what does convex ground mean?

The sides of the blade curve outward slightly, like the lens on a magnifying glass, but less pronounced. There are also blades that are flat ground, and hollow ground, where the sides of the blade curve inward.

Sharpening a convex ground blade is usually done with a leather strop, in order to preserve the convex shape. Another method is to put sand paper on top of a mouse pad and sharpen the knife on the sandpaper.
 
That's a good definition/explanation of a convex grind. However, how that is interpreted by any particular knifemaker or manufacturer is a completely different matter.

I like to draw a few lines of distinction, like this:

"convex bevel" - only the bevel has been convex ground
"convex grind" - any part of the blade face has been convex ground
"full convex grind" - one continuous arc from spine to edge, bulging slightly in the middle.


You may have guessed by now, it's the last one I prefer most. ;)
 
The major benefit of the convex grind is that it has the greatest edge strength of all of the grind configurations.

Field sharpening can be done with a regular stone like any other knife but reshaping is best done with the sandpaper technique to reprofile the edge.
 
george tichbourne said:
The major benefit of the convex grind is that it has the greatest edge strength of all of the grind configurations.
Click to expand...

The strength depends on the cross-section much more so than the type of grind, especially in regards to edges. I have a convex ground blade, full from edge to spine and the edge is about 3-5 degrees per side. This is obviously far less durable than a v-ground at at 15 degrees per side.

-Cliff
 
I do what Pendentive refers to as a convex grind on my blades. I grind almost halfway up the face. I've been able to chop saplings about the size of the average forearm, split the pelvic bone on deer and cut six inches into a deer breastbone with no damage to the edge. Edge holding holds up excellent.
 
I make a convex ground moose knife that has been used to cut off 3/8" steel bolts with no damage to the cutting edge...ok, a hammer was used to drive the knife through the bolts and it bruised the spine. I don't think that I could do that with a flat or hollow grind.

Whenever severe abuse is contemplated I suggest a convex grind.
 
george tichbourne said:
[cut off 3/8" steel bolts]

I don't think that I could do that with a flat or hollow grind.
Click to expand...

I have cut 3.5" nails with A. G. Russell Deerhunters in *stainless* steels, 0.010 - 0.020" thick edge, ground at < 10-15 degrees per side. These are about the thinnest production ground knives.

To cut 3/8" bolts, pick a decently tough steel, grind the edge 0.035" thick, at 20 degrees per side (flat). Primary grind geometry doesn't matter as the stress is all on the edge anyway and the primary only ruptures if the edge fails.

That is probably a gross overestimate of what is needed as well, you could probably get away with 0.025" in thickness and a more acute edge if you use a dual 15/20 grind or similar. I don't think INFI would fracture specifically at that level at that task. I have chopped it into nails which is worse than using a hammer with a much thinner edge and it only slightly rolled.

It depends on how many hammer hits you want to take of course. I whacked through 1/4" tension bar in very few hits (less than 10) with some knives with thinner edges grind than that awhile ago. If you do it slow enough you can do it with just about any knife of course.

But back to the convex knife, just grind a v-ground bevel on another one which has a similar angle up to the 0.035" point and the durability will be near idential. Again 0.035" is probably a gross over estimate, I use that because that is the point at which decent steels start to become near-invunerable to fracture as the strength is so high.

-Cliff
 
So, if you have a knife that you'd like to put a convex edge on, does it make a difference whether it's a full flat grind, hollow grind, or whatever from the factory?

For example: would there be a difference in performance, or maintenance, or general characteristics between the convex edge on my CRK Shadow Vs. a Spyderco Perrin? (that weren't there before)
 
I'm sure Cliff Stamp would say "There is no performance difference between a multi-bevel and a convex edge".

Personally, for me, I like convex edges. Easy to maintain and keep super sharp.

You can put a convex edge onto any grind, flat, hollow or convex....even a chisel grind.

The grind makes more of a difference in the material you are cutting and frankly, blade thickness is often more of a factor than the grind.

Specifically, regarding your comparative question, there would be a difference. 2 different blade types, geometries, steel, etc. Load of variables. Best thing to do is decide what application is best for that knife, for its particular cutting duties and then go from there.

I would rather strop than hit a stone, so I have convex edges on nearly everything.
 
Food for thought:

I have not figured out a way to test it. But, I wonder if the full convex grind has a different energy curve then either the flat, or hollow ground blade. As the edge digs deeper into the material the convex blade requires a fairly even amount of force. Compare that with a flat gound blade in which the force is all front loaded until you achieve the full width of the blade, or, the hollow ground blade which required an ever stronger investment in force as your cut deepens.

To me the convexed blade is to the compound bow: as the flat ground blade is to the straight recurved bow. This may be why compound blades feel "sharper".

n2s
 
You could be on to something there. I noticed while using my convex ground knives they do cut deep without alot of force. If look down the cutting edge of the knife you might think man that's pretty thick will it cut, yes they cut very well. I think it's the shape of the grind in which the material your cutting flows up the blade.
----------------
Scott
www.knifeshows.com/gossman ScoGo@aol.com
 
Interesting analogy, N2S.

I have invested a lot of energy and thought into full convex grinds - with very good results so far. Of course, much of my testing included impact tests, which clearly won't be performed on a small folding knife. So, the results have limited application.
 
not2sharp said:
I have not figured out a way to test it.
Click to expand...

Cut into something and measure the amount of force applied, just use a bathroom scale and a cutting board. You can then note how the force depends on the height. Of course you can use more precise measurements if you want, if you have a TI calculator you can buy a cheap force probe and use that as well, or a digital balance, etc. .

Concerning the high cutting ability of convex grinds, these are usually horrible skewed arguement which result from doing something like radically thinning an edge and then attributing the performance to the fact that the grind was changed to convex. Well yes - but this wasn't the reason the cutting ability was so amplified. Yes an Opinel will out cut a Buck Strider, this isn't because the Opinel is convex ground and the Buck/Strider flat ground.

Here is a trival through experiment which shows the fallacy in the convex blade arguement :

Imagine an Opinel cutting through something, now take an idential opinel and cut a hollow grind in between its spine and edge, leaving the exact same edge geometry and blade thickness and simply creating a large relief in the grind.

Obviously the friction along the blade will be less because there is less contact area, obviously the binding forces will be less because the material doesn't need to get spread apart (if you don't cut full depth it will be a *LOT* less).

Thus obviously the blade will cut better.

[this doesn't mean hollow ground blades cut better, if you took that blade and convex ground it until the hollow grind was gone it would cut better again, then you could hollow it out again it would cut better again, then you could convex it again, etc. ]

There are of course lots of blades actually ground that way. Again look at the cross sections of the blades, don't simply focus on the nature of the grind. You can easily grind a convex blade which cuts horribly (a splitting axe) or one which cuts very well (Opinel). Note the difference there is the *cross section*.

[note I am not promoting the idea that you can simply ignore primary grind geometry - it needs to be considered as well, but it is secondary to cross section, except in extreme cases such as the cross sectional strength of an I beam which is actually *FAR* greater than a convex beam of the same weight]

-Cliff
 
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