Convex Vs Flat Vs Concave, and why?

Which edge profile do you prefer on a pocket knife blade? Why?

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gamma_nyc

gamma_nyc

Joined
Dec 1, 2007
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Which edge type do you prefer on your pocket knife? Choose in the poll and feel free to support your choice in a message below.

I’ve never consider concave before, but it looks like it would do well in some cut tests I like to do.

2341EF61-ECC8-472C-8046-202ED786C8EA.jpeg
 
I like slicers - effortless slicers that fly through materials being cut. A thin, full-height hollow grind is beautiful for that, with a very thin edge and no friction in the 'hollow' of the primary grind behind the edge. Also a breeze to resharpen, with minimal metal removal to fully apex again.

If using a thicker blade for tasks better reserved for thinner ones, like cutting cardboard, a shallow & polished convex helps reduce friction. That's sort of a special circumstance calling for that profile.

Flat grinds are OK, if they're THIN. Also easy to thin further and refinish to remove scuffs & scratches by laying flat to sandpaper.
 
The problem with your poll is that it is meaningless without exact dimensions.

For example, a flat grind can be virtually identical to a convex or concave grind, or it can be more acute or less acute. It can have more or less metal behind the edge. On and on.

The nice thing about a flat grind is that it is easy to describe and achieve. If I say I put a 15 dps flat edge on a blade, anyone can reproduce that exact edge angle. If I also specify the edge width (behind the edge) the whole edge can be replicated exactly by anyone. The other edges are way more difficult to describe and replicate.

A flat edge can be acute or brutal. You can easily modify it with a microlevel or, as 000Robert says, by cutting back the shoulders.

All of these grinds, depending the exact geometry you give them, can be excellent performers or any given task.
 
Twindog said:
The problem with your poll is that it is meaningless without exact dimensions.

For example, a flat grind can be virtually identical to a convex or concave grind, or it can be more acute or less acute. It can have more or less metal behind the edge. On and on.

The nice thing about a flat grind is that it is easy to describe and achieve. If I say I put a 15 dps flat edge on a blade, anyone can reproduce that exact edge angle. If I also specify the edge width (behind the edge) the whole edge can be replicated exactly by anyone. The other edges are way more difficult to describe and replicate.

A flat edge can be acute or brutal. You can easily modify it with a microlevel or, as 000Robert says, by cutting back the shoulders.

All of these grinds, depending the exact geometry you give them, can be excellent performers or any given task.
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I could not concur more with this.
 
It is not clear to me whether you are talking about primary or secondary (bevel) grinds. My take on the terminology is that the picture of a "flat" grind looks more like a Scandi grind, which would typically have a 10 to 11 DPS primary grind and no secondary grind. If you put a 15 to 25 DPS secondary grind (bevel) on it then you would have what is commonly called a flat grind. A flat grind with a narrower primary angle that extends all the way to the spine is a full flat grind. You could also have a flat grind with a convex bevel, or a convex primary grind with a flat bevel.

Of the pictures, a concave bevel would be my last choice. My wife asked me to sharpen a linoleum knife that has a concave bevel. I have not yet figured out how to do that without turning it into a hollow grind as in the pictures.
 
As they're pictured in the OP, my idea of a standard hollow grind usually matches the diagram labelled as 'hollow' - leaving the edge grind itself essentially a standard V-beveled edge - same as the factory edge grind on the vast majority of knives anyway, regardless of their primary grind shape behind it. A hollow grind is still 'concave' in the basic geometric sense, even if the 'concave' grind on each side doesn't extend all the way to the cutting edge. Taking the concavity on each side all the way to intersection at the apex would obviously leave the edge extremely thin and very sharp, but also weak and vulnerable to damage. I couldn't see much practicality in that, in my own uses.

