Andrew, you don’t need that fancy shape. Just a cylinder shape,
except with facets, so the thing ends up as a hexagon, or other
polygon. The fewer facets, the stronger the action. You don’t need
to make them either. Look in the Stuller tools catalog, and you can
find these rotary burnishers made in carbide, ready to go.
However, be aware that these are a poor imitation of the action of a
hammer handpiece. No matter how you make the rotary tool, the action
is a wiping/side to side burnishing action, rather than an inline
impact. The effect on the metal is quite different. The rotary
burnishers sold are usually presented as good means with which to
burnish out porosity in castings, and they work well for that. For
setting stones, they’re less effective.
The simplest rotary burnisher (or hammer if you like) is simply a
bent bur. Take an old dull busch bur, remove the original working
end. Turn it over, heat the last quarter inch or so of the back end
of the bur and bend it to a short L shape. round off and highly
polish the end of the short leg of the L. In use, the long end goes
in your dremel or flex shaft chuck, and the short end, flying around,
can burnish out pits and defects in a casting. If you want to get
fancy, make one where the working end is a bit of carbide soldered
on, equally rounded and highly polished. That gives less friction
with the metal, and lasts longer. With some care, and lubricant, you
might be able to move bezels too. But I doubt that this will be a
strong improvement over just using a burnisher, or hammer and punches
for heavier metal.
You might consider one of the air hammers. There’s a tool Harbor
Freight sells for engraving on glass, etc, which is air driven off of
any compressor. Made under their Chicago Electric brand, I think…
Intended for stipple/impact engraving, some people use them for stone
setting as a hammer. I’m not a great fan of them, but they’re not
that expensive, and perhaps that would serve your needs if you’ve got
compressed air available.