Making Contact
Before polishing can begin, the newly made lap must be immersed in hot water for three or four minutes (five minutes or more for the channeled lap) in order that the pitch may be slightly softened. In the meantime, the barrel top should be fitted with
three wooden cleats (Fig. 27) ; and a clean sheet of paper, with cutouts to fit over the cleats, might be placed on top. Put a spoonful or two of rouge or other polishing agent in a clean jar, add water and stir.
Use a small paint brush to apply the mixture (a creamy application for the initial charge) to the face of the mirror. (Watch out for loose bristles.) Remove the lap from the hot water and wedge it firmly between the cleats. Immediately place the charged mirror on top and work it back and forth with pressure for a few seconds in order to embed the polishing compound in the waxed surface of the lap.
Then, with the mirror centered on the lap, place a weight of about 20 pounds on top of it, and allow it to press for about 15 minutes, or until all of the heat has gone out of the tool. This process is known as hot-pressing, and should precede each day's polishing operation, the purpose being to bring the surfaces of lap and mirror into contact at every point.
Incongruous results in figuring sometimes occur, traceable to a temporary deformation or bending of the mirror induced by unequal loading during the pressing period. The deformation is impressed into the lap, and therein lies a source of unforeseen trouble. To avoid this and insure equal distribution of the weights, they should rest on a 6" square or round board which has three pegs, such as rubber-head tacks, driven into it 120° apart in a centrally inscribed circle of 21/8" radius. The pegs, in turn, rest on the mirror.
In putting the lap away for the night, the free polishing compound should be flushed off, and a cover placed over the lap. Nothing, not even the finger tips (save perhaps a clean sheet of paper intended as a cover), should be allowed to come into contact with the surface of the lap.
Next- Chapter IV Polishing, Testing, Correcting
Theory of Polishing |
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