Ball Grades

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ISO 3290 Standards govern the dimensional characteristics of metallic balls. The ABMA (Formerly AFBMA) now uses ISO 3290. MBA classifies balls under the ISO 3290 standard when possible. Some ball types are still made to the old AFBMA standards, and the relevant standard will be specified in our listings.

There are many factors involved in ball grades, but for most purposes the ovality tolerance should suffice. The ovality tolerance and the surface finish are the main factors determined by ball grade, with further specifications of the diametrical tolerance within a ball lot.

The diametrical tolerance variation only applies to balls supplied as part of a ball lot. For general supply, a much larger variation is possible. The table lists the acceptable diametrical tolerance variation for general supply, and also the tolerance for special orders in ball lots. Orders for ball-lots are by special arrangement, and will be based
on the required ball gauge for the lot.

The ISO 3290 diametrical tolerance does not mean that all balls will be within the tolerance of the nominal size. The tolerance applies to the ball gauge size, not the nominal size. On manufacture, ball lots can be supplied with various ball gauges. The ball gauge may be specified as 0 which is the nominal size exactly, or it might be below or above nominal size.

The ISO 3290 diametrical tolerances will apply to all the balls within a ball lot to ensure that the balls are as close to identical as possible. The table below compares ISO 3290 with the former AFBMA grades. It should be noted that a low grade number is higher precision, and a large grade number is lower precision. Each grade on the charts also meets the specifications of all the grades below it (for example, a Grade 10 ball can rightly be classified as Grade 40, because it exceeds all the requirements of Grade 40.

There is more available to this article. Please download the Ball Grades PDF for the complete article.

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