Several years ago, I, a boy of 11 or 12 who was just getting into metalworking, stopped by a yard sale looking for tools, materials, anything interesting. This particular yard sale had a box of needle files, all of various shapes. I bought five or six, left, then came back later to pick up more. The owners gave the me the nickname of "The File Kid".
I still have those files. Most of them, anyway (hard to keep track of those little things) . I never thought they'd be really all that useful, after all, I doubt I paid more than 75 cents for them, and they seemed even more useless after I got a dremel, but any artist who works in metals will tell you otherwise. Files in general are priceless, enabling you to remove material in clean, utterly precise swipes of motion. They're not fast, granted, but it's tough to make a mistake with a file. Once they dull, bigger ones can enter a new life as blade. Basically, if you see files for sale on the cheap stock up. You'll need 'em.
Moving on however, this week's subject is filework. As you've come to know, handmade knives aren't just physically superior to most mass-produced blades (barring some higher-end brands), they are an art form themselves, falling between tool and art, or "functional art". Filework is a huge part of that, essentially carving different areas of the knife, particularly the spine near the guard, to make beautiful patterns.
Really, all the tools you need are a sharpie, vise, and at least two needle files, one square and one round. The greater variety of files you have, both in shape and in coarseness, increases the ease and complexity with which you do the work. For the demo piece below, I only used one file, a half round.
There's a large variety of patterns you can do, going from very simple to complex. Vines, branches, and ropes are common choices. Often it's just a sequence of marks, say a round indent, then a sharp triangle, then a round indent and so on. Look at online images for ideas, often just searching "knife filework" will get you what you want. Mark on the spine exactly where to file away at, and make sure it's consistent, with equal distances and sizes. I like to do one side first, then the other side. This helps me keep track of the correct sizes, and helps pace it so the left side corresponds to the correct place on the right.
The first few strokes should be in a single direction (file a stroke downwards, lift, bring up, place down, repeat), and with the most minimal pressure. Pressing hard and making back and forth motions will cause you to lose control and mark up the spine in areas you don't want, resulting in your having to regrind the knife in order to fix.
I file at about 45 degree angle. If it's too steep, the marks become too prominent when viewing the flat of the blade. Too shallow, and they don't look distinctly separate from the "raised" portion (the vine itself).
This is just a personal preference, but I prefer to do all the round indents (you'll notice the pattern is composed of half-circular indents and wedge indents) on one side first, then the other, and then the wedges. This helps me get into the groove of things (pun intended), and can moderate the sizes. You'll notice I don't match the sharpie exactly; turns out my filework is way more exact and I can moderate it in a more precise and uniform manner than I can with a sharpie.
After filing in the wedges, the filework is all roughed in. I went in afterwards and deepened the rounds, to give a more flowy look. This is something I do quite a lot; leave room for tweaking as I go. The trick is to keep a constant angle while filing; if it's convex, even slightly, they'll reflect light unevenly, catching the viewer eye's attention to the filed portions and away from the raised vine, where it should be focusing.
With the filing done, it's time to give the filework a finish. Wrap the sandpaper (220 grit or so, then moving up to at least 400+. I only did 320 as this is a demo piece) around the round handle of the needle file, and use it to work the rounds, then the file's blade itself for the wedges. Once you've brought it up evenly to the finish you want, you're done!
Overall, there are hundreds of possible patterns you can use, and you'll find they can get exceptionally beautiful when polished or when the steel is sandwiched with other metals (in a full tang or folding knife construction) to make even more complex patterns.
Some absolutely stunning filework by bladesmith Alistair Bastian, showing a fantastic glimpse of the possibilities