Measuring Internal Spherical Diameters

Bluing Master

Applying Prussian Bluing to the surface of a precision ball and rubbing it into a spherical cavity is a widely used practice. Prussian Bluing is a very vivid blue pigment suspended in an oil base. It may be purchased from many tool supply companies. By using several balls of varying sizes, a fairly accurate assessment of the spherical cup may be made. The accuracy of this technique is closely associated with the experience and personal ability of the inspector.

For high quality results, the bluing must be applied in a very thin even layer. The use of a satin finished, rather than a highly polished ball will greatly facilitate the process. To aid in manipulating the ball for the bluing process, it is usually equipped with a handle. This will aid both in applying the bluing and in contacting the ball with the test part.

There are a few limitations that apply to this technique. For very finely finished surfaces, rubbing the ball in the cavity will seriously scratch the surface. Constantly rubbing the test ball in the spherical cavity will quickly wear the ball undersize. The accuracy of this process is limited by the thickness of the indicating coating of bluing. The ball has a tendency to wobble around in an oversize cavity, thereby applying some bluing in the oversize area, thus making the cavity look smaller than its true diameter.

Ball Tangency Method

The original space shuttle engine blew up on its test stand in Mississippi. This failure was traced to the cold welding of a spherical joint used to prevent the bellows from opening like an accordion. NASA approached us to devise a foolproof method of measuring these spherical diameters eight inches down inside the bellows. Measuring the male sphere was easy. We built a simple spherometer, consisting of a ring with an electronic probe in the center. We set the gage to zero with a master ball and then compared the male sphere. Measuring the concave spherical cup was a little more complicated. If a ball diameter is smaller than the spherical diameter of a concave cup, that ball will sit tangent in the very bottom of that cup.

First we set a test gage to read zero on top of this small diameter master ball. Then we begin substituting a series of slightly larger balls. With each of these balls the gage will read progressively larger in the same increments as the increases in the ball diameter until a point is reached where the ball diameter is ever so slightly larger than the spherical diameter of the cup. At this point the reading of the gage will make a sudden dramatic jump up as the ball is now sitting up on the very top rim of the spherical cup.

This same exact procedure can be used to check the absolute diameter of most spherical cups. It should be recognized that the sensitivity of this procedure is strongly influenced by the relative depth of the spherical cavity. A full hemisphere will greatly magnify the jump in the reading, while smaller percentages of spherical depth will be progressively less sensitive.

Keeping the master balls in order during this inspection procedure is a constant problem. By fixing a very small diameter piano wire to the ball, a tag can be applied to the other end. This tag is also an aid in removing the ball from the spherical socket when you get close to size.