Mike Stewart: Convexed blades with convexed edges hold an edge much longer!

[FortyTwoBlades]

Right here:



"highest" would make no sense here for a V edge, because with a V edge there is only ONE spine height over the stone: There is no "highest"... So you are talking here strictly about a convex edge, and you are saying everything beyond the initial apex edge angle ("highest spine height") is "relief"...

It isn't: The "relief" is most critical near the apex: It becomes increasingly less important after that... In splitting some materials the convex may have less drag, but some other materials will also offer less drag to V-edge shoulders... Drag is not an advantage a convex edge can claim consistently, because cutting is mostly not about fluids... So the only convex "advantage" left is a stronger more open apex angle for a given profile thickness...

What most convex edge fans say is that the convex edge has 1-more apex strength/durability and then 2-less subsequent drag...

What they get wrong is that you need to reverse that order: You need 1-less drag and then from that follows 2- More edge strength/durability.

The reason they get the order wrong is they mistakenly assume more "strength" in front will buy them less drag.

The actual reality, as tested, is that sharper (up to a point) apex angles have a sharpness that is more durable, as if the steel was better, because sharper apexes transfer side loads more readily to their stronger parts, which also reduces drag.

You have to start with the "relief": So there is no "relief" after that...

Gaston

You dramatically misconstrue my statement...

When forming a convex edge against a flat stone, you start the angle you want the edge to be, and drop the angle as you progress over the stroke, which lifts the apex off the stone and abrades the shoulders, blending the shoulders into whatever grind (if any) is behind the edge. The apex is only being abraded at the greatest angle out of that range during the stroke, and as such it is that which is the edge angle. Everything behind that angle is a relief in the geometry because of its lessened angle. You claimed:

You think that for any given apex strength you gain easier penetration with convexing.

[emphasis added]

I did not say that. I've discussed only geometry, not strength.

 

[me2]

No. If everything is the same, the convex blades has less steel and is weaker. To illustrate this, the easiest way to keep things the same is to have a full flat ground blade and then convex the edge. This leaves the edge thinner than the flat ground blade for the same edge angle.

In reality there is so much variety in the way things are ground that the "all else equal" qualifier is extremely difficult to apply.

 

[FortyTwoBlades]

No. If everything is the same, the convex blades has less steel and is weaker. To illustrate this, the easiest way to keep things the same is to have a full flat ground blade and then convex the edge. This leaves the edge thinner than the flat ground blade for the same edge angle.

Only if you don't remove any steel from the apex itself, as that would reduce the blade height, shifting the edge deeper into the geometry of the blade, effectively thickening it. This is something that many folks end up accidentally doing because of the deflection of soft surfaces such as strop leather or mouse pads with sandpaper over 'em. The more force you use on a flexible backing, the more the deflection of that material, and the thicker the apex angle produced, even though your blade angle relative to the horizontal plane isn't any different. So to keep your edge angle the same when convexing a flat geometry using a flexible backing (one of the most common ways of producing a convex) you have to hold a "lower angle" to maintain the original apex angle.

 

[Mikael W]

Only if you don't remove any steel from the apex itself, as that would reduce the blade height, shifting the edge deeper into the geometry of the blade, effectively thickening it. This is something that many folks end up accidentally doing because of the deflection of soft surfaces such as strop leather or mouse pads with sandpaper over 'em. The more force you use on a flexible backing, the more the deflection of that material, and the thicker the apex angle produced, even though your blade angle relative to the horizontal plane isn't any different. So to keep your edge angle the same when convexing a flat geometry using a flexible backing (one of the most common ways of producing a convex) you have to hold a "lower angle" to maintain the original apex angle.

True!

I always avoid sharpening the apex and instead I focus on the area above the apex.
Then the apex is easy to finish off, with a leatherhone on hard backing.


Regards
Mikael

 

[Ed Fowler]

No. If everything is the same, the convex blades has less steel and is weaker. To illustrate this, the easiest way to keep things the same is to have a full flat ground blade and then convex the edge. This leaves the edge thinner than the flat ground blade for the same edge angle.

In reality there is so much variety in the way things are ground that the "all else equal" qualifier is extremely difficult to apply.

It is a very simple experiment, just get some bar stock, try different grinds, make a jug for a torque wrench and bend them.
You may find some surprises, then your challenge is to explain them.

Have fun with it!