Convex vs Flat grind?

[Ryansknives]

Does one get sharper than the other? I’ve always been under the impression sharpness has a lot to do with blade thickness behind the edge. Hence the reason kitchen knives are made from thin metal.

I just sharpened a Kukri with a belt sander using a convex grind. Just easier with the curve. I went to town wacking some wood. The edge held up really well. It also chewed through stuff like never before. It was the first time I’ve sharpened it.

I sharpened 2 chef knives one with a convex grind one with a flat grind. Both cut very well. However I definitely noticed with slicing tomatoes and a couple other veggies the flat grind seemed to work ever so slightly better. The convex grind felt like it had a slight drag to it. But not enough to say one knife was sharper than the other.

Let me clarify a few things. The belt sander I’m using is a harbor freight 1X30. I’m actually planning on upgrading that. I’m using the same belts for both grinds. The flat is done on the platen with an adjustable angle guide. I did not measure the angle just used the presets of about 20 degrees. For the convex edge I used the slack portion and just gustimated what felt right as far as the angle. I’ve also just recently started playing with convex grinds. So my skill with sharpening a convex edge may have had a factor in my experience.

 

[lonestar1979]

Depends on thickness of the blade and thickness behind edge....nothing wrong with either,id rather take flat grind,and full convex is great for chopping.Thickness is key!

 

[Obsessed with Edges]

Ditto for thinness being the bigger influence, rather than the grind profile alone.

A very thin, polished & subtle convex is great in the kitchen, BTW. A 'V'-bevelled profile can sometimes bind up at the crisp shoulders of the bevels in foods that pinch the blade (like apples, for example). That's where a thin polished convex does better with such foods. Polished convex also does well in cutting tough materials, like heavy cardboard, for the same reason. But again, THIN is a big part of that.

 

[chiral.grolim]

Does one get sharper than the other? I’ve always been under the impression sharpness has a lot to do with blade thickness behind the edge. Hence the reason kitchen knives are made from thin metal.

I just sharpened a Kukri with a belt sander using a convex grind. Just easier with the curve. I went to town wacking some wood. The edge held up really well. It also chewed through stuff like never before. It was the first time I’ve sharpened it.

I sharpened 2 chef knives one with a convex grind one with a flat grind. Both cut very well. However I definitely noticed with slicing tomatoes and a couple other veggies the flat grind seemed to work ever so slightly better. The convex grind felt like it had a slight drag to it. But not enough to say one knife was sharper than the other.

Let me clarify a few things. The belt sander I’m using is a harbor freight 1X30. I’m actually planning on upgrading that. I’m using the same belts for both grinds. The flat is done on the platen with an adjustable angle guide. I did not measure the angle just used the presets of about 20 degrees. For the convex edge I used the slack portion and just gustimated what felt right as far as the angle. I’ve also just recently started playing with convex grinds. So my skill with sharpening a convex edge may have had a factor in my experience.

Good job sharpening your knives
I use the HF 1x30 sometimes as well and, just like you, "gustimate". The main things with convex sharpening is 1) don't hold the blade at too high of an angle or you'll just round over the cutting edge and 2) don't hold it at too low of an angle or you'll only remove the shoulders of the previous bevel and never actually sharpen the apex. I trend toward #2 and simply apply a red sharpie to mark the edge, then grind, and then look at the blade afterward to see if my angle is appropriately removing the marker all the way to the edge. If your edge is REALLY messed up, it should be obvious whether or not you are sharpening the apex without the aid of a marker

Sharpening a knife on a solid flat hone will produce a thinner edge than sharpening at the same angle on a flexible hone (the 1x30 belt). HOWEVER that doesn't mean that one is necessarily "sharper" than the other since that is really determined by how "keen" your edge becomes at the apex. Straight-razors, for example, are typically honed flat and then stropped to a convex final apex that is keener and cleaner than what the stone achieved, though the apex geometry may be thicker behind that apex. Here is a link to a really really neat blog by ToddS with SEM images of blades sharpened and stropped in various ways: https://scienceofsharp.com/

Something else you can vary to achieve more/less "bite" is the hone grit to produce a toothier or a crisper edge at a given angle. In most cutting tasks, I've found I prefer a toothy edge for the "bite".

 

[MtnHawk1]

Ditto for thinness being the bigger influence, rather than the grind profile alone.

When I first started to freehand sharpen, I got too hung up learning about all the many different edge types and angles, BTE thickness, blade grinds, steel types, etc., and all the many different theories and opinions about them. I didn't pay enough attention to the simple and obvious: that thin edges cut and chop better than thick edges. Now if a knife, including big wood-choppers, isn't durable enough with a thin V-edge and microbevel (relative to the blade thickness and task), I don't want the knife.

