The C.M.M. Evaluation Primer

THE BALL BAR

1.0 A Ball Bar consists of two very round spheres of exactly the same diameter securely attached to the opposite ends of a rather long rigid bar (see Fig. #1.).

The center to center distance between the two spheres is absolutely constant. This means that no matter where the Ball Bar is positioned within the Coordinate Measuring Machine envelope it should measure exactly the same length. In reality what is measured is the sphere center to sphere center dimension. This distance should measure exactly the same because the intersphere dimension of a Ball Bar is the length of an infinitely small center of one sphere to that of the other. For this reason, there are absolutely no chances for any of the alignment or cosine errors that are so common to all flat parallel artifacts such as gage blocks, end standards and step gages.

Standard Ball Bar
Figure # 1.

INSTALLING THE BALL BAR

1.1 Most Ball Bars are rather long so they are very sensitive to the heat transmitted through hand contact by the calibration technician. Any hand contact should be limited to a very minimum and the technician should wear gloves to slow down heat transfer. After any hand contact, a thirty minute soaking period should be allowed for the Ball Bar to achieve thermal equilibrium with the C.M.M. environment, before any serious measurements are made. In the early days, elaborate multilayered insulation of the bar was provided. We have since found that this insulation so disturbs the thermal equilibrium pattern of the Ball Bar that it actually defeats its purpose. We now supply the insulation on special order only.

The Ball Bars and their positioning device should be kept stored in the temperature controlled C.M.M. room; or they should be moved into the room the night before their anticipated use.

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