Examining for Pits
Hold the clean, dry mirror before a light, and with the magnifier examine the texture of the surface at the center. The pitted surface should appear uniform with no intermingling of large pits. Compare it with an area out near the edge.
There should be the same uniformity of surface appearance, indicating that grinding is going on evenly. It may be helpful to cut
up four small squares of plate glass and grind them together in this fashion: 1 and 2 with No. 400; 2 and 3 with No. 220; and 3 and 4 with No. 120. The worker is thus provided with comparison surfaces of these grades, but the same texture should not be looked for, as it should be remembered that there is a difference in the hardness of pyrex and plate glass. Prevention here is the best cure. If the worker thoroughly grinds down eight charges of each grade of abrasive, with pressure continuously applied, he need waste no time looking for .pits.
Chapter IV The Pitch Lap - Making the Lap |
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