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What happened to the convex edge?

So far the discussion has been limited to cutting ability, sharpening, and impact resistance. What about toughness, ie resistance to lateral bending. In my own mind, it seems like a flat ground blade would not offer as much resistance to bending as a knife with a convex or concave arc/grind. Of course there are many variables to consider, but let's just say all else is equal for the sake of argument. Would a convex grind, hollow grind or flat grind be toughest? Would there be any perceptible difference? Any engineers out there that could shed some science on the subject?

Rick
 
I still use convex edges, not on knifes as much though. Unless im doing a samurai style tanto. Ive noticed a drop in convex edges even with the katanas being produced, even the "authentic" ones. strange because it has the best geometry for a cutting/slashing sword blade.
as far as I know the convex edge works best because there is nothing angled on the blade for material to catch on. The edge just glides through cutting and the body of the blade seperating the material apart.
 
I'm a freak about "sharp" grind lines and "cleanliness" in my work. Because of that I strayed from any kind of convex grinding for a long time.

I always thought that to get a convex grind a guy simply slapped the blade up against a slack belt.

I suppose some do, but it will leave you with a horrible cross section (i.e BAD blade geometry).

I have found that a THINLY ground blade with a very slight convex is something that can be done with an exceptionally "clean" execution, and also offer GREAT blade/edge geometry.

I flat grind against my platen up to a ~320x Norax belt. I take the flat grind to the spine, and the edge down to 0.004"- 0.020" depending on the knife. The 20 thou is the very thick end for me, on a camp knife.

Then I switch over to the Rob Frink rotary platen....aka the best thing since sliced bread.

With a 120x I take the blade down to an edge, or at least VERY close. The go up to a ~320 Trizac Gator belt.

This process can produce a whicked cutting blade, and it will be sharp and crisp so long as you pay attention to what you're doing.

There are a lot of guys that only flat grind, or only hollow grind, that would over generalize, and tell you, "a convex is too thick." I would bet a wooden nickel my blades would be thinner than almost any of theirs.

BUT, I ALSO do only full flat grinds on some knives, and only hollow grinds on some knives.

It's all about finding the optimal steel, heat-treat, blade and edge geometry for what the knife is intended for!!!! :D :) :D

BTW- I don't like sending blades to customers sharpened on a belt sander, because unless they have one at home, they will never be able to reproduce that edge. I do my final sharpeing on a India Fine stone, and then 1 or 2 ceramic stones.

Plus I offer free re-sharpening. That way, when they TRY to get their edge back with a stone and screw it up...it can be fixed ;) :p

-Nick-
 
I flat grind and polish to a convex edge.
I just delivered a custom bowie to a martial arts instructor.The knife had an 11" edge.It was arm and leg severing sharp.The buyer said that he had to resharpen it because it had no edge on it.The edge was there,he just couldn't see it.I have heard this same comment at shows when someone was looking at a knife.If they don't see that secondary bevel,they think it can't be sharp.I polish the edges on a felt belt, which works perfect for the convex grind.A variable speed belt grinder is a must.
SA
 
Brian, yea, I sharpen free-hand. There are a lot of gizmo's out there that work really well though. Several of my friends have the full, whiz-bang Edge Pro, and it will put a scarey edge on anything. I suppose the only reason I don't have one is the $300 price tag.

I got the Spyderco Sharpmaker because that way I could get two grits of Spyderco ceramic stones for $50 total, instead of $50 per stone for the bench stones. I don't usally pay a whole lot of attention to the set angles they have...but it works great either way.

I set the edge with a fine India using kerosene as a lubricant, and then go to the med and fine Spyderco stones. You can get an edge that's very aggresive, but will shave as well.

I know exactly where SA is coming from. That's the kind of edge somebody walks up and cuts themself on, because they don't see a big secondary bevel. Most of the time they probably needed a good cut ;)

-Nick-
 
Rick Baum said:
So far the discussion has been limited to cutting ability, sharpening, and impact resistance. What about toughness, ie resistance to lateral bending. In my own mind, it seems like a flat ground blade would not offer as much resistance to bending as a knife with a convex or concave arc/grind.
Click to expand...
Of course it offers more toughness, it is thicker, for more of the blade than a flat or hollow grind.
 
All of you made some good points.

It sounds like there is a lot of combination grinding going on...i.e. flat grind with a convex edge. so how far would you guys say your convex goes back?

I really appreciate how Greg and Nick described their technique to achieving their geometries. Thanks guys.

