Epiphany-Geometry-Retention

[scdub]

I believe that I just understood, for the first time, why a thinner edge geometry will maintain a sharper edge while cutting most materials.

I’d like to apologize to anyone/everyone here that has already made this point dozens of times - for some reason I didn’t quite understand.

Ok here goes: Everything else being equal, it seems to be a commonly accepted fact that a blade with thinner geometry (the shape of a blade in cross-section), will retain an edge longer through most commonly encountered materials that folks expect knives to cut (plant/wood fibers, meat, plastics, and compounds like cardboard).

This never really made sense, but I believe I just finally understood that this is because a thinner geometry takes less pressure to push through the material, and less pressure = less edge damage.

That’s it right? Is there anything else going on or simply that a thin edge doesn’t need to get pushed so hard to make the cut?

 

[Guy McVer]

This is news to me.

A finer edge will slice more efficiently because it will encounter less push resistance in the form of friction. I don't know anything about it necessarily lasting any longer. That will depend on what you do with it and how well you handle it, but if it's anything then it's more fragile because it's more vulnerable to lateral (sideways) damage due to it's thin profile. If you want to cut wood with it then you should keep a steady hand with even pressure, be careful to follow the grain, and avoid twisting the blade.

 

[Crag the Brewer]

I believe that I just understood, for the first time, why a thinner edge geometry will maintain a sharper edge while cutting most materials.

I’d like to apologize to anyone/everyone here that has already made this point dozens of times - for some reason I didn’t quite understand.

Ok here goes: Everything else being equal, it seems to be a commonly accepted fact that a blade with thinner geometry (the shape of a blade in cross-section), will retain an edge longer through most commonly encountered materials that folks expect knives to cut (plant/wood fibers, meat, plastics, and compounds like cardboard).

This never really made sense, but I believe I just finally understood that this is because a thinner geometry takes less pressure to push through the material, and less pressure = less edge damage.

That’s it right? Is there anything else going on or simply that a thin edge doesn’t need to get pushed so hard to make the cut?

Thin Wins!!!

If it's thin Enough, it doesn't even Need an edge. haha

 

[Crag the Brewer]

This is news to me.

A finer edge will slice more efficiently because it will encounter less push resistance in the form of friction. I don't know anything about it necessarily lasting any longer. That will depend on what you do with it and how well you handle it, but if it's anything then it's more fragile because it's more vulnerable to lateral (sideways) damage due to it's thin profile. If you want to cut wood with it then you should keep a steady hand with even pressure, be careful to follow the grain, and avoid twisting the blade.

It's not friction (I don't think?) (I could be wrong?) It's something else besides friction relating to the thin edge

Because, surface area has nothing to do with friction. I know...... Most people would disagree with that. But it's true.

 

[Guy McVer]

If it's thin Enough, it doesn't even Need an edge. haha

...or a blade. That's why a wire cheese slicer > knife.

 

[Crag the Brewer]

...or a blade. That's why a wire cheese slicer > knife.

Or a Garrote....... Good catch!

 

[Twindog]

The extra friction of a wide edge doesn't happen at the apex, but from having to push material apart that doesn't want to be pushed apart.

Not sure myself, but I think that as the apex dulls, it doesn't initiate a cut as well, making the wider edge harder to push through the cut.

 

[scdub]

The extra friction of a wide edge doesn't happen at the apex, but from having to push material apart that doesn't want to be pushed apart.

Not sure myself, but I think that as the apex dulls, it doesn't initiate a cut as well, making the wider edge harder to push through the cut.

Right - which continues to increase the force needed to make the cut, and presumably increases the amount of dulling occurring at the apex. (?? but again just spitballs here…???)

 

[Obsessed with Edges]

I believe that I just understood, for the first time, why a thinner edge geometry will maintain a sharper edge while cutting most materials.

I’d like to apologize to anyone/everyone here that has already made this point dozens of times - for some reason I didn’t quite understand.

Ok here goes: Everything else being equal, it seems to be a commonly accepted fact that a blade with thinner geometry (the shape of a blade in cross-section), will retain an edge longer through most commonly encountered materials that folks expect knives to cut (plant/wood fibers, meat, plastics, and compounds like cardboard).

This never really made sense, but I believe I just finally understood that this is because a thinner geometry takes less pressure to push through the material, and less pressure = less edge damage.

That’s it right? Is there anything else going on or simply that a thin edge doesn’t need to get pushed so hard to make the cut?

Bingo.

