Question about convexing

[davjau]

I am trying to convex a ka-bar short knife and it seems that it is getting sharp, but even after the 1000 and 2000 grit sand paper it still will not glad through paper very easily.

Should I be forming a pretty good burr like when sharpening a bevel edge knife? I am wondering if I should start back over with a more coarser grip paper and get up a bigger burr before moving to the finer grits.

I did get up a small one to begin with, but maybe not enough to really get the edge going...any thought?

 

[HandofCod]

Ha! I tried doing the very same thing!

It was the first knife I bought and I'd turned it into a spoon from stone sharpening with zero experience so I thought I'd try convexing. The short story is that it's still a spoon. That knife is just too thick at the belly for my skills because the grind terminates so close to the edge. Every now and then I try to resurrect it but I don't think it was the best choice to convex in the first place.

It's not all down to my skills either as I just finished convexing my Recon Scout and that will cut fine slivers of paper no problem consistently along the edge and that is arguably a much thicker blade.

 

[lambertiana]

Have you convexed the entire edge, or just the bevel where it meets the primary grind? When you start with a thick edge bevel, it takes a while to get the entire edge truly convexed.

If you have a true convex edge, it is possible that you are putting too much pressure and/or are using too high of an angle. Try a lighter touch at a lower angle.

 

[davjau]

I removed the black coating up to where the grind starts....trying to convex it starting from that point...eventually hoping to finish off the entire grind with 2000 grit paper and make it mirror polish

 

[banana-clip]

Looks like you got a nice edge on it so far.

 

[Rob_Angerer]

Should I be forming a pretty good burr like when sharpening a bevel edge knife? I am wondering if I should start back over with a more coarser grip paper and get up a bigger burr before moving to the finer grits.

You might try using rubber cement to temporarily attach varying grits of wet/dry sandpaper (e.g. the type used for autobody work, available at auto stores and elsewhere) to a firm foam pad (e.g. to a mouse pad).

Start somewhere like 600 grit, and work up a good burr on one side pulling the knife towards you with the edge trailing. You should be able to feel a good burr with your fingernail along the full edge length... then switch sides. The second side will largely be easier, as the burr will just kind of bend/roll around to the other side. But get a good solid burr on 2nd side.

Then go to a 1000 grit. Repeat.

I don't know that you need to go to 2000 grit.

From there, use a leather strop loaded with some fine rouge, or something similar, and mechanically work the burr back and forth, one side then the next, and it will "work harden" and eventually just snap off (work harden, like when you bend a piece of wire repeatedly to break it).

Then you can keep stropping both sides lightly until you get the desired results. Keep approx the same angle, else you'll dull your hard-won edge somewhat.

The pros use a slack belt and progress from medium to a finer grit, or a worn out belt. I've accomplished about the same thing on a cardboard wheel mounted to a rotary bench grinder, but be careful of heat (can inadvertently overheat the edge, especially the tip). Discoloration (heat coloring) is a good clue you didn't dunk the blade often enough to cool it down.

Hope that helps.

 

[davjau]

Ha! I tried doing the very same thing!

It was the first knife I bought and I'd turned it into a spoon from stone sharpening with zero experience so I thought I'd try convexing. The short story is that it's still a spoon. That knife is just too thick at the belly for my skills because the grind terminates so close to the edge. Every now and then I try to resurrect it but I don't think it was the best choice to convex in the first place.

It's not all down to my skills either as I just finished convexing my Recon Scout and that will cut fine slivers of paper no problem consistently along the edge and that is arguably a much thicker blade.

i am going with you. after comparing with my bark river fox river, the edge is just way too thick to convex. you would have to take off an absurd amount of metal, but I am still contemplating it.

Both knives are the same blade thickness but the difference is the bark river starts going to the 0 grind from the spine all the way down to the edge. The Ka-bar tries to do this starting half way down the blade (from the beginning of where I started removing the black finish). That is just too short of a distance to get to a nice thin edge to begin convexing. You would have to use 220 grit paper and just take the entire side down a good bit before trying to actually work on the edge.

Maybe I should just turn it back into a bevel edge since that is the way it came...any thoughts?