help with sharpening convex

[40swg23]

so I have a fallkniven A1, with a convex grind. I am using sandpaper glued to a mousepad to sharpen. I have p220, p400, and p600 sandpaper. I spent a good bit of time with this knife trying to get it sharp. I mean it's not even shaving sharp, after all the time I spent. I know that these grinds can get scary sharp, as I have read. I just cant seem to get there. How do I know when the angle is right? Do you get it shaving sharp before you move on from the 220 grit?

Thanks in advance

 

[thombrogan]

Yep. Get it shaving sharp with the 220 grit before moving on. Also, if you use the sandpaper over a harder surface, the edge will still be convex, but you'll usually get better results quicker. It takes patience, but it's easier than it seems.

 

[Dog of War]

Agree with thombrogan about needing something harder backing the sandpaper. Mousepads IMO have too much give and over time your edge becomes more and more obtuse. Slight changes in angle and too much pressure when using something soft like mousepad can quickly undo work you've already done. A harder backing lets you use more pressure and gives better control ..... I like a rubber sanding block like used for auto body work.

You can also convex on a regular stone, in fact that's exactly what a lot of guys who freehand have done for years whether intentional or not.

 

[Cliff Stamp]

I know that these grinds can get scary sharp, as I have read. I just cant seem to get there. How do I know when the angle is right? Do you get it shaving sharp before you move on from the 220 grit?

Edge trailing can produce significant burrs depending on the steel, you may have to make 1-2 elevated passes into the abrasive to remove the burr before you raise the grits. All grits can be made shaving sharp, it is in general easier to get the coarse ones sharper because they do so much more work per pass. You judge the angle by how it feels, until you get that knowledge you outline the edge with a marker and check it after a few passes to see where it has been abraded.

-Cliff

 

[David Schott]

I always do convex on a slack belt..never tried the pousepad thing. Recently when I wanted a really great edge on a cutting comp knife, I put a normal bevel on with a lansky, then turn to the slack belt. anything thicker than that primary bevel is just hurting cutting power, so I slack down to the point i can hardly see that lansky bevel, which thins the profile a bit more and then finally barely let the edge contact the belt. This leaves me with a convex'd cutting bevel and a flat ground blade. The main attraction to me of a convex is how it moves through materials, not how sharp it gets. The difference is between driving a triangular wedge through things or driving an appleseed-like shape through things....which is far easier.

Without a belt grinder, i dont have a lot of experience on convex edges. I think you may be overthinking them...Think about a normal edge, only the very tip of that edge cuts, the rest of the sharpening bevel doesnt even come into play other than it sliding across what you are cutting. Convex removes that thicker material, but the edge itself shouldnt be all that different angle-wise at the apex.

 

[A C Richards]

I use a piece of 9oz leather glued to a 10 - 12 inch long x 1.5 inch wide micarta block. Then I use sandpaper over that cut in strips and hold it in place at each end. I can go to whatever grit i want and even polish it with the leather and polishing compound rubbed into it. It will dafinitly mess up a finish but it gets the blade scary sharp, scares the hair off your arm sharp.

Chuck

 

[Mtn Hawk]

The main attraction to me of a convex is how it moves through materials, not how sharp it gets. The difference is between driving a triangular wedge through things or driving an appleseed-like shape through things....which is far easier.

It seems to me that the optimum edge would depend on the material being cut, sliced, chopped, or whatever, rather than a specific edge geometry or angle. I have not done any in-depth testing of the cutting and chopping ability of convex edges vs. V-edges, although I've played around a bit with both. What I found in my limited experience is that a V-edge cuts, slices, and chops (in hard wood, I don't usually chop soft or green wood) significantly better than a convex edge. However, I believe it was Cliff Stamp who mentioned in another thread that the convex edge is superior for chopping soft wood.

Think about a normal edge, only the very tip of that edge cuts, the rest of the sharpening bevel doesnt even come into play other than it sliding across what you are cutting.

True, and a good point. I don't see this mentioned too often even though I think it's very important.

Convex removes that thicker material...

The profile of a convex edge clearly shows that it is thicker above the edge than a V-edge.

Logically, it seems that the less metal behind the edge the less resistance there is and the better the blade will cut or chop, at least in most materials. I think you are saying the same thing here

...anything thicker than that primary bevel is just hurting cutting power...

Unfortunately the terms "primary bevel" and "secondary bevel" are often used interchangeably so I can't be sure exactly what you mean. Again, looking at profiles, a V-edge obviously has less metal behind it than a convex edge so it seems like it would have to cut and chop better.

I have never run across any extensive comparison testing of the two edge types, just read a lot of forum posts about how great convex edges are. They very well might be, but if they really are superior to V-edges I'd like to see solid and objective proof of it.

Anyone know of any honest and unbiased tests regarding this?

 

[Cliff Stamp]

It seems to me that the optimum edge would depend on the material being cut, sliced, chopped, or whatever, rather than a specific edge geometry or angle.

Generally yes, the basic consideration is wedging forces, examine soft and hardwood axes, one has a very thick convex profile and the other a thin hollow ground profile. Both do the exact same thing, chop woods, however the type of wood changes the optimal grind significantly. This same basic difference in property of soft vs hard wood and how it effects optimal geometry is the same basic consideration for any profile. On some materials you want enough metal to wedge the material apart (convex grinds), other times you want to reduce wedging and side friction to a minimum (hollow grinds), a flat grind just goes inbetween and isn't optimal at either, but also isn't horrible at either as well.

However, I believe it was Cliff Stamp who mentioned in another thread that the convex edge is superior for chopping soft wood.

Not as much the edge, but the primary profile, the actual way the blade itself is shaped :

That blade has dual convex primary grinds, it sweeps down and up in arcs from the 3/4" point on the blade. The actual edge (the part that does the cutting) on any knife is v-ground regardless of how it is sharpened because the abrasives themselves lack curvature.

The profile of a convex edge clearly shows that it is thicker above the edge than a V-edge.

For a given thickness, a convex edge will retain more metal than a v-ground edge, however in general, people who run convex edges tend to alter the profile dramatically so this presupposition no longer holds. Generally someone takes a stock profile, say 0.035" thick by 20 degrees per side, then puts it on a belt sander and applies a a bevel which sweeps from 8/10 degrees. This is then used as "proof" that a convex edge is superior.

Of course what it actually shows is that a 8/10 degree edge cuts better than a 20 degree edge (assuming the durability isn't overloaded). The same thing would be seen if a straight relief grind was applied as recommended by J.J. of Razor's Edge. You would in general see better performance if the primary profile was adjusted to reduce the edge thickness to a minimum instead of adjusting the edge angle to a minimum because that makes the edge very wide and sharpening efficiency is vastly reduced.

There are some durability issues as well, at the same level of cutting ability a thinner edge with a more obtuse angle is more durable for cutting than a thicker edge with a more acute angle, however in extremes of chopping and really heavy dynamic cutting, the thicker edge with the more acute angle will reduce gross damage.

-Cliff