A Better Lap
The channeled pitch lap just discussed is the conventional one used by both amateur and professional mirror makers in the past. The one about to be described (Fig. 28) originated at Pennsylvania State College. Introduced into the Amateur Astronomers Association workshop at the Hayden Planetarium by the
author, it has been coming into general favor. The amount of effort involved in making the rubber mold may appear to be unreasonably out of proportion to the result, but experience has demonstrated that the time spent will pay dividends.
Contact is easily made and kept; the lap polishes faster than the channeled lap; and it responds more agreeably to the figuring strokes. The reservoir of water and polishing agent surrounding the facets tends to equalize temperatures and retard evaporation, with the result that turned-down edge rarely occurs. Because of the considerably smaller areas of the individual facets, a somewhat harder pitch than that used for the conventional lap is employed. Less than a third of the usual amount of pitch is needed, and the rubber mold will make innumerable laps.
Sheet rubber, about 7" square and 1/16" thick, will do for the mold, the pattern for which is shown in Fig. 29. A saddler's punch, of ⅜" to 7/16" diameter, or a No. 4 cork borer, is a good size for making the holes. The radius of the horizontal arcs shown is 24", but straight lines, rather than curved, might just as well be used. The separations given are for ⅜" holes, and should be suitably varied for a punch of different size. The pattern is first laid out on a sheet of thin paper, and a 6" circle then drawn, encompassing as many whole small circles as possible. The pattern is then pasted to the rubber mat with mucilage. When dry, the mat is laid on a thick magazine and the holes are punched out. To cut out the partial holes on the periphery of the 6" circle, lay the mat over the edge of a board, with the periphery of the circle and the board's edge coincident.
a better lap part 2
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