This is the same thing verified in testing by CATRA, with those test results relied upon by major mfrs. like Buck Knives. It was the reason Buck narrowed their default edge geometry to 26°-32° inclusive - narrower than what they used to do with their knives.

Years ago, I had the same sort of epiphany in testing a newly-reprofiled edge on a traditional pocketknife in use as a simple steak knife. It occurred to me that so many knives used in cutting food on ceramic plates are bound to be damaged by hard impact, driven by the use of heavy pressure in pressing a thick-edged blade through food. A much thinner geometry goes through steaks and other foods like a laser with no need to drive it through with heavy pressure. This is how it hit home for me, when I saw it with my own eyes and felt it with my own hands in trying out my newly-thinned edge for my dinner.

It's also more apparent in cutting tough-skinned fruits & vegetables that tend to pinch & grab the blade like a vise. I've noticed this in cutting apples with thicker blades and with large thick-rind fruits like watermelon, which can really stop a thick blade in its tracks, mid-cut. In cross-section, the 'wedge effect' is obviously worse with a wider geometry, making it difficult to push the blade through tougher materials.

Thick edges handle lateral stresses better, like twisting, prying or heavy chopping with the possibility of oblique-angled impacts against the edge. But I tend to avoid such things in the cutting I choose to do with my blades, if at all possible.

 

[ferider]

Given you push and you cut (sawing motion), there are two things:

A thinner angle will create less resistance (more cutting force / red above). Plus, geometrically, the worn thinner edge will stay thinner (dark blue line), at least for the initial abrasive wear.

 

[FortyTwoBlades]

The simple version:

Thinner geometry = better resistance to dulling from abrasive wear because as they wear down the geometry behind it remains thinner.
Thicker geometry = better resistance to dulling from plastic deformation because it has more lateral support to resist rolling.

 

[Spey]

scdub ,​

Welcome to your bright new world Seriously !##!

For those that don't already know; a typical felling axe for example, in the hands of an experienced user, will be running bevels to the edge of 30°inc or less (30° inclusive aka 15dps on a double bevel) for chopping in cross grain applications with MUCH greater Force (Mass x Acceleration) than anyone typically uses on a knife. Many run shallower aka more acute bevels. Many in the knife world believe they need bevels steeper than an axe ... ;-/ It is unfortunate for many who choose to simply accept what the masses have pushed in recent years related to obtuse edge bevels on knives.

A number of years ago, Cliff Stamp (R.I.P.) did a fairly good job illustrating the importance of geometry by showing how supposedly inferior blade steels at shallow bevels outperformed the newer "super-steels" in sharpness and edge retention running current day obtuse bevels. He even went so far as to give assignments out for those interested in first hand participation & learning A small segment of the industry learned a great deal about past knowledge somehow lost (knives historically used much thinner geometry than current day, and this can be found researching various archives pre YouTube culture).

Part of a Cliff post from a 2020 discussion (note Cliff also references the historic Buck example that David mentioned in Post#9 above) that covers some of what is being discussed in this thread.
"I have also quantified this increase in edge retention as a function of angle in the past. Larrin has done the same more recently. Buck did it with CATRA in the 2000's, however care has to be taken as CATRA and similar work isn't really showing an increase in edge retention in that kind of comparison as they are confounding edge retention with force required to cut something (a fillet knife will cut rope longer than an axe even if blunt because the axe will fail due to requiring too much force even when it is sharper, CATRA doesn't separate these two measurements).

As a point of history, full size chopping axes have apex bevels of ~15 dps, and that is a 3 lbs head swung by a man slamming into a piece of wood, so even without any data, it would not be surprising that a knife made out of a much stronger steel, which is subjected to far lower forces, should be able to sustain them without damage and hence blunt by slow wear (for which lower angles -> higher edge retention).

In general, very general :

-knives blunt by wear, deformation, fracture and corrosion resistance
-corrosion resistance isn't sensitive to angle
-the goal to maximize edge retention is to minimize the angle which prevent deformation (and to a lesser extend fracture)

As to what is the minimal angle, well it depend on :

-what is being cut
-skill
-force

A skilled user, low force, cutting soft targets can use a much lower angle than the opposite.

(the reason why it is to a lesser extent fracture as edges rarely fail by toughness related issues, because the rate of strain application is always very slow compared to the ability of a steel to deform plastically in cutting, assuming you are not cutting hard targets like rock, metal etc. and if you are, well edge retention isn't a concern anyway)"

Personally, I do not carry/use knives, axes, tomahawks, etc. with bevels to the apex greater than 30°inc (15dps), unless there is a specific reason.

