This may sound idiotic, but are you sure you don't have the platter installed upside-down?
A slightly tapered bearing spindle matched to a slightly tapered hole in the platter (both narrower at the top) would provide an inherently stable fit, assuming good machining of course.
And/or what Bob said...
A slightly tapered bearing spindle matched to a slightly tapered hole in the platter (both narrower at the top) would provide an inherently stable fit, assuming good machining of course.
And/or what Bob said...