Pete Macke wrote:
If a shaft is turning at about 800rpm and you want to use a drive wheel to
friction spin a 12" disk at 30rpm what size should the drive wheel be?

Have you seen the carnage when such questions were asked previously? Are
you /mad/?

Ok, assuming the drive wheel was at the very perimiter of the 12" wheel,
the size would be 12*(30/800) or about 0.45", ballpark 1/2" category
unless it is rocket science, in which case I recommend other means...

PK