Why is Convex bad?

stoffi said:
Edges or blades..? If it's just the edges then it's like a few minutes to convert them to v-grinds.
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Not for me unless I use a sharpening jig and back way off of the pressure - my V grinds are still convex.

d.weglarz13 said:
I love to bring my knives sharp on those, now that i have practiced a lot. But, they just never look as nice as those who use the systems(or maybe those with years of skill).
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Pretty-looking freehand edges don't take so much skill as they take tedious work and back-tracking (as in "have to go back to the 600 grit stone AGAIN!" :grumpy: type of back-tracking).

If you have free time, a variety pack of wet/dry from the automotive supply store, and some hubcap polish, you can take this:

grindsugu.jpg


And make it into this:

mirsugu08.jpg


Danged thing ended up convex. :grumpy:
 
Convex bevels and convex grinds are two different things, while convex Bevels will help to improve cutting performance of some flat and saber ground blades its still not even Close to what a full convex grind can do. If your going to talk about convex vs ______ it needs to be full convex because convex bevels on other types of grinds adds too many variables.

Many primitive cultures convex their cutting tools with curved rocks, I would be surprised if their stones were higher than 400 grit or if they could even pass one of our simple sharpness test but when you see these tools in use they cut like they are razor sharp. These people use this method of sharpening because they know the advantage to the shape, then again their stones might Be magic.........

Many can say what they want about convex but for me I've seen the results and can't denie what has been witnessed repeatedly.
 
Thombrogan, could you possibly take a second to tell me how exactly you went about this? Do you start by laying the knife on the paper flat on its side, and then roll the edge as you do the stroke? Any time I lay my knife down on something, the side of the blade always shows little tiny scratches. (maybe because i use sand paper too rough?) If anyone could maybe post any links as to how to go about this and how high in grit to go to, could be great. I would really like to try this, but i don't have a sander or buffer, just 2 hands!
 
The convexing is easy - if you have a human skeleton and compressive tissue (skin, muscle, ligaments...), you, too, will make convex edges and often convex grinds when going above the edge.

The first grind was on an 80 grit wet grinder (the knife was muy gordo), but the mirror-polishing process used wet/dry sandpaper in 220, 320, 400, 600, 800, and 1,000 grit followed by polishing paper (1,500 or 2,000 grit wet/dry would've worked better, but it wasn't here) and hubcap polish. The sandpaper was layed flat on a piece of glass (I used restickable gluestick to keep it in place and any hard and flat surface will work so don't think it's glass or nothing) and the blade was rubbed perpendicular to the edge and spine in sections. The natural arc of my elbows convexed the edge.

Once a uniform layer of 220 grit scratches covered one side of the blade, 320 was used to put a new layer of scratches and so on. After finishing with the polishing paper, a dab of hubcap polish (I used Meguiar's because it doesn't have an ammonia smell) was worked into the blade with a scrap of shop towel (a little of either goes a long, long way) and rinsed and repeated three or four times. Never do so when tired or your thumb will gash open and you'll have to wait until you've stopped bleeding to polish more.

After the third or fourth application of hubcap polish is done, you'll see the blade is shiny, but not quite mirrory. Simply repeat the process, but start with either the 320 grit or 400 grit depending on how mirrory the blade was or wasn't. And when that almost works, repeat with the next finer step and you'll have quite the reflective blade when you're finished.

And a few uses of that knife later and you may lose all interest in mirroring more than just the edge.
 
People need to take into account what is being cut. Some things cut more easily with a beveled edge and some cut more easily with a convexed edge. For your own experiments, try cutting a piece of medium hard cheese. You'll see that it sticks to one type of edge and slides off the other. Then try cutting a tomato very thinly and see what happens. Then cut something like soft raw meat. I think you might find some very big differences between your edges based upon what you will cut with them.

We need different tools for different jobs.

Stitchawl
 
the real deal filipino and japanese swords are convex ground, the advantage of a convex grind over a v grind is there is less resistance in chopping and slicing. The profile of a V grind has a "ledge" where the blade turns into the edge, and this catches on the material being cut, producing friction. A convex grind has no ledge so there is less resistance, profile grinds and scandi grinds also have this property, but there is more material on the edge of a convex due the arch shape, so it won't chip and break as easily as a profile grind, and can chop with it.

Most knives before industrialization were convex ground, we see more V grinds now-a-days because there is no machine to do a convex, convex are done by hand. And before machinery a perfectly flat rectangular sharpening stones was hard to come by, knives were often sharpened on whatever large stone was around, boulders, walls, ...etc. and then steeled or stropped. and simply by free handing it, knife edges evetually become convex

I'm sure you've seen videos of people severing bamboo in one stroke with a japanese sword, the lack of friction is slicing is what allows this, you can't do that with a machete or european sword, but if they were convex ground you could.
 
Bevel shoulders do not contribute anything, they just get in the way. Best thing to do about them is smooth them off.
 