I view all truly sharp edges as having essentially a V-grind nearest to the apex itself, no matter how narrow the 'bevels' of that V-grind might be (down to microscopic in width) and no matter what the overall (primary) grind is - flat, convex or hollow/concave. So long as the steel behind the apex is adequately thin enough to provide good cutting geometry through many resharpenings, and adequately thick enough to provide decent edge strength in normal uses, I feel the primary grind and its thickness to the spine will have the more noticeable impact on how the blade as a whole functions in cutting.

Every knife I've ever REALLY liked for ease of slicing all have the thinness of the primary grind in common, at least, with the very thinnest and best examples usually being a very thin, full-height hollow primary grind up to a relatively thin spine. But very thin & flat primary grinds (like a Victorinox paring knife), or very thin & shallow convex primary grinds (like an Opinel, even with its standard V-bevelled secondary grind) have also impressed me nearly as much for ease of slicing.

And for thicker blades, a polished convex to round off and smooth out the shoulders of what would otherwise be a thick V-ground edge can make a big improvement in cutting. Buck's old 440C blades were notoriously thick behind the edge, in spite of the fact they were also ground to a hollow profile for some width behind the cutting edge. Picture below, of one of Buck's old 112 blades, a '2-dot' in 440C. The best thing I ever did for that knife was to thin, convex and polish the grind below the 'hollow', which turned that blade into a vicious, slick-slicing cardboard cutter. Prior to doing that, the thick-edged, V-beveled shoulders of the edge grind were terrible about binding up and getting pinched in heavy cardboard.
rxT9k8t.jpg
 
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I will agree with some of the other guys posts that the picture the op is using is not an accurate depiction of the blade grinds. The hollow grind and convex grind are a fair representation but the flat is what I call a scandi grind. In my opinion a flat grind goes up to near the spine or a full flat goes clear to the spine and they will still have a secondary bevel at the edge. The concave grind I’m not familiar with and by the picture it appears that the primary grind goes all the way to the edge. I would think it would be very difficult to maintain an edge unless it had a secondary bevel and it looks to be very delicate at the edge.

I have a few scandi grind blades that are thin blade stock but most of my knives are either hollow grind or flat grind. I’ve had a few convex edge knives but they were to much trouble to maintain a good edge. Flat or hollow grinds are much easier to maintain and more practical in my use. For general use I prefer the hollow grind but for some tasks I prefer a flat grind so I usually carry both. 😆
 
Obsessed with Edges said:
As they're pictured in the OP, my idea of a standard hollow grind usually matches the diagram labelled as 'hollow' - leaving the edge grind itself essentially a standard V-beveled edge - same as the factory edge grind on the vast majority of knives anyway, regardless of their primary grind shape behind it. A hollow grind is still 'concave' in the basic geometric sense, even if the 'concave' grind on each side doesn't extend all the way to the cutting edge. Taking the concavity on each side all the way to intersection at the apex would obviously leave the edge extremely thin and very sharp, but also weak and vulnerable to damage. I couldn't see much practicality in that, in my own uses.

I view all truly sharp edges as having essentially a V-grind nearest to the apex itself, no matter how narrow the 'bevels' of that V-grind might be (down to microscopic in width) and no matter what the overall (primary) grind is - flat, convex or hollow/concave. So long as the steel behind the apex is adequately thin enough to provide good cutting geometry through many resharpenings, and adequately thick enough to provide decent edge strength in normal uses, I feel the primary grind (and its thickness) to the spine will have the more noticeable impact on how the blade as a whole functions in cutting.

Every knife I've ever REALLY liked for ease of slicing all have the thinness of the primary grind in common, at least, with the very thinnest and best examples usually being a very thin, full-height hollow primary grind up to a relatively thin spine. But very thin & flat primary grinds (like a Victorinox paring knife), or very thin & shallow convex primary grinds (like an Opinel, even with its standard V-bevelled secondary grind) have also impressed me nearly as much for ease of slicing.

