Let me put out a disclaimer of sorts to start with here before delving into the actual step-by-step. I know there's likely a ton of other methods out there for making rivets and there's nothing especially earth-shattering about my own method. Three other techniques I'm familiar with are: using small dots of PVA glue to build up a "rivet" shape, gluing small plastic bead-shaped "spacers" from a water purifier filter into place in shallow drilled divots, and simply using round styrene rod thinly cut into measured "rivet-height" lengths and glued where needed.
I think this method gives a reasonably realistic-looking end result and I've found it to be a bit more durable than glued-on rivets, where I've frequently had my rivet work scrape off or easily get knocked off - especially when working with metal figures where the plastic-to-metal bond isn't so strong. I've also found the "rivet setting" part of this writeup to be equally useful for other things like small studded spikes or even horns on figures. The downside to my rivet-making method, and I'll lay it out plain now, is that the "setting" part is slightly more time consuming than some of the other approaches mentioned above - on the other hand, if you make a whole lot of rivet "blanks" in batches during down time (like when watching a movie you've already seen a hundred times), then that's the bulk of the work done and the rest is just placement.
I would suggest to anyone trying this method of making rounded rivets that they take all safety precautions necessary - there, that's my public service announcement duties covered for the month. No, really, I personally check over all my equipment and tools before I start any work to make sure that nothing is damaged or compromised, that everything is firmly tightened and nothing is out of place and that my eyes are shielded from possible flying debris. As a general rule, I always try to use my Dremel immediately over (if not slightly within) a small, portable wastebasket so as to minimize the odds of something being flung loose as well as to collect as much waste, dust, and stray particles as possible.
- To get started, take a length of the 0.4mm diameter round styrene rod and set your Dremel on the lowest speed setting with a medium sanding disc firmly in place. You want to hold the length of styrene rod perhaps 1/4" up the rod from the end to be shaped and be sure that you always shape the end of the styrene against the portion of the medium sanding disc spinning away from you. (I'm left handed, so for me that's the bottom.) You might want hold the styrene rod with hobby pliers, forceps, or even tweezers - anything so long as you can maintain a firm grip on it. Myself, I'm comfortable pinching the rod between my fingertips with the remaining length pointed outwards away from me while shaping it, but I also have very steady hands and maintain a constant focus on my work to avoid injury. (At the lowest speed setting, my particular Dremel can be used for polishing, light finish woodwork, or even shaping and buffing fingernails in a slightly different hobby; I wouldn't use my hands to hold material at any higher RPM settings, however.)
- Even at the lowest speed setting, the medium sanding disc is capable of melting the styrene rod through friction - we actually want to take advantage of this as it makes the shaping process easier. With very light taps of the end of the round rod against the portion of the disc spinning away from you, twisting the rod slightly at every tap to expose a new face to the sanding disc, holding it at about a 60° and you'll start to see friction melt the styrene in the shape of a rough "bullet" point. I should note that at a low speed setting and with about 1/4" of the styrene rod exposed, you'll find that the end "jumps" constantly away from the disc while shaping - this isn't necessarily a bad thing as it limits the amount of time and friction that the styrene is exposed to in contact with the sanding disc, but it does take some getting used to.
- Gradually you want to move the very end of the now-semi-pointed styrene rod to more of a 45° angle in relation to the disc, and maintain the process of touching the very end against the spinning disc with very light taps while making sure to twist the rod slightly at every tap to expose new surfaces to the disc. You'll quickly note that it begins to assume more of a rounded aspect at the end.
- At this point, you'll likely be able to finish off the rounding process with extremely light taps of the very end of the now-shaped styrene rod end against the sanding disc, again making sure to continue twisting the rod with every contact - if done lightly and carefully enough throughout the whole process, you should essentially have a smooth, natural rounding to the end of the styrene rod with a minimum of wasted material. If the shape is slightly off that's not necessarily a bad thing, as the appearance when eventually mounted on the figure can help to contribute to any "battle damage."
At this point you cut a short length off of the styrene rod in addition to the rivet - I'd suggest about a millimeter or so, but it all really depends on how much depth you have in the area you're wanting to place your rounded rivets.
The next part is relatively straightforward: for the sake of consistent spacing and level appearance, you'll likely want to take a ruler and a soft lead pencil with a good point and go along and mark where you're going to place your rounded rivets - you'll know before you start the next step of drilling if things look good appearance-wise, and you can always adjust your planned placement at this point since everything is simply markings on the figure. I prefer to use mechanical pencils with 2B grade lead for as they make clear marks even on metal figures, erase fairly easily afterward and are soft enough that they don't gouge sheet styrene if you're doing layers of custom conversion.
Now you take your drill bit that matches as closely as possible to the diameter of the round styrene rod you've used to make your rivets, and sink placement holes to at least the depth of your length of rounded rivet (remember I suggested cutting them off the rod in 1mm segments or thereabouts). You might want to drill slightly deeper than the length of your rivet cut-offs, since you can always put a little extra glue in the hole and simply hold the rivet in place to the correct depth until the glue sets up a bit.
Once you've got your holes drilled for placing your rounded rivet lengths, you'll want to use tweezers or something similar with a narrow-but-firm grip to hold the length of rivet near the top, apply a small amount of cyanoacrylate glue (CA glue, Krazy Glue, Zap-A-Gap, etc.) around the lower end of the rivet length and, placing the end into the hole you've pre-drilled, hold it at the height you want the final rivet to stick out (or course, this assumes that you may have been a little off on the depth of the hole - if you're able to drill precisely right or have the foresight to test fit and/or shorten the length of the rivet "insert" before gluing and slotting it home. I would suggest CA glue over polystyrene (plastic) cement, since an excess of plastic glue on the slotted end of the rivet when mounted in/on a plastic figure could creep up and deform the rounded rivet shape at the visible end that you've spent your time making. CA glue also allows a bit of leeway in terms of adjusting the fit of your rivet before it sets, tends to fill any gaps better if your holes weren't able to be the same diameter as the styrene rod you used, and can be wiped away without melting/distorting the final appearance of your rounded rivet if you did happen to use a little too much glue at the base before slotting it home.
Here's an example of rounded rivets made following the guidelines above set into a sheet styrene "plate" over the skull of a converted figure:
As you can see in this close-up picture of some finished ones I've left some of the rivets imperfectly shaped and in a few places not put anything into the holes I've drilled for them, all with the idea of getting across battle damage, or perhaps "wear and tear" on the figure.
Here's more of the same with finished pictures of the rounded rivets in several places across a full figure, the same as seen in the top-down head picture above: