Sphere Grade Guide
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.
Sphere Material Guide
Ruby (Al₂O₃):
Synthetic ruby known for its high hardness and wear resistance, ideal for general inspection touch probe applications.
Silicon Nitride (Si3N4):
A hard pressed ceramic with similar properties to ruby, suitable for various applications, ideal for scanning of soft materials such as aluminum.
Zirconia Oxide (ZrO2):
Sintered zirconia oxide, offering a high level of hardness and wear resistance, ideal for aggressive scanning on cast iron components.
Alumina (Al2O3):
Commonly called ceramic, sintered alumina is known for its hardness and wear resistance similar to ruby, ideal for general inspection touch probe applications.
Diamond (C):
The hardest substance on earth, diamond is unsurpassed for all touch probe and scanning applications. Diamond spheres will not scratch polished surfaces, and will not wear, even when used on highly abrasive materials.