Which brings me to another thought that I heard mentioned. The idea of customers needing to see grind lines and big edge bevels. Why is that?

As far as I can tell it comes down to what a maker does in his shop during production. If he stops to test his edge performance before finishing, he should be able to build a good edge.

Which brings me back to the idea Rick mentioned...toughness. I heard metioned many times in Atlanta..and here on the forum...that a knife is not a prybar.(Which sort of explains the current trend in handmades and blade thickness.) At the same time, from the same mouths, I have heard that you never know what your knives will be used for. These are two seemingly contradictory statments. I always wondered if Jerry Shipman had a flat ground knife if he would have made it out of his truck.

Hey, a big thanks to Dan Koster for posting some curvy knives. Dan is a convex edge guru from the HI Cantina...he knows some cool stuff.

Does anybody have any performance notes on flat ground blades?

SHane
 
Here's my thoughts (I'm the only one that cares, but what the heck).

Regarding Bevels: I believe one of the troubles with the perfectly flat grind is friction. (For this description, picture cutting a block of cheese in half.) As the blade cuts thru a material the cut material is pushed to the side. The more the material is in contact with the steel, the more friction and the harder to complete the task.

If the bevel is convex it pushes the material away faster (than flat or hollow) and the blade surface recedes back away. If the blade is hollow ground the material is not pushed away as fast, but the blade stays back out of the way. If the blade is flat all the way to the edge the cheese is going to stay in contact with the steel. In theory, not only is this the maximum amount of friction, but there is no air space and you have vacuum to over come as well.

For this reason I've been doing kitchen knifes like Nick Wheeler said. I go flat all the way to near the end then convex for the last 3/8" or so. Or I use a hollow grind. Also, like Nick I make the thinnest edge possible for the intended use of the knife.

Either convex or hollow it makes slicing chores go better than flat.

(BTW the most insane cutter I've made to date is a small paring knife with a hollow grind on a 20" wheel taken to almost sharp, then a polished edge.)

RE: convex edges

Peter, thanks for the note about not feeling as sharp but lasting longer. I haven't gotten a convex edge to feel as sharp so I've never done edge holding testing on them - generally giving up before finishing the test. I'm gonna have to re-visit that now. Thanks,

Steve
 
I have an article that Shackleford is editing now on blade grinds, he has had it for over a week. I just wanted to mention it so you would not figure I was using words from this thread.
I will comment here later, got to get a Bowie in the mail today.
 
For sharpening convex, slack belt works good but most people you might give/sell a knife to doesn't have access to that. Stones work, but unless you're just touching up the very edge, it's hard to make it not look ugly and uneven along the top of the sharpening line. I'm a big fan of the sandpaper on soft backing method, mousepads and thick leather both work well for the backing. It's much easier to keep the sharpening line even, and on an etched blade even looks bevel-like enough that people can "see" it's sharp.

Here's a good article about convex's in general with a section on sharpening both on hard and soft surfaces. http://home.nycap.rr.com/sosak/convex.htm

edit: I wanted to note that I don't follow the exact instructions in that link, I only do about the last 1/4" down to the edge, not the whole side, that's how I get the faux bevel look.
 
Not all convex grinds are equal, all too often we preceive convex, flat or hollow as single events. Actually each is a blend, as some great thoughts in this thread have indicated, they are all different and should be headed in the direction of What For? Many good thoughts in this thread! I feel that we are all learning and this is good.
 
As far as a convex grind being to thick for fine cutting, I feel if you need a thin cutting blade and still want a convex grind get a knife made from thin stock, for example an 1/8" thick blade with a convex grind. If you want a blade for chopping or heavy duty outdoor use cutting go with a 3/16 or 1/4" thick blade. I don't think one particular grind is better then the other. Blade thickness dictates what type of cutting you are going to do. It seems the convex grind takes alot of criticism and gets beat on alot.
Scott
 
Larrin, I previously deleted my response but thought about it and I feel that it is warranted.

Don't you think that Rick Baum had a valid point? Afterall he didn't ask about just a convex grind, he also asked about a concave arc/grind. I believe that it might also be called a hollow grind which would have less material than a flat grind.

Let's try to keep things in a learning environment and keep the "Duh" type of comments to ourselves. I for one would like to hear from someone with an engineering background that could verify or refute Rick's theory. Sounds interesting to me. Unfortunately I'm not qualified to answer Rick's question but maybe someone will come up with something.

Keep up the good questions Rick ;) .

Dallas
 
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