Making the Diagonal
By the testing procedure just described, we find the exact condition of the surfaces of three pieces of plate glass. Let us keep the best (A in the above case) for testing, and undertake to correct one of the others, say piece C. Other pieces
of the same thickness should be cut up so that they can be fitted on the flat side of the glass tool on which the mirror was ground, as in Fig. 54. Or, if the preferably smaller sized diagonal, 1 3/16" x 1 5/8" (2" long if it is to be made elliptical), is going to be used, four or five pieces of this size can be grouped at the center and surrounded with "filler"' pieces. By this arrangement, upon completion of the work, you will have the choice of the best of several pieces.
As will be seen from Fig. 59, to avoid needless obstruction of light, one edge of !he diagonal should be ground off at a 45° angle. This is done by holding the diagonal between thumb and fingers, at a 45° angle to the piece of scrap glass on which the grinding is to be done. The bulk of glass can be removed with No. 80 carbo, terminating with No. 220. The other edges can be ground squarely and neatly, also
with No. 220, and a very small bevel ground on each corner with No. 400.
This last operation may best be deferred until polishing and figuring are complete, but then great care must be taken to keep the fingers from contact with the polished surface, or scratches are almost sure to develop. If the arrangement of making several diagonals in the same block is followed, then the grinding of the edges will have to be postponed until final selection of the piece for the diagonal is made. If the diagonal is to be made elliptical, the operation described in this paragraph is replaced by that outlined later in this chapter under the section on An Elliptical Flat.
The back of the tool on which the mirror was ground often consists of waves and ripples; this also applies to any piece of plate glass, and unless it is first corrected it will not make a satisfactory foundation on which to cement the diagonal or diagonals. In fact, both surfaces must be made to match each other quite exactly, and it is with this aim that we now proceed.
The second 6" disk mentioned among the additional materials needed for the diagonal will be required for the diagonal polishing lap, so by grinding one surface of it and the back of the mirror tool together with No. 400 carbo, the back of the tool can be reasonably well corrected. In order that the tool will not rock on its convex surface when inverted, it should be balanced in a large hole 4½' in diameter, cut in a thick piece of cardboard.
making the diagonal part2
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