edge. But I'm not, and I can't.
Absolute agree 👍, using dished stones sucks! I don't like it and I don't do it!!
 
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I bet in many cases more material from the regular stones gets used up from flattening than from sharpening, especially when one tries to be very careful in having flat stones. Depends on how hard the stone matrix is of course. Another benefit of using diamond stones, very little flattening needs to be done.
 

ian

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I tend to convex my stones. Not intentionally. Need to make an identical set as @stringer so we can switch stone sets?View attachment 167116

Been thinking about this. How do you manage to convex yours unintentionally? Is the SG4k usually dished? If so, I could see the two stones reaching an equilibrium where one is convex, the other concave. But you mention that you use an Atoma to flatten the SG4k. Interesting, in any case.
 

M1k3

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Been thinking about this. How do you manage to convex yours unintentionally? Is the SG4k usually dished? If so, I could see the two stones reaching an equilibrium where one is convex, the other concave. But you mention that you use an Atoma to flatten the SG4k. Interesting, in any case.
I guess using the ends a lot more than the middle? I watched some random sharpening video that was in Japanese. What I got out of it was "Draw a line, real or imaginary, in the middle of the stone. Each half is for one side of a knife." I've been mostly doing that, with finishing strokes across the whole stone.
 

psfred

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That is why if you are flattening stone with other stones you need to use three of similar composition -- that evens things out and you can get dead flat fairly easily.

I've had this discussion several times, once with someone unwilling to listen, but a convex bevel is not a bad thing unless the convexity gets in the way -- that one time we were talking about hand plane blades, where a convex bevel past a very small limit means the bevel hits the work first, not the edge. Sort of like cutting carrots with a too thick blade. You get pieces, but they aren't sliced, they are broken. A beefy carpenter or cabinet maker can get a plane iron riding on the bevel to cut with excess pressure, but it's gonna give poor results.

At any rate, I've seen people around here sharpening butcher knives on well worn stones with minimal problems -- you just need to get a fresh edge on carbon steel for cutting up hogs, it's not a beauty contest.

Eventually a saddle shaped stone is going to give a fat enough blade it won't cut well, and I suspect the original owner then replaced the stone and left the worn one on the junk shelf for "antique tool" enthusiasts to find.
 

ian

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That is why if you are flattening stone with other stones you need to use three of similar composition -- that evens things out and you can get dead flat fairly easily.

I've had this discussion several times, once with someone unwilling to listen, but a convex bevel is not a bad thing unless the convexity gets in the way -- that one time we were talking about hand plane blades, where a convex bevel past a very small limit means the bevel hits the work first, not the edge. Sort of like cutting carrots with a too thick blade. You get pieces, but they aren't sliced, they are broken. A beefy carpenter or cabinet maker can get a plane iron riding on the bevel to cut with excess pressure, but it's gonna give poor results.

At any rate, I've seen people around here sharpening butcher knives on well worn stones with minimal problems -- you just need to get a fresh edge on carbon steel for cutting up hogs, it's not a beauty contest.

Eventually a saddle shaped stone is going to give a fat enough blade it won't cut well, and I suspect the original owner then replaced the stone and left the worn one on the junk shelf for "antique tool" enthusiasts to find.

Convex bevels can be pretty nice! FWIW, a lot of the discussion above (e.g. in the OP) is about other drawbacks to dishing, not about convexity.
 

psfred

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Biggest problem I've had with dished stones is that they are never the same shape, and when you move from a coarse one to a finer one that is flatter, you can't get to the apex any more. Woodworking tools are the very worst, can't have a bow in a 2 1/2 wide blade from a coarse stone if you every want to get even full width shavings from it.
 

Mr.Wizard

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I have seen a video where a parang (or something similar) is sharpened on a heavily dished (half-pipe) stone to produce a nice full height convex. If you can keep the blade "flat" against the stone like that it could be quite convenient. The stone would need to be banana-curved for the radius to work out.
 

Pie

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I’m sure there’s tons of people using the same stone for years and it’s just a means to an end. That stone is probably crazy dished, but works perfectly for what they use it for.

As hobbyists most of us sharpen different kinds of knives, and need some sort of standardization between stones. Maybe that’s why we’re all so neurotic about keeping them flat. If you only ever sharpened one petty on one stone, I’m sure you’d just muscle memory into sharpness forever.
 

Kawa

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I flip my stone regularly during sharpening: up becomes bottom and visa versa (so left becomes right). I believe (haven't tested versus not flipping) this helps anti-dishing, since I also believe that the pressure and movement (and therefore the wear of the stone) when sharpening the left or right side of the blade changes. Doesn't matter if you flip the knife or change hands: your left and right hand never act the same and for flippers, I believe you use different parts of the stone while doing left or right side of the bevel.
If you flip the stone you might use more of the stone 'with both hands way of pressure and movement'. You use both halfs of the stone (left and right) as if you sharpen the left side and the right side of the knife on it.

I try to flip the stone intentially on moments when i dont change the bevel side. Especially when you change hands: you still use the same part of the stone mainly.




too autistic?
 

psfred

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I wear soft stones much more on the right side than the left, and have always had issues using the center and not the ends. Trying to get better, it's really frustrating when a blade becomes too convex (or a plane blade isn't straight across the cutting edge).

Thin knives with very small bevels are much easier to sharpen on worn stones I think, especially fairly soft carbon steel. Single bevel knives made of very hard steel are a royal pain on flat ones, I cannot imagine how it's possible to sharpen one on a badly dished stone. But then I'm no expert, either!
 

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