An Elliptical Flat

With a not excessive amount of labor, a rectangular diagonal can be converted into one of elliptical shape. A simple diagram will show that the length of the rectangular
diagonal as prescribed by formula will have to be increased by the thickness of the plate glass used. Therefore, the 1 3/16" x 1⅝" diagonal made of ⅜" glass should be made from a rectangular piece 2" long, as already mentioned.

A stick of hardwood about a foot in length should be turned up to a diameter equal to the minor axis of the diagonal, and given two or three coats of shellac. One end is cut off at a 45° angle, and the figured surface of the diagonal cemented to it
with hard pitch, thus giving it protection in the grinding to follow. In order to make a good bond, both the end of the stick and the diagonal should be preheated before applying the hot pitch.

The corners are ground all around on the scrap piece of plate, using No. 80 for the bulk of the work, and finishing with No. 120. Hold the stick horizontally, and work all over the plate, rotating the stick the while. Five to 10 pounds of pressure can be applied at the diagonal end of the stick. One to two hours will suffice.

To free the diagonal, insert the edge of a razor blade into the pitch binder and .strike it a light blow; any pitch adhering to the glass can be dissolved in turpentine. The result is a neat- looking elliptical diagonal, offering a minimum of obstruction to
the light. With thick glass, it is most unlikely that its figure will be in any way impaired through removal of the corners.

The above grinding can be done in a lathe as well, holding the stick in the chuck, running at slow speed, and using a sheet of brass or band iron held against the edges of the rotating diagonal, with carbo and kerosene or water fed into it. Particular care must be exercised to protect bearings and ways from abrasive.

There is another way of making the elliptical diagonal, available to the possessor of a drill press. A 6" glass disk, about %" thick, and a second one of equal diameter, but at least ¾" thick, are required; the tool on which the mirror was ground will also be used. The surfaces of the ⅜" thick disk, and the one to which it is to be cemented after perforation, will no doubt be at considerable variance with each other, so they should be first ground together, using two charges of No. 400 carbo, working the second one down well. During this operation, the positions of the disks should be alternated several times. This will bring the two surfaces quite flat and into close harmony.

Fig. 56. Cutting an elliptical diagonal from
a glass disk. Stand the ⅜" thick disk on edge diagonally (45° angle) inside
a rectangular box 4¾" or 5" to the side, and completely surround it with plaster of Paris, filling the box. When it is thoroughly dried out, which may take several days, tack a cover on the exposed side, and remove one of the adjacent sides. With a suitable length of thin-walled brass or copper tubing of correct inside diameter, mounted and held in the drill-press chuck, running at slow speed, and using No. 120 carbo and kerosene or turpentine, bore through the plaster and glass (Fig. 56).

Boring will proceed more rapidly if several V-shaped nicks are made in the edge of the tube with a three-cornered file. When the boring is completed, the box can be opened up and the plaster broken loose.

After perforation, the surface fit between the tool and the diagonal with its surrounding glass disk will have been upset by the relieving of strains in the glass. The surfaces should accordingly again be ground together, separately of course, but this time with No. 600, as the differences will be slight. The perforated disk and elliptical diagonal are now cemented with paraffin to the flat back of the "mirror-tool," in the manner previously described. For surfacing, fine grinding and so on, proceed by methods already described.

Chapter 8 Tube Parts - Alignment- The Finder