Please explain how, with all else equal, the edge on a full hollow grind is keener than a half or quarter hollow grind.

[rbscebu]

I have read in quite a few places discussing SR grinds, including on B&B, that the more hollow the grind, the keener the edge can be honed. Hollowness of the grind doesn't affect the bevel angle (does it?).

Assuming that all other factors are equal, how can the hollowness of the blade's grind affect the keenness of the edge?

Please keep your answers simple. I'm only an engineer.

 

[Kyle89]

Because it says so on the internet so it HAS to be true...

Always figured the hollowness couldn't ever affect the relationship between the spine and the apex because it's just open space. Not at all knowledgable enough on this but just drawing inferences. Perhaps blade flex can come into play here with using pressure in earlier stages of a progression? *shrugs*



Looking forward to reading this discussion.

 

[Wid]

All I know is; when I hone a razor no matter the grind, I use the same methods and HHT. So either will have a very similar edge.

 

[RayClem]

The keenness of the edge depends upon the thickness of the spine relative to the width of the blade. That sets the apex angle. That can have a significant affect on keenness as well as blade comfort.

Then there is the fineness of the hones and any abrasive pastes.

The hollowness of the grind should not have any affect on keenness.It may affect the way the blade hones. For example, a full wedge razor (no hollow at all) would be difficult to hone because you have to remove steel from the entire face of the razor. Otherwise, you have to hone with a few layers of tape so the hone only touches the edge, but that then changes the apex angle. An extra-hollow-ground razor hones easily as the bevel is quite narrow.

The hollowness does seem to affect the way the razor shaves. I have one quarter-hollow ground razor; I did not like the way it shaves, so I never purchased another one. I have a couple of half-hollow ground razors; they are OK but not my favorites. The razors that seem to work best for me are full-hollow ground, extra-hollow ground and especially bellied hollow ground. The bellied blades work very well for me as the design of the blade makes the edge a little stiffer so it stands up better to my coarse beard. With proper honing and stropping, any of the blades can be made quite keen, even the Gold Dollar razors. Now the durability of the edge is another matter.

So, with the exception of a full-wedge razor, the hollowness of the grind itself should not affect the keenness.

 

[Ice-Man]

Ok here goes the blade on a Bellied hollow is thinner on the edge compared to a true wedge, as you have flex so you can hone it thinner and that's what gives you a keen edge see the pictures bellow.

This next picture is of my Koraat 8/8th full-bellied hollow custom,

the line on the left is a human hair strand hope this explains better for you. So the thinner the blade gets on the ridge of the hair and it digs in and you get a clean cut, so the thicker the blade the harder it is to dig in to cut see next picture.

 

[Titleist]

I appreciate the discussion. But I don't think I'm smart enough to understand the subject entirely. For the last decade or so, I've honed razors of mostly all grinds except wedges or framebacks and shaved with them.

I'm a simple Man with a simple mind. But I admire those who are able to dive in this deep.

 

[Darth Scandalous]

So, there are two factors at play in what makes a blade sharp. One is the width of the apex, and one is the width of the bevel behind the apex. The first determines how the blade catches the hair and the second determines how it actually cuts through it.

Two identical bevel angles honed identically ought to have the same dimensions at both points regardless of the way the rest of the razor is ground. I can't rule out that flexibility may change the actual width behind the apex, but I don't see how grind could affect the actual apex width itself, with the same native bevel angle.

 

[Butzy]

I dont know that it is keener, rather, it's easier to make a shave-ready edge on a full hollow than a half or wedge in my experience.
something else to consider:
It's sort of like the "what is heavier a pound of feathers or a pound of bricks" discussion. They're both a pound, you're just a lot more likely to see a pound of bricks from day to day... similarly, it's easier to see a keen full hollow razor and as such you tend to see people shave with them more often it seems.

 

[Eastcoast30]

As Rayclem has stated, it's all about the thickness. And as you continue to hone a wedge, the edge continues to move towards the spine. Eventually, you are shaving with an axe. That's the quick explanation. And while the edge section keeps getting thicker, it will still be sharp. It will probably be years or decades before it will affect shaving and require a regrind.

 

[Doc226]

For the same bevel angle it should feel the same.

The feedback will be wildly different

I would bet if a barber was shaving a person wearing headphones with a wedge vs a hollow grind, the shavee would not be able to sense a difference.

 

[cotedupy]

So, there are two factors at play in what makes a blade sharp. One is the width of the apex, and one is the width of the bevel behind the apex. The first determines how the blade catches the hair and the second determines how it actually cuts through it.

^ This, in a nutshell, is the answer.

The things to do with the blade itself which affect cutting are; edge angle, edge refinement, and the thickness of of the blade 'behind the edge' or at the shoulder.

 

[H Brad Boonshaft]

Why would the thickness of the blade behind the cutting-edge matter for cutting hair, if the edge thickness, refinement and bevel angle are the same?

I could see it affecting a knife, where you are cutting and cleaving objects larger than the edge bevel.

A beard hair is a fraction of the bevel width and should not matter, the hair is cleaved long before it has passed the bevel width.