And for thicker blades, a polished convex to round off and smooth out the shoulders of what would otherwise be a thick V-ground edge can make a big improvement in cutting. Buck's old 440C blades were notoriously thick behind the edge, in spite of the fact they were also ground to a hollow profile for some width behind the cutting edge. Picture below, of one of Buck's old 112 blades, a '2-dot' in 440C. The best thing I ever did for that knife was to thin, convex and polish the grind below the 'hollow', which turned that blade into a vicious, slick-slicing cardboard cutter. Prior to doing that, the thick-edged, V-beveled shoulders of the edge grind were terrible about binding up and getting pinched in heavy cardboard.
rxT9k8t.jpg
Click to expand...
I'm sure this will horrify some folks, but I think your observation about Buck's grind applies (albeit to a lesser extent) to Chris Reeve knives. The Sebenza has a beautiful hollow grind, but the thinnest part of the hollow is a fair bit north of the edge. To make the Sebenza a better slicer, I put a dual-grind on it, with a 12° secondary bevel and then a 15° edge bevel.
 
Blade Lab said:
I'm sure this will horrify some folks, but I think your observation about Buck's grind applies (albeit to a lesser extent) to Chris Reeve knives. The Sebenza has a beautiful hollow grind, but the thinnest part of the hollow is a fair bit north of the edge. To make the Sebenza a better slicer, I put a dual-grind on it, with a 12° secondary bevel and then a 15° edge bevel.
Click to expand...
I have three older Sebenzas myself - 1 large one and 2 of the smaller ones, all in S30V. I did find the original edge grind to be kind of unimpressive, in terms of the geometry. Aesthetically, they were finished beautifully at the edge, but somewhat wider and more polished than I liked - didn't have much bite in the edge. I did thin the large one out a bit, by freehand means, to something a little less than 30° inclusive. The two smaller ones I haven't touched - they've been kept essentially as safe queens, while I decided to keep & maintain the larger one as a user. Just thinning the edge a little bit and using nothing more than a Coarse or Fine DMT to set the working edge, I've liked how it's cutting much better since doing that.
 
A high or to the spine flat grind with a secondary bevel is my favored choice. I am boring, but this geometry is very effective for my purposes. Hollow grinds with secondary bevels would be my second choice, but something about the delicacy of this design, or perceived delicacy, rubs me wrong. I have a few hollow ground blades, but not that many. I have one scandi ground Morakniv, but have not yet sharpened it. I kinda do not look forward to it when the time comes. I need to read up on that.
 
My favorite are thin, almost full height hollow grinds on a small to mid size folder with a standard V secondary bevel. I also like a full flat grind as long as it's not too thick behind the edge. On fixed blades I like a flat grind, V edge, or a scandi.
 
Blade Lab said:
I'm sure this will horrify some folks, but I think your observation about Buck's grind applies (albeit to a lesser extent) to Chris Reeve knives. The Sebenza has a beautiful hollow grind, but the thinnest part of the hollow is a fair bit north of the edge. To make the Sebenza a better slicer, I put a dual-grind on it, with a 12° secondary bevel and then a 15° edge bevel.
Click to expand...
That is about how I handle most knifes. I usually change the angle at a grit progression, kind of an approximation of a convex edge.
 
On pocket knives I really like full-height shallow hollow grinds like on Douk-Douk knives where it's very nearly a full flat grind with a little extra material removed between the edge and the spine. Slices like a demon while remaining robust enough for things like wood carving.
 
I'm a convex or flat grind guy.
I can upkeep these types at my house with limited tools for the entire usable life of the knife.
I don't mind shallow hollow grinds, like are found on case spey blades, but I do notice that they feel a little awkward to me when I'm cutting through an apple.
 
Full flat grinds almost always have a secondary bevel as it would be impractical to grind the full face of the blade on every sharpening. Secondary bevels on all sorts of grinds end up being convex to some degree unless a fixed angle system is used. When free handing it is virtually impossible to not introduce a little bit of convexity at the very edge.
 
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