How round is that ball? Put more accurately, how spherical is it? If you're in the measurement business, you better know.
The American Bearing Manufacturers Association (ABMA) places a rating on balls, known as ball grade. Ball grades define several important attributes of balls.
ABMA grades run from Grade 3 (highest grade) to Grade 3000 (lowest grade). Therefore, the lower the grade, the higher the quality of the ball. The Grade number identifies, in millionths of an inch, the allowable deviation from spherical form. In other words, a Grade 3 ball can deviate no more than 3 millionths of an inch (.000003") from a perfect sphere. A Grade 5 ball can deviate 5 millionths, Grade 10 can deviate 10 millionths, and so on.
At Q-Mark Manufacturing, Inc., we use only Grade 5 or Grade 10 balls (depending on ball size) for our catalog styli. Our calibration spheres are always Grade 5. Grade 3 balls are rare, and very expensive!
For some special probes, especially larger diameters (>15 mm), we use Grade 25, but we always get the customer’s OK first. And for odd-size balls, Grade 25 is often the only grade available. Please review the ball grades chart below for more information.