Fine Grinding
About six to 10 charges of each grade of carborundum, depending on the pressure used, should be thoroughly ground down, but for the beginner it is recommended that not less than eight charges be used. About five minutes is enough for each charge,
but if one is unable to maintain heavy pressure a charge should be worked for a longer period.
The purpose of each successively finer grade is to remove the pits left by the preceding grade, and to be assured of this not less than 40 minutes should be spent on each. Before leaving No. 120, work the last charge down well, then follow with a charge of No. 400 (worked with the mirror on top), and again test for focal length.
This time we have a surface which, when wet, will be capable of producing a fairly sharp image of the window of the Foucault lamp. Use a piece of ground glass, held before the eye, to pick up the reflection from the mirror, and adjust the lamp until it and the sharply focused image of the window are equally distant from the mirror. Half this distance is the exact focal length, which should be about 49" at this stage.
If it is found that a slight correction must be made, of an inch or two, it should be done on this grade. Any greater correction should be
made with No. 80. It may be more convenient to use a piece of
bright tin, rather than the ground glass, in picking up the reflection,
as the worker may then stand closer to the mirror and can more
quickly reach and adjust the rack. The eye has to be placed in back
of the ground glass, but a more critical determination of sharpest
focus can be made with it.
fine grinding part 3 |
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