Convex edge on a full flat grind

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maximus83

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What is the advantage of doing a convex edge on a full flat grind (FFG)?

I've seen a couple makers doing this. Interested to find out whether this could be a good-performing type of edge and what the reasons would be to try this, versus going with a standard v-grind.
 
Convex edges can best be thought of as "V" edges with the shoulders knocked off. So for equal edge angle they're a thinner geometry.
 
Is this a worthwhile thing to do? If it's a great performing edge, I'm wondering why more makers, or individuals here in the forums, aren't using it and discussing it. Obviously in the case of knife manufacturers, this could be for a lots of reasons that have nothing to do with the quality of edge, such as customer demand, and manufacturing costs of doing things at scale. You do see a lot of knives being offered with some combo of flat grind plus v-edge, and also you see a lot of full convex. But I don't see a lot of this FFG + convex edge combination.

ETA: here's a recent example I ran across.
 
I do it on many knives (not all). Essentially what 42Blades said. When I have the geometry of the primary bevel ~95% done (the blade bevels...not the edge bevel), usually almost to zero, I will "sharpen" it creating the secondary bevel (edge bevel) at a low angle. Maybe 7° per side, apexing at this stage. Then I knock off that shoulder as I do the rest of the final bevel shaping/polishing. This creates a VERY slight convex just above the edge. The apex is then ground back just enough to reflect a little light and allow some "meat" for the cutting edge, sharpen as usual (usually 15° per side). I don't always include that info when posting for sale, but sometimes will mention it.
 
samuraistuart said:
I do it on many knives (not all). Essentially what 42Blades said. When I have the geometry of the primary bevel ~95% done (the blade bevels...not the edge bevel), usually almost to zero, I will "sharpen" it creating the secondary bevel (edge bevel) at a low angle. Maybe 7° per side, apexing at this stage. Then I knock off that shoulder as I do the rest of the final bevel shaping/polishing. This creates a VERY slight convex just above the edge. The apex is then ground back just enough to reflect a little light and allow some "meat" for the cutting edge, sharpen as usual (usually 15° per side). I don't always include that info when posting for sale, but sometimes will mention it.
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Thanks, sounds cool. Have not heard anyone describe in this much detail.

Do you see a noticeable improvement in slicing task performance with that approach, due to the thinner geometry behind the edge? Do you recommend your customers who are manually maintaining these to just maintain the 15 dps edge (so this is essentially a micro-bevel on your convexed edge)? If that's the case, then I assume at some point after a lot of sharpenings, the end user has to either send the knife back to you, or learn how to thin and maintain that convexed shoulder area.
 
I have a number of blades with convex grinds -- a SRK that I re-profiled as the factory edge was too obtuse, and it just sort of turned out convex by accident, not intent. Another is a Marble's Campcraft in 52100. Both are extremely efficient slicers now, but any difference between a shallow V grind and the convex edge would be difficult to detect, IMHO. In theory, convex should be slightly more durable, but in real life, I'm not sure it makes a whole lot of difference.
 
Alberta Ed said:
In theory, convex should be slightly more durable, but in real life, I'm not sure it makes a whole lot of difference.
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Not if edge angle is held constant. Two edges, each 15° per side, the convex will be thinner, though the visual height of the bevel will be increased. It's like adding another little grind in the area that was previously the shoulder between the primary grind and the edge, just curved instead of flat. If a convex edge is more durable than a V edge, it's because it's a thicker edge angle. :)
 
The most obvious benefit I see, to convexing an otherwise 'flat' grind blade, is that the absence of hard, crisp shoulders behind the apex makes for much slicker cutting in thicker/tougher materials, like cardboard or wood. More than once, I've noticed how the crisp shoulders of a V-grind edge end can sometimes dig in and bind, in cardboard especially. You can actually see where the cardboard hangs up on the shoulders and deforms to the exact profile of the shoulders. A polished convex behind the apex, on the other hand, makes a world of difference in such materials, as it eliminates anything that can catch and bind, behind the edge.

I don't put a lot of stock into claims of a convex being more durable than a V-ground edge. To me, it's a moot point. The overall durability of the edge is basically all about the geometry at the apex, and the thinness of the steel behind it. Doesn't really matter what the shape of the transition between the edge and shoulders is, in that regard, other than the as-mentioned point earlier that the thinness of the grind will be changed one way or the other, in changing the shape of it from one to another. Making a V-grind convex, by knocking down the shoulders, inherently removes metal behind the edge, which makes it thinner behind the edge. By itself, there's no reason to believe it'll magically make the edge more durable, by thinning the steel behind it.

