# Convex sharpening



## crlums (Jun 15, 2020)

I just traded for a Tanaka blue 2 Damascus. This will be my first knife with a convex edge. What are the main differences between sharpening something like this versus something like my Tanaka b2 nashiji which is wide bevel and hallow ground? Does the term hamaguri apply here or is that specific to wide bevels.


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## M1k3 (Jun 15, 2020)

You only have to worry about the convex bevels when it comes time to thin. Unless you want to actually sharpen your edge convexly.


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## crlums (Jun 15, 2020)

M1k3 said:


> You only have to worry about the convex bevels when it comes time to thin. Unless you want to actually sharpen your edge convexly.


Thanks for the advice. Would having a convex edge improve performance at all, or does the performance benefit come from the convexity above the actual cutting edge?


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## M1k3 (Jun 15, 2020)

Mainly from the bevels. Geometry cuts it's a common saying. The edge initiates the cut, beyond that geometry and friction, play a major role.

If you're just picking up sharpening, I wouldn't worry about trying to convex the edge. Just focus on getting a good quality edge first. 

Learn to walk before trying to do cartwheels.


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## crlums (Jun 15, 2020)

Got it. I’m not new to sharpening but I am definitely new to convex grinds. It sounds like I’ll do best to just keep a consistent bevel angle when I sharpen the edge. If/when I need to thin it I guess I’ll need to learn how to maintain the convex geometry though. Thanks again for the input!


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## ian (Jun 15, 2020)

A convex edge isn’t really advantageous in my opinion. It’s nice to soften the shoulders if you make a largish bevel, but I kinda think that having the convexity all the way to the edge makes for a subpar cutter. I’m not sure who explained this to me, maybe @gman(?), but if there’s convexity at the edge the knife can behave more like an axe, splitting rather than cutting the food, especially with hard food like carrots, etc.... the point being that during the cut, as soon as it penetrates the outer layer the very apex is no longer in contact with the product and instead the convex sides are splitting it apart. So yea, hold a constant bevel angle.


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## kayman67 (Jun 15, 2020)

Have you put this to a test? I experienced the opposite for years for most types and applications.


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## ian (Jun 15, 2020)

kayman67 said:


> Have you put this to a test? I experienced the opposite for years for most types and applications.



You may be right, too. I’m not positive, of course, and I haven’t tried to run a rigorous test. But the explanation made sense to me, and explained why sometimes when I’m not careful with my angles it seems like the knife stops “cutting“ after it enters the product. The discussion I was referencing was really about “why is it necessary to hold a constant angle?”. It also seems to be something you can verify in a macro scale by trying to push cut something with an axe, although of course there are other factors there.... You have more experience than I do, though, so I’d be happily corrected. Any other details on your experience?


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## WPerry (Jun 15, 2020)

Just for clarity, we're talking about a convexity at the edge, along the lines of a secondary bevel, as opposed to convexity further up the sides, correct?


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## ian (Jun 15, 2020)

Yes, convexity near the edge.


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## Barmoley (Jun 15, 2020)

The claim to fame of convex edges is that they are stronger than straight edges of the same angle at the very edge and cut better than the increased angle that would be required to increase durability to the level of the convex edge, axes, choppers, swords, etc usually have convex edges. So they are a compromise when your straight edge fails due to durability. One could also argue that the transition from the very edge to the rest of the bevel is smoother with convex, but I am not sure that is much of the factor if we are talking about the very edge and not the overall grind. For precision cutting or ultimate cutting ability straight, acute edge will win, wood working tools, machine cutters, precision cutters usually have straight edges. If durability is an issue then convex edge might help. Consistent convex edge is hard to do on hard, flat stone, much easier on a somewhat slack belt, or sand paper on top of a mouse pad or any other number of techniques. Probably not something to worry about. Just try to keep a consistent angle on your stone, it will be somewhat convex anyway, just from the hand wobble.


