Double / no bevel - theory question

[pe2dave]

Reading this, I'm puzzled, my thinking is that a long tapered grind will make for a very weak edge,
hence the need for a secondary grind?
This piece suggests: "I would always assume there is a micro bevel when there is a scandi grind. Otherwise the edge would be to vulnerable. The visibility of it depends on how far up on the secondary it is placed and if it is straight or convex bevel. " So perhaps a tiny (finishing?) bevel is the only means of strengthening the edge?
Any thoughts?

 

[D_W]

scandi grinds are already a bit steep. If you add 5 degrees to each side freehand (which is probably at least what people do), they'll be in the ballpark of 30 degrees or so.

Whatever the case is, the damage occurs right at the edge and there's no real way to cheat it with different grind shapes, but you can get less cut resistance by strengthening only the edge to the amount that's needed and keeping the bevel thin behind it.

The scandi grind and partial hollow bevel grinds are just a way to make a knife cheaply out of flat stock, but at least the hollow grind gives some jigging to sharpen and clears some of the steel behind the edge.

 

[Jorny]

I think they are overtheorising it. Traditionally most grinding of knives in scandinavia was done on large sandstone grinding wheels and so were somewhat hollow ground. Using a wet sandstone grinding wheel (or tormek without jigs) makes it easy to grind them freehand since you can look at the flow of water to make sure you hold it at an correct angle.

The so called scandi grind is also a way to ensure that the knife is suitable for carving with. Adding a secondary bevel to a knife makes it unsuitable for food carving as it makes it impossible to use many techniques where you use the bevel to add control.

A typical recommendation for a slöjd knife is to grind it to about 27 degrees.

 

[D_W]

I've made a few knives - I think the introduction of the short hollow grinds and the "Scandi" types probably coincides with very inexpensive grinding processes (a blade of those types can go into a jig/holder and be ground by an automated process and trued up easily after hardening without someone needing to do it by skill.

The plus side of that is that you can get very inexpensive knives that are very good. It gets a little more odd when it's requested on expensive knives.

Buck knives had an interesting segment on youtube where you can see someone (unskilled loading blanks on a rotating wheel and the entire grinding process is automated and they come out one after another. IT's marvelous in the sense that it makes the same neat grind every time and the knives are relatively inexpensive (rosewood handled knives made in the US with brass fixtures and a stainless blade for about $45, and they're not small).

Convex grind and hollow ground knives were generally both made on wheels, but older knives were rocked on the wheel by a cutler to keep a hollow from forming. The method to get the hollow is obvious - just leave the blade against the grinding wheel and don't move it up and down. Small makers have moved toward belt grinders, of course - no need to have a mist of filthy water flying around the shop from a wet wheel, and far less cost to get into making.

 

[Jorny]

I don't know that much knives outside traditional swedish crafts (and there I am by no means an expert!). However, I do know that grinding without a secondary bevel has a very long history in the nordic countries. Probably because a knife was not a specialised tool. Especially among the sami people. Since knives were used to create all kinds of implements they needed to be suitable for shaping wood, and then a secondary or a convex bevel creates all kinds of problems. To be clear I am talking about is the ordinary everyday beltknife used for all kinds of things. The ancestor of the Mora knives so to speak. Nothing fancy.

You are probably right that it is a process very suitable for industrial processes.

 

[pe2dave]

Thanks @Jorny - If general purpose, a delicate edge wouldn't be much use?
The Mora knife is the example I'm talking about. Almost no secondary, but there is a 'micro bevel' if you want to give it a name.

 

[D_W]

Thanks @Jorny - If general purpose, a delicate edge wouldn't be much use?
The Mora knife is the example I'm talking about. Almost no secondary, but there is a 'micro bevel' if you want to give it a name.

knives and woodworking are somewhat different. If you're using chisels and planes, you can't tolerate deflected or damaged edges. If you're using a knife, it doesn't matter that much.

