Chapter 3 Mirror Grinding

It is assumed that a cellar or other room in which the temperature is fairly constant is available. Grinding, even polishing, might be done almost anywhere, but testing and figuring can be carried on only under conditions of uniform temperature.

Strokes. There are three motions that the optician must employ in order to preserve a surface of revolution on his mirror.

First, the back-and-forth grinding stroke produces the curve.

Second, the mirror must be rotated in order to produce this curve on all diameters.

Third, the worker must walk around the barrel (or the tool must be rotated) in order to employ all diameters of the tool. While the directions of these last two motions are not important, they must not occur in unison, and for this reason it is perhaps safest if they are counter to each other.

It is not necessary that any of the motions be performed with machine-like regularity, nor is it necessary to run oneself dizzy in their execution. Take six or eight strokes, rotate the mirror a trifle and shift to a new position where another six or eight strokes are taken, and so on.

The lengths of the strokes at the disposal of the mirror maker are described with reference to the diameter of the mirror or tool. A full-diameter stroke is one in which the mirror travels a length of 6"; its center passes completely across the tool (Fig. 22). A one-half diameter stroke carries the mirror's edges halfway in to the center of the tool, resulting in a travel of 3".

A one-third stroke carries the mirror's edges 1" past the edges of the tool. In using these strokes, the center of the mirror need not necessarily pass over diameters of the tool; much of the time, especially in the early grinding, and finally, in polishing to figure, chordal strokes are taken. But the length of the stroke —- one-third or one-half, or some other length — is constant, whether performed diametrically, or over chords of the tool.

What an abrasive does