From flat grind + secondary to full convex?

[WhittlinAway]

I've been wondering about convex grinds lately. I don't know much about how they are achieved. I know that you can convex a secondary bevel using sandpaper over a backing like leather or a mousepad or by sharpening with a flexible belt, but not much beyond that.

Starting with a factory full flat grind with a secondary bevel, does taking those techniques to the extreme and laying the full surface of the blade against the sandpaper or belt yield the same result as a factory full convex grind? Or is that something that can only be created during the initial profiling of the blade? Or is that a modification you can achieve, but requires a different technique?

Or am I just babbling at this point? I realize my terminology may be imprecise and acknowledge that my knowledge of the subject is lacking. Corrections are greatly appreciated, as are shared knowledge and pointers to where I can learn more.

 

[Obsessed with Edges]

It's actually pretty easy to do with wet/dry SiC sandpaper. More so, if the backing under the sandpaper is somewhat firmer than a mousepad, which I consider too soft for doing this quickly, and it also tends to round over the apex at the edge. A firmer backing will allow the sandpaper's grit to dig more aggressively using more pressure, therefore hogging off metal faster and leaving the apex crisper when working near the edge (lighten the pressure there). I've used either plate glass or a granite surface plate for this, with maybe 1 to 3 sheets of plain printer paper to add just a little bit of 'give' under the sandpaper. Just lay the blade flat and 'strop' in linear passes along the length of the sandpaper - the convex will form automatically this way, with the compression of the backing under the sandpaper doing all the shaping of the convex. Start with something like 150-220 grit for the shaping of the convex. Remove some or all of the printer paper under the sandpaper when working to refine the area near the apex - this will ensure the apex remains crisp and can be made even more so. Then refine with 320 and beyond. Grits beyond ~ 800 or so will start to polish, with a bright mirror finish coming somewhere beyond 1000 - 2000 grit.

Depending on the steel and size & thickness of the blade, it can happen very quickly. Simple steels like 1095, CV, 420HC and 440-series steels will shape very fast this way, with traditional pocketknife blades taking maybe 30 minutes or less to shape the convex. More carbide-heavy steels will take longer - especially steels with a lot of vanadium content. And very big & thick fixed blades will obviously take much more time as well, no matter the steel type.

Most of the benefit is in convexing the steel just behind the edge - enough to gently ease the crisp shoulders of a typical secondary bevel on a flat-grind blade, without significantly altering the edge itself, which becomes too obtuse & rounded off if you spend too much time there or raise the angle too high. And you don't necessarily have to convex all the way to the spine, to get the benefits of a convex. When convexing, keep the angle LOW and the backing under the sandpaper relatively firm, so the convex is shallower and the resulting angle at the edge won't be too obtuse. A thin, shallow, subtle convex can be a wicked slicer.

 

[Pensacola Tiger]

Starting with a factory full flat grind with a secondary bevel, does taking those techniques to the extreme and laying the full surface of the blade against the sandpaper or belt yield the same result as a factory full convex grind? Or is that something that can only be created during the initial profiling of the blade?

A convex grind, as opposed to a convex bevel, can only be created when the blade is initially ground. A full flat grind has had material removed so that is is not possible to create a convex grind.

 

[WhittlinAway]

Obsessed with Edges , thank you for the detailed explanation of the technique and the reasoning behind it. The way you described mousepad vs. printer paper makes a lot of sense.

Pensacola Tiger , thanks for clarifying that. I'd been wondering whether that was the case.