Measuring the absolute diameter of plastic balls

Measuring the absolute diameter of plastic balls presents a couple of rather testy problems. Unlike the very fine surface finish produced on precision steel balls, the surface of plastic balls have a tuff or even a nap on the surface. At high magnification the surface of most ground plastic materials looks a lot like the bristles of a toothbrush.

Where does the surface texture end and the true body of the ball begin? In order to standardize the measuring procedure, the diameter of plastic balls is determined by measuring them between flat parallel anvils under 18 grams of force, and compensating for the Hertzian elastic deformation. Using an optical comparator for evaluation is also satisfactory as long as you understand the limitations. On translucent plastics there will be diffraction or halo of the optical beam at the surface. This will tend to make the ball measure slightly larger than a physical contact measurement. The fuzz or knapp on the surface can exacerbate this problem.

When using optical measuring C.M.M.s or edge finding devices, a different problem arises. These devices usually analyze five pixels of an array. Depending on the algorithm used, the surface texture will look like one half of a sine wave to the system. When it averages both sides of the ball, this can add considerably to the apparent diameter of the ball. When very critical applications arise, we suggest the use of a master ball that would be compared on a plus or minus basis to the production parts. It must be remembered that these materials are "plastic", so their very nature limits the immediate and long term accuracy of parts made from it.

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