Convex edges

[andrew7978]

I am making a few knives with convex edges. Unfortunately, I do not have a belt grinder at this current time so I am doing them with a file. My question is how do I know how wide to make the edge. I understand that the wider the edge, the stronger it is, but the less cutting ability it has. I just need to know how to find a balance between these two things. Thanks!

 

[koyote]

It depends on the convex edge- you can do a flat, saber, or scandi type grind and then put a convex final on it.

For the taper convex like I usually do on thinner stock you can get away with roughing it out a bit at ... say 8-10 degrees and then use a larger version of the 'mousepad' method.

Take a piece of 1x1 about 2 feet long and glue 14-16 inches of 1/8 inch or so closed cell foam (camping pad, weatherstripping stuff, whatever) leaveing several inches of bare wood on each side. Then get shop roll sandpaper in 1 inch width in 100-800 grit and start working it like you would a mousepad sharpener.

remember that you are dragging the edge behind when doing this method.

the magic of the convex edge is that you get the best of the both sides of the edge thickness coin

 

[andrew7978]

Thanks for the suggestions, I should have been more clear, I was referring to a full convexed knife and was curious about the thickness about half way up the knife. Or is only the edge thickness that really matters?

 

[john0270]

I would say its all a matter of personal preference. Lately I have been a fan of doing a flat grind down to 20-30thousands edge thickness, and then doing a convex edge starting about 1/4-5/16" up, this is for a few short swords I have been working on. My thinking is that it has the cutting ability of a slim flat grind blade, but the edge stoutness of a convex. It cuts through flesh and bone quite nicely(for a short sword).

 

[DustinHall]

I think you're asking about how high the grind should go up on the blade, right? If so, then it depends on the blade but, usually 1/2 or 3/4 is good for a full convex IMO... if you want a balance between strength of the edge and good slicing ability, I'd go half way up the height of the blade.

 

[koyote]

(Qualifier. I've been doing this for like 3 years now? And I'm far from an expert, but full convex on larger blades is something I'm really working on- it's my preference for heavy choppers and boar knives and such)

For a full convex to the spine I generally either forge a saber or put a rough saber grind on and then slack belt grind. I'd say down to around 1/8 on the flat and I end up with a 1/16 wide edge for heat treating.

To do this wthout a belt grinder I'd draw file a shallow saber grind and then work it with a long sanding stick using 1 inch shop roll- my sanding stick for cleaning up a convex by hand is 12 inches long and a thinner piece of a flexible hardwood that gives a little (but not too much) as I work it. all "draw work" from the hilt to the tip.

It's nice if you leave the belly only 70% done and the drop/clip/etc about 60% while you do this, it gives a better distal when you finish the belly and spine.

And because everyone wants photos, here's a WIP: (nominally 3/16 stock in the tang)

 

[andrew7978]

thanks for the suggestions everyone. I will try and implement a few of them. I guess to really figure out what will work I will just have to test one out and reshape it if it doesn't cut like I want it to.

Pics to come in a few weeks once I finish up another few blades!