 

[Guy McVer]

the worn thinner edge will stay thinner (dark blue line), at least for the initial abrasive wear.

With the caveat that you are going to have to be more careful and deliberate with that thinner edge.

I love the picture you included. It's very clear conceptually and it's great in theory, and I do love theory. In the real world, that thinner edge isn't going to hold up as well unless you keep it in the relative safety of your kitchen where you only feed it the best ingredients.

 

[davek14]

It's not friction (I don't think?) (I could be wrong?) It's something else besides friction relating to the thin edge

Because, surface area has nothing to do with friction. I know...... Most people would disagree with that. But it's true.

It's not surface area, it's how hard the blade is pushing to spread the material. If the blade has to spread the material further in a shorter cutting distance then it obviously has to push harder. Friction is indeed a function of force.

 

[FortyTwoBlades]

With the caveat that you are going to have to be more careful and deliberate with that thinner edge.

I love the picture you included. It's very clear conceptually and it's great in theory, and I do love theory. In the real world, that thinner edge isn't going to hold up as well unless you keep it in the relative safety of your kitchen where you only feed it the best ingredients.

Again, often axes are run at about 10° per side. Scythes used for cutting woody growth are typically 7-9° per side. Knives can handle thin edges 15° per side or less and hold up fine under hard use.

 

[elasmonut]

Im no physics professor, but the way I undetstand a inclined plane or wedge, like most knives.... is the the thinner the wedge the less force it takes to seperate the material being cut. But the more force and abrasion foccused on the the fine edge, the quicker it wears or degrades.
Steeper angles wear slower but cut less efficiantly or have more drag. Hardness and wear resistance come into play, thats why heat treating and alloy selection complicate the process...
Thin cuts, thick is tough
Hard lasts, tough is easier to repair..
It is always a trade off..
Most modern steels can be hard, tough, or stainless...pick two!

Magnacut is a good balance but none of the the three strengths really stand out if is IMO a good compromise

 

[eKretz]

Give yourself a quick and easy demonstration by slicing some potatoes. A thin blade glides right through while a thick one hangs up and requires a lot more force to get through, even if they're equally keen at the apex. Part of that is related to edge angle and part of it is the narrower blade itself. For another sort of example, one can easily push a thin piece of steel banding (say .010") edge-on through something that an 1/8" thick piece of steel won't but dent with all the force you can muster.

 

[MrStabby7]

I believe that I just understood, for the first time, why a thinner edge geometry will maintain a sharper edge while cutting most materials.

I’d like to apologize to anyone/everyone here that has already made this point dozens of times - for some reason I didn’t quite understand.

Ok here goes: Everything else being equal, it seems to be a commonly accepted fact that a blade with thinner geometry (the shape of a blade in cross-section), will retain an edge longer through most commonly encountered materials that folks expect knives to cut (plant/wood fibers, meat, plastics, and compounds like cardboard).

This never really made sense, but I believe I just finally understood that this is because a thinner geometry takes less pressure to push through the material, and less pressure = less edge damage.

That’s it right? Is there anything else going on or simply that a thin edge doesn’t need to get pushed so hard to make the cut?

Makes sense to me.
I imagine this was the idea behind the hollow grind.

 

[FortyTwoBlades]

Makes sense to me.
I imagine this was the idea behind the hollow grind.

Hollow grinds retain rigidity due to the thick spine while minimizing the the thickness of the rest of the blade. Shallow hollow grinds are done because both sides can be ground at once by drawing a blade between two wheels set in opposition to one another and get the geometry just behind the edge (where the primary grind geometry has the most pronounced effect on performance) very thin. However, small diameter hollows create significant wedging once you hit the upper region of it, which is why kitchen knives with short hollows on flat stock kind of suck at cutting anything resistant whatsoever.

 

[Guy McVer]

It's not friction (I don't think?) (I could be wrong?) It's something else besides friction relating to the thin edge

Yeah, I have been thinking about this and I believe you are right because the key concept here is actually distribution of force.

Picture someone laying on a bed of nails. They're safe as long as their weight is being distributed between all the nails. If you were to concentrate the same downward pressure on just one nail, it would pierce their flesh instantly.

A thicker edge distributes the downward force of the cut over a larger area. A thinner edge concentrates the same force into a smaller area.

It is also geometry to some extent as well. Picture a needle vs a sledge hammer for an extreme example. The sledge hammer has no chance to pierce a sheet of fabric no matter how hard you swing it, while the needle slips in between the fibers easily.