Making the Diagonal 2
For this grinding, two charges of carbo should be used, with a one-quarter diameter stroke, alternating the positions of the disks frequently in order to preserve, as nearly as possible, uniform flatness. Next, the several pieces of glass that are to be mounted on the tool (see Fig. 54) should be separately ground, very briefly, against its flat surface with No. 400 carbo.
This will bring the surfaces into close harmony, the intention being to eliminate any
danger of warping or deformation resulting from the cementing together of ill-fitting surfaces.
Now lay the tool on a board and place it in an open oven, or near a source of radiant heat. Heat slowly and uniformly until the glass is hot enough to melt the paraffin (about 150° F.). The diagonal and the other pieces should also be heated, but not to a degree that they cannot be handled.
Remove the tool from the oven, and wipe its flat surface with a stick of paraffin (or brush on melted paraffin), leaving it coated with just a very thin film. Lay the diagonal, ground side down, in the center, and squeeze it down firmly and evenly. Then place each of the other pieces in position, pressing them down also, so that only a very thin film of paraffin binds them.
A space of about 1/16" should be left between edges. If, when the assembly is ready for polishing, any abrasive grains lie at the bottom of these spaces and cannot be dislodged, a few drops of shellac will seal them in. Some workers prefer to fill in this space with paraffin or beeswax, but in doing so there is danger of disturbing the seating of the diagonal. If a filler is used, it should then be recessed with the point of a knife, so that it cannot collect abrasive which may later become dislodged and cause scratches. The grooves should be thoroughly scrubbed out when changing grades, and the filler further recessed before polishing.
The over-all surface of the cemented pieces will hardly be found to be in the same plane, but the differences can be corrected by grinding, when cool, against the back or yet unused surface of the second disk. Use No. 400 carbo, a short stroke, and invert positions for each charge in order to forestall the natural tendency toward curvature. By doing the leveling off on the back surface of the second disk, we avoid "grooving" the surface already made nominally flat in the initial operation. To avoid confusion in identity, the first surface should be marked in some way.
The Lap
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