Tiny Ball Sphericity Measurement

To begin with, we live in the real world! Measuring the spherical geometry of really small balls is a major problem. If you are making millions of one size ball, you can afford Swiss watch like tooling that can cost tens of thousands of dollars for each individual ball size. When you are working with a wide variety of small diameter balls, in a lot of different materials, you have to develop a practical method, for measuring, small ball sphericity. In order to do this, we take a two pronged approach. The first challenge is, how do you physically hold a very small ball while you make the sphericity measurement? You can’t use a vacuum because the holding force is far too low. Magnetism is not practical, and besides many small diameter balls, such as tungsten carbide, tantalum, titanium, aluminum, and brass are all non magnetic.

Physical force as a clamping method is impractical partly due to the bulk of the parts involved. Fixturing by using physical clamping force ends up being restricted to elaborate tooling that is limited to a single ball diameter. What ends up being the only practical solution is to glue the test ball into a socket in the end of a very small diameter tube. Even the small measuring force exerted on the ball by the measuring probe of the roundness measuring machine will bend ordinary small diameter tubing. Tungsten Carbide ( T.C.) tubing that is four times as stiff as stainless steel, should be used. This T.C. tube is in turn held vertical in a standard drill chuck, mounted on a heavy base. Developing a test probe that will work on these very small parts is the second part of the problem. Now that we have the ball rigidly mounted and exposed to probing by the measuring machine, what does the actual probe look like. A sharp pointed probe that is at right angles to the axis of the ball, would seem to be the logical approach, but the accuracy requirements for such a probe and the problems of mechanically aligning this probe with the ball make it totally impractical.

What can be used is a very small diameter, tungsten carbide cylinder that is mounted vertical to the rotating axis of the measuring machine. The outside diameter of this very small cylindrical diameter is precision lapped to provide an excellent surface texture, to provide non destructive contact with the test ball. When the outside diameter of this tiny cylinder is brought into tangent contact with the equator of the very small diameter ball, mounted on the end of the T.C. tube, that is rigidly held vertical in the drill chuck, and rotated around it, a normal roundness measurement can be made. The cylindrical tungsten carbide tool is held vertical by an instrument quality, hand tightened drill chuck that is mounted on a heavy duty four inch (4.00”) [101.6mm] diameter platform.

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