I need to know the application (size, tolerence)to recommend the best method. Typically for checking parts the CMM or the Sunnen are good for +/- .0001". Some Cmm's can do better then others. In a production application the Sunnen wins hands down. If you are trying to calibrate plain ring gages, I use a Federal internal comparator. our uncertainty is +/-.000009"+1.5 X L.
------------------