How the Curve is Obtained

How the Curve Is Obtained. In order to produce a concave curve on the mirror blank, it is ground face down on top of another glass disk of the same diameter, called the tool. By using diametric strokes of a length that will bring the edge of the

mirror almost to the center of the tool (approximately the full-diameter stroke), then, at the end of the stroke (Fig. 22, top), the pressure per unit area of surface, and hence the abrasion, is greatly increased at the edge of the tool and the center of the mirror.

Thus, a greater amount of grinding takes place in these regions, with the result
that the upper disk becomes concave, and the lower one convex. It is interesting to note that gravity alone would bring about this condition without the application of pressure, but at a much slower rate. On account of the long stroke used (a shorter one would also work, but the action would be slower) the surfaces are not in contact, and are not spherical. The reason for this is that the edge zones of the tool comprise an area that is approximately three times greater than that of the center zones of the mirror, so there is a proportionately greater removal of glass from the center of the mirror.

They "fit" together somewhat as shown in Fig. 23, which is exaggerated to show the hyperboloidal figure of the mirror, and the oblate spheroidal figure -of the tool. After the curve has reached the proper depth, the surfaces are then brought into contact and made spherical by wearing back the edges of the mirror. :

This is accomplished by means of a short stroke, in the early use of which the radius lengthens slightly. The reason for this is apparent from a study of the diagram. It does not matter that the pyrex mirror blank and the plateglass tool are of unequal hardness, and of different expansion coefficients. A more-or-less uniform "fit" of surface is maintained, depending on the stroke used, the plate glass merely being worn away at a more rapid rate. And for all practical purposes, the expansion difference is insignificant.

The foregoing is an old and reliable method of obtaining the curve, but requires that an unnecessary amount of glass be removed. The worker should grind his mirror by the quicker, more conventional method about to be described.

We go to work