>Maybe using a bench grinder
I tried a small one which I had access to on a piece of scrap metal and I thought it was quite difficult to work with, handling the piece and controlling how much material was removed. As the metal got thinner it heated up unpredictably fast, often resulting in rapid and sudden degradation of the material. Obviously a matter of experience, I know, still I think I would be better at moving the tool rather than the piece I work with.
Actually I was more successful just using a metal file to make a short knife, although that's not what I would use for a bigger project like a sword or a machete.
>old files could be a little brittle, maybe a piece of leaf spring?
yeah, good point, probably impossible to get the blade to a decent sharpness, however I know some old bayonets are often pretty stiff and not sharp, either. Using a garden shear is pretty neat, though. thanks for the idea.