How spherical is a sphere? If you're in the measurement business, you'd better know.
The American Bearing Manufacturers Association (ABMA) ranks precision spheres by grade number.
Grade numbers run from 3 (highest grade) to 3000 (lowest grade). The grade number identifies, in millionths of an inch, the permissable deviation from a perfect sphere. A grade 3 sphere is no more than 3 millionths of an inch (0.000003") from perfection. Grade 5 spheres are within 5 millionths. Grade 10 are within 10 millionths, and so on.
At Q-Mark, we use only grade 5 or 10 spheres on our catalog styli. Our calibration spheres are all grade 5.
For stylus sphere diameters over 15 mm, we offer grade 25. And for unusual sizes, grade 25 is often the only available.