Convex Bevels - are they better? If so, why don't I hear much about them.

L

leozinho

Member
Hi all,

I've been watching some videos of knifemaker "Gavko" videos on Youtube. He's a knifemaker that seems to have tested a lot of blades.

He's adamant that a convex bevel is the best grind. I've read the arguments for a convex edge (and I'm not asking to open a debate on edge geometry), but I haven't heard nearly as much talk about convexing the bevels.

If you are curious, here's the video I watched. After establishing his flat bevel with a 50 grit belt, he glues a thick piece of felt to his platen to convex the bevel with a 50 grit then 120 grit. Then it is heat treated. I'm sure he didn't invent the technique, but I've not seen anyone else doing it.

http://www.youtube.com/watch?v=FY1vpF0ZgMA

So...

Is it 'better'? If so, in what way? Stronger or cuts better? Any reason why more people aren't doing this? I have seen videos of people using the KMG rotary platen for convex grinds., but it doesn't seem to be nearly as popular as flat or hollow grinds.

I would think it's not as aesthetically pleasing due to the lack of crisp grind lines. Seems like it would have to be a full grind up the the spine or blended into the flats. (But I haven't seen a good picture of Gavko's finished knife with a convex bevel.)

Thanks.
 
Yes,
In my humble opinion and the opinion's of Bill Moran, Ed Flower and many other greats of custom knife making.

The con vexed bevel separates as it cuts, also there is more sectional density behind the cutting edge so there for a stronger blade. I have made these slack belt and with a rotary Platen.

Jeff from Bush Money Knives has a link to from a man in England that shows the actual mechanics of how a convex bevel works. Perhaps Jeff will chime in here and link it for us?

My thinking on why the convex isn't is popular is most that have tried to make them come out too thick and they don't cut well. You are correct that the may not be as aesthetically pleasing and I do find they work best ground up to the spine.

Laurence

www.rhinoknives.com
 
Better - yes.

Curves are almost always better and that is why raindrops, ships, pretty girls and airfoils have curves. Curves are more difficult to fabricate than angular designs and so far, there is no machine that will mass produce a convex bevel - they must be done by hand. Generally speaking, curves produce relatively low parasite drag and in the case of a blade, the convex bevel is superior in terms of strength and performance.

Jeff

[video=youtube;cFTV4ZgeLiY]http://www.youtube.com/watch?v=cFTV4ZgeLiY&feature=player_embedded[/video]​
 
You have to remember also that there are more than one kind of convex grind which is good for different things depending on where the bevel starts. From the same thickness of steel a higher grind would be weaker but cuts more easily. There are a lot of people who say that it is only good for choppers and what not but a convex bevel can cut better than a flat grind sometimes. All it is really is a flat grind with the grind lines rounded over. It cuts just as well if not better so the myths aren't really anything to go by. I have little experience but I reckon I know enough about this to talk about it.
 
The only variance in convex grinds is how high up the spine you go as the with and thickness of the blade sets the curve. The traditional saber grind was used on ... sabers and the quality pieces were taken to the spine. These were not for cutting down trees, and were wicked sharp. I have a great field knife that is convex ground, and its a hair popper all the way around to the tip, and the edge will pierce as well as the point.

I think the reason you don't see them as often is they're more difficult to grind. Mass production has opted for the double bevel grind because its easily done in an automated process. Over time the user will round out the transition with his/her sharpening skills, but that's still not a real convex. I've convex ground two blades; my first was on a wharny just to make it simpler, and it was a pain in the butt - all free hand, no jig. Lots of patience.
 
Rob, you said what I did in a few lines.
I tried to do a flat grind on the knife I am making and it didn't work, I found that a good convex is easier than a good flat grind. I did it freehand with files though and the grind line washed over into a convex by itself.
 
LiamLynch said:
Rob, you said what I did in a few lines.
I tried to do a flat grind on the knife I am making and it didn't work, I found that a good convex is easier than a good flat grind. I did it freehand with files though and the grind line washed over into a convex by itself.
Click to expand...

Liam, I'd read somewhere that to start a convex you set a steep flat bevel along the entire edge to the same angle, so on a piece of 3/16" thick I set it at 15 degrees, then gradually tapered it back in. Its not as elegant as some of the ones I've seen, where the curve is constant from edge to spine; mine sort of hurried up to about 1/2 height, then slowed down gradually to the spine. And then after HT, it was alot of back and forth to keep the edge from meandering.

How did you start off your bevel before you started rounding it back in?

Thanks, Rob
 
I like convex edges and put them on many of my blades. They are a bit stronger and the geometry opens up the material being cut.

Some thing to keep in mind is, the cutting is not done with the side of the bevel it done with the secondary bevel or edge. I make camp knives and machetes using a full convex; but all others are flat ground with a convex edge.
 
Thanks all.

I realize it's old news to a lot of the folks here but the "convex vs flat" is new to me. Again, thanks for indulging me. After doing further research, it appears that some think the convex bevel is both stronger (for the weight) and cuts better, though that's hardly agreed upon by everyone.

(I do feel like there is good evidence that the convex edge is superior to a flat edge for my purposes.)

Next question:

What are the ways to go about grinding the convex bevel?

So far I've read about the following:

1) freehand by moving the angle of the blade against a flat platen or contact wheel.

2) Using the slack portion of the belt or a KMG rotary platen

3) Placing a thick piece of felt or 'PVC foam' (I think that came from Bark River) against a flat platen. This is done after establishing a flat grind. This would be akin to the mousepad and sandpaper method of sharpening a convex edge, but instead it's done for the whole bevel.

I'd imagine that drawbacks would be:
1) More difficult to produce consistent grinds (for me, at least.)
2) More difficult to have those pretty, sharp grind lines (especially when there's a swedge or false edge on the top.)

Am I on the right track? I'm making progress on my flat grinds (big thanks to Fred Rowe. I'm using his Bubble Jig and it has helped tremendously), but the ability to make a convex bevel is something I'd like to add to my arsenal.
 
I started by filing in a bevel that had a height of around 5/8" on 3mm stock. It was a flat grind which you could do on a grinder. I then used a sanding block to knock of the grind line (mostly done already due to inferior filing skills) and then I used a soft sanding block to round the bevel completely. Not the best way but I'm sure it could be modified to work.
 
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