The apex should ALWAYS be essentially a V-grind anyway, at least to some microscopic degree, else it'll be thick-angled and dull if it's rounded ('convexed') all the way out to the apex itself.
 
Obsessed with Edges said:
The most obvious benefit I see, to convexing an otherwise 'flat' grind blade, is that the absence of hard, crisp shoulders behind the apex makes for much slicker cutting in thicker/tougher materials, like cardboard or wood. More than once, I've noticed how the crisp shoulders of a V-grind edge end can sometimes dig in and bind, in cardboard especially. You can actually see where the cardboard hangs up on the shoulders and deforms to the exact profile of the shoulders. A polished convex behind the apex, on the other hand, makes a world of difference in such materials, as it eliminates anything that can catch and bind, behind the edge.

I don't put a lot of stock into claims of a convex being more durable than a V-ground edge. To me, it's a moot point. The overall durability of the edge is basically all about the geometry at the apex, and the thinness of the steel behind it. Doesn't really matter what the shape of the transition between the edge and shoulders is, in that regard, other than the as-mentioned point earlier that the thinness of the grind will be changed one way or the other, in changing the shape of it from one to another. Making a V-grind convex, by knocking down the shoulders, inherently removes metal behind the edge, which makes it thinner behind the edge. By itself, there's no reason to believe it'll magically make the edge more durable, by thinning the steel behind it.

The apex should ALWAYS be essentially a V-grind anyway, at least to some microscopic degree, else it'll be thick-angled and dull if it's rounded ('convexed') all the way out to the apex itself.
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Steering edge through cutting material is much better with convex , and for that reason convex edge is less prone to chipping and that make them more durable ....
 
maximus83 said:
What is the advantage of doing a convex edge on a full flat grind (FFG)?

I've seen a couple makers doing this. Interested to find out whether this could be a good-performing type of edge and what the reasons would be to try this, versus going with a standard v-grind.
Click to expand...
After years of use and many time sharpening you will must regrind whole (FFG) blade to get edge where was when was new .With convex edge you will ALWAYS have same geometry
 
Natlek said:
After years of use and many time sharpening you will must regrind whole (FFG) blade to get edge where was when was new .With convex edge you will ALWAYS have same geometry
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No...no you won't. It'll thicken with time just like any other edge geometry as it wears back into the primary grind.
 
If you know what you have to do once in a while when you sharpening convex edge it EASY to maintain same geometry .. . . at least to me :)
 
The same could be said for a full flat grind. Just grind the flats a little bit every time you sharpen. It'll make the blade a little scratched up cosmetically, but you'd be preserving the cutting geometry. However, you'll also be reducing the spine thickness a little bit each time, as well.
 
Natlek,

I think this is what you mean: A convex secondary grind of a FFG primary will need thinning down the primary if same performance (ease of cutting) is required when it gets sharpened into thicker stock.
 
It's actually really difficult to truly maintain a consistent geometry with a convex because most methods of producing one rely on a flexible backing, which will vary in its deflection in accordance with pressure as well as angle. Just because you're creating a finish that seems to be hitting the full surface doesn't necessarily mean you're maintaining the same geometry, and inherently as spine thickness and edge angle reduce, the degree of convex that is possible, presuming a full-height convex, becomes increasingly flat.
 
FortyTwoBlades said:
The same could be said for a full flat grind. Just grind the flats a little bit every time you sharpen. It'll make the blade a little scratched up cosmetically, but you'd be preserving the cutting geometry. However, you'll also be reducing the spine thickness a little bit each time, as well.
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I would like to see someone to do that :) Here I speak like user /minimal sharpening tool / not knifemaker .
 
FortyTwoBlades said:
It's actually really difficult to truly maintain a consistent geometry with a convex because most methods of producing one rely on a flexible backing, which will vary in its deflection in accordance with pressure as well as angle. Just because you're creating a finish that seems to be hitting the full surface doesn't necessarily mean you're maintaining the same geometry, and inherently as spine thickness and edge angle reduce, the degree of convex that is possible, presuming a full-height convex, becomes increasingly flat.
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That is correct , but in any case after many sharpening I will have MORE usable knife then FFG
 
Natlek said:
I would like to see someone to do that :) Here I speak like user /minimal sharpening tool / not knifemaker .
Click to expand...

Yeah, man...it's called laying the blade flat on the sharpening stone. I wasn't talking about using anything mechanical. Grinding is the shaping stage of sharpening.
 
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