Dear Meike Williams, there is a concave cutting burr which comes in a
range of sizes. It comes on the standard 2.35mm shank which fits
into any hand-piece chuck and are listed in (my) Grobet USA catalogue
on page 56 as three varieties: Busch Cup, Grobet Cup, and Busch
fast-cut cup. This would do the job and, by the time you’ve polished
them, leave you with a smooth rounded finish. If I’m just making a
single pair, I simply smooth round the post-end with a fine needle
file, a quick fine emery, then polish. Kind regards, Rex Steele Merten