 

[FortyTwoBlades]

Any edged tool should be kept with as thin of an edge as is consistent with requisite strength. The advantage of a convex vs. flat edge geometry is that it's essentially a flat geometry with the shoulders taken down, reducing drag while maintaining an equal edge angle.

 

[lonestar1979]

Subtle convex ,almost like flat is best....also flat grind with convex edge.Thickness matters the most at the end.

 

[MtnHawk1]

The advantage of a convex vs. flat edge geometry is that it's essentially a flat geometry with the shoulders taken down, reducing drag while maintaining an equal edge angle.

I'm not disagreeing with you, just saying that, looking at profiles of convex and flat edges, the convex edge looks like it would have more cutting resistance due to its more rounded profile, with more mass.

 

[Wowbagger]

Any edged tool should be kept with as thin of an edge as is consistent with requisite strength. The advantage of a convex vs. flat edge geometry is that it's essentially a flat geometry with the shoulders taken down, reducing drag while maintaining an equal edge angle.

Totally this ^
and it's an excellent excuse to carry more than one knife

I don't know anything much about convex, really, but the knives that are lightly convex over the entire sides of the knife and the spine not being the thickest part has been fascinating me this year. I "ground" one like that for my self but have not got it thin enough nor spent enough time with it (something to look forward to in the new year).

I use quotation marks on "ground" because the way I go about it is so half fast . . . I just watched a vid by Big Brown Bear (BBB) last night of him grinding a REAL KNIFE, very thin behind the edge, and . . . it is a beautiful thing to watch !
Keep it up BBB !

 

[Wowbagger]

I'm not disagreeing with you, just saying that, looking at profiles of convex and flat edges, the convex edge looks like it would have more cutting resistance due to its more rounded profile, with more mass.

I find this really shows up while cutting a tomato or other soft skinned item (grapes ?).
As it does if the edge geometry is just too wide angle and thick. The skin just wraps around the edge and stops the cut. Soft rolls and buns like a hot dog bun resists this type of edge even when sharp. Thin it out and close up the angle and it slips right in and down.

One example is this fruit tester's knife. For some reason the factory thought it needed the geometry of a fire wood spitter or masonry tool. Rolls, tomatoes and such just laughed and ignored it's attacks. I thinned the edge and knocked the sharpening angle back and as if by magic I had a useful light long bladed knife.
What that has to do with convex I don't know . . . I'm just saying . . .
Maybe I'm saying I'm not convinced a convex edge would have improved it's performance.
Maybe a guy needs to carry a knife with geometry to cut tomatoes no convex AND a knife to cut apples with convex. Because you never know . . . you really never know.
(the knife on the right)


One doesn't need a lot of durability in an edge that just cuts soft skinned fruit, at least until the knife clacks down on the cutting board. White soft plastic cutting boards forever for me !

 

[MtnHawk1]

Wowbagger, I don't have any convex-edge kitchen knives but your descriptions of cutting with both types of edges are what I would expect would happen. I don't give a lot of attention to sharpening kitchen knives. I just keep a thin V-edge on and call it good. Almost every food item I cut is soft, or relatively so. I am more concerned about performance in terms of clean, fast slicing than I am in other things such as edge durability, so I agree with you there, too. I use a high grit "steel" to keep the edge sharp as long as possible before sharpening.

Most of my heavy knife use is in the wilderness. I have to keep my pack weight down so I don't have the luxury of bringing a lot of edged tools, stones, and other sharpening tools and supplies. To keep things simple, I also put a V-edge and microbevel on these knives, including big wood-choppers. I've read a lot of threads and posts about the advantages of convex edges over V-edges, especially for chopping tools, and maybe that is so. I've never done a strict comparison test in the real world to find out for myself. My best guess, though, which seems logical, is that a thin edge will always outperform a thicker edge, no matter what the edge profile or geometry.

 

[FortyTwoBlades]

I'm not disagreeing with you, just saying that, looking at profiles of convex and flat edges, the convex edge looks like it would have more cutting resistance due to its more rounded profile, with more mass.

For equal edge angle and stock thickness, there will be less mass in a convex, and the bevel shoulder will be higher up on the blade. If a convex edge has more mass behind the apex than a flat-ground edge, the convex must also have a thicker edge angle (it's a geometric impossibility for it to be otherwise.) And if that is true, then a flat-ground edge of equal edge angle to the convex would be even thicker than the convex would be, and therefore also thicker than the geometry of the thin flat-ground knife. You can keep stacking thicker and thicker geometries inside each other for days, but the only way you can have one inside the other and have the same angle at the apex is with the convex having less material in the shoulder than the flat grind.

