How spherical is that 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. In other words, 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 25 are within 25 millionths, and so on.
At Q-Mark, our ruby, silicon nitride, and zirconia spheres are all grade 5. All of our calibration spheres are grade 5.
Our ceramic and carbide spheres are grade 25. Finally, most spheres larger than 12.0 mm diameter are grade 25.