Q: Is it worthwhile to try to sharpen jewelry-sized drill bits? If so, how should I go about that? If there’s rust on them, or on my burs, does that mean they need to be replaced?
A: Take out a standard hardware store type drill bit and look at its larger, easier to see tip. The tips on your little ones should generally look the same: two planes coming to a chisel-like narrow point at the center. The two planes defined at the front end by the spiral cut into the drill, and are inclined away both from the center, and the flute of the spiral. The leading edge of each of those planes is sloping up, in the direction of rotation, so it can cut into the work. A dull bit will either have bits of these surfaces broken off, or the center chisel or leading edges/cutting edges will be dulled and rounded over.
Small drill bits can be easily resharpened, once one has a good feel for what those angles need to be to cut to, as well as having something to sharpen the bit with. I like the very thin (.009″) “flexible” separating disks. Their sides have a much smoother surface than the more common gray ones (which can also be used, but which cut much more coarsely, and on tiny bits, this makes a difference). Those nice flat sides will cut a nice clean tip on even the tiniest bits, if you can use good enough magnification to see what you’re doing. A good loupe or, at the least, a high-power Optivisor may be needed. Once this is mastered, you can use about half the length of the flutes on the small drills with several resharpenings after breaking off points. The flutes get shallower as you move down the shank of the bit, so after you’ve got a ways down the bit, resharpening leads to a wider and wider center “web”, the area in the center of the bit between the flute, which leads to a wider and wider center chisel. It still works, but not as well. With the somewhat less minuscule bits, you can thin the web right at the point by grinding the flute a bit deeper, but this gets tricky to do. Until this is mastered, take comfort in knowing that the small drill bits are pretty cheap. And the factory points are generally more uniform than what you’ll get resharpening them yourself.
So it may be easiest, at first, to just buy your bits new. And if they seem dull, they probably are. Even slightly dull drill bits just don’t work well enough to bother with. When dull, even a bit, they will heat up, destroying the temper of the bit. Even high speed bits get messed up by this, getting more brittle. So then the dull or break even more easily…As to your other question, a bit of rust on old burs might not kill them. It’s relative. How much rust? And more important, what was the condition of the bur first? Small jewelers’ burs are often just a high quality carbon steel, which are wonderfully sharp, and not too costly, when new, but they burn out and dull somewhat quickly. That’s why they often get sold in six-packs. If the burs you have were sharp before they rusted, then they’ll likely still cut OK, though perhaps not quite as smoothly. If they were dull to start with, they won’t be any sharper now.