Tthis is a pretty simple detail that you can use to add some variety to your models.
Start with a small brass tube. You can use any size you'd like for different looks. I used a brass tube with an outside diameter of 1.6mm.
Once you've cut a small length of tube, secure it into a pin vise to better handle it.
Depending on your brass tube's diameter, use a triangular diamond file to file "teeth" into the tube. I cut three "teeth" into my tube.
Once you're done filing the "teeth" into your brass tube, you've essentially got a small circular saw.
Take the the part you want to add the detail to and mark the spot of your detail to see how it might look.
Once you're happy with the detail's location, slowly start to drill in the detail. Make sure to go slowly as the drill can slip from the location very easily. Once you've got it set in, you can start to go a little faster to deepen the detail.
This is what it should look like after you've finished drilling. It will probably be a bit rough.
Lightly sand the surface to smooth it out a bit.
Here's the finished engraved circular detail.
If you have small chisels, you can add other engraved details for different looks!