Imagine starting with a flat-ground edge and then lightly flat-grinding just the edge shoulder without touching the edge itself. This is essentially what is going on with a convex edge, except after grinding that shoulder, you then grind the two shoulders made by that grinding, then the four shoulders made by that grinding, and the eight made by that grinding, and so on and so forth until you have a flawless, smooth convex. If converting a knife from a flat-ground edge to a convex on a flexible abrasive surface, it's very easy to accidentally increase the edge angle, which will result in a thickened geometry at the expense of cutting performance, but it will still be thinner than if you put an edge of that same thickened angle on a flat-ground bevel without the blending done behind it. Does that make sense?

 

[lonestar1979]

Bbb knives have amazing geometry,thats how all knives should be ground,and also heat treat is higher on rockwell scale.All edges off stones are slightly convexed,obtyse like convex should be left for heavy choppers and axes,not slicers.

 

[sickpuppy1]

Bbb knives?

 

[lonestar1979]

Triple B, big brown bear knives.

 

[bucketstove]

Hi

slicing tomatoes and a couple other veggies

Cut the cheese. Mozarella/cheddar... one blade will jam more than another.

Looking at words

Imagine words

Oh my word

 

[FortyTwoBlades]

Hi

Cut the cheese. Mozarella/cheddar... one blade will jam more than another.


Oh my word

Cute, but it misrepresents a lot of aspects of why they work the way they do and why they can be useful.

 

[MtnHawk1]

FortyTwoBlades, thank you for taking the time to detail this out (Post 12). It's the best and most concise explanation I've ever read contrasting convex and flat edges, and I've read many. Your post helped clarify a lot.

One of the main problems I've had is trying to conceptualize how a convex angle is measured. I never understood how a somewhat curved surface could be thought of, and measured, as an angle. Where on the curve is the "angle" supposed to be?

I think another problem in my understanding, which you mentioned in both posts 6 and 12, is regarding "equal edge angle". Often the diagrams and drawings of flat and convex edges do not appear to be scaled the same, with the convex appearing thicker.

I relief grind at a very low angle, as taught by John Juranitch, so there is little or no shoulder on or near my knife edges, except for a small low angle microbevel. This being so, maybe my thin flat edges and thin convex edges are so similar that there is little or no real-world difference in how they actually perform.

What helped me to understand convexity better was when I read that, since it's impossible to repeatedly hold the exact same angle when freehand sharpening, some convexity will naturally occur. You described a similar process, with a similar result, very well and clearly in your second paragraph.

The advantage of a convex vs. flat edge geometry is that it's essentially a flat geometry with the shoulders taken down, reducing drag while maintaining an equal edge angle.

After reading your explanation, I re-read this quotation with much more understanding than the first time. This one sentence pretty much says it all, if I understand it correctly, and I think I do now.

 

[bucketstove]

Cute, but it misrepresents a lot of aspects of why they work the way they do and why they can be useful.

more words

 

[FortyTwoBlades]

FortyTwoBlades, thank you for taking the time to detail this out (Post 12). It's the best and most concise explanation I've ever read contrasting convex and flat edges, and I've read many. Your post helped clarify a lot.

One of the main problems I've had is trying to conceptualize how a convex angle is measured. I never understood how a somewhat curved surface could be thought of, and measured, as an angle. Where on the curve is the "angle" supposed to be?

I think another problem in my understanding, which you mentioned in both posts 6 and 12, is regarding "equal edge angle". Often the diagrams and drawings of flat and convex edges do not appear to be scaled the same, with the convex appearing thicker.

I relief grind at a very low angle, as taught by John Juranitch, so there is little or no shoulder on or near my knife edges, except for a small low angle microbevel. This being so, maybe my thin flat edges and thin convex edges are so similar that there is little or no real-world difference in how they actually perform.

What helped me to understand convexity better was when I read that, since it's impossible to repeatedly hold the exact same angle when freehand sharpening, some convexity will naturally occur. You described a similar process, with a similar result, very well and clearly in your second paragraph.



After reading your explanation, I re-read this quotation with much more understanding than the first time. This one sentence pretty much says it all, if I understand it correctly, and I think I do now.

Thank you again!

Yeah, it's technically possible to measure the angle of the intersection of two arcs, but it requires mathematically modeling those arcs first, which is kind of a pain in the butt, making it impractical for all but theoretical purposes in this context.

From a practical perspective, you can use a hard material (that's softer than steel) such as plexiglass, to find the effective angle by laying the blade flat on the surface of the material, and slowly tilting the spine up until a forward push on the blade causes it to bite. The reason to use a hard material as the test medium is to reduce how much the material will deflect under pressure (which should be kept light in any case with this test) so you get a closer result to the actual edge angle. Since an edge will only bite at any angle greater than the bevel angle, you can find the angle by approximating just the tiniest bit under the angle at which the edge will actually bite.