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## M1k3 (Jun 15, 2020)

Barmoley said:


> The claim to fame of convex edges is that they are stronger than straight edges of the same angle at the very edge and cut better than the increased angle that would be required to increase durability to the level of the convex edge, axes, choppers, swords, etc usually have convex edges. So they are a compromise when your straight edge fails due to durability. One could also argue that the transition from the very edge to the rest of the bevel is smoother with convex, but I am not sure that is much of the factor if we are talking about the very edge and not the overall grind. For precision cutting or ultimate cutting ability straight, acute edge will win, wood working tools, machine cutters, precision cutters usually have straight edges. If durability is an issue then convex edge might help. Consistent convex edge is hard to do on hard, flat stone, much easier on a somewhat slack belt, or sand paper on top of a mouse pad or any other number of techniques. Probably not something to worry about. Just try to keep a consistent angle on your stone, it will be somewhat convex anyway, just from the hand wobble.


I always remembered Murray Carter saying something along the lines "we're humans, not machines". One vs the other


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## kayman67 (Jun 15, 2020)

Usually a convex edge will be made in form of a higher thinner grind bevel with the benefit of added strength and less side pressure. The knife cuts on its own with ease. This doesn't mean that someone can't make a thicker convex that might have some negative effects to some extent in some applications. I was curious what those might be, though. I haven't seen any downsides so far, but who knows. 

Talks about this aren't new in other types of knives, though convex always worked better as a cutter.


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## crlums (Jun 15, 2020)

Ok, so lots of opinions and the consensus seems to be that I should just go for a consistent angle at the cutting edge. Thats where I'll start. The discussion of how and if/why to create a convex cutting bevel is super interesting, but beyond my skill level at this point.

When it comes to thinning above the sharpening edge, what is the strategy for maintaining the convexity? Do you vary the pressure to try to match the convex curve that already exists?


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## kayman67 (Jun 15, 2020)

To some extent it will be convex anyway. Even with the big boys there were laser tests done and most scandi grinds proved not to be zero, so some convexity was there.


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## crlums (Jun 15, 2020)

kayman67 said:


> To some extent it will be convex anyway. Even with the big boys there were laser tests done and most scandi grinds proved not to be zero, so some convexity was there.


What about for something like the Tanaka B2 damascus that is known for having a pronounced convexity? I'm sure that when I thin I don't get a perfect zero grind, but I suspect I would reduce the convexity some. Is it worth trying to avoid that or is that just to be expected if I decide to thin it?


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## kayman67 (Jun 15, 2020)

Now, for that knife thinning would require a lot of work and if it's not abused a lot, you might not need it at all for a very long time.


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## crlums (Jun 15, 2020)

kayman67 said:


> Now, for that knife thinning would require a lot of work and if it's not abused a lot, you might not need it at all for a very long time.


Got it. The Tanaka I traded for hasn’t actually arrived yet, and probably won’t need to be thinned based on the pics I saw. Just trying to think ahead to how to care for something with different geometry than I’m used to.


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## Nemo (Jun 15, 2020)

@Benuser has an interesting method for making a convex edge. IIRC, h starts at a conservative thinning angle (to essentially knock the shoulders off the bevel). He then gradually raises the angle until he gets a burr.

As regards thinning a convex blade face- you can cut in a flat thinning bevel but you will gradually distort the grind.

My approach to maintaing the convexity while thinning would be similar to hamiguriba sharpening. Remember that steel is removed beneath where you are putting pressure. Start with pressure just behind the edge then gradually move up the blade until you are as high as you feel comfortable with. You will essentially be creating a convex wide bevel. And will obviously need to refinish the blade if you don't like this look (especially if it doesn't create even looking bevels). On a damascus blade, etching would also be a good idea.


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## ian (Jun 15, 2020)

If the knife comes in good condition, though, you will have forgotten about this thread by the time you need to do substantial thinning.

edit: that’s assuming home use. I suppose if you use it at work you might need to do that sooner, but still not for a long while. edge sharpening plus thinning right behind the edge will take care of you for a while.


