At that point you need a stiff metal (titanium?) which was precision made
(extruded, rolled, milled, etc).  The stiffer the metal, the better anyway,
since you aren't losing as much energy in flexing the rod.  You could add
balancing mechanisms, but the added weight and cost are surely going to be more
costly than starting with a precision part in the first place.

BTW, why 1/8"?  Why not use 1/4", or even thick-wall tube at 1/4"?  Either of
these will certianly reduce vibration.

-Adam

Andrew Kunz wrote:
>
> My problem is how to figure exactly how to balance a 12" rod 1/8" dia rotating
> at 20K-30K RPM.
>
> Andy