In most cutting tests, a knife with a very thin edge lasts longer through the test (the test doesn't "feel damage" or care about it, it just cuts until cut resistance is too much to continue cutting)

The micro bevel that you see on thin bevel new knives is generally just the least expensive way to sharpen them, but it's not a dictate that the user won't hone it off.

If you use one of those knives for soft/wet wood whittling, meat cutting and vegetables, it's not going to fail that easily if it's at 20 degrees and when it does, what's left behind is still a thin edge.

The knife industry folks who do standardized testing don't really get the woodworking thing as they have tests where a thinner edge (that's allowed to fail and that isn't finished to a bright polish in some cases) will do better than what our edges (in woodworking tools) do.

 

[Awac]

https://morakniv.se/en/morakniv-stories-en/edge-angles/
My mora carving knifes have a flat bezel. You want a flat reference behind the cutting edge when you carve (Convex is the worst as the knife rocks when you carve, SLIGHT concave is ok, as it guarantees you are not convex ).I find mora excellent, keep a strop near and use often. Start using the knife as a lever and problems will occur.
My general purpose knifes have a micro bezel so the edge is a little tougher.

Axes also work on the same principle, hand carving with a convex grind axe can cause it to skid off target as the bezel is held away from the wood.

 

[Jacob]

https://morakniv.se/en/morakniv-stories-en/edge-angles/
My mora carving knifes have a flat bezel. You want a flat reference behind the cutting edge when you carve (Convex is the worst as the knife rocks when you carve, SLIGHT concave is ok, as it guarantees you are not convex ).I find mora excellent, keep a strop near and use often. Start using the knife as a lever and problems will occur.
My general purpose knifes have a micro bezel so the edge is a little tougher.

Axes also work on the same principle, hand carving with a convex grind axe can cause it to skid off target as the bezel is held away from the wood.

I think you would find that all sharp edges are effectively "convex" ground for the last mm or fractions thereof. A concave grind all the way to the edge would be vanishingly thin at the edge by definition, and would break down immediately it was used.
The shape of the rest of the blade is another thing altogether and more to do with the manufacturing process than sharpening.

 

[Awac]

I think you would find that all sharp edges are effectively "convex" ground for the last mm or fractions thereof. A concave grind all the way to the edge would be vanishingly thin at the edge by definition, and would break down immediately it was used.
The shape of the rest of the blade is another thing altogether and more to do with the manufacturing process than sharpening.

The finer the steel grain structure the finer the edge can be made. My knifes for carving are flat. I do not have a secondary angle nor a hollow grind. I sharpen them flat.

What happens at atomic level?You might be right as the edge follows a grain of steel around the edge.

 

[D_W]

we hone the grains - the edge itself will be finer than a grain of steel (obviously well different than an atom).

iron carbides are about as hard as arkansas stones, so they can be cut. Carbides that are much harder than that can be cut in methods other than scratching, but we generally rely on abrasives that are just much harder than the carbides if it matters.

The way I read the steel chart, really fine steel has particles around 2-3 microns or so, and relatively fine is somewhere around double that (from reading elsewhere, it was assumed at one point that tool steel wouldn't be much less than the measurement that corresponds with about 6 microns, but a knife guy - ed fowler - demonstrated forging and then refining grain size to something that seems to me corresponds on a chart to 2-3 microns.

But the astm grain size charts and formulas aren't that easy to use. Whoever is using them is probably using attached software to do calculations and give them an idea of size.

Particle steels have grain sizes that are small numbers of microns, like 2 or 3, but in normalizing and quenching, some larger particles probably are made (but you can abrade them to visual perfection or close). What's more important for fine edge holding is whether or not the grains will stay in place (when steels have big carbides, or poor uniformity - like japanese super blue often does, but the super high carbide particle steels and esp. pre particle steels do the same - you get visible edge issues immediately with use, and sometimes even due to things like stropping). the people who use those super high carbide steels don't care about that, though -they end up in things like sailor's knives and 10V turning tools (that are great for turning rough filthy wood full of dirt).

(and I'm sure a lot are sold to white collar people who read about the steels and think they must be better if the same knife that's made of 154cm costs 3 times as much when it's made of S125V)