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## crlums (Jun 15, 2020)

Nemo said:


> @Benuser has an interesting method for making a convex edge. IIRC, h starts at a conservative thinning angle (to essentially knock the shoulders off the bevel). He then gradually raises the angle until he gets a burr.
> 
> As regards thinning a convex blade face- you can cut in a flat thinning bevel but you will gradually distort the grind.
> 
> My approach to maintaing the convexity while thinning would be similar to hamiguriba sharpening. Remember that steel is removed beneath where you are putting pressure. Start with pressure just behind the edge then gradually move up the blade until you are as high as you feel comfortable with. You will essentially be creating a convex wide bevel. And will obviously need to refinish the blade if you don't like this look (especially if it doesn't create even looking bevels). On a damascus blade, etching would also be a good idea.


Thanks for the description. I’m going to hold off on doing this on the Tanaka. I have a wide bevel I want to thin too though so this description is very helpful. I’ll also watch jons videos again.


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## crlums (Jun 15, 2020)

ian said:


> If the knife comes in good condition, though, you will have forgotten about this thread by the time you need to do substantial thinning.
> 
> edit: that’s assuming home use. I suppose if you use it at work you might need to do that sooner, but still not for a long while. edge sharpening plus thinning right behind the edge will take care of you for a while.


Yeah I totally agree. I think I got ahead of myself on the Tanaka. I still appreciate all the tips and onions though. I’m more likely to apply this to thinning a wide bevel.


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## ian (Jun 15, 2020)

Np, if you ever run out of onions we got you covered.


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## Nemo (Jun 15, 2020)

crlums said:


> Thanks for the description. I’m going to hold off on doing this on the Tanaka. I have a wide bevel I want to thin too though so this description is very helpful. I’ll also watch jons videos again.


I agree, if the Tanaka arrives in good condition, this could be a long way off.

Which wide bevel do you want to thin? Some have convex edges which will respond well to this techniqe but many have flat or slightly convex bevels that should be thinned flat. Often you will be able to thin just behind the edge and just below the shinogi, leaving a gap in between. This indicates a convex wide bevel. Unless you have a very big grinding wheel, you will need to grind this flat. If you want an aesthetic result, you can grind the convexity out on the first thinning but it seems to me that this is a waste of steel and the end result doesn't look too bad, especially once a patina sets in.


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## crlums (Jun 15, 2020)

ian said:


> Np, if you ever run out of onions we got you covered.


Haha, what an appropriate typo for this forum. (I meant opinions if it wasnt clear)


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## crlums (Jun 15, 2020)

Nemo said:


> I agree, if the Tanaka arrives in good condition, this could be a long way off.
> 
> Which wide bevel do you want to thin? Some have convex edges which will respond well to this techniqe but many have flat or slightly convex bevels that should be thinned flat. Often you will be able to thin just behind the edge and just below the shinogi, leaving a gap in between. This indicates a convex wide bevel. Unless you have a very big grinding wheel, you will need to grind this flat. If you want an aesthetic result, you can grind the convexity out on the first thinning but it seems to me that this is a waste of steel and the end result doesn't look too bad, especially once a patina sets in.


I also traded for a cheap yamashin funayuki. I figured it would be a fun (and low stakes) knife to practice wide bevel thinning and sharpening on. Would you suggest starting with a compound wide bevel approach or would be more strait forward to start with more of a zero grind approach where I thin the entire wide bevel evenly?


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## M1k3 (Jun 15, 2020)

crlums said:


> Haha, what an appropriate typo for this forum. (I meant opinions if it wasnt clear)


Either way.


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## Nemo (Jun 15, 2020)

crlums said:


> I also traded for a cheap yamashin funayuki. I figured it would be a fun (and low stakes) knife to practice wide bevel thinning and sharpening on. Would you suggest starting with a compound wide bevel approach or would be more strait forward to start with more of a zero grind approach where I thin the entire wide bevel evenly?


I would follow the existing bevel. Convex if it's convex, flat if it's flat or concave. Depending on the steel, you can grind to a zero grind if you want, then add a microbevel. Otherwise stop just short of raising a burr and add a primary (edge) bevel of 10-15 degrees per side (depending on the steel).


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