That's what I meant by the ledge on the V catching on the material, grinding them off gives you a profile grind :)
 
So why then are full convex the best? I mean, if it were convex beveled but saber ground, then wouldn't the convex edge push the material away enough where the shoulder of the saber grind wouldn't make contact? And, lets just say we were comparing the best convex grind to a regular v grind. What would be better if I only had sandpaper/mousepad method and something like Edge Pro for the v grinds?(as opposed to power machine or wheels). A lot of people say that the mouse pad thing gets it sharp, but not perfect like a EP. So, in this case would it be better to go with a V? thanks
 
you can get a edge sharp as you could with a edgepro with mouse pad/sandpaper, you'll just have to go all the way to 2000 grit polish paper, and then strop with black, then green compound. Any sort of abrupt change to the profile of the blade could cause more catching or friction. You won't really notice that much difference in slicing with a V grind vs convex until the material you are slicing engulfs the blade past the edge shoulder on a v-grind ( unless the V is sharpened at a really steep angle, then it just plain won't slice well at all ). It depends on what you are doing that makes a particular edge type the best for that job. If you were decapitating people or severing limbs then you'd notice a big difference between a V or convex, that's why the samurai swords were convex. Also in chopping you'd notice a big difference.
 
also, those guys that do sever bamboo with one stroke and the like: do they use full flat ground convex ground? or do they use convex beveled? thanks
 
all the swords that I've seen that are able to do that are full convex, pretty much zero bevel. The whole blade is the edge.
 
Straight surfaces stick more.

A straight cut produces flat surfaces on the cut material. Chop in deep, and you get flat surfaces to the left and right of your blade. If the blade also has flat surfaces, all the flat surfaces can mate up perfectly, and you get a lot of friction. But if the surfaces of the blade are radiused-- a flat surface can only contact a rounded surface at one point-- you get less binding.

Anyone who has converted a machete to a convex grind will know what a difference it makes.

Boxy cars went out in the 90's, not just cause they were ugly, but they were just bad at cutting through the air. Protruding angles and flat surfaces do not help knives either.
 
DocNightfall summed it up really well

DocNightfall said:
Straight surfaces stick more.

A straight cut produces flat surfaces on the cut material. Chop in deep, and you get flat surfaces to the left and right of your blade. If the blade also has flat surfaces, all the flat surfaces can mate up perfectly, and you get a lot of friction. But if the surfaces of the blade are radiused-- a flat surface can only contact a rounded surface at one point-- you get less binding.

Anyone who has converted a machete to a convex grind will know what a difference it makes.

Boxy cars went out in the 90's, not just cause they were ugly, but they were just bad at cutting through the air. Protruding angles and flat surfaces do not help knives either.
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Katanas are technically a convex zero saber grind. So are scandi knives, although the convexity is much milder.

As to full-height convex vs. convex saber-- full convex is better optimized for cutting performance. The saber is a compromise that puts more metal on the sides of the blade, to add lateral strength and to put more weight behind the edge without increasing blade height.

Figuring out how high up the blade the edge grind should go another of many balancing acts that depend on the knife's usage.
 
DocNightfall said:
Katanas are technically a convex zero saber grind. So are scandi knives, although the convexity is much milder.
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So mild as to be non-existing, perhaps. Scandi grinds are perfectly flat. I certainly go along with saber grind, but without any convexity involved. The sides are perfectly straight from edge to hamon and beyond. With a convexed edge there is a rounding of the metal as you move away from the edge. Nothing rounded on Japanese edges, despite what the 'Cold Steel' company would like us to believe.

Stitchawl
 
DocNightfall said:
Boxy cars went out in the 90's, not just cause they were ugly
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I agree with all your points except this one. Much to my dismay, boxes on wheels are seemingly all the rage...but to your point, it has nothing to do with performance. There is a reason that curved surfaces are so prevalent in nature. Nature likes to erode things into curves. It is sort of like digital vs analog. Nothing in nature is digital but everyone thinks digital is perfection.

Horses for courses.
 
stitchawl said:
So mild as to be non-existing, perhaps. Scandi grinds are perfectly flat. I certainly go along with saber grind, but without any convexity involved. The sides are perfectly straight from edge to hamon and beyond. With a convexed edge there is a rounding of the metal as you move away from the edge. Nothing rounded on Japanese edges, despite what the 'Cold Steel' company would like us to believe.

Stitchawl
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There is some controversy as to what exactly a Scandi grind is. Some Sacndinavian knives come with a very small secondary bevel (Moras come from the factory like this), many modern examples come flat. But the few used vintage Scandinavian woodcraft knives that I've seen all had a mild convexity from edge to shoulder. Perhaps this was a result of sharpening on dished stones, but regardless, I am of the opinion that this is how they used to be.

Japanese blades traditionally are convex.
http://www.ksky.ne.jp/~sumie99/shaping.html
http://www.bugei.com/niku.html
 
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unit said:
I agree with all your points except this one. Much to my dismay, boxes on wheels are seemingly all the rage...but to your point, it has nothing to do with performance. There is a reason that curved surfaces are so prevalent in nature. Nature likes to erode things into curves. It is sort of like digital vs analog. Nothing in nature is digital but everyone thinks digital is perfection.

Horses for courses.
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If nature erodes things into curves, it is because the curves are less obtrusive. On the cross section of a knife, any angled portion (other than the edge itself, of course) will just get in the way of things. This has a lot to do with performance.
 
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