We Go To Work

The upper edges of the tool must first be beveled all around for about 1/16", with the coarse carborundum stone. The stone can be applied vigorously to the tool, and if a few small chips are knocked off, which is unlikely, it does not matter. Later, as

the surface is ground down, causing the edge to become sharp again, the beveling should be repeated. This is important, so that during grinding chips will not break off and get between the disks, causing scratches.

Have the pail of water placed conveniently near. Shake a few drops of water on the barrel top, and place on it several sheets of newspaper, with sprinkles of water applied between them, and lay the tool in the center. The wet paper will provide enough adhesion to prevent the tool from sliding about during grinding.

Apply some No. 80 carborundum around the edge zones of the tool, as that is where the grinding will begin. Add water, and place the mirror on the tool so that its center is about ½" inside the edge. The one-third stroke is to be used, and grinding is to take place on chords of the tool, so that the center of the mirror is carried around the edge somewhat as in Fig. 24a. After taking six or eight
chordal strokes (counting as one stroke the combined back-and-forth motion), rotate the mirror, shift to a new position, and work on another chord.

The pressure should be applied over the center of the mirror (as shown in Fig. 24b), which should be sufficiently far in from the edge of the tool so that it will not teeter on the edge at the end of the stroke. If pressure were applied over the center of the tool, hollowing out would be slowed down considerably, although gravity would eventually do it for us. After two or three trips around the barrel, wash and dry the mirror and try the template. When the curve in the small central area of the mirror seems right, take the next series of strokes on chords
farther in on the tool. Spread the abrasive now over a wider zone,
move the mirror in so that its center is about 1" inside the edge of
the tool, and grind as before. Apply plenty of pressure for quickest
results.

Frequent renewal of abrasive is necessary in the early stages. as much of it is pushed over the edge without being used. This residue can be scooped up later, placed in a jar of water and stirred, and the muddied water poured off. The unused abrasive at the bottom of the jar can then be reclaimed. When the widened curve on the mirror fits the template, the mirror is moved inward another ½" or so, and ground until the extending curve again conforms to the template. If at any time the curve becomes too deep, move the mirror in considerably, and
continue with the one-third strokes.

The curve should extend to within 1/16" of the edges of the mirror when it is grinding center over center on the tool; the curve will have been achieved by the
steps shown in Fig. 25. Of course, we can hardly expect the mirror to be spherical. The concave surface will probably consist of a series of waves, with its general curve almost inevitably slightly hyperbolic, somewhat like the condition shown in Fig. 23. This must be corrected by continuing with the one-third stroke, working center over center until contact has been established. As the abrasive grains lend to rolled in hollows and depressed zones, this stage of the work can be greatly facilitated if the disks are frequently separated and the carbo on both surfaces redistributed with the finger tips.

At this stage, too, it is advantageous to prolong the time spent in grinding with each charge of No. 80 until it is thoroughly broken down. It may be observed, as the charges are ground down fine, that the mirror has a tendency to stick or grab whenever the grinding strokes bring the centers of the disks coincident. This sticking tendency occurs at the "edge zones, and is the suction effect of the central gap between mirror and tool. Repeated breaking down to a fine consistency of each charge of No. 80, together with frequent respreading of the abrasive, and use of the one-third or shorter stroke, is the quickest way to grind the edges back and bring the surfaces spherical. When contact is established, the curve should extend practically across the whole diameter of the mirror, there being enough of the original bevel present to prevent a sharp edge. A more exact determination of the radius should now be made.

Flush off the disks in the pail of water, and wash your hands as well. Deposit about 1/8 teaspoon of No. 400 carbo at the center of the tool, and puddle and spread it around the surface with the finger. Grind this down for about five minutes, stopping now and then to respread it. Then flush the mirror thoroughly and stand it, wet, on the testing rack. Using the window of the Foucault lamp, or any light source held alongside the eye, locate its reflection on the wet surface of the mirror.

Move the light from side to side. If the reflection is seen to move in the same direction, you are inside the center of curvature of the mirror; if it moves in an opposite direction, you are beyond the center of curvature. When a position
is found where the reflection just seems to appear, and from no determinate direction, as the light is moved from side to side, that is the center of curvature of the mirror's surface, and the distance from mirror to eye is to be measured exactly. One half of this distance is the focal length, which it should be possible to determine within ½" at this stage.

The charge of No. 400 enables the surface to hold the water for a longer period. Even then you may have to interrupt the test to splash more water on the mirror. If the focal length is found to be about 49", you are ready for No. 120, but if it is too long, continue with No. 80 until you bring it right. It is preferable to use the one-third stroke for this shortening, but if the radius is too long by several inches, a longer stroke will speed up the process, after which some lime must be spent in again restoring contact.

If the focal length is less than 49", invert the positions of the disks, and grind with the tool on lop, using the one-third stroke. This stroke will lengthen the radius gradually, without deforming the figure. Use of a longer stroke, in any upside-down grinding, might flatten the mirror's edge to an extent that it might refuse to polish later on, unless contact were thoroughly restored by means of a
short stroke, used with the mirror on top. About two hours of actual effort will be expended in achieving a curve of the proper depth, with the surfaces brought into contact.

While the difference of a few inches in focal length either way is of no material consequence, we have decided on a length of 48", and the worker may as well begin now to develop a skill in working to precision, and endeavor to arrive exactly at that focal length for the finished mirror.

If through prolonged grinding the edge of the mirror has become sharp, it should be carefully beveled with the fine carbo stone. In doing this, rest the mirror on the edge of the water pail, and bevel at a 45° angle, using the stone as though it were a file. Rotate the mirror slowly, and continue the filing stroke until the edge
is smooth and rounded to the touch. A coarse stone can be used on the tool; because of the wider angle of its edge there is little danger of chipping. Beveling will in all likelihood have to be repeated during the fine grinding, or whenever the edges become sharp and there is danger of scratches from chipping.