Reversible Roundness Machine Calibration Sphere

Reversal technology is an important tool in the dimensional metrology arsenal. If the geometry of an artifact is measured, and then the artifact is reversed and measured again, the errors in the part can be mathematically separated from the errors in the measuring machine.

The artifact of choice for evaluating the performance of axis of rotation devices is a very precise sphere. The master sphere is usually attached rigidly to a rugged base plate. This facilitates the holding and centering of the sphere on the rotation axis of the measuring machine. The roundness of the master sphere is measured. The base plate is rotated and the roundness is measured again. Any errors in the sphere rotate with it, and any errors in the spindle remain in the original location. This approach works very well for applications of ordinary quality.

When the base plate is rotated, the original reference axis of the sphere is lost, so that there is a small loss in the accuracy of the second set of data collected. This very slight increase in uncertainty is acceptable in all but the most exacting applications.

If the Master Sphere could be rotated on its own axis instead of rotating the base plate, even this small problem would vanish and state of the art evaluations could be made. In order to achieve this ultimate accuracy, the master sphere is mounted in a three ball kinematic coupling so that it can be rotated to achieve perfect reversal. There is a rather intricate preloading and stop system built into the base plate. In order to avoid any heat transfer from the calibration technician to the master ball, there is a thin plastic sprocket mounted just above the three ball kinematic coupling that is used to rotate the sphere. In addition to the temperature isolation, this also keeps contamination form being transferred to the surface of the master sphere from the hands of the technician.

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