Good site. I've made a couple over the years using files. As a matter of fact, a file knife was supposed to be one of my wife's Christmas presents this year. I heated it the first time to soften the metal, took it out of the fire and started to straighten it......that's when a crispy coating shattered across the file. It was only hardened on the outside, the inside was a very soft steel.
I worked it, rehardened it.........it's ok. Not great, but it will work for potatoes and such at reenactments. I just have to put a handle on it before Christmas and build her a neck sheath.
I have another file I'm working on now.
BTW, the file with the crappy surface hardening was a Craftsman, who'd have thought it?
I saw a site similar to that a couple years ago and took the plunge, still grinding and grinding... haven't finished a blade but have gotten darn close and I've got a good number of profiles done .
The older files use good steel for making knives. As Flinter found out, a lot of the modern ones are just case hardened and are worthless for a decent blade.
The tricky part about using files or automobile springs is that, as you know, the steel is already hardened. You will need to anneal (soften) it by heating it up to critical or nonmagnetic for the simple steels. Usually this is just a little after the steel is glowing red and all the "shadows" have left. It's a lot easier to see this at dusk.
In order for the steel to properly anneal, it needs to cool down as slooooly as possible. Usually what people do is bury it in the middle of a pile of vermiculite and pack it down and let it sit for a day. If you simply let the steel cool in the air, it's called "normalizing" which is great to reduce the stresses built up in steel (smiths normalize many times when forging a blade) but the steel doesn't get as soft as it could. This means you'll spend a LOT more time filing the blade.
Unless you feel comfortable annealing the steel, it's recommended to start with annealed steel. The places that manufacture automobile springs (not the places that just stock them) will stock annealed steel. Fortunately, due to the booming auto/truck customizing trend, there is an increasing number of places that do such work. I bought a 22' bar of 3/16" 5160 steel from a guy and it was a good deal for me.
If either of you want a 1' piece or so of annealed 3/16" 5160 or 1/8" 1084 steel just let me know and I'll send it out free (happy holidays and all).
both #4 and #5 (from the left) were forged. I had #4 almost totally finished, but the edge cracked when I quenched it. I ended up putting it in a vice and breaking the tip off about 2" back to check for grain structure. Was very nice... too bad it died!