 

[Darth Scandalous]

Why would the thickness of the blade behind the cutting-edge matter for cutting hair, if the edge thickness, refinement and bevel angle are the same?

I could see it affecting a knife, where you are cutting and cleaving objects larger than the edge bevel.

A beard hair is a fraction of the bevel width and should not matter, the hair is cleaved long before it has passed the bevel width.

I'm not talking about the full distance of the bevel from apex to shoulder, just the cross-section of the blade directly behind the apex. The narrower it is (within reason) the better it will cut. I would guess it doesn't matter much past about the equivalent of a third to half the diameter of the average hair. So, a distance of 35 to 50 microns from the apex, maybe?

Isn't that one of the things that we're refining when we strop?

Edit: meant to say, the width of that cross-section is determined by the bevel angle, but my limited understanding is that we can further refine that by stropping.

 

[H Brad Boonshaft]

Good point Doc, a lot of what we “feel” when shaving is in the hand, not the face at the time the beard is cut.

A heavy blade cuts easier, (to the hand), but the shave is not always better/cleaner at the skin.

Take the hand, out of the equation, with a non-shavee shaver, can the shavee feel any difference? Doubtful. Isn’t that the point, the actual shave at the skin, not how it feel in the hand.

Stropping is more about refining,(lightly polishing the leading edge of the bevel) and more importantly, straighten the actual edge. The effects of stropping are dramatic with HHT.

 

[APBinNCA]

According to scienceofsharp, keenness below a certain width can only be achieved by plastic flow from stropping. I remember reading somewhere that heavier grinds strop different, that it is harder to affect the edge and improve keenness beyond a certain level. It supposedely has something to do with how the whole razor interacts with the strop. So that seems to suggest the above mentioned flex plays a part.

 

[HazMat Shaver]

With all things, the same including bevel angle, The grind cant make it different. Different angles or metal or... Lots of ways to be different but with all the same...
NOPE!

And if you are talking about the flex in the blade while honing that is no longer not the same bevel angle.

 

[SparkyLB]

I have read in quite a few places discussing SR grinds, including on B&B, that the more hollow the grind, the keener the edge can be honed. Hollowness of the grind doesn't affect the bevel angle (does it?).

Assuming that all other factors are equal, how can the hollowness of the blade's grind affect the keenness of the edge?

Please keep your answers simple. I'm only an engineer.

I'm one with little experience, but I've picked up on much you nice folks have shared with me. Your statement, "Hollowness of the grind doesn't affect the bevel angle, right?" I agree with. From what I understand, it could only affect it in so far as the final thickness of the blade at the thinnest point. Once THAT edge is honed on a concave stone, the hollow edge that results would be a micro "hollow ground" on the edge. I'd think this thins out the cutting edge and that could only make things better--so long as it's not so thin as to fold when it hits a whisker.

I equate keenness with width. The thinner, the more effective.

.....You're only an engineer, gimme a break!! You crunch numbers for breakfast!

 

[Gamma]

I have not found grind to affect sharpness. I prefer to shave with heavier grinds actually, but not because they're sharper.

On paper, a 17˚ bevel is a 17˚ bevel, regardless of the grind. The bevel might be skinnier on a hollow, than it is on a wedge, but the geometry of a triangle that terminates in an apex with a 17˚ angle behind it will stay the same regardless of what happens behind where edge width is measured.

If you draw out a triangle that mimicks the side-view of a razors bevel, and you map it to have the peak angle at 17˚, you will never be able to change the edge-width dimension without first changing the angle.

Edge-Width at a typical distance behind the apex of two blades with identical bevel angles will also always be identical, regardless of grind. The hollow grind past the bevel's top line will be thinner than what you'll find on a wedge but that part of the blade does not factor into cutting whiskers. We are really only concerned with the first 100 µm behind the apex. Most bevels are at least 1/16" across, about 1600 µm.

Decrease 17˚ to 14˚ and sharpness can improve.
Add compound bevels into the equation and things get juicy.
Edge condition, such as sharpening quality, apex condition/refinement/etc is another topic - but the query is based on 'all else being equal' so that becomes irrelevant. Sorta.

Things like burrs and actual edge geometry factor in too. Measuring a bevel angle is sort of a generalization. Nothing is as perfect as it is assumed to be by the edge wizards of the internet. So you can calculate 17˚ in once place along the edge but without having a microscopic view of that edge you never know, for sure, what the actual geometry or conditions up by the edge's apex are. So it's hard to say 'all else is equal' because the data we collect is usually not complete. We operate in a less-than-perfect system. Things are averaged, and compared in a relative sense.

Hell, if I have two steels, one softer one much harder and both are formed into identical blades with 17˚ bevels I will probably find that one will take a sharper edge, regardless of the grind.

 

[Ravenonrock]

Regret skipping math class but glad I attended shop.

 

[Frank Shaves]

My face feels smoother after shaving with a near wedge. I do not understand why. And I have lots of full hollows and near wedges with varying bevel angles made from from different types of steel.

This is the only pattern I can find. I suspect it’s